• The site migration is complete! Hopefully everything transferred properly from the multiple decades old software we were using before. If you notice any issues please let me know, thanks! Also, I'm still working on things like chatbox, etc so hopefully those will be working in the next week or two.

Blown motor? Head gasket? HELP PLEASE!!!

fpgt17

fpgt17

New member
Okay so my 1999 Grand Prix which is very well taken care of and in great shape is having some major problems.I started the car last week and it fired up perfect like always and then two minutes into it running (warming up) it started chugging as if a coil went out. I immediately shut my car off. I then tested the coils, ICM, changed plugs (they were shot), tested fuel pressure and was running just over 50PSI. All looked and tested great. Today i then decided to pressure test/ leak lease my cylinders (was affraid to at first). I have 150psi on all the back cylinders, 150psi on one in the front, 90psi on another and ZEROOOO on the front middle one!!!!! When i did the leak down test on the cylinder with zero compression it was at 100% leakage. I cant tell where it is going.

Heres what i have come up with/ been told.....

- Blown head gasket (but no coolant in oil or oil in coolant) Maybe didn't run long enough to get any in tho.... But usually with a blown head gasket i at least get more than zero compression!

- Blew a hole in my piston and that's why i have zero.

- crank sensor. (Off time and valves are open when shouldn't be and its letting air escape? idk......

HELP!!!! let me know what you guys think! THANKS!
 


01GTP is on the right track. Re-do the cylinder leak down test. Check the intake plenum, pull the oil fill cap and listen, the dipstick tube and listen, listen to the tail pipe (sign of a valve hung open or a burnt valve), check the rad for any signs of bubbles. Use a stethiscope and probe anythign and everything, especially down the dipstick tube.

Jay
 
Well i retested the front three after i did the leak down test.... Now the front three are all giving me ZERO! Could it be possible that when i pressurized the front to do the leak down test it made the head gasket go out more.... i am leaning twords it being that now. :( I don't feel any air coming from the oil filler cap or exhaust. But there is no way all the front valves could be off giving me a compression of zero on three different cylinders right?
Man my gp is not making me happy.....
Thanks for the reply's tho guys! helps allot!!!!
 
I just dont see how the headgasket or valves could go bad in 2 minutes of running it. Plus is was running perfectly fine the night before.... but i was told that the CPS wouldnt give me zero compression tho. The piston and rings will still create the seal.... AHHH idk :( What does everyone think?
 


Are you opening the throttle blade wot when your doing it? Are you only pulling the plug for that cylinder? Is the engine up to operating temp?
 
Last edited:
When doing a compression test, you need to have ALL the plugs out, and the throttle at WOT while cranking. Test one cylinder at a time, and record your results. The engine doesnt necessarily have to be at operating temperature, makes it a little hard to do without burning yourself a few times while installing the test hose.

The other thing I suspect is the schrader valve in your compression tester is acting up. Not an uncommon thing. Pull the schrader valve out and make sure its working properly. Replace it if necessary. Its the same schrader valve that your tire valve stem uses. You could borrow one from a valve stem if you dont have spares. Generally a small flat screwdriver will remove the schraders if you dont have a valve core (schrader) tool.


Hope this sheds some light.



Jay
 
Another thing, just to be sure your doing it right...we dont all know eachothers mechanical abilities, and as a diesel mechanic I like to help out where I can..

anywho, when you were doing your cylinder leakdown test, did you have each cylinder at TDC (Top Dead Center) on the compression stroke while testing? If not, you will most deff get a zero reading for that cylinder.


Jay
 
you should also dump a little oil into the cylinder just to help seal similarly to how it would be when the car is running.
 


Well i quess i was doing it wrong. Its been a while since the last time i had to do a compression test. I had all the plugs in and only took out the one that i was putting the test hose in. So the other five were in while i did the test.... Also idk if it was at TDC. i am not the best with testing motors/ figuring things out but i know how to fix allot of it. How do i make sure/ put it at tdc. Last time i did that the motor was out of the car and i could see the piston heads and i could turn the crank.....
 
Check your compression tester. I've done quick tests with all plugs but the one, heck even let the car start.... IT's not the right way, but to test for some compression, it works.

There's no understandable way that you lost two cylinders during testing. Taht's why I suspect the tester.
 
Well the tester is brand new. I have zero compression on the front three cylinders. I also did a leak down test on the front three which were 100% leakage......So i dont think its the tester. Both brand new testers were saying zero for the front three. But i had the rest of the plugs in and the throttle body was not open. I want to re-try it the "right way"
 


Now there's two compression testers or one compression, one leakdown?

Here's what I'm seeing. A newer guy, that hasn't had time on the forums to see how nearly indestructible these factory head gaskets are. Out of a million of these motors, I'd be surprised to see more than 1 HG issue.

Popped pistons.. heck..that's common, but not on 3 cylinders.
 
Yes i am new on this forum and know the HG are very strong and that is why i am making sure everything is right before it tear into it....
There is one compression tester, one leak down cylinder tester. Both bought new.
I did the test without the plugs out and throttle not open. Gotta re-test it with all the plugs out and throttle open.

Also i was told that it could be a CPS (Crank Position Sensor) Or even timing off.
Could either of those cause zero compression? i know they can give you less compression but idk about zero..... like you said these hg's are very strong and i would not blow out three pistons at once. I mean it only ran for 2 min... didn't overheat and wasn't even warmed up. It just started shaking after 2 min like i lost a coil. But all fuel and spark tested good.
 
Dont really know how to test at tdc.... Only done that when engine was out of car and could see the pistons..... well if timing couldnt do that and neither could my CPS....... wouldnt that lean twords my head gasket? idk what else to check.....
 
You must log in or register to reply here.
Back
Top