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Login to post Forums » Off-topic discussion » That sinking feeling on the oil pressure gauge...

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Dec. 21, 2010 4:22 p.m. BoxheadTim SuperDork

Otherwise known as "just how accurate are GM oil pressure gauges in their pickup trucks"?

I've noticed for a little while now that the (hot) oil pressure seems to be slowly migrating towards the wrong end of the gauge so some input is appreciated.

'95 GMC K1500 w/ 350 TBI, 242k on it. Not really leaking or using oil.

When started from cold, the oil pressure is around 40-50psi depending on outside temp. Once the engine is warm, it goes into "windshield wiper" mode for a few minutes, basically oscillating between ~20 psi and ~35psi as if the pressure relief valve can't make up its mind. Once it's properly warm - ie, after the long slog up the pass on the way to work - the gauge settles at 20psi at about 1200rpm upwards (no change until 3k rpm, didn't rev it any higher) and around 10psi or less at idle.

Now if this was a Fiat or a Ferrari, this would be a sign I need a new oil pressure sender but on an SBC?

Any suggestions what to check apart from the balance on the 'buy another 4x4 beater' fund?
Dec. 21, 2010 4:40 p.m. Travis_K Dork

You are running 20w-50 oil and a decent filter right? Probably needs rod bearings, but it will probably last many years the way it is.
Dec. 21, 2010 5:05 p.m. BoxheadTim SuperDork

Decent filter - well, it's a GM OEM filter at the moment and it's running 5W30 dino juice as the GM dealer couldn't be bothered to check with me what sort of oil I wanted.

Supposedly the truck has been run on synthetic for most of it's life (according to the PO) so I guess I could ask the local quick lube place to stick some of their high mileage synthetic in (10W30 or 10W40, have to check what they have on tap) and see what happens.

TBH I don't really want to run it on something as 'thick' as 20W50 but that's probably the next step.

Of course the alternative would be to learn how to (re)build an SBC and stick something in that actually manages to pull the skin of a rice pudding. SBCs aren't that hard to find on CL out here but I don't really want to dump another 2-3 grand into the truck - it's already cost me close to 2.5k in repairs in the past three months due to me needing it back in service badly and not being able to do the work myself for that reason.

Shoulda bought a Dodge with a Cummins and learned how to change gearboxes .
Dec. 21, 2010 5:09 p.m. Toyman01 SuperDork

I'm going to say the bearings are loose. A lot of the time it's the main and rod bearings. My Roadmaster has the same problem at 238K. Run thicker oil. 20w50 does a pretty good job of keeping the pressure out of the basement. It's a Chevy, if it isn't knocking it probably isn't a problem. There are a lot of old SBC that run a long time with 10psi or less at idle. I used to have a 78 truck that at idle the factory gauge would sit a 0. Drove it two years in high school like that. It never knocked so I ignored it. If you are going to keep it forever, put a set of bearing on your to do list.
Dec. 21, 2010 5:17 p.m. BoxheadTim SuperDork

I'll keep an ear out for knocking noises - I thought I noticed something this morning on startup but I'm not 100% sure.

I wasn't planning to keep it that long - it's mostly a winter beater that occasionally gets to haul stuff - so I don't want to pour even more money into it than I already had to.

Guess I'll try slightly thicker oil, run it for a few thousand miles more and if that still doesn't help I guess it's either a new beater or Costco's finest 20W50. Or both.
Dec. 21, 2010 5:29 p.m. iceracer Dork

Low oil pressure sometimes will cause a lifter tap.
Dec. 21, 2010 5:33 p.m. Toyman01 SuperDork

Rods kind of rattle cold, mains are a deeper noise. The 78 I had sounded moderately horrible on a 20 degree morning. It would stop after a few seconds. The knock you really need to worry about is at warm idle. If it starts that it's done.
Dec. 21, 2010 6:10 p.m. Travis_K Dork

Run Valvoline VR1 20w-50. Its not that much, and in my truck with the worn out bearings it helped the oil pressure quite a bit. Honestly I think your oil is way too thin, and thats the problem. I made that mistake when I first got the truck too, and it had oil pressure issues, but with 20w-50 it was fine for several more years, then i replaced the bearings.
Dec. 21, 2010 6:30 p.m. HiTempguy HalfDork

10psi per 1000 rpm is the general rule of thumb, no? Sounds to me like you have at least another 50,000miles to go!
Dec. 21, 2010 8:59 p.m. Rustspecs13 New Reader

My grandpa left my dad an I a 94 chev 2500. Unfortunately it had the 305. It randomly uses oil, and the pressure is close to what you describe. And it has 125K on it.

Ive been told the # 7 oil control rings fail a lot because they weren't assembled right at the factory.

I'm going to rebuild it, mainly just to do it. The hard part will be if i decide to get a 350 and rebuild that. Not really for performance, but just a little extra kick. If its going to get 14mpg, it might as well have some power.

But then Id have to tune it or something. Maybe a cheap basic megasquirt setup....

~Alex
Dec. 22, 2010 7:41 a.m. WilberM3 HalfDork

i ran rotella t diesel oil in my jeep and it seemed to like it. its 15w40 dino or 5w40 synthetic. reasonably cheap too.
Dec. 22, 2010 10:30 a.m. iceracer Dork

Toyman01 wrote:

Rods kind of rattle cold, mains are a deeper noise. The 78 I had sounded moderately horrible on a 20 degree morning. It would stop after a few seconds. The knock you really need to worry about is at warm idle. If it starts that it's done.

