GM 8.1L truck motor any problems?

Been contemplating a more powerful tow rig lately in the GMT800 chassis and see trucks with the 8.1 listed at a fair price.  Do these motors have any major problems?  What are the trouble spots?

Thanks.

Biggest trouble spot is mpg lol. It will be at best maybe 10mpg under the most perfect circumstances. My friend had one in a suburban and it got worse gas mileage then my family’s 72 26 ft Aquarius motorhome that has a built 454.

MPG is a huge downside, i worked for a guy that was getting 7mpg city with it

I have one in a 2003 Suburban, mpgs aren't great, but not as bad as the 79 GMC Dually I had. Towing power is darn good. On the highway with just me in it, I have hit 14-15 mpg, around town is usually 10-11.  Towing drops it a bit, usually 8-10 mpg when I pulled a 28ft enclosed trailer. Edit: mine has 3.73 gears.

They seem to be quite reliable, most things that need fixing are the stuff around it, like the plastic fittings on the heater hoses, oil cooler lines leak as they get older, etc... Mine has 248K on it, doesn't burn oil, but does leak a bit.

Early engines had issues with intake gaskets (pre 03 I think).  That and most burn a little oil.   Other than that a great engine.  Have one in our 2006 Suburban.  It has 4.10 gears, and we normally get 13 MPG highway 9-10 city.  The trucks had the Allison transmission, which is great, but does cost a little in the MPG department.

The 8.1/Allison is one of the all time great combos as far as just feeling right. I had a friend with one and always loved borrowing his truck. I kind of wanted to keep the drivetrain and throw the whole rest of the truck away. He was getting between 11 and 13 mpg mixed which is why he sold it. Made me sad. 

AIUI, the 8.1L/Allison combo is great until you drive the Duramax/Allison combo and then you never want to go back. :)

I had an ‘04 Suburban with the 8.1. They have a rep for burning a little oil but I never had to add any in between changes. It didn’t do as bad as I thought it would on gas but had the 3.73 rear. Damn stout engine. 

In the Suburban, the 8.1 gets basically the same mileage as the 6.0, so might as well get the big block.  Sooo many torques. My 6.0 Sub 2500 gets 13mpg empty on the highway.  11.5 in mixed use, 10 or less towing.  Might as well have the big block for that. 

AIUI, the 8.1L/Allison combo is great until you drive the Duramax/Allison combo and then you never want to go back. :)

I recently rebuilt most of the front end on an early duramax/allison truck and the engine noise actually bothered me enough that i constantly thought about potential ways to make the truck 'not annoying' instead of being impressed that the dash showed 19-20 avg mpg. Newer ones are quieter, i know. I liked the Allison ok but i like the later ones that have a manual shift mode a lot better as i felt the stock upshifts were kinda high/late with no load. I also didn't like how much throttle it took to get the full torque of the engine, which is usually one of the main ways that turbo cars are satisfying (lots of torque at mild throttle). 

Anyway, I've driven the same truck in 8.1 and disliked it for other reasons (mostly that it needed a tune to remove its torque management behavior), but i'm just saying there's definitely a way of thinking that doesn't fall in love with the duramax over the 8.1.  

As far as 'major problems' on the 8.1, replacing the crankshaft position sensor has been known to turn into a major problem. I once watched a coworker cut through the floorboard of a tow truck over it. 

The crank sensor is like a foot long.  I broke one trying to remove it, ironically, because I was R&R'ing the driver's side head and I didn't want to risk damaging it.

Lifters go bad from time to time, which eats the camshaft.  Seems to mainly be a problem on vehicles that idle a lot like tow trucks.

The PCV system is pretty neat.  There is no fresh-air breather hose, or even any external lines at all.  There's just a 1/8"-ish hole in the bottom of the intake manifold.

Really, the whole engine is neat.  It's like a 5/4 scale LS made in iron.  Symmetrical ports, coil near plug, no coolant goes through the intake manifold (separate iron water manifold that bolts to the FRONT of the heads), flat sealing surfaces everywhere and modern captivated O-ring type metal gaskets instead of cork/rubber/cardboard.  About the only throwbacks are that it still has a camshaft driven oil pump (drive is only accessible with intake off) and there's no plastic manifolding or tinwork, it's all iron and some aluminum.

So how does this hole act as a PCV ?   

iceracer said:

So how does this hole act as a PCV ?   

It pulls air from the crankcase into the manifold like a PCV.  Some PCVs are just a fixed orifice and not a valve anyway.  There's gotta be some kind of fresh air return though, I'd be shocked if there isn't. 

