1997 Possible HG Failure

Ok so I bought a 1997 pontiac Grand Prix the other day, it has the 3800 series II, 160,XXX on it,

Here is the problem It mis-fires on cylinder #3.

I did a compression test and got the following:

1-150psi
2-165psi
3-100psi:confused:
4-163psi
5-165psi
6-165psi

It also overheats, and looses coolant. And I have no heat and I can hear air being pushed through the cooling system.

Sounds like the HG to me but When I tested that damn #3 cylinder It would hold 100psi for 10+ mins without bleeding down????

It would just not climb past that, and there is no milkshake in the oil pan but the coolant smells like shit.

I am a mechanic but i dont wanna pull the head if I dont have to. I know about the UIM problems with this engine as well as the LIM gasket problems.

Sure sounds like a Head Gasket to me. With the car stone cold, start it up, and immediatly give that upper radiator hose a squeeze. If its got high pressure right away you defenatly got a HG issue or a cracked head/block.

Since it holds pressure, I would bet this car had a UIM go and it hydrolocked due to coolant getting sucked into the cylinder. This can bend a connecting rod because coolant can't be compressed like air, which in turn will enlarge the cumbustion chamber (as the piston no longer comes up as high in the cylinder) and lower static compression. This is a pretty common occurance when people let the UIM issue go to long, especially in 96-97 engines. It's usually cyl 5 or 6 (as they are closest to the throttle body) but not always.

hi I believe it is the HG now because if I start the engine after sitting all night (freezing cold) it builds pressure real quick in the cooling system.

I have a feeling it is the HG but it could possibly also be a bent con rod.

The car has 170,000 on what appears to be the original UIM and LIM.

It doesnt appear to be burning coolant However, Is it possible that the HG is leaking and just wont hold any more than 100psi??

Either way the head is comin off to investigate.

100PSI would blow all your hoses apart in a split second.

Why not just tear it down. Once the heads are off you'll be able to turn the motor over by hand and make sure all the pistons reach TDC. If you do have a bent rod, then just scrap the block.

Ok so i removed the UIM, LIM and front head today, first of all WOW!!!
The front cooling passages were 100% blocked and the other ones werent far behind.

The LIM gaskets were SHOT, let me re-phraise that, BEYOND SHOT!!

I cant tell if the UIM is warped or not but it was deff leaking and all the gaskets on it were shot.

The #3 cylinder that only has 100psi of compression reaches TDC just like the other two next to it so the only place the compression could have been going is into the cooling system.

I will post up pics of the head, and the old gaskets tomorrow probably.

The fact that the car ran and drove as well as it did is a testament to the 3800's robustness. If it were a ford it would have thrown a rod by now lmao.

So I will pull the rear head next and see what condition that is in.


Quick question, what is the best thing to use to clean the heads and block of all the old gasket material?? I was going to use some solvent and a brass brush but maybe someone has something better to use.

No solvent, and no brass brushes.

I usually use a sharp putty knife/gasket scraper, or else I also just bought a little tool at o'reilley's that is basically a handle with a razor blade attached (1.99). I use this to clean all my gasket surfaces. It works really nice. Plus with it its hard to damage the head because the razor bladse will bend before the head gets nicked. I have also used scotch bright pads (the kind without soap on them). Just take your time and after you get it all cleaned up, spray it down with some brake clean.

Dont forget to replace the head bolts and let me know if you need torque specs.

I have also used scotch bright pads (the kind without soap on them).

I would strongly recommend against scotch brite pads on engine parts unless you can do a very thorough cleanup back to perfectly clean and dry parts which are not assembled and then blow off with a strong flow of compressed air.

Reason: I bought a GM-reman 4.3L V6 long block (not a 3800 I know but bear with me, at least a 90 deg V6 :disappoin ) some years back and it was taped with no less than 6 pink pieces of paper inside and outside of the crate urgently warning against the use of scotch brite products to cleanup engine covers. In fact this voided the 50k warranty too...any claim engines would require oil samples and teardowns to verify.

