Hi all,

Truck is a 1991 pickup, ex cab 4x4 with 256k miles on the chassis (engine unknown but rebuilt at some point).

I ran a compression test today out of curiosity and would like some help interpreting the results. I did the test warm, throttle wide open as you're supposed to. I tested each cylinder 3-5 times because I kept getting a variety of results, so I'm not sure which results to trust. I will list the highest reading I got for each cylinder, and then the other results.

Cylinder 1

• High: 140
• Other Results: 100, 90, 110, 115

Cylinder 2

• High: 120
• Other: 90, 90

Cylinder 3

• High: 130
• Other: 90, 90

Cylinder 4

• High: 130
• Other: 75, 120

As you can probably imagine, I was left scratching my head after each test. I cranked the engine for about 6-8 compression strokes per test, so that could explain some of the variance. Even so, I'm not sure if I should just take the highest reading for each, or if I should take the 'average' reading...thanks for any help in advance!

Also, did not run a wet test as I didn't have an oiler available.

 

A helper really goes a long way when doing compression tests, but in most cases you can rig the guage where you can see it from the seat or use a "hot shot" (remote starter).

You want to crank untill the guage quits climbing any significant amount, while keeping in mind the duty cycle of the battery and starter or you can over heat things like the starter or fusible links. There should be a good rest period between cranking each cylinder that allows everything to cool back down. Also having a "good" jump box attached will alleviate any concerns of battery voltage "sag".

From here I'd say you probably had a leak in the testers fittings.

 

Was it hot or cold?
go with the highest readings.
it looks fine to me for 256k.

agree with CO 94 PU, maybe tester leaky fitting.

 

I just did this in the spring too, crank until you don't see it move and then check it. I had it setup so I could see it through the windshield while cranking but also had a buddy come by a verify. I checked it in cold weather and had 150+ across all six not sure if that makes a difference, odo reads 270k no known rebuilds.

 

Another variable is location (altitude above sea level). If you're in Denver at 5000 feet 130-140 psi is an excellent reading.

I would agree that the highest reading is probably the most meaningful. Basically watch the gauge and crank the engine until the needle stops going up.

 

Thanks to everyone for the replies. I have updated information for those who are interested!

I ran another compression test today because I wanted to add the wet test to check piston ring wear but decided to get a baseline dry test again for the sake of consistency. What I did differently is that I only cranked the engine 10 times per cylinder and made sure to keep strict count of how many compression strokes I hit (10). Additionally, I took one test per cylinder and immediately moved on to the next. My theory was that last time I tested, the first test yielded the lowest psi and the following tests were higher due to residual pressure in the cylinder even after releasing the pressure valve on the tester. So I tested each cylinder at 10 compression strokes once, then went back through and tested again.

Results:
Cylinder 1: 105
Cylinder 2: 105
Cylinder 3: 107
Cylinder 4: 105

When I tested my theory about residual pressure, I did it on #1. First test yielded 105 psi. I released the pressure via the release valve but didnt disconnect the tester from the cylinder and immediately tested again at 10 strokes. Result was 150psi. Unscrewed the tester and tested again and got 105 again. So basically I determined that I couldn't test one cylinder more than once without disconnecting the tester or results would be skewed.

Moving on to the wet test results. I added 6ml oil to each cylinder and got:

#1: 110
#2: 120
#3: 107
#4: 118

I then added another 4ml oil to each cylinder and checked again in case the 6ml wasn't enough to distribute around the rings fully.

#1: 110
#2: 120
#3: 105
#4: 112

So overall, the wet test increased pressure by a maximum of 15 psi. Variance across cylinders was a total range of 2 psi dry and 15 psi wet.

Now, I forgot to mention: when doing these tests initially, I forgot to open the choke by depressing the accelerator pedal...smh...I felt like an idiot. So I tested again, with throttle wide open this time:

#1: 105
#2: 105
#3: 105
#4: 103

Oddly, the test yielded the same result. Now, out of curiosity, I decided to crank the engine to get 15 compression strokes per cylinder. Results:

#1: 170
#2: 175
#3: 175
#4: 165

Total of 10 psi range and the needle basically stopped moving around 13-15 compression strokes, so that is what I would call max compression for those cylinders.

I'm curious about what some of you said though about cranking until the needle stops. I've read a few different places that doing that can give a false representation of the cylinder health since eventually you will crank the cylinder to its max compression, which is why they say to choose a number and crank each cylinder the same number of times...thoughts?

Also, on the issue of altitude--I am at about 2k ft. I found these compression conversion factors on another website...don't know how accurate they are but here they are:
Altitude Factor
1000 .9711
2000 .9428
3000 .9151
4000 .8881
5000 .8617
6000 .8359
7000 .8106
8000 .7860

So at my elevation according to this, my 105psi would be equivalent to 111psi at sea level.

 

Further breaking it down, here's the compression value at each compression stroke of the #1 cylinder when I cranked it to the max pressure where the needle stopped.

30psi, 40, 50, 60, 70, 85, 95, 107, 120, 130, 152, 165, 170.

Across all cylinders, the first stroke yielded about 30psi and the following strokes yielded about 10-15 psi.

 

Tests were all done after bringing it up to operating temp. Also, I've heard you only want to do a set number of compression strokes per cylinder and just keep a constant number since if you keep cranking, you will eventually arrive at the max pressure regardless. Thoughts?

