1 2 3
Opti Dork
3/14/21 3:17 p.m.

Ive owned multiple generations of Miata and now a GMT400 hence the title.


Backstory: I have an 03 Dodge Ram with 360k miles. It has been the most reliable truck Ive ever owned. Unfortunately it is a 2WD and a 4.7, and its getting difficult to find parts that arent your run of the mill service items. I needed to recover the drivers seat last year and according to my upholstery guy and the local dodge dealer, I got the last OEM seat cover in the nation. I think no big deal Im working on a 17 year old truck thats just the way it goes. A few weeks later Im working on my dads GMT400 Suburban, I go to removed the headlamp switch and the bracket that holds it has been broke and glued back together, I think "Damn Ill never find one of these for a 27 year old truck." I do a little research just to see if its out there and the local Zone has an ACDelco in stock for like 7 dollars. That got the wheels turning. I thought Ill always be able to get parts for a GM truck with places like LMC, GMs support and all the people modifying all the different generations. So last year I started look for a GM truck. Originally it started as a TBI, Vortec or LS, just buy the nicest one I could afford. I realized the GMT400s have been going up in value the last few years and I was going to be hard for me to swallow paying the premium for a TBI truck which would immediately needed to be modded to not be a slug and do what I want it to do. Then I was looking at vortecs and LSs. I had a 96 Vortec 5.0 Z71 in high school and Ive regretted selling it ever since, so I decided to just look for a nice vortec 5.7 truck. That brings up to about 6 months ago. I was looking almost everyday I missed probably 7 or 8 of them, with a couple being sold right out from under me. It got to the point where if a clean one would pop up, I would just message the seller "I have cash in hand, I will pay full asking, when would you like to meet." I remember not being able to give mine away 10 years ago, and now there were selling within hours of being posted. Tuesday i had arranged to pick one up for 5K. Had 182K miles and was spotless. I showed up at the arranged time and he sold it to someone else. I kept looking and was seeing clean ones in DFW on car lots for 13-15K for a ECSB Z71. So i decided they are too hot here and needed to look elsewhere and up my budget a little. I checked all of Texas, Oklahoma, Arkansas, Louisiana, and New Mexico. I found one Weds in AK, for 7K. It looked really clean. Ad said it was clean, but had clear peeling on hood and roof. Its 90s GM white paint so no surprise. We make arrangements and he agrees if he has it Fri night he will hold it until I can get there Saturday.


So we head out 7 am Saturday, get to Texarkana about 12 and check the truck out. Im expecting terrible steering and brakes, you know the GMT400 trademark, oil leaks, who's seen a gen 1 SBC that doesn't leak oil, and a myriad of other GMT400 common problems. The truck actually looks super clean. The clear on the hood is worse than expected, but the color hasn't started coming up so at least its still all white and doesn't have the speckled look most of the white ones do. Its pretty dry under the car which is surprising. Pretty much everything works except the AC is only cool. I jack it up to shake the front end down and its not terrible. So we go for a test drive and it has the best brake pedal I've ever felt in a GMT400. Its drives and runs nice, so we go back to negotiate. It has terrible tires and he tells me it had an o2 code, which I verified to be a heater circuit code for a downstream, but he had already put a sensor in. He shows me an invoice for 3K miles ago that includes a new EGR valve, plugs, wires, cap, rotor, o2 sensor, fuel filter, and an AC charge. We agree to 6500 and we head back to DFW.

It made the trip with only the slightest hiccup. We hit Arlington, got stuck in traffic and I got 3 or 4 really hard upshifts and downshifts. No check engine light, but I check codes right quick and have a pending P1870. Which is essentially saying something in the trans is slipping. Commonly the TCC. It clears up I keep driving and make it home. I fill it up and it managed 16.5 MPG. Pretty much what I was expecting.


Its a 1998 Extended cab (3rd door) short bed K1500 Z71. It has the 5.7 L31 Vortec/4l60E/10 bolt with 3.73s. It has 138K miles.


Pictures of the truck. Guy asked if I wanted the camper, I said yes assuming its worth something, and I could sell it. Its not my favorite but its growing on me. Matches the old man vibe the truck gives off.

