Low compression

[MUSTANGJOE]

Hey guys ..

just did a compression test this week. My lowest # was 115 psi and the highest was 125psi. My engine has 79 000 miles. What can cause this low score?..should I rebuild or not? Thanks

 

[srothfuss]

Depends on what you are looking to do.

If you add a Charger without a tune, you will surely chew up that motor in no time. But if you are just planning on adding new heads, cam, etc... You should be OK if your car doesn't see track useage and you take care of it.

But, if you got the money... Might as well do things right the first time and get them over with.

 

[maverick0716]

Could be worn rings or something like that. Those miles are fairly low, but they could have been hard miles.

 

[jerry beach]

Did you have the throttle plate wired wide open when you ran the test? Was the engine warm? Did you have all the plugs out when you spun it?

You can put a few ounces of oil in each cylinder and do the test again. If the #'s come up alot, then that points to bad compression rings. If you read the directions and did it properly, then fine. If not you should do the dry test over.

 

[Daggar]

Hey guys ..

just did a compression test this week. My lowest # was 115 psi and the highest was 125psi. My engine has 79 000 miles. What can cause this low score?..should I rebuild or not? Thanks

There's nothing wrong with it the way that it is. All of your cylinders are within 10%. The Ford shop manual shows a maximum difference of 25% between the highest and lowest cylinders to be acceptable. I'd say that you're WELL within that range. That motor will likely run for another 100,000 miles.

 

[BornInAFord]

Sounds ok, and like Daggar said, the most important thing is the consistency. Do you have the stock cam? An aftermarket cam often will change the dynamic compression, so your test can't be compared to other tests. In fact, often the higher perfomance cams will have even less cranking compression than the lower performance ones. Reason? Most cams have some overlap, and most aftermarket performance cams will have more overlap so they can use it to better fill the cylinder at higher RPMs. This means that at cranking speeds (400-800RPMs?), both the exhaust and intake will be slightly open at TDC.
Daniel

 

[HISSIN50]

Did you have the throttle plate wired wide open when you ran the test? Was the engine warm? Did you have all the plugs out when you spun it?

You can put a few ounces of oil in each cylinder and do the test again. If the #'s come up alot, then that points to bad compression rings. If you read the directions and did it properly, then fine. If not you should do the dry test over.

Well said, and what came to mind to me. Trying to compare (or look at compression numbers absolutely) is hard - too many variances in testing methods. and doing a wet test after the dry (like Jerry said) reveals a lot more info.

good luck.

 

[MUSTANGJOE]

The motor was cold when they did the compression test. I dont know if all the plugs were in or out.

 

[HISSIN50]

if you have forged slugs, the fact that the motor was cold can matter (numbers can be down on a cold motor with forged slugs).