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Compression Testing


KYRangerDude

Well-Known Member
Joined
Sep 24, 2010
Messages
85
City
Whitley City
Vehicle Year
1992
Transmission
Automatic
92 Ranger 4x4 2.9

Since I have nothing else to do, I'm gonna do a compression test on my 2.9
What's an acceptable number? I've done this on my chevy 305 and it was 120lbs, don't know if it's the same for the 2.9 or not. I'll be looking for variances as well.
 
They tend to vary engine to engine. I am not sure what the 2.9 is "supposed" to be, but as long as they are equal across the line and not horribly low, then you are fine. That is unless you are throwing a turbo on it.
 
Results: #1= 110 #2= 115 #3= 110 #4= 105 #5= 110 #6= 115

While I realize compression tests in themselves doesn't indicate a "healthy" engine. I suppose they would give an indication of possible issues.

I don't know if these are "good or bad" results. They seem fairly consistent.

I did however see on #6 plug some gas. Excess fuel being injected into that cylinder, could it cause my rough idle and getting a #12 code?
 
those numbers look good, as stated before good rule of thumb is that as long as they are all within 5%-10% of each other and above 90 psi it should be fine.

the number itself will vary massively from engine to engine, my truck's 350 has 225-235 across the board
 
IIRC think 110psi is around the minimum of what it should be. Rebuilt one and it got 175PSI, my Tempo gets 130-135psi w/ 180K on the clock. Looks pretty consistent, so that's the main thing. A little low, but should run.

Excess gas in #6 cyl can sometimes be a ruptured fuel pressure regulator - the vacuum line for it is directly above this cylinder. If it ruptures and starts leaking gas through that vacuum line....
 
Ideally you want 130 - 175-80psi.
The lowest compression cylinder tested must be with in 75% of the highest cylinder tested or if it runs at all it will run like doo doo.
By all accounts you should be good. A little worn and a little power lost but your truck has decent compression for a well used 2.9L
The reason why the PSI varies so massively from 2.9 to 2.9 is a combination of things just like in any other engines paired together. BUT especially true for the north american 2.9L. Meaning, The cam thrust plate, valve train oiling and cam shaft oiling problems due to relying on only TWO oiling ports that were fed by TWO cam bearings, lack of coolant flow etc etc etc that may or may not be present and or noticeable yet to their respective owners at the time of testing. Also the year of the 2.9's being tested matters as well.
Now a 92' does have the revised heads BUT still suffers from poor oiling and cooling flaws. All the revised heads were, were basically heads with extra material by way of squared pedestals versus oval pedestals, a better sealing valve cover gasket surface, and more material in the valve stem guides. They solved the head cracking problem but left you with the inevitable cam and valve train destruction by way of poor oiling and cooling problem. :-)
While the use of synthetic oil, timely changes, and coolant flushing regimens help prolong their life span ( like any other engine ) they will not stop the inevitable valve train carnage.
 
Did you do the test hot or cold. Compression/leakdown and vacuum pressure will tell you the health of the engine. Valve lash will affect compression also but ur numbers look ok put a vacuum guage on it if it holds above 17-20 at an idle and steady run it till it pukes. Oil pressure will tell you the condition of the lower end bearings. I know what you mean about being bored looking for productive stuff to do. In my opinion you cant do too much preventive maintenance on your vehicles. Fixing leaks and thorough inspections can save alot of money in the long run. May as well get it ready for winter.
 
do it again with the engine hot and the throttle wide open and you'll get higher numbers.

nothing to worry about with those numbers though
 
I did the test on a cold engine. I didn't want to deal with getting my fingers around a hot manifold. I may not have done it correctly either....after screwing in the hose and connecting the gauge I turned the motor just a few cranks...the engine almost running. Once it did run and I got a 145 on that cylinder. I reset the gauge and cranked it the same as all the others.
 
When it is cold the havent expanded and there is little oil around the rings Do you have a vacuum guage.
 
No vacuum gauge. I can do the opposite tho...blow air into the cylinder. I've done this on a car engine and depending where it comes out tells what/if anything is wrong...I don't know about an EFI engine tho....
 
Unplug your coil to keep it from trying to start.
 

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