Came from Dale Earnhardt's dealership #doitfordale

Interior is super clean

I did a little research last night about the P1870, some guys are just rebuilding the trans, some are replacing the valve body and TCC valve with an upgraded part, and some are having luck with just a fluid change. I decide to do a pan service and flush today and start there since the truck only has 138K miles. This morning I picked up a filter kit and an O2 Sensor.


Got it in the air, and saw a new o2 sensor.

Oh man. I really dont feel like chasing the circuit and finding the problem. Then I realize thats the bank 2 downstream and the code if for bank1 downstream. Sure enough that one appears to be OE.

I put a little PB on it and it comes right out and I put a new one in.

While it was up I gave the front end a better shake down and surprisingly the only loose part is the idler arm, even the idler bracket is tight. I had already planned on a front end rebuild, so Ill probably just do the whole steering linkage anyways. Right now the plan is proforged or the mevotech TTX line for the whole linkage with solid tie rod adjuster sleeves. Eventually I plan on the jeep steering shaft (ujoints instead of rag joint), redhead gearbox, k2500 forged lower arms with press in ball joints (stock is stamped steel with riveted in joints), new upper control arms, and bilsteins shocks. On that note, surprisingly it already has billies on it. Ive seen them on a few other GMT400s but not many.

I flushed the trans. Fluid was dark but not terrible. It always looks worse than it is in the machine.

Then I drop the pan. There was some clutch material but it didnt seem excessive to me.

Inside the trans looked pretty clean


I put a a new filter and pan gasket on and filled it up.

Next I checked the brake fluid.

It looked terrible. I was going to do a full fluid change but I plan on doing the GMT800 master swap anyways and the brakes are great the way there are so I didnt want to mess with them too much. I was aggravated that there was so much grime in the reservoir that you couldnt see how much fluid you had with the cap on. So I sucked out the reservoir and wiped the inside down and refilled it. No after picture because the fluid was immediately black again. I can see throught the reservoir now, so thats nice.

A/C system had about 9 oz, should have 32, no obvious leaks, so I let it sit under vacuum for a while and put some dye in it and charged it. Its working great now. Ill check for dye when it quits working. Im sure its leaking somewhere.

After that I went for a drive and on the first test drive the pending P1870 went away so maybe I lucked out. We will see.

Immediate plans are to get it inspected and transfer the title, then new tires. Ive always advocated for the KO2s, I think a set will run me about a thousand, but through some friends I can get a brother in law discount on the Bstone Revos or Fstone Destination AT2s and save three to four hundred dollars.. I like that they are less aggressive and I dont. I have to realize this is supposed to be a capable DD. I dont need the most aggressive AT, I need it to drive nice and quiet, but personal experience is The KO2 is very well mannered for how aggressive and capable it is. So Im torn. Replace all the steering linkage with uprgaded parts, and in the near future is an MPFI conversion. It has a slight rough idle, nothing crazy, but Ive seen a spider fail  and dump fuel and take out a main, so I dont want to leave it too long.

New York Nick
New York Nick GRM+ Memberand Reader
3/14/21 3:28 p.m.

Congrats on the sweet score. That is a clean looking truck. I did the gmt800 master cylinder a few weeks ago and I was shocked at the difference. Best sub $80 upgrade I have ever done. 

edit: darn mobile picture issue. I'll replace this on my computer so it's not skewed. 

Dusterbd13-michael (Forum Supporter)
Dusterbd13-michael (Forum Supporter) MegaDork
3/14/21 4:07 p.m.

Hey now! Im no cliche! I haven't owned THAT many miatas or gmt400s.....


Oh. Crap. I have....


I did the gmt800 master in my regular cab. Have another for the extended cab, but really don't feel like messing around with the most fantastic factory gmt400 brakes ive ever had.

Good looking truck, and congrats! The 4l60e has a reputation of being a hand grenade with the pin pulled, and ive killed two now. Went 4l80e in my extended cab this last month after the 60 turned into a bucket of neutrals. 

Opti Dork
3/14/21 4:29 p.m.

In reply to Dusterbd13-michael (Forum Supporter) :

I've killed a few 4 l60es and I've had a few be absolutely trouble free for tons of miles.


