Compression Check/Numbers?

[jabGT]

Did a compression test on my 95 GT with 74,000 miles:

4-158 / 8-150
3-160 / 7-151
2-152 / 6-154
1-150 / 5-153

Are these numbers normal? Engine is shaking bad at idle- reason for the test.

Thanks

 

[crunchie12268]

Those numbers look pretty good to me. I would think that the reason for the vibration at idle is do to some kind of ignition problem. Do a tune up, the plug wires could be arching, porcelin on the plugs could be crack, rotor might be bad, cap could have a small crack. It could be a lot of other things, but I wouldn't get to worried until you do a tune up.

 

[281pony]

those are nice compression numbers.

as long as the cylinders are within 10% of each other they are usually fine also. 160-150 are your high-lows, so you seem to be fine.

run the codes on it yet?

 

[Michael Yount]

Pull the codes first - always.

 

[HISSIN50]

when you pull codes, dont omit doing the balance test. it will test the injectors to ensure they are all working.

i might check the spark out of each wire.

also while playing around, check out your balancer. it might be on the outs. look for elastomer coming out, the balancer wobbling, etc.

good luck.

 

[jabGT]

I didn't pull the codes. I am thinking it has to do with ignition, however I have just replaced all plugs, wires, and caps but still the same. balancer looks fine. Plugs from two cylinders looked dark and yellowish like they are getting fuel soaked, but not corresponding to the cylinders with slightly lower compression. Just resently installed a supercharger along with the vortech boost retard, I pressure checked the BTM and all worked good.

Is it possible I have a spark output problem, how would a check this?
Could I have an injector problem?
Could it be my coil?

Switching to 42lbs injectors and getting a custom tune, but I want to figure this out before taking to the dyno.

Thanks for the help

 

[crunchie12268]

wow, you need a signature... it sounded like this was a stock engine.

Well now there are some possiblities that I am not familiar with, because you have a supercharger and all. Good Luck

 

[jabGT]

yeah, sorry about that, should of been more specific. Have the problem with the blower off to.

 

[HISSIN50]

what are the plugs gapped at? stock ignition? then look at Michael's post and the first part of my above post (balance test).

good luck.

 

[Michael Yount]

jab - the engineers built diagnostics into the computer so that we wouldn't have to just guess when things weren't working correctly. The first step is always to pull the codes to see if they can point you in the right direction, or help you eliminate things. Unless you just like guessing at the answer and spending needless money while doing so, pull the codes. Here, Autozone will do it for you for free.

 

[Michael Yount]

Is it at all possible you got the wires mixed up when you were replacing them? You have the proper firing order for the car? You wouldn't be the first to do it. For me, 35 years of working on cars and I still try to put the t'stat in backwards everytime.

 

[90mustangGT]

Is it at all possible you got the wires mixed up when you were replacing them? You have the proper firing order for the car? You wouldn't be the first to do it. For me, 35 years of working on cars and I still try to put the t'stat in backwards everytime.

Yeah, sometimes the little things cause the biggest problems. The HO and non HO firing order is different, I have made that mistake more than once. The car will run but will run rough with the firing orders from one to the other.

Compression checks are good to a point, I got similar numbers, but when I did a leakdown test it told a different story.

 

[jabGT]

I doubled check firing order and all correct according to manual. I don't have insurance on the car right now so I can't take it anywhere to get codes pulled, I'll see if my buddies have a scanner or just buy one. I don't know much about scanners, does the car have to be running to pull the codes or does the compter store info? How much do scanners cost and what kind should I get?

Thanks

 

[HISSIN50]

no need to drive anywhere. links for pulling codes are all over in here. Jrichker has links and i think the SN95 sticky does too.