compression question


Rangermilitia

Rangermilitia

15+ Year Member

Joined
Nov 12, 2008
Messages
60
Points
3,101
City
San Ramon, Ca
Vehicle Year
1987, for the m
Transmission
Manual
So I have 150-160 compression on all 5 cylinders and one cyl has about 120-122psi. Can this cause a fairly noticeable missfire? As the motor heats up it seems to get better. This is a 93 4.0 explorer motor in a 87 ranger
 
Let me get this straight... You have done a compression check on an engine that CAME with used wires and plugs? That's a joke, right?
Big JIm
 
To all these 'experts' here - what does plugs / wires have to do with a compression test?

Bad plugs / wires can cause a misfire, but an injector can cause one also.

To the OP - please describe your testing procedures for the compression test - we can proceed from there.

Bird
 
Last edited:
Agree on description of test(s)

Wet? Dry? All cylinders with plugs out?
 
Thanks bird for answering my origional question.. I'm not a dummy and know what good plugs and wires are... just for a piece of mind I did replace plugs and wires and.... same results, thanks for the good info.... not a joke, right? Anyway I did the compression test with a cold motor, all plugs in except for the cylinder I was testing. I cranked the motor until the needle on the tester stopped moving. I am kindof leaning towards the coil pack.. any ideas?

People only replace plugs and wires for a missfire when..... they don't know what to look for.
 
Thanks bird for answering my origional question.. I'm not a dummy and know what good plugs and wires are... just for a piece of mind I did replace plugs and wires and.... same results, thanks for the good info.... not a joke, right? Anyway I did the compression test with a cold motor, all plugs in except for the cylinder I was testing. I cranked the motor until the needle on the tester stopped moving. I am kindof leaning towards the coil pack.. any ideas?

People only replace plugs and wires for a missfire when..... they don't know what to look for.
 
Compression test is usually run with all the plugs out, throttle blade blocked open and fuel injectors disabled - all cylinder pressures should be within 10% of each other to be considered 'good'. That one cylinder may be part of the misfire problem, but still can't rule out other things.

You might run a compression test cold, then try it hot to see if expansion due to heat is changing things.

Bird
 
Cool ill do that tomorrow n report back my findings. Can I just pull the fuel pump relay to disable injectors?
 
Wet Test!

If you do get lower compression, then do a wet test to separate rings from guides.
 
Oil In Cylinders!

Yes. On the low one. See if there is a deference. Increased compression says rings. No change is valve leakage.
 
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