The plan is if it dies soon when I'm sorting it out, it's going to a local trans shop that's really good and I'm going to tell them to build me a good one with a long warranty and I'll pay what it costs, if it dies later I really want to do an 80 swap with the trans go kit and never have to worth about it again.


What all was involved in your 80 swap?

Dusterbd13-michael (Forum Supporter)
Dusterbd13-michael (Forum Supporter) MegaDork
3/14/21 4:51 p.m.

In reply to Opti :

It was in my 95. Had to do a shorter drive shaft, new cooler lines, relocate the trans crossmember, different flexplate, change a little bit of wiring, Andre Flash the PCM for the different transmission control. I personally did it by buying a parts truck with a good transmission because it gave me everything I needed and a bunch of extras

solfly Dork
3/18/21 12:41 p.m.
Run_Away GRM+ Memberand Dork
3/18/21 1:14 p.m.

Nice truck, I have it's blue interior twin.

Azryael Reader
3/18/21 5:16 p.m.

Besides the '73-91 squarebody trucks, the GMT400 is the last best looking Chevy truck in my opinion until the GMT K2XX platform.

I've always loved this interior color, and the blue, that GM offered back then.

BigIron New Reader
3/19/21 1:48 a.m.

Being a Z71 it came with Bilsteins, so it likely has the original shocks on it yet.

Opti Dork
3/19/21 12:24 p.m.

Parts are starting to roll in. As long as my idler/pitman combo get here in time, I've got a little work to do to it tomorrow. I did manage to ready it up and get it inspected and the title transfered. The trans and o2 code have not come back.


When I was driving it around to ready it up I went over to a friends house whos a big gmt400 fan, and when I pulled up and he started checking it out he says "I smell coolant." I told him no way, it's clean underneath. About 3 seconds later I see a drop on the ground and he spots the leaking water pump. This thing has done 3k miles in the last year and I just put 1k on it in a week so I expected a few things to pop up. No biggie. I have a water pump and Tstat on the way. It likes to hover around 177 degrees so I'm thinking someone put a colder tstat in it or the one on it is getting lazy.


Just got my tires. I decided on the Firestone Destination AT2s. I really wanted the KO2s but I also wanted a lower weight P rated tire for comfort and fuel economy, and my buddy discount made the Fstones half the price of the KO2s, so that sealed the deal.



I'm thinking about putting the white letters out, to go with the old man motif. I've always done Black wall on everything, but I think it fits on these old trucks. What say GRM? 

Black or White out?


A coworker let me know GEEZER was available as a plate. It's tempting

Mr_Asa GRM+ Memberand UltraDork
3/19/21 1:32 p.m.

In reply to Opti :

White letters always out

Opti Dork
3/21/21 1:00 p.m.

I spent the day working on the old truck yesterday. I noticed my fuel trims were creeping lean on the hwy. They were better at idle and worse at higher RPM, may me look at the fuel system. The first thing I did was check fuel pressure.

And it was low. Imagine that a GM truck with a dying fuel pump. I wasnt surprised, with a full tank its louder than my old warlboro in my camaro on empty. I know regulators are common failures on these, but I figured if the regulator was leaking in the intake I would see rich fuel trims not lean. Also already has a new filter. Plan is to put a pump in, recheck it, and if its not better than MPFI conversion.

Parts. Idler and bracket,  pitman, water pump, thermostat, door lock actuator, and door ajar switch.

Old stuff is smoked.

Proforged pitman arm installed. It is obviously much better quality. Has a real rubber boot instead of the foam washer on cheap ones. Even the zerk fitting holes are nicer. New fitting threaded right in, when I get other stuff I always have to fight them from cross threading. Im excited to see longevity. Id like to not have to throw a front end at this thing every year or two, which unfortunately seems to be the norm on these now that most replacement parts seem to have gone down in quality. I forgot that these pitmans are a PITA to install. I took the whole gearbox out. Every other manufacturer says "It would be dumb to put a non removable frame crossmember under this huge and very tight nut that holds on a very fast wearing but crucial item." not GM, they are like "Screw them, make them pull the box to change the pitman, and if you can do something with the steering design to make them wear out every few months thatd be great, and call the normal parts manufacturers and tell them to only build really E36 M3ty replacement ones."

