HELP HELP HELP low compression!!
Thread Starter
Intermediate
Joined: Mar 2015
Posts: 48
Likes: 1
From: Beirut
HELP HELP HELP low compression!!
Hello guys
I spent all my savings restoring my 86 corvette to its original state but it always finds a way to disappoint me
Today I installed a new 700r4 transmission (old one was burnt), new spark plugs and leads. I also done a compression test while doing my plugs. Finds out that my engine is running low on compression, 7 bar each cylinder (100psi) which is pretty low I guess (7:1).
About a year ago, the timing chain snapped off while driving so I had to replace that and rebuild my top end. Car seemed to run fine until today I found out that it is running weak.
I have no idea what to look for and how to start digging into it? It gave me a hard time lately and im over spending more money on it.
Can someone please lead me to the right way so that I wont blow my money everywhere? Could it be a bad bottom end? Top end? I dnt know.
also the car is not using oil or even smoking.
Thank you for your help, really appreciate it
I spent all my savings restoring my 86 corvette to its original state but it always finds a way to disappoint me
Today I installed a new 700r4 transmission (old one was burnt), new spark plugs and leads. I also done a compression test while doing my plugs. Finds out that my engine is running low on compression, 7 bar each cylinder (100psi) which is pretty low I guess (7:1).
About a year ago, the timing chain snapped off while driving so I had to replace that and rebuild my top end. Car seemed to run fine until today I found out that it is running weak.
I have no idea what to look for and how to start digging into it? It gave me a hard time lately and im over spending more money on it.
Can someone please lead me to the right way so that I wont blow my money everywhere? Could it be a bad bottom end? Top end? I dnt know.
also the car is not using oil or even smoking.
Thank you for your help, really appreciate it
Last edited by eliebarcham; May 13, 2015 at 08:15 PM.
Melting Slicks
Joined: Jan 2014
Posts: 2,183
Likes: 381
From: Lunenburg MA
Did you test it with all the plugs out, at operating temp, with the throttle wide open, for at least three compression cycles? Are the sure the gauge is accurate and was in tight? Are they all close to each other? Does adding a teaspoon of oil to the cylinder raise it?
If they are are close to each other, it's probably fine.
If they are are close to each other, it's probably fine.
Safety Car
Joined: Aug 2009
Posts: 3,745
Likes: 275
From: Melbourne, Victoria, Australia
Did you test it with all the plugs out, at operating temp, with the throttle wide open, for at least three compression cycles? Are the sure the gauge is accurate and was in tight? Are they all close to each other? Does adding a teaspoon of oil to the cylinder raise it?
If they are are close to each other, it's probably fine.
If they are are close to each other, it's probably fine.
Don't worry about it, are you sure the valve lash is ok? factory specs say one turn from zero lash. (when you can no longer turn the pushrod by hand is zero lash)
I know some lifters only need half a turn, just something to look at,
if they are all the same don't worry about it.
Have you adjusted the TV cable correctly, a new trans can slip and burn if the cable is too loose. Check the cable tension with full throttle travel, there should be minimal slackness in the cable.
Check your timing, 6 dbtc?
Do not rely on the harmonic ballancer, the outer ring can slip making the 6degrees actually 20 degrees retarded.
Having retarded ignition will make the corvette feel like a slug.
You really need to be sure that the no.1 piston is at top dead center with timing mark on tdc.
How is the Throttle position sensor? set 0.54 volds DC, check it goes to 4 volts dc at full throttle travel.
The ecm can get itself in knots trying to compensate for any problems, reset the ecm (disconnect / reconnect the ecm cable near battery) or disconnect reconnect battery and see how that goes.
A major problem will have a check engine light code pop up, have you checked for any stored codes?
Last edited by gerardvg; May 13, 2015 at 11:21 PM.
Thread Starter
Intermediate
Joined: Mar 2015
Posts: 48
Likes: 1
From: Beirut
Did you test it with all the plugs out, at operating temp, with the throttle wide open, for at least three compression cycles? Are the sure the gauge is accurate and was in tight? Are they all close to each other? Does adding a teaspoon of oil to the cylinder raise it?
If they are are close to each other, it's probably fine.
If they are are close to each other, it's probably fine.
He didnt try adding some engine oil to check if bottom end is bad.
Maybe I should redo the test as you told me. Might get different readings.
Thank you very much
Last edited by eliebarcham; May 14, 2015 at 06:06 AM.
Thread Starter
Intermediate
Joined: Mar 2015
Posts: 48
Likes: 1
From: Beirut
Don't worry about it, are you sure the valve lash is ok? factory specs say one turn from zero lash. (when you can no longer turn the pushrod by hand is zero lash)
I know some lifters only need half a turn, just something to look at,
if they are all the same don't worry about it.
Have you adjusted the TV cable correctly, a new trans can slip and burn if the cable is too loose. Check the cable tension with full throttle travel, there should be minimal slackness in the cable.
Check your timing, 6 dbtc?
Do not rely on the harmonic ballancer, the outer ring can slip making the 6degrees actually 20 degrees retarded.
Having retarded ignition will make the corvette feel like a slug.
You really need to be sure that the no.1 piston is at top dead center with timing mark on tdc.
How is the Throttle position sensor? set 0.54 volds DC, check it goes to 4 volts dc at full throttle travel.
The ecm can get itself in knots trying to compensate for any problems, reset the ecm (disconnect / reconnect the ecm cable near battery) or disconnect reconnect battery and see how that goes.
A major problem will have a check engine light code pop up, have you checked for any stored codes?
I will double check if my tv cable is tight enough at full throttle but it seems fine.
I have also adjusted my timing properly. I still need to look at my tps although I replaced it few years ago.
Also the only check engine code im getting is 32 which is my EGR. I guess it is a bad sensor or solenoid.
Thank you
