weird compression test, too high?? (plus lead up story)
08-22-2007, 10:35 PM
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weird compression test, too high?? (plus lead up story) So I have been having some problems with my car not running right. This is on a '98 LS1 with an all stock longblock. Here is the back story.
It got over revved on a 2-3 shift that turned into a 2-1 shift. At the time it probably revved to ~8-8,500.
Of course, it bent a few pushrods. I changed them out and it bent another one on one of the same cylinders that got bent from the first over rev. This was just in normal driving? I revved it to maybe 6,000 and it bent a pushrod. So I changed that pushrod and after that it ran fine. Perhaps a very slight tick that was not there before the over rev, but otherwise it seemed normal.
Over time (5-7,000ish miles?) the noise has gotten louder. This noise is in the top end. Not a deep sound, but a fairly light metallic clacking sound. It is there pretty much all the time no matter if its hot or cold. Most notable around 2,300 rpm.
Oil changes show little to no metal. Engine is not using much oil at all. Oil pressure is good. When oil is changed, very very little metal is on the magnetic drain plug.
So the car is not running well lately. There seems to be a very light miss at idle and through the revs. And... the car does NOT like to rev over 4,300 RPM. When it does it totally breaks up and runs rough as if it is down a few cylinders. Once you get back below 4,300 for a few minutes, then it will smooth back out. I'm telling you it is like a light switch. 4,300 rpm, BAM, engine runs like ****. Its instant. I had a buddy hook up HpTuners and we took it for a ride. He says there is no sign of anything wrong, even when the engine is running like ****. O2's show ok, ignition is not retarding, etc. There are no codes when it happens either.
So this has left me wondering what the hell is going on. So tonight I did a compression test on the engine. The test was done after I drove the car home from work and it sat for 2.5-3 hours to cool off. No oil in the spark pulg holes. Here are the results:
Driver side bank (front to rear) 165, 165, 170, 200
Passenger side bank (front to rear) 165, 165, 175, 205
What in the hell is that all about?! The rear cylinders are like 40 psi higher than the other 6?
I have read 165 range is a little bit low but not really too bad. 200+ psi is higher than it should be according to the specifications right? What is causing this? Or am I wrong and have 2 good cylinders and 6 f'ed up ones?
It may be noteworthy that the 2 rear cylinders are the 2 that bent the pushrods the worst. To about a 45 degree angle. One of the other cylinders (can't remember which) just had a slight bow to one of its pushrods. It was also the driver rear cylinder that bent a pushrod not even going to the rev limiter.
I am wondering if I don't have some colapsed lifters in those clyinders that are causing it? Would that make sense?
Any thoughts on this??
It got over revved on a 2-3 shift that turned into a 2-1 shift. At the time it probably revved to ~8-8,500.
Of course, it bent a few pushrods. I changed them out and it bent another one on one of the same cylinders that got bent from the first over rev. This was just in normal driving? I revved it to maybe 6,000 and it bent a pushrod. So I changed that pushrod and after that it ran fine. Perhaps a very slight tick that was not there before the over rev, but otherwise it seemed normal.
Over time (5-7,000ish miles?) the noise has gotten louder. This noise is in the top end. Not a deep sound, but a fairly light metallic clacking sound. It is there pretty much all the time no matter if its hot or cold. Most notable around 2,300 rpm.
Oil changes show little to no metal. Engine is not using much oil at all. Oil pressure is good. When oil is changed, very very little metal is on the magnetic drain plug.
So the car is not running well lately. There seems to be a very light miss at idle and through the revs. And... the car does NOT like to rev over 4,300 RPM. When it does it totally breaks up and runs rough as if it is down a few cylinders. Once you get back below 4,300 for a few minutes, then it will smooth back out. I'm telling you it is like a light switch. 4,300 rpm, BAM, engine runs like ****. Its instant. I had a buddy hook up HpTuners and we took it for a ride. He says there is no sign of anything wrong, even when the engine is running like ****. O2's show ok, ignition is not retarding, etc. There are no codes when it happens either.
So this has left me wondering what the hell is going on. So tonight I did a compression test on the engine. The test was done after I drove the car home from work and it sat for 2.5-3 hours to cool off. No oil in the spark pulg holes. Here are the results:
Driver side bank (front to rear) 165, 165, 170, 200
Passenger side bank (front to rear) 165, 165, 175, 205
What in the hell is that all about?! The rear cylinders are like 40 psi higher than the other 6?
I have read 165 range is a little bit low but not really too bad. 200+ psi is higher than it should be according to the specifications right? What is causing this? Or am I wrong and have 2 good cylinders and 6 f'ed up ones?
It may be noteworthy that the 2 rear cylinders are the 2 that bent the pushrods the worst. To about a 45 degree angle. One of the other cylinders (can't remember which) just had a slight bow to one of its pushrods. It was also the driver rear cylinder that bent a pushrod not even going to the rev limiter.
I am wondering if I don't have some colapsed lifters in those clyinders that are causing it? Would that make sense?
Any thoughts on this??
Last edited by theantirotor; 08-22-2007 at 10:42 PM.
08-24-2007, 08:33 AM
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I overrevved my 98 stocker a couple months ago (nitrous mishap) I bent 3 pushrods. did a compression test . Pulled the heads and every intake valve had hit the pistons. it only bent one though. that was the only cylinder to have poor compression. From the sounds of it you need to pull the heads. I will bet that you know have fancy custom valve releifs in your pistons.
08-24-2007, 06:11 PM
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Have you checked your puchrods again? I guess that a bent pushrod or collapsed lifter that is allowing the intake valve to close sooner could give more compression.