Proforged Idler and Bracket installed. Same thing with noticeable quality improvement as the Pitman

I must really love this truck. I was actually torqueing everything to spec. Normally a practiced only reserved for the most crucial engine components....and even then I mean who am I kidding, didnt happen as often as it should.


Then I put the water pump on. Easy as I remember, EXCEPT those fiber gaskets. I HATE fiber gaskets. Someone needs to make a Gen 3-esque aluminum gasket for gen 1 and 2 SBCs. There is not enough room in there to scrape them off. Its a PITA.


I also threw a Tstat in. Its not super hot here in TX right now but the truck was running at mid 170s running down the road. Id rather have it running closer to 200 for better heat and fuel economy. I put the OE 195 in it. I assumed someone throw a 180 or 160 in it, when I pulled it out it was a new motorrad with almost no discoloration but no marking to identify its rating. It was also incredibly soft. 

Next I pulled the door panel off to replace the stuck door lock actuator. GM really secured the door panel on these. Those clips are no joke. On a newer car I pull a couple screws and yank and the panel comes right off. For this thing I had to get both my trim tools and work my way around slowly. Made me appreciate how quiet this thing is on the inside.

The lock actuator was bad but I dont think it was the whole problem. I popped it off and actuated it and it would move but just barely. So I installed the new one and unhooked it worked fine but hooked up it did the same thing. Someone had greased the plastic clip the rod rides in and it was super hard and sticky. I think they are normally dry but that ship had sailed and I couldnt get everything off so I cleaned up what I could, added just a little syl-glide and its working fine now.

I noticed the courtesy lights werent working and the fuse was missing the other day. Put the fuse in and the come on and wont turn off. I figured it was a door switch, so I threw two in because the cost liek 4 dollars.

Well I was wrong and it didnt fix it, now I actually have to diagnos it. UGH


I also threw some nice wipers on


Finally I put the tires on it and aligned it. Well tried to align it. The eccenetrics are frozen to the bushing sleeves. I set the toe. Camber is all messed up and caster is way to low, just means I have to order control arms sooner rather than later.


They were bigger than I expected. No rubbing yet, but who knows once I fix camber and caster. White letters out won the poll by one vote, with exactly one vote by Mr_Asa, and I think it was the right choice for sure.


Truck looks great, and drives so good. Super smooth and quite and doesnt pull even with a jacked up alignment. Steering is way tighter with just the smallest dead spot. My buddy drove it and said it has the same sized dead spot as his 2016 Chevy 2500. Braking and traction are greatly improved. I also changed the tire size in the computer and the speedo is now correct. Was about 10 percent off when I picked it up for some unknown reason.

No coolant leaking and operating temp went up to high 180s, which I think is pretty good for a cool day like yesterday. We will see how she does during the super hot summer.

I also managed to break one door clip which now rattles and I can not deal with, it was so solid before, so new clips are being order with new door handle bezels (since I have to take them back off)

I broke my perfect glove box. One of the hooks broke when I was trying to reinstall it. I am going to try some plastic weld first, if not new glove box.


I am loving this truck. Off to order more parts for next weekend.

Run_Away GRM+ Memberand Dork
3/21/21 1:42 p.m.

I had some dome light/door switch issues on mine that I ended up wasting a bunch of time on because I didn't realize there are two different lengths. Make sure the replacements you put in are the correct ones.

solfly Dork
3/23/21 7:10 a.m.
Run_Away said:

I had some dome light/door switch issues on mine that I ended up wasting a bunch of time on because I didn't realize there are two different lengths. Make sure the replacements you put in are the correct ones.


Floating Doc (Forum Supporter)
Floating Doc (Forum Supporter) GRM+ Memberand UberDork
3/23/21 8:10 a.m.

What did you pay for the door lock actuators? 

Opti Dork
3/24/21 7:36 a.m.

In reply to Floating Doc (Forum Supporter) :

I think they were like 32 dollars. Just got them at the local parts house. Probably cheaper online

Opti Dork
3/25/21 8:40 a.m.

I found a quick test procedure online for the courtesy lights. I followed it, and it told me to change the headlight switch. I had a few minutes at work so I popped it out.

I didnt even have to completely remove the trim panel just popped it out, pushed the switch out and popped a new one in.