That sounds more like lifters. Easy way to check for main brng wear. With the oil warm, at low speed, watch the gauge. Accelerate and decelerate. pressure will change. Probably will work only on a manual transmission.
Dec. 22, 2010 10:45 a.m. dj06482 Reader

Check out the sending unit. When the one in my '89 C1500 went bad, the pressure readings were all over the map (and would drop to 0 while waiting at a light). Getting a new oil pressure sending unit solved the problem for me. It's a longshot, but might be worth a try!
Dec. 22, 2010 10:52 a.m. HalfTrac, Lord of the FWD deathbox SuperDork

WilberM3 wrote:

i ran rotella t diesel oil in my jeep and it seemed to like it. its 15w40 dino or 5w40 synthetic. reasonably cheap too.

What should a Cherokee show at warm idle for oil pressure? Ours seems to hang around 25psi, but will hit 40-50psi on acceleration/cruising at speed.
Dec. 22, 2010 11:18 a.m. tuna55 Dork

Here is how I see it, that old adage that a GM vehicle will run badly longer than most cars will run at all is pretty true. It will be fine unless you try and do something silly. Not a GM, but my Dad ran a Ford 300 with a rod knock for years, towing a car to the dragstrip back and forth on the thruway. Having said that, this might just be lifter knock. The oil passageways can get clogged over time. Pull the valve cover and try to adjust them to make sure they don't bleed down weirdly. Pulling the intake isn't exactly easy on a non-vortec 350, so I'd just drive it until you wanted to swap the intake anyway (because stupid cheap aluminum Edelbrock intakes are everywhere and will really wake it up).
Dec. 22, 2010 11:51 a.m. BoxheadTim SuperDork

Well, I stopped by the local Jiffy Lube on the way to work and had them put in the only W40 oil they had in stock - Rotella VW-spec 5W40.

Oil pressure, I can haz. About 10psi more than yesterday (idle about 20, about 35-40psi from about 1500rpm on with the engine hot).
Dec. 22, 2010 5:27 p.m. iceracer Dork

HalfTrac, Lord of the FWD deathbox wrote:

WilberM3 wrote:

i ran rotella t diesel oil in my jeep and it seemed to like it. its 15w40 dino or 5w40 synthetic. reasonably cheap too.

What should a Cherokee show at warm idle for oil pressure? Ours seems to hang around 25psi, but will hit 40-50psi on acceleration/cruising at speed.

As I said on your other post, "Your oil pressure is fine".
Dec. 22, 2010 8:04 p.m. Jensenman SuperDork

BoxheadTim wrote:

Well, I stopped by the local Jiffy Lube on the way to work and had them put in the only W40 oil they had in stock - Rotella VW-spec 5W40.

Oil pressure, I can haz. About 10psi more than yesterday (idle about 20, about 35-40psi from about 1500rpm on with the engine hot).

There ya go. That motor was not designed to use the thin synthetics. Like a Harley you measure the bearing clearances with a tape measure. (ducks and runs for cover )
Dec. 23, 2010 6:30 a.m. tuna55 Dork

Jensenman wrote:

There ya go. That motor was not designed to use the thin synthetics. Like a Harley you measure the bearing clearances with a tape measure. (ducks and runs for cover )

As I said before in another thread, this is not true. Not at all.

tuna55 wrote:

This is a bit of a misnomer...

SBC 350 bearing to journal clearance: 0.0008-0.002

S2000 FC20 bearing to journal clearance: 0.0007-0.0016

The clearances are really not all that different from a "state of the art" zingy engine built by Honda and an ancient 50's design luggy engine made by GM.
Dec. 23, 2010 9:04 a.m. Toyman01 SuperDork

tuna55 wrote:

Jensenman wrote:

There ya go. That motor was not designed to use the thin synthetics. Like a Harley you measure the bearing clearances with a tape measure. (ducks and runs for cover )

As I said before in another thread, this is not true. Not at all.

tuna55 wrote:

This is a bit of a misnomer...

SBC 350 bearing to journal clearance: 0.0008-0.002

S2000 FC20 bearing to journal clearance: 0.0007-0.0016

The clearances are really not all that different from a "state of the art" zingy engine built by Honda and an ancient 50's design luggy engine made by GM.

The difference being a SBC will still run with .125 clearance, the Honda will be strewn all over the parking lot.
Dec. 23, 2010 9:11 a.m. tuna55 Dork

Toyman01 wrote:

The difference being a SBC will still run with .125 clearance, the Honda will be strewn all over the parking lot.

tuna55 wrote:

Here is how I see it, that old adage that a GM vehicle will run badly longer than most cars will run at all is pretty true.

Agreed.

However, it does not mean that a small block is somehow incapable of using synthetic fluids whilst a Honda is.
Dec. 23, 2010 9:50 a.m. tuna55 Dork

Why does a GRM Superdork not change his own oil? You actually stopped back at another quick lube place to get them to just add more oil for you? I am confused.
Dec. 23, 2010 9:54 a.m. HalfTrac, Lord of the FWD deathbox SuperDork

tuna55 wrote:

Why does a GRM Superdork not change his own oil? You actually stopped back at another quick lube place to get them to just add more oil for you? I am confused.

I usually don't change my own oil unless:

1) I'm already doing something else to the car
2) I'm REALLY bored and have lots of beer to go with the job

In most cases i'd rather pay someone the $10 to change the oil and not get dirty.
Dec. 23, 2010 9:56 a.m. Ignorant SuperDork

I don't change my own oil, but thats because I sold all my tools and now live in an apartment.. Going to be moving again come June, The tools and stuff was just too much to keep around.
Dec. 23, 2010 10:15 a.m. tuna55 Dork

I even changed mine in an apartment. Carried my Sears two ton jack and jackstands down three flights just to avoid the "I wonder what kind of oil this guy wanted" and the "whoops, Clem, weren't you supposed to fill that car after I drained it?" scenarios. My father even once had a small block come back with eight quarts and the same filter. It's way too easy to get that way too wrong.

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