1988 C1500 blown head gaskets

[Mike's money pit]

Hi guys and gals,
First, thank you for taking the time to read my long post.
I bought a 1988 C1500 about a year ago to have a cheap driver. It was supposedly a 77,000 original mile truck that the owner’s grandad bought new and then passed down to his grandson (the previous owner). It was parked for around 20 years before I bought it. The body is in good shape and everything worked. The truck had a mystery coolant leak ever since I bought it and also air bubbles would come up into the radiator. The radiator would also sometimes overflow when I turned the truck off. My mechanic suggested replacing the radiator cap which seemed to help a little. About a month ago, the truck overheated and started knocking. It knocked for about 15 seconds as I accelerated into traffic and then I pulled off the road. I added 2 quarts of oil 😬 and ended up adding about a gallon of water. The truck ran fine for about 2 miles then started to overheat again. I added about 2 gallons or water this time and was able to get it home (about 15 miles) without further problems. Several hours later I went to crank the truck and it was hydro locked. The next day I took out the spark plugs and water ran out of cylinders 3 and 5 (a lot from #3) and also from 4 and 6. I am not particularly mechanically inclined, but I decided to replace the head gaskets myself so I wouldn’t get too deep in this truck. A friend looked at the heads after I got them off and suggested taking them to a machinist shop to get checked. One was cracked so I ended up exchanging them for another set of casting #193 rebuilt heads.

I just got everything back together today (I removed the AIR pump pipes from the exhaust manifold but left the smog pump in place for now). and cranked the truck. It runs but will not idle without a little gas. I drove it about 10 miles. The oil pressure stayed around 30-40. The water temp stayed right about 210*. My battery might be a little weak from sitting for a month but the engine seemed to be a little hard to crank. I could hear a clacking sound when the truck is under load. It sounds like a valve clicking or maybe detonation. The truck feels like it is not making good power and feels like maybe it is missing. I’m not sure where to start. I am thinking maybe taking the plugs out to see if a cylinder isn’t firing. I have not checked the timing but the distributor slipped right in when I reinstalled it. Vacuum leak? Mis-adjusted valve? Timing? Any help will be appreciated! Thanks again for reading my post.

[AussieinNC]

1/ Did you change the thermostat?
2/ Did you have the radiator cleaned and pressure tested?
3/ If it wont idle it will be a vacuum leak....check every rubber line that plugs onto the engine, especially into the intake manifold....I am assuming this is a TBI system...
4/ Timing should be 12 BTDC at idle, and all in at 35 BTDC around 3500 rpm
5/ Did you check the water pump? Sitting for that long the impeller could have corroded away...
6/ Check the heater core for leaks....

These engines at 80000 should not have cracked heads unless something else has caused the overheating...

[Old Truck Man]

I suspect you have at least one bent rod. When water gets into a running engines combustion chamber it will not compress. So the connecting rod bends. And then that piston becomes lower down in the bore at TDC and doesn't have the proper compression ratio. You should have rotated the engine over when you had the heads removed and made certain every piston came up to the proper distance.

[Mike's money pit]

Thanks for the replies. I may have shot myself in the foot by trying to go cheap. I was really reluctant to spend much on the truck and had hoped the head gaskets would be all I needed to address. I am definitely learning as I go, though.

Old Truck Man: what is the best way to check for a bent rod, now? The good news (I think) Is that the truck was not running when it was hydro locked. I tried to start it but it wouldn’t turn over. Before turning the truck off the last time, it was blowing some steam out of the exhaust, though. It looked like the steam when it is cold in the morning but it kept exhausting steam until I shut it down
Even after running for 30 minutes.

AussieinNC: I came to the conclusion that it must have been 180k or 280k on the truck. It is on it”s 3rd transmission I think. I had to replace the aftermarket transmission that was in the truck about a month after I bought it. I forgot to mention I replaced the leaky water pump 6-7 months ago. I did not replace the thermostat or have the radiator cleaned and pressure tested. I flushed it with clean water several times right after I bought it and I was always amazed at how rusty and muddy the water looked after just a day or two of running it Surprisingly, the truck does not seem to leak any fluids. I have had the cover off of the heater core and inspected it. I have not noticed any water at all inside the cab. I reconnected all the vacuum lines and checked over them. I need to research how to find a vacuum leak.

[AussieinNC]

Originally Posted by Mike's money pit

Thanks for the replies. I may have shot myself in the foot by trying to go cheap. I was really reluctant to spend much on the truck and had hoped the head gaskets would be all I needed to address. I am definitely learning as I go, though.