AND......It didnt fix it. I say Im going to review the wiring diagram and actually chase it down, but Im pretty sure Ill just end up crossing my fingers and throwing an interior lighting module at it. I really hate chasing electrical stuff.

The grain on the switch is slightly different, which i like, but the backlights are green and the rest of the dash is yellow. I dont know if the original had burned out, or were yellow and I just never noticed, but Im not a huge fan of that.

Ive got a fuel pump, control arms, and door handle bezels for tomorrow.

Mr. Lee
Mr. Lee GRM+ Memberand PowerDork
3/25/21 7:00 p.m.

Sadly I've got a GMT 800 2500 6.0 4l80 excab short bed I would swap for that in a heartbeat. 

Opti Dork
3/27/21 11:21 p.m.

I ordered some parts this week. I planned on putting them all on today, but I had a friend call in a favor and I had to go hang drywall this morning. I did end up working on the truck, but I got started late and didnt get everything done.

The fuel pump has been SCREAMING, and the long term fuel trims have been hanging around 10-12 when driving, and as shown earlier the fuel pressure was a little below spec. So I ordered a pump, most manufacturers make you swap to a different plug style which I didnt want to do. the fuel gauge was actually accurate so I wanted to do just the pump and leave the sending unit. They seem to be available up to 97 but I couldnt find any for a 98. So I went with a whole module from Bosch. They have the OE connector, and they say they updated the pump to a turbine style which is quieter and more reliable. I dont know if thats a real thing and benefit or just marketing bs.

I got the old pump out

The tank looked pretty clean inside

New pump in


I also ordered some upper control arms. Proforged doesnt make them, and mevotech doesnt make them in the TTX line, so just some normal OE style replacements from the local parts house.

Got them installed.

On the second one I realized the eccentric bolts came out way too easy, and the reason I was replacing these was I thought the bolts were frozen to the bushing sleeve, and wanted to recheck to make sure the knockouts had been removed. When I aligned it the other day I slid back an eccentric and it had the slotted hole. So i checked again, and Im dumb. The knockouts on the inner holes are still there. Thats why I couldnt adjust camber and caster last week. So I took the arms back off to remove the knockouts.


All better now. 3 of them came out real easy like normal. 1 of them ate my lunch. Good news is I can actually adjust camber and caster.

Old control arms. The boots were torn, the ball joints didnt have play but they were definitely worn after 23 years, plus the bushings were starting to crack, and the new ones are all clean and pretty so Im happy.

Then I aligned the truck.

I only had to adjust the driver's side. Ideally Id like a little less positive camber, and more caster with a slightly bigger split, but it was getting late and I was tired, and I know Im gonna end up doing more front end work in the next few weeks, so once its all done, Ill really dial it in. For now it seems to drive nice and straight, and no tire rubbing so far.

Fuel pressure was at 60 and went up when I go WOT, so thats an improvement. Truck does seem to idle better, still isnt perfectly smooth. Hopefully the MPFI conversion helps. Truck does seem to have picked up some lost hp.

After that I called it a night. Initial impressions are steering feels slightly better, and truck feels a little more solid over bumps, Im assuming from the new bushings.

I checked fuel trims on the way home. They came down but arent perfect.

Ive got a few more small things to do tomorrow. Today I noticed Ive got a P/S leak but I forgot to check and see what it was.


Opti Dork
3/28/21 4:45 p.m.

I got a few small trim things done.


I went ahead and ordered an interior lighting module. It was like 27 dollars from GM. The courtesy lights are supposed to stay on for like 20 seconds after you close the door. There is normally a little click and then they go out. I noticed I never heard the click so I was thinking the module was bad.

Its up under the dash but nothing to bad. I also popped open the old one to look for damage. I didnt see any, but I already had it so I threw it in.

I get it back together and close the door and wait about 20 seconds and I hear the click......but the lights stay on.

I was going to give up and pull the fuse back out, but when it also powers the mirrors and its a real pain to see at night when im grabbing my stuff.

A bunch of people have problems with the dome light so I thought Id pop it out real quick.

When I popped of the lense i could barely see something behind the housing. Looked like scotch locks.

