1991 GMC 2500 5.7 - need some advice

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sr17

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I have an older GMC 2500 PU with the 5.7 TB engine. The mileage is unknown. I bought the truck a couple of years ago with the counter not working and from what I can tell, it has not been working for many years. The overall condition of the vehicle would indicate that the mileage is not "very high"

Truck starts and runs strong - uses about a qt of oil between oil changes of 3K miles.

Truck does have some fairly loud piston slap (at least that is why I think it is) upon start up. More when cold, but it is also there when starting warm. It runs quiet after that.

When engine is warm, especially after towing a trailer, low oil pressure light will sometimes come on when idling in gear (i.e. at a traffic light) and it will sometimes want to stall.

Considering the fact that the truck does not smoke, burns oil modestly and pulls strongly, I have a hard time imagining that the engine is worn out.

I have been using the prescribed 5W30 vicosity and for the last couple of oil changes 5W40 - with no noticeable difference one way or the other.

Hoping some of you expert BITOG'ers will be able to give me some pointers as to what might be going on.

Finally - I have a gallon of Valvoline MaxLife 10W40 sitting on the shelf. Would that be ok in my truck?
Thanks!
 
Use the Maxlife 10w40 now and then switch it to Maxlife 5w30 or even 10w30. Clean the fuel system with techron, replace the fuel filter and maybe add the top off with rislone.

Then keep driving it.
 
This sounds EXACTLY like my old '94 TBI 350 Suburban-the hotter it got, the lower the idle oil pressure would get. 215K, no blowby whatsoever, no sludge visible in the valve covers-but the oil pressure would get low enough that lifters would start ticking. 20W50 put an immediate stop to that problem, & it would even start OK at 0F with it-first time I'd used 20W50 since the '80s! It's sold now, I always assumed it was cam bearings or worn oil pump, no bottom end noise.
 
Don't worry about it. I had 4 of them. I scrapped every van [G20] because of rust, but the engines never wore out. Try a thicker oil, maybe 10W40?
 
You can get a rebuilt engine for around $1,000, or a brand new one from GM for like $1500

Depends on the rest of the condition of the truck whether it's worth it or not.
 
I would hook up the oil pressure gauge, check that. If it checked out OK, then I would just drive it and not worry too much. To quote the manual of my Buick: "Sometimes when the engine is idling at a stop, a chime will sound and the light (oil pressure) may blink on
and off. This is normal."
 
First I'd run thicker oil. You don't know what the bearings clearances are and they could be getting a bit bigger
laugh.gif


The Maxlife 10W-40 should be fine as a test and maybe the long run solution...

Second, I'd fit the biggest oil filter I could. There are some 2 qt units that will work fine. Any Napa store can look one up for you. That will give you a bit more capacity and it'll cold flow easier (more media area) so the thicker oil will not be an issue at any temp you care to drive it.

Third, I get a couple of bottles of Kreen from Kano labs and add 8 oz per oil change. If there is a lifter on the edge of sticking/ticking, it'll clean it out gently as you drive.

Lastly, you can get that same Napa store to look up the oil pressure sender. It's likely a 10 psi unit and it could be off a lb or two. You can get a I-H unit that is either 3 or 5 psi and replace it. No more flicker, but it will still come on if the pump runs dry for some reason.

Nice truck. Should you have a reason to get into the bottom end, an HV oil pump will extend your run a long ways. May be due for a timing chain... Should go a long time. And yeah, run a big dose of Techron in the gas with each fill-up for a while
laugh.gif
 
Last edited:
Could you take the valve covers and have a look?

I dont know what this would show you, or even if there are any adjustments. But, it probably isn't difficult to pull at least one valve cover.


Then maybe, you could spray carb cleaner on the pushrods or???


All of the above is blind guessing.
 
Thanks for all the great feedback guys!!
I do have a can of Sea Foam in the tank right now, and I have another full can on the shelf, so I was thinking to tun that in the next tank. I also have a can of Sea Foam Top Engine Cleaner & Lube that I was thinking to run through the intake.
I really like the truck, tranny is smooth shifting - no rattles, very little rust and interior is like new. I'm hoping to keep it running a few years.
 
Originally Posted By: Merkava_4
I think that one guy is right; you should give 20W-50 a try.


Will do that next time - I guess the MaxLife 10W40 is a stepping stone in that direction - since I already have a jug of it.
 
Originally Posted By: Nick1994
You can get a rebuilt engine for around $1,000, or a brand new one from GM for like $1500

Depends on the rest of the condition of the truck whether it's worth it or not.


Nickl, this peaked my interest!
I am not sure I need to do anything drastic right now, but the truck really is in very good shape so if the cost for a new engine is as low as you say, that might be just the thing for the long term.
How would you go about getting a new engine from GM for 1500? Do the dealers sell them for that price?
 
Originally Posted By: sr17
Originally Posted By: Nick1994
You can get a rebuilt engine for around $1,000, or a brand new one from GM for like $1500

Depends on the rest of the condition of the truck whether it's worth it or not.


Nickl, this peaked my interest!
I am not sure I need to do anything drastic right now, but the truck really is in very good shape so if the cost for a new engine is as low as you say, that might be just the thing for the long term.
How would you go about getting a new engine from GM for 1500? Do the dealers sell them for that price?
Yes, I've been in GM dealers where they advertise them for that price. Just look it up on Google. You can find other sellers of the engine too, here's 1 link.

http://www.summitracing.com/search/part-...all-block-gen-i
 
You realize you will have to switch a lot of stuff over from the old engine to the new? It can depend on how much money you want to spend and/or how much of the work you want to/can do yourself.
 
The ABC of your situation:

A: Install mechanical oil pressure gauge to verify oil pressure.

B: Pop oil filter off and change your filter adapter (which contains the filter bypass valve) for a new one.

C: Try 20/50. (Marine 5.7's run on this stuff for 2000+ Hours)

D-Y: Anything else.

Z: Consider a new engine.
 
Fantastic reply DoubleWasp!
I need to invest in a oil pressure gauge - time to visit Harbour Freight.
Is the oil pressure sending unit easily accessible on the 5.7?
 
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