what should compression numbers be on a 2.9l


Clay46

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I just picked up a 1987 ranger 2.9, its missing a few things so i decided to do a compression test before i fixed it up. my results were 120 on 3 cylinders, 115 on one cylinder and 110 on two cylinders. I was wondering if these numbers are ok. I know the general rule is no more than 10% difference between max and min but i can't find anything specific that says what the compression numbers should be on a 2.9. so do these numbers seem good?
 


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Psi doesn't matter. More things effect cylinder psi readings than just how "good" the motor is. Not all gauges are even equal.

Do a wet test then compare all the numbers together. But only 5 psi difference between cylinders on a dry test is a good start.
 

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Those numbers a bit to the low side, but not horrible.

Dirtman is right for the most part, the only thing I'd add is that starting around 90 PSI there isn't enough pressure in the cylinder to get the fuel mix to burn when cold, so if your compression drops below 90 PSI then it won't run.

That's still 20 to 25 PSI difference, and it can take a long time to loose that much.
 

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Well my main point is most cheap gauges are never very accurate so 115 psi could really be 150 psi... put 3 different gauges on you'll get 3 different sets of numbers. Which is why I rarely pay attention to the specific number (unless its drastically off) and just go by what they are between cylinders, and between wet/dry.
 

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Well my main point is most cheap gauges are never very accurate so 115 psi could really be 150 psi... put 3 different gauges on you'll get 3 different sets of numbers. Which is why I rarely pay attention to the specific number (unless its drastically off) and just go by what they are between cylinders, and between wet/dry.
I was using my roommates harbor freight compression tester which i know is a cheap tester so probably not very accurate, ill do a wet test later today along with i want to do a leak down test in the next few days. this was my parts truck and i know no history of the engine just that some things like the distributor and idle air control valve were taken off, still going through it to see whats missing. just trying to test it to see if its a good engine before i go ahead and put new parts on it. only reason for trying to fix it is because i totaled my daily last week which is the exact same truck just different color so if worst comes to worst ill just pull the engine from my totaled one which i know was a good engine.
 

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Its very rare you see a truely worn out 2.9.

They are pretty hardy little bastards.

Even with those compression numbers id say its fine.
 

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You need to test all 6 cylinders at the same time with all 6 spark plugs removed for the test, and hold throttle wide open, gas pedal to the floor, when cranking

Crank speed effects compression, because metal rings on/in a metal cylinder and metal valves on metal seats DO NOT hold air very well :)
Its a temporary seal at the very best of times, so the speed of the piston compressing the air matters a lot

So all cylinders except the test cylinder should be open, no spark plug, so their compression doesn't slow down the crank speed

Test all 6 and write down results
Gauge accuracy then won't matter too much because you have a larger sample, 6 numbers not just 2
Then do the WET test and write down results
WET test numbers will always be higher, thats normal since oil seals the rings better, but how much higher determines if the rings in one or more cylinders are a problem

A Vacuum gauge($20) can also tell you if compression is lower that it should be on a running engine
 


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