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1991 GMC Sierra

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1991 GMC Sierra

Knowfear99 Knowfear99
New User | Posts: 2 | Joined: 12/12
Posted: 12/09/12
04:45 PM

5.7l TBI vin K. odometer reads 248k

I recently had a large amount of coolant in my oil, and it was smoking pretty bad under load. (losing coolant as well). As far as I know, the truck has never overheated. (At least not since I've owned it). I bought the truck from a kid who supposedly "rebuilt" the engine. I believe by "rebuilt", he means he painted everything and put some chrome tins on it. Anyway...

I assumed it was most likely a bad head gasket. I pulled the engine apart and took the heads into a shop to have them resurfaced and get a valve job. They told me everything looked really good on them. I replaced the head gaskets. Neither of them looked like they were damaged anywhere when I took them off. I got it all put back together and running. I ran some motor flush through it twice and changed the oil and filter. Also flushed the cooling system. After driving it for am hour or so, I checked the oil and it still looked like goop. I haven't driven it more than a mile or 2 since, and always just basically idling around my house. Fast forward two weeks. Coolant level is still right at the top of the radiator. Doesn't seem to be losing coolant any more. Today I bought two more jugs of oil, another filter, and some seafoam. I drained everything. Put in half the can of seafoam and 2.5 quarts of oil. Ran it for 5 minutes and drained it. Put in the rest of the seafoam and the rest of the jug of oil. Ran for 5 minutes and drained. Changed filter and put in correct amount of fresh oil. The smoking seems to have gone away some. After running for 30 minutes, the oil still looks a little fouled, but no where near as bad as it was.

Then I did a compression test on it.


My guess now is leaning towards bad piston rings. I am going to putter around in it for a day or so and keep an eye on the oil. If it gets goopy again, could it be a cracked block somewhere around the number 3 cylinder?

Go easy on me, I am in no way a mechanic. I am just poor and need to try to fix things on my own. I have already spent upwards of 800 bucks on this mess in the last 2 weeks. Any insight would be great.  

waynep71222 waynep71222
Enthusiast | Posts: 448 | Joined: 03/12
Posted: 12/09/12
07:36 PM

since you have a compression tester...  and you are able i guess to remove and install the cylinder heads...

do you have an air compressor???
do you have a remote starter switch???

does your compression tester hose have the proper coupling to hook to your air compressor hose???

if you *** run another test...   take the valve core from the end of the compression tester hose..  remove ONE spark plug at a time...   ignition off.. throttle blocked open somehow...
radiator cap open.. and filled to the top of the filler neck.. if you have a hollow stopper and some clear vinyl hose..
thread the compression tester hose into the open spark plug hole..

use the remote starter switch to bring the compression up..  OR mark the damper in 4 positions 90 degrees apart..
you can align them with TDC to position the crank in the 8 different positions..

warning.. sometimes the engine will SPIN...
with each piston at TDC apply shop air..  see if the radiator bubbles...  or the coolant level changes.. see if there is air coming out the throttle body... see if there is air coming out the oil filler cap if the breather opening is plugged..   see if there is air coming out the exhaust..

there are specialized tools called cylinder leak down testers... and they do work.. but they have a restriction at the end of the hose to limit the amount of volume of air escaping the end of the hose.. i like the increased flow and pressure build up to check the gaskets .. the combustion chambers.. the pistons and rings.. the valve sealing..

i use this test if i think i have a blown head gasket...  yes  most of the time the motor will spin.. it only takes seconds to reset it..  

Knowfear99 Knowfear99
New User | Posts: 2 | Joined: 12/12
Posted: 12/09/12
08:21 PM

My tester doesn't do the leakdown test, but I can get one. Also, I can't really see the head gasket being bad since I just replaced them 2 weeks ago and haven't put more than 3-5 miles on it since then. That was why I was thinking maybe it was bad rings. The heads and valves were just redone 2 weeks ago.

I will try to get some leakdown results tomorrow though. The only thing I didn't didn't do while I was doing the compression test was to put oil in the cylinder and test it again to see if it gained. I can do that tomorrow as well and see what it does there.

I'm really to a point where I just think I should pull the engine and have the block  reworked and just do a complete rebuild. I was just hoping it wouldn't come to that this close to Christmas.  Frown  

pepsi1 pepsi1
Guru | Posts: 1718 | Joined: 09/11
Posted: 12/09/12
09:00 PM

KnowFear99 First welcome to CHP:

  If all else fails with Wayne's suggestion. With the money you have put into your engine already, (I.E.) All your time and machine shop parts and labor.
  If #3 still shows a low pressure. I would pull the engine out, and start looking for a Crate engine. New engines are $2000 for your year. I'll do some research for you and you can decide what you want to do.


pepsi1 pepsi1
Guru | Posts: 1718 | Joined: 09/11
Posted: 12/09/12
09:33 PM


Jegs has your engine although you didn't say if you had a 1500 or 2500.

Chevy Performance engines carry 3 year 100,000 mile warranty. Pass Car/Lt.Duty truck
P/N 809-12568758  $1769.99 Free Shipping. This engine has 200HP/300ft-lbs tq.1987-1995 Grin  Cool

P/N 809-12530283  $2029.00 Free Shipping. This also 3 year 100,000 mile warranty.
I don't know if this engine will go into your truck. Not because of fit, they are all interchangable. This engine has more HP/Torque. If you do that talk to a Tech. Rep. at Jegs... 1996-2000 L-31R

If you decide to go that way by the time you pull yours out this will be in your driveway.... Of course these are suggestions. I made going by what you have said and the mileage on that block. If your going to use the truck and keep it. Thats the way I would go. By the way these are long blocks. All you will need to do is reinstall your intake and TBI. Thats it....They are all new engine parts not rebuilt.