compression check

GWH4.7 · Sep 22, 2008

I changed out the spark plugs on my 2003 Dakota QC 4.7 today. The truck has 60,000+ miles so I also did a compression on each cylinder. Here are the results.
cylinder Psi
1 133
3 138
5 135
7 135
2 125
4 130
6 130
8 125
I did try putting a little oil in each cylinder but it didn"t change much. My repair manual says it should be 140-180psi. So should I be concerned about these numbers or are they normal for an engine with 60,000+.

mtlcafan79 · Sep 22, 2008

I've always seen it as you are looking for cylinders that are out of line with the others. Those all fall within 10% of each other which is fine.

99dakota5.2 · Sep 22, 2008

Did you take all the plugs out and do the comp tests or did you leave them all in and then pull one by one checkin the comp?

GWH4.7 · Sep 22, 2008

one by one

99dakota5.2 said:
Did you take all the plugs out and do the comp tests or did you leave them all in and then pull one by one checkin the comp?

99dakota5.2 · Sep 22, 2008

Try takin all the plugs out then re check.

GWH4.7 · Sep 23, 2008

I can try taking them all out and retesting this weekend, but why would this change the readings?

sltdak · Sep 23, 2008

I too am interested in how checking with all the plugs out changes the reading vs checking with only one plug out.

Bad Sport · Sep 23, 2008

You also need to hold the throttle blade wide open for an accurate compression reading.

GWH4.7 · Sep 23, 2008

Bad Sport said:
You also need to hold the throttle blade wide open for an accurate compression reading.

I did hold the throttle blade wide open. But i still don't understand why all the plugs should be out when you do the test?:huh:

GWH4.7 · Sep 26, 2008

Dose anybody know why I should take all the plugs out to do a compression test? Are the numbers I have bad because they are below factory specs?
Thanks for any info.

caulk04 · Sep 26, 2008

Perhaps your gauge isn't calibrated correctly.

Since they're all quite close, I wouldn't worry about it.

TazRango · Sep 27, 2008

GWH4.7 said:
Dose anybody know why I should take all the plugs out to do a compression test? Are the numbers I have bad because they are below factory specs?
Thanks for any info.

I'm not 100% positive, but I believe it just allows the engine to turn faster. It's not fighting the dead cylinders on their compression strokes, and the test cylinder can pump up better.. It would also be a lot less strain on your starter and battery. At least that's what makes sense to me.

Since your readings are all well within 10%, and they didn't change when you did the wet test, your engine sounds fine. If you take the plugs out, the readings will go up across the board.

GWH4.7 · Sep 27, 2008

TazRango said:
I'm not 100% positive, but I believe it just allows the engine to turn faster. It's not fighting the dead cylinders on their compression strokes, and the test cylinder can pump up better.. It would also be a lot less strain on your starter and battery. At least that's what makes sense to me.

Since your readings are all well within 10%, and they didn't change when you did the wet test, your engine sounds fine. If you take the plugs out, the readings will go up across the board.

Thanks, that would make sence that with all plugs out it would be easier on the motor. I might give it another try if time permits. Thanks again TazRango

AIR_RAM · Sep 28, 2008

TazRango said:
I'm not 100% positive, but I believe it just allows the engine to turn faster. It's not fighting the dead cylinders on their compression strokes, and the test cylinder can pump up better.. It would also be a lot less strain on your starter and battery. At least that's what makes sense to me.

Since your readings are all well within 10%, and they didn't change when you did the wet test, your engine sounds fine. If you take the plugs out, the readings will go up across the board.

Yes... thats just about it.

You do want to rig the throttle body open as well. The object is to allow the engine to spin as freely as possible and reach as high as an RPM as it can while cranking. This will allow more air to be trapped in the cylinder when the intake valve closes which will raise the CR reading.

SPEED SAFE, AIR RAM

howardk · Aug 14, 2014

In my opinion, DO NOT take out all the plugs. . . go one by one. Blow out around the plug before removing, and NEVER crank with an open plug hole. You are just asking for trouble, getting dirt or something in the combustion chamber by the air drawing in to an open spark plug hole.

Yes, Wide Open Throttle, and kill the ignition system. . . the compression will build to get a good reading in a handful of complete cycles (within 10 rotations), so 5 seconds of cranking on a good battery will do it. . .

I don't know the spec for this engine, but suspect the gauge is out, or maybe the hose is crazy long, which increases the overall area and reduces compression readings. . .

TazRango · Aug 15, 2014

The standard procedure for a compression test is - and always has been - to remove all the plugs.

And that's besides the fact that this thread is 6 years old.

howardk · Aug 15, 2014

It is my opinion and experience to go one by one. I gave my reason and you will find no difference in readings either way.

bballbrownie34 · Aug 15, 2014

:forehead:

Arisley · Aug 15, 2014

If you don't remove all the plugs, you will not get an accurate reading. Proven by the readings he got above. Clean the engine with compressed air before removing the plugs, and you don't have to worry about anything getting sucked down the motor.

I have been doing compression tests for fifty years and have never sucked anything down a cylinder. Always have removed all the plugs. Follow TazRango's advise and you will have no issues.

Oop's, Sorry TazRango, missed the part about being a six year old thread.

howardk · Aug 15, 2014

Thanks for your opinions guys.

Lots of people get information from old threads, so not sure why the age of the thread is relevant.

I built my first race engine in 1976, so you have me beat by more than a decade, and it is never to late to learn something new. Although I have never had a problem getting full readings one-by-one on any engine in the past I have not tested a Dodge Dakota engine before, but can't see why it would be different. I am about to do a H.L. Adjuster change out on a 4.7 engine and will see how the compression check goes at that time. In my experience the cranking RPM is not a lot different plugs-in versus plugs-out, basically because compression stroke is followed by the decompression stroke, which adds energy back(power stroke when running).

compression check

GWH4.7

mtlcafan79

99dakota5.2

GWH4.7

99dakota5.2

GWH4.7

sltdak

Bad Sport

GWH4.7

GWH4.7

caulk04

TazRango

GWH4.7

AIR_RAM

howardk

TazRango

howardk

bballbrownie34

Arisley

howardk

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