There is a fresh air return, it is in the throttle body, small passage before the throttle blade. 

The 8.1L does have a bit in common with the older Big Block Chevy too. Same valve cover bolt pattern, same valvetrain design, older exhaust manifolds bolt up, it has the motor mount bosses to bolt it in where a BBC was and the 8.1 crankshaft will fit into a Gen 5/6 BB.

Thanks.  Sounds like a decent option if I need the torques for towing and don't drive it around town too much

• I like mine but it is thirsty, even with the 6 speed.
• the suburbans never got the allison because the trans is so big the floor space doesn't accommodate without a body lift so you can only get that combo in the trucks
• the eat oil LIKE A LOT. spec per GM is 1qt per 1000 gallons of gasoline, which at about 10mpg
• parts are expensive for no reason if its unique to the 8.1. For example 6.0 harmonic balancer is 50 bucks, 8.1 is about 125
• 00-03 are less desirable for cam sensor issues (NLA parts kind of thing) and 04-06 do not have the EGR and then get floating pin wrist pins unlike the earlier gen do not
• not a lot of aftermarket but honestly its 460ft lbs out of the box...
• it tows way better than a small block - you do not need to wind it out to the moon to go up a light grade
• pan leaks like every GM v8 can be a a nuisance
• its a stroked 454 with LS tech on it
• makes good big block rumbly noises
• these trucks go for stupid money for no reason what so ever. I see a lot of good shape 10+ year old trucks go for 10-15k expect a haggard one around 8k the equivalent 6L will be about 4k

more details on the PCV here:

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1st-ever-big-block-gmt900-frankenburban-20/130992/page8/#post2469366

I put one into my GMT900

How long do those Allison transmissions last?

In reply to amg_rx7 :

Probably longer than the truck. They are beefy and sap some power too. Thats why the same truck w the 4l85 will get better fuel economy and be faster. But its negligible. 5k miles a year for towing/adventures not much in mpg difference to your pocket over the course of a year imho.

In reply to amg_rx7 :

The Allisons are sweet.  I have one behind my Duramax.  It's creepy good at always being in the right gear - up AND down hills when towing.  And they're stout.

Duramax trucks got the 6 speed Allison with tap shift in 06.  Did the 8.1L trucks get the same, or did they stay with the 5 speed?  I had an 06 Duramax with the Allison and it was great, never felt like it was searching for gear or in the wrong gear, the tow/haul mode was excellent at holding the gear for grade braking.

Had 450k miles on it when I sold it, and I am sure it is still going strong.  Really wish I still had that truck, but the 8.1L in the Suburban is a good compromise to get the extra interior space the wife wanted (and a Duramax swap is in order for when it is no longer her daily driver).

Going to look at an 8.1+Allison truck soon.  Was looking at fuelly to get an idea of MPG compared to the 6.0 and 5.3. 

Am I nuts to look into this truck for a once a month tow rig 400 miles round trip, occasional weekend errands?

Seems like I can pick one up for ~$8k which is whole nother truck cheaper than a diesel but...

amg_rx7 said:

Going to look at an 8.1+Allison truck soon.  Was looking at fuelly to get an idea of MPG compared to the 6.0 and 5.3. 

Am I nuts to look into this truck for a once a month tow rig 400 miles round trip, occasional weekend errands?

Seems like I can pick one up for ~$8k which is whole nother truck cheaper than a diesel but...

My buddy that had the 8.1 suburban had an enclosed trailer for longer trips with his street rod was going to go to the la roadster show in ponoma but realized he could afford to that trip with the suburban so I got rid of it and got a 2006 gmc crew cab sle 2500 duramax which he still has. Note that this was from the edmonton area so much longer then 400 miles. The truck has been to primary tow vehicle for his dirt late model aswell since he got back into racing them.

For a once a month 400 mile round trip, the fuel mileage difference will not be enough to matter unless you're really living paycheck to paycheck and can't afford to chew gum... lol

I have an 8.1 liter '02 Suburban that is my trailer towing pig and it's stout. 12 mpg unloaded on the highway and 10 mpg pulling my 9400 lb trailer. And it doesn't care about hills, either it seems. But I only use it once or twice a month for travel and the difference in fuel mileage between it and a diesel really doesn't overcome the cost differential between a diesel and the gasser. And it's not as loud or smelly in the campground like a typical diesel tow rig, and I like that fact.