The basic premise is that the green abrasive particles can get shredded off the backer material and into the oil side of the engine. It doesn't take many blips on the filter bypass valve to send these particles directly to main and rod bearings where you can guess that they do exactly as they were designed.

Anyway, I used the scraper technique which apparently worked well...84k since the replace and all is well.

yeah but if he's got the heads/LIM/UIM off, he'll be able to blow/vacuum or wash all these surfaces free of and of them green particles before he puts the motor back together.

No problems on my vehicles to date. if your carefull, block everything off with rags and vacuum it our before you put it back together, you shouldnt have any problems. Plus if its a high quality oil filter, then anything the size of the "green particles" shouldnt make it past the filter.

Plus if its a high quality oil filter, then anything the size of the "green particles" shouldnt make it past the filter.

Agreed...but there's a filter bypass valve...that little black thingie off center inside the filter adapter casting.

The general idea is that a plugged filter won't grenade the engine...but my information is the valve is quite active with cold oil, as it is simply the "path of least resistance".

Just like anything...proper due dilligence with cleanliness goes a very long ways. I'm just giving out my opinion and reasoning that the useful green pads aren't as benign as they seem.

Just an update, I installed a new HG and re-installed the head only to have the same 100psi of compression on that damn cylinder, so i put 100psi of shop air in the cylinder and low and behold I have a bad intake valve. You can hear the air rushing past the valve into the manifold.

Now the question:

Do I get a junkyard head??

Do I lap the valve and see how good I can get it?

Do I have a professional valve job??

Also can I re-use the HG being that it was brand new untill I installed and then removed the head??

Just a question...did you do this test with the rocker arms bolted down? If so you could have a bad lifter which is holding the valve open.

If you did test with those rocker arms present, I'd try the test again with just the valve stems up to the keepers as the only parts on the problem cylinder.

Hi I un bolted the rocker arm and tried it that way too, it still leaked so i removed the valve spring and it continued to leak, its the valve without a doubt.

What would cause an intake valve to not seal properly??

Its not like an exhaust valve thats always hot so i dont understand why it would get damaged.

The intake valve may be bent or the rotator stopped working (producing uneven wear), or the seat or casting is cracked.
I have had intake valves get stuck slightly open from an engine sitting too long unused, (oil deposits accumulating on the valve stem)

Just an update, I installed a new HG and re-installed the head only to have the same 100psi of compression on that damn cylinder, so i put 100psi of shop air in the cylinder and low and behold I have a bad intake valve. You can hear the air rushing past the valve into the manifold.

Now the question:

Do I get a junkyard head??

Do I lap the valve and see how good I can get it?

Do I have a professional valve job??

Also can I re-use the HG being that it was brand new untill I installed and then removed the head??
Respectfully, HG's sometimes blow for a reason... some other part has failed, often in the head, such as a warp. The new HG may blow again soon, unless the original problem is found and fixed.

Therefore, every time I have changed a HG, I send the head(s) to a machine shop so they can hot tank it to clean it up, check for straightness, pressure test it for cracks/leaks, look at valve spring tension etc,
If anything looks suspect, I get them to fix it, before the head goes back on.

Even if they find nothing wrong, at least I get peace of mind. Doing this has been very successful, I have never had to re-do a HG job. :)

In this case, a once -over of the head might have saved you from having to do this job over again, because they would have found the leaking valve.

Since you are taking the head off again, send it out for a once- over. They do not have to do a valve job or, if it's not too bad, or they can just do one valve, if cost is an issue. See what they say about the condition, first.

Don't bother with a junkyard head, unless you find that yours is cracked.
And no, the HG cannot be re-used.

Ok sweet, I will bring it to a machine shop and have them look at it.

On a side note, I am not so shure that the HG was actually leaking to begin with, I am starting to think it was the damn valve all along, and the UIM and LIM gaskets were also shot with out a doubt.

So I am just trying to repair the car and then i am going to be selling it. If they can just fix the one valve and verify the head is not warped/cracked then awesome!

If they cant then i will go to the yard, do a compression test on a car and if all the cylinders check out its mine lol.

tough break man,

I would pull the head off again, take it apart, check it out (whether it be a burnt valve, kissed the piston, foreign material in the combustion chamber ie spark plug tip, or like magic rat said)

Either way, you should be able to salvage the head, just replace the valve, have a machine shop do a once over and slap it back together.