Old Truck Man: what is the best way to check for a bent rod, now? The good news (I think) Is that the truck was not running when it was hydro locked. I tried to start it but it wouldn’t turn over. Before turning the truck off the last time, it was blowing some steam out of the exhaust, though. It looked like the steam when it is cold in the morning but it kept exhausting steam until I shut it down
Even after running for 30 minutes.

AussieinNC: I came to the conclusion that it must have been 180k or 280k on the truck. It is on it”s 3rd transmission I think. I had to replace the aftermarket transmission that was in the truck about a month after I bought it. I forgot to mention I replaced the leaky water pump 6-7 months ago. I did not replace the thermostat or have the radiator cleaned and pressure tested. I flushed it with clean water several times right after I bought it and I was always amazed at how rusty and muddy the water looked after just a day or two of running it Surprisingly, the truck does not seem to leak any fluids. I have had the cover off of the heater core and inspected it. I have not noticed any water at all inside the cab. I reconnected all the vacuum lines and checked over them. I need to research how to find a vacuum leak.

I use WD-40 with a spray tube,,,,with the engine running, gently spray each of the vacuum lines and fittings...you will hear the engine note change if the WD-40 seals off a leaky pipe, even for an instant...

I also use small cable ties to tighten down vacuum lines if needed.

Also check where the injector hat mounts onto the manifold...

I would suggest using a radiator flush cleaner in the system to clean the radiator...but with a warning...doing so may cause weak freeze plugs to start leaking....

[Mike's money pit]

I’ll try the WD40. Is the injector hat the whole throttle body that is mounted to the intake?

I did notice the vacuum line on the EGR valve is missing. Covering the port on the EGR valve and on the other end didn’t seem to make any difference.

[AussieinNC]

Originally Posted by Mike's money pit

I’ll try the WD40. Is the injector hat the whole throttle body that is mounted to the intake?

I did notice the vacuum line on the EGR valve is missing. Covering the port on the EGR valve and on the other end didn’t seem to make any difference.

"Injector hat" is the same as "throttle body".

Pull that EGR valve and install a block off plate...I have seen EGR valves leak down the stem and can be a really difficult to diagnose.

[Old Truck Man]

Best way to check for a bent rod on a running engine. Do a compression check. pull all the sparkplugs. with the throttle plate wide open crank the engine. a cyl with lower compression will be the culprit.

[gmc684x4]

Time to pull her apart low milage
You should be able to get away with simple ring and bearing the motor and replace the bent rod
Unfortunately hydro locking and engine usually bends or breaks a rod

Pick up a how to rebuild a small block book from Sa books

Parts that do not come off easy look for missed bolts

I would pay the money to have a good machince shop hot tank and inspect the block for cracks and hone it for you and inspect the rods as well if ther can not match the piston on the bent rod i would replace all 8 as a set
I would not re use that piston as it had grest force applied to it

Good chance your block is roller csm ready find out from machine shop how big a cam you can use to big a cam for your head setup will eat the cam lobes quickly

The more you dissasemble your engine for the machine shop will save you money and talk them about it too first some things are better left for them to deal with

[Dead Parrot]

An exhaust leak can sound like your original noise description. Get yourself a piece of hose to use as a stethoscope and try to narrow down where the noise is coming from.

[gmc684x4]

A word of advice on auto tranny when replacing them if re using the torque converter have it flushed out by a shop and flush the lines and rad out very well

The reasoning is there will be metal mixed in the lines and rad from the failed transmission which will cause quick failure on new tranny
Rebùilt tranny will have voided warranty if not properly flushed out lines and rad

[Mike's money pit]

Thank you guys for all the replies. I hadn’t checked the thread in a couple of days due to work. I started out checking for a vacuum leak. The truck is pulling around 20 lbs of vacuum at a slightly fast idle. It wouldn’t idle at the point so I was feeding it a little gas pedal. I couldn’t find a vacuum leak anywhere so I decided to check the timing. I guessing I dropped the distributor in slightly off because the timing was pretty retarded (guessing maybe 30 degrees?). I turned the distributor clockwise and the engine became progressively smoother. I turned it a little too far but at 8° advanced the engine would run and idle well. I took it for a test drive but it felt like it didn’t quite have the perfect power. I dialed it back to 0° which is what the sticker on the shroud calls for and it runs great

I appreciate all the advice. I’m going to run it for a few weeks and see how it does.
Mike