Significance of Compression test results


#1

What do the compression test results tell me about my 318 Dodge Ram engine? The low figure is 100, the high is 150 and the others vary from 110 to 130. I’m trying to figure out if the engine has enough life in it to justify replacing the transmission that just dropped (212K miles on both). Engine seems to run smoothly. Trying to stretch $$ in this tough economy.


#2

I don’t know off hand what the readings SHOULD be, but if the test was done correctly, there should be no more that 15% differnce between the best and worst readings if the engine was in good shape. In your case that’s much more, indicating you may need engine work (valves, rings?). In US cars with fairly large engines, if the engine gradually wears out, all cylinders may drop at the same rate, and if the difference between the best and worst does not exceed 15%, I would just keep driving such a vehicle until it would not pass the emission test due to excesive oil consumption.

Based on the readings you present, I think they are suspect, and I would get another test. In any case a large V8 will keep running even with 2 bad cylinders, unlike a 4cylinder car.

In a car like yours , I would go for a good used transmission, or a rebuild from a reputable non-chain shop.


#3

You should perform the compression tests (and, leakdown tests) with the engine as warm as you can stand it. Naturally, a cold engine will leak much more than a (very) warm engine.
It seems that a lot of “mechanics” do the compression tests on cold engines.


#4

Doc’s post was excellent as usual. However, it the engine is running smoothly and well with good power and reasonable oil usage (1qt per 1500 miles or less allowing for the 212K) and no disappearing fluids, and the rest of the vehicle is in good shape, then I truthfully don’t think a compression test is important in the decision.

A boneyard tranny of lower mileage would be the way I’d go.


#5

Was this “compression test” performed electronically (by shorting cylinders) or with an actual pressure gauge? That 100 number is not good…But a valve job on a 318 is no big deal either…


#6

Thank you everyone! Engine is running smoothly with power, oil usage around 1qt per 800-1000 miles. Mechanic just said figures meant “wear” but didn’t explain beyond that. So, now I’ve got a better idea. Used tranny is $1000 and installation is $1000, so I was trying to make a decision based on fact rather than emotion (love that truck!). I think I’ll get the tranny fixed and drive into the sunset. PS: I couldn’t find anyone who could answer this question, so thank you all.


#7

I would hope for a rebuilt automatic transmission for $1,000. That figure of $1,000 for labor, sounds high, to me, also. Why don’t you shop around?


#8

Many tuneup shops now do just that, and the computer will show % of normal compression without event taking the spark plug out. This is not as accurate, but often good enough.

The last test I had done was on a small block Chevy with 200,000 miles on it. The results showed 6 cylinders at 100%, and the other two at 93% and 94%. This basically proved the engine was wearing normally and had a lot of life left in it. I gave the car to my son who drove it till 300,000 miles and it still was not burning oil at that time. Today, 5 years later a kid somewhere is still driving that car, since we see it every now and then. Still no blue smoke!


#9

Yes, the engine is worn, It’s due to either rings, valves, or both. A wet compression test could be done to determine how much of a factor the rings are in this.

The 150 is somewhat tolerable; the 100-130 is low and it appears to run smoothly because all of the readings pretty much suck.

The compression readings should be roughly 20 X the compression ratio. (9 to 1 means 9 x 20= 180 for example).
This will vary a bit based on altitude, barometric pressure, engine wear, etc.

As mentioned, the readings should all be close to each other.


#10

Hey, I took your suggestion and found a transmission shop in town who quoted $575 - thanks for the alternative.


#11

Pickup trucks are never scrapped unless there is a lot of body damage. Somebody always puts them back on the road. It may not be economical for you to keep it but it usually is. Depends on what year truck and condition. The older the better to keep, especially if it is needed for work. You don’t want to buy a newer one and lose the transmission in it.