One of the OE wires was a little close to the other, probably because there isnt much room and when you jam 4 wires and 2 scotch locks in a spot made for 2 wires everything get shifted around, and it was a little dark like it was burned. I pulled the scotch locks off, put the wires were they go and wrapped everything in electrical tape and checked it. 20 seconds later I hear a click and the lights turn out. SUCCESS.

Then I put my new door handle bezel on the drivers side.



I also stuck a new bulb in the door panel courtesy light.

Then I popped the passenger side door panel off to address the rattle I had created when I did the door lock actuator.

The culprit

I dont know how one broken clip made so much noise but before it was nice and quiet. I pull the panel to change the actuator, break one clip, and then over bumps the door panel rattles. I ordered some on ebay and replaced all the clips and the female pieces.

Got it back together, and put the new bezel on. I didnt notice til after the door handle piece is also faded so I need to order one of those.

Opti Dork
4/11/21 11:21 a.m.

This thing has terrible headlights, so I ordered some parts.


Here is the problem

1 bad hotspot and you cant even see if the left light is doing anything, and almost no cutoff. I know they are halogen bulbs in a normal reflector housing but I think they could be better. I know I need to clean up the leaves, Im doing that after I type this out.

I think this is part of the problem. The lenses are super foggy, which is probably why the left light is so bleh. and has no discernible pattern or hotspot

This is another part of the problem. The power the bulbs runs through the headlight switch, which leads to melting switches and common failures, and voltage drop. This is power at the bulb with engine running. The guys on the candle power forums, say a small drop in voltage leads to a large drop in light output.

I got a headlight harness which draws power directly from the battery, so I should see much closer to alternator voltage at the bulb

Im not sure what language the top directions are in, but I definitely cant read it. Mmmm quality

This is the harness if someone wants to build one. I doesnt seem too complicated, but for 27 dollars I let LMC have my money

I also got some new headlights. Just went with TYCs stock replacements. They were like 35  dollars each. I also modified the locking tabs on some 9012s to replace the low beams. The 9006 should put out like 870 lumens. I was obviously seeing less becuase of the lenses and lower voltage. I think the 9012s are like 1700 lumens, so combined with the better lenses and voltage I was hoping for a big improvement,

Back together. It wasnt as dark as the before picture, but there was an obvious improvement.


I wanted to let it get darker and drive around but i had to leave, and my buddy just bought a Telluride, so we drove it around and I got to check it out. Those things are sweet, I definitelty wouldnt mind having one. I just have to get SWMBO to replace her volt with one, which is easier said than done she loves that thing.


Overall Im very impressed. Its exactlly what I wanted. I originally looked into a projector retrofit, but the cheap ones apparently arent very good and the good ones arent very cheap, and I think they dont look as good as the clean stockers. I wanted a big improvement but didnt want blue or white lights. I wanted it to not seem out of place on an old truck, so I was stuck with halogen bulbs and stock style headlight for looks.


I have heard some complaints about the TYC holders shaking, so I have saved the OEMs and will swap them out if need be, but I have to drive it at night first

Dusterbd13-michael (Forum Supporter)
Dusterbd13-michael (Forum Supporter) MegaDork
4/11/21 5:53 p.m.

Here to give a link to that headlight wiring harness? I maybe like you and just buy it instead of making it at that price.

Opti Dork
4/11/21 7:06 p.m.

In reply to Dusterbd13-michael (Forum Supporter) :

This is what I used


I did basically all the same stuff to the headlights on my '96 GMT-400.  New housings, LMC relay harness, and upgraded bulbs.  The headlights are decent now, but not amazing. Of those mods, the cheap LMC harness made the biggest difference. Totally recommended. 


Next step is some version of the 4-HI mod (all 4 bulbs on when hi-beams are engaged. Yeah, I know, near field vision blah blah blah. I don't care.). 

Opti Dork
4/11/21 10:10 p.m.

I thought about the 4 hi mod, but I almost never use the high beams, and I hate messing with electrical stuff. The upgraded harnesses I found with it built in were all 150 or more. I would build one but I don't think it would improve much for me since I never use them and even setting up one relay isn't something I care to do.


I think I'm going to do some more steering repairs/upgrades or a gmt800 master and brake hydraulic refresh. Not sure yet. Still chasing this problem on my pops tbi suburban

1 2 3

You'll need to log in to post.

Our Preferred Partners