I'd buy a new head gasket. SOmetimes you can find a shop that will sell you just 1.

I pulled the head already, the valve sounds like sandpaper if you spin it in the seat. The combustion chamber looks good tho.

Autozone and rock auto will both sell me just the one HG for about $20

I am pissed at myself for not doing a leak down test BEFORE I pulled the head to see where the compression (65 #'s of it) was going.

So now I costed my self $20 and alot of agrivation.

yeah, its ok. Happens to the best of us. Heck, my friend and I had to pull a motor apart twice, once because of a blown head gasket, and the second time because some loser threw a bolt into the LIM port while we were working on another vehicle waiting for the right injector o-rings to be delivered. The second time around it needed two new valves, and the piston was pretty marked up, but we still used it.

In my younger days, we use to pour bb's down the carburetor. The noise the car made it was so funny. I wonder how much damage that did to the engine. Would that put dents in the top of the pistons?

Interesting thread with a lot of speculation so I guess I'll add my two cents worth since I have been modding and building engines for over 35 years.

If the intake valve is bent the only way that is going to happen is if the valve hits the top of the piston. The most likely explanation is due to overheating caused by the original issue which was the bad UIM and LIM gaskets. With overheating valve stems can swell which will cause the valve to stick in the guide. If the valve sticks open then it will hit the top of the piston. Also, in most cases you get coolant contamination in the crankcase and engine oil. As a result it will cause issues with the lifters and do the same thing. :2cents:


In my younger days, we use to pour bb's down the carburetor. The noise the car made it was so funny. I wonder how much damage that did to the engine. Would that put dents in the top of the pistons?

:rofl: :rofl: :rofl:

I am going to just get a head from a junkyard. i dont wanna wait to have the head fixed when I can just go and get one that will work.

Will any 3800 series 2 head work on my car???

Will any 3800 series 2 head work on my car???

Not any. Only one from a L36 VIN "K. The VIN "1" L67 (supercharged) heads will not work since the fuel injectors are located on the cylinder heads along with some other minor changes.

Ok but any N/A series II head would work??

Hey a little update......i bought a new intake valve and lapped it into the head, bought and installed a new HG and New Head Bolts.

Got it all back to gether and did a compression test, 180psi on all three cylinders...YAY.........or maybe a little too early to celebrate............

Let me explain:

I did a compression test wile cranking the engine, wile the cylinders did build compression I neglected to do a leak down test with shop air BEFORE I installed the LIM, UIM, ect.....

Well the *** valve leaks as does the other two cylinders on that *** head!!!!!

SO it runs on three cylinders 2,4,6 AGAIN!!

So now I am out another HG and 8 More bolts, i am going to get a re-man head from auto-zone and a new HG AGAIN and new bolts AGAIN and hopefully this will be the last time this happends.

The head is $200 comes with new valves, springs, seals and a 1 year warranty.

The HG is $20 and the Bolt set is $20

That will bring me up to $825 spent on the car so far.

Not bad except that I bought this car to fix and sell and now i will have to sell it for more in order to make a profit from it.

Still I have fun learning and I am now an expert on the upper part of a 3800 Series II engine lol. :)

Well the fucking valve leaks as does the other two cylinders on that damn head!!!!!

SO it runs on three cylinders 2,4,6 AGAIN!!

WHats this mean? I thought it ran on 5 of 6 cylinders initially, and did you buy a used head or just put a valve in your bad head?

Why would replacing a valve on one cylinder cause the motor to not fire on 1/3/5? I guess I dont understand.

Sorry it was running on 5 cylinders orrignally however the front two that were fireing were not 100%.

Now I lapped in a new valve and tried to lap the others as well to get a better seal.......that didnt work as well as I hoped and now none of the front three cylinders are contributing.

I am gonna get a used head form a junkyard like I should have done to begin with. $75 with a 3 month warranty but I gotta get ANOTHER *** HG and Bolts but w.e. at least this time it will be the last time.