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compression question


Rangermilitia

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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
 


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badtxz

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Are the wires and plugs new?
 

Rangermilitia

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?

Fairly new.. but bought the motor with them..
 

Big Jim M

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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
 

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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
 
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Blueox

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Agree on description of test(s)

Wet? Dry? All cylinders with plugs out?
 

Rangermilitia

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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.
 

Rangermilitia

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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.
 

Rangermilitia

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Sorry for the double post... damn cell phone tweakin out on me
 

Bird

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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
 

Rangermilitia

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

Blueox

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Wet Test!

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

Rangermilitia

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Ok. Just put some oil in the cylinders?
 

Blueox

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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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