My interpretation of my results so far is that compression I'm getting is pretty low, but it's not the cylinder rings. The wet test didn't cause a big spike in compression for the same number of strokes as the dry test, which it would have if the rings were failing to seal. Even though I did those tests without throttle wide open, I also did the dry tests without wide open throttle. So referencing the baseline dry test against the wet test with all other constant variables, the greatest increase in compression was 15 psi. And an avg increase of 7.25psi. The reason I think the average psi jump is valuable is because if it were a ring issue, you would expect to see significant wear across all cylinders since they should all wear about the same (same reason psi should be within 10% of each other). So deviation from the average might be explainable by getting one more compression stroke in when taking the test (which is entirely possible since I wasn't able to watch the gauge until I got a helper and was going off of sound for those tests).

So further analysis would point toward a possible leaky valve issue. Some amplifying information that supports my theory that it is not a ring issue is that I had an oil analysis done by Blackstone labs. I took an oil sample at about 800 miles and they tested the contents of the oil. They found 0 fuel in the oil, and low amounts of steel (rings) compared to what is typical. So rings look to be in good shape and no obvious blow by.

Would love to hear your thoughts.

 

There is a lot of data and points here to answer on a mobile device, I'm probably gonna miss lots.


"Also, I've heard you only want to do a set number of compression strokes per cylinder and just keep a constant number since if you keep cranking, you will eventually arrive at the max pressure regardless. Thoughts?"

It's a bit of a grey area for some reason. Essentially the engine is a pump, agreed? We want to measure the maximum pressure achieved by this pump in as short of a time as possible to maintain a consistent block temperature.

Scene 1
Stroke
1, psi 25
2, psi 50
3, psi 75
4, psi 100
5, psi 110
6, psi 115
7, psi 117.5
8, psi 120
9, psi 121
10, psi 121.5

Scene two.
1, psi 25
2, psi 50
3, psi 75
4, psi 100
5, psi 110
6, psi 115
7, psi 120
8, psi 125
9, psi 130
10, psi 135

Which one of these do you think properly represents the maximum cylinder pressure achievable? (The first one has essentially stopped climbing and its safe to say it's not going to go much higher. The second one seems to want to keep climbing.)

...

"My interpretation of my results so far is that compression I'm getting is pretty low"

But you're still not holding the throttle open! This forces the pump to have to try to suck all its air through a small straw. Imagine trying to go snorkeling with a soda straw vs a proper snorkle. You understand this essentially strangled the pump, regardless of the pumps health it has a finite amount of time to breath in set by the valve duration.

...
"but it's not the cylinder rings. The wet test didn't cause a big spike in compression for the same number of strokes as the dry test, which it would have if the rings were failing to seal. "

Not holding the throttle open skews this interpretation also. When the pistons move down it creates a low pressure area in the chamber which is then filled by the higher pressure surrounding it. Primarily this will be through the idle air bypass and the PCV valve, but there is a third path through the piston rings. Holding the throttle open ensures there is a path of least resistance that vastly out scales the PCV valve and the oil channels of the piston. (IE, your dry test really isn't dry if your not holding the throttle plate open)


...
"no obvious blow by"

There should always be some blow by, it's specifically why we have vented crank cases on ICE motors, it's governed by the ring gap primarily. If the gaps are overly small you risk damaging important bits like cylinder walls and pistons when the rings start the heat expansion process.

Is this relevant, maybe not but it's an important part of the engineering of internal combustion engines.

..
I'm intentionally skipping over the "data" in that last post since the test methods are flawed. I will go back and read the prior post in detail, it's still possible to tease out some hypothesis even from poor methods sometimes.

 

Well done!

You've established a base line compression range of 175-165 psi, which is well within the 10% variance.

The next step is a leak down test which will give you insight into the health of the valve seat, if you're inclined to continue establishing a base line for later comparison.

..
Now we're to the sticky bits.. What prevoked you to have said whimsical curiosity about your compression?

 

First, thank you for the quick and detailed reply.

"Which one of these do you think properly represents the maximum cylinder pressure achievable? (The first one has essentially stopped climbing and its safe to say it's not going to go much higher. The second one seems to want to keep climbing.)"

I agree, continuing to pump in scene 2 will yield higher compression results. However, again, if you crank until the pressure stops climbing, isn't this just measuring the max compression of that cylinder? I have seen many sources that say to crank for a specified, consistent number of compression strokes. Just curious...also, we should be able to tell the health of the cylinder and its ability to compress based on the increase in psi per stroke. My understanding of the compression test is that you are looking for the characteristics of the cylinder's compression abilities, not necessarily just the max psi. I certainly am no expert so please tell me your perspective on it.

Moving on. "There should always be some blow by, it's specifically why we have vented crank cases on ICE motors, it's governed by the ring gap primarily. If the gaps are overly small you risk damaging important bits like cylinder walls and pistons when the rings start the heat expansion process."

I meant nothing out of the ordinary in this case. The oil test results yielded <0.5 units of fuel, the allowable limit for 800 miles was <2.0 units. So well within allowable tolerances.

"Not holding the throttle open skews this interpretation also. When the pistons move down it creates a low pressure area in the chamber which is then filled by the higher pressure surrounding it. Primarily this will be through the idle air bypass and the PCV valve, but there is a third path through the piston rings. Holding the throttle open ensures there is a path of least resistance that vastly out scales the PCV valve and the oil channels of the piston. (IE, your dry test really isn't dry if your not holding the throttle plate open)"

The results from the throttle wide open dry test and throttle closed dry test were the same in my case. I agree, my test initially was flawed. However, when using the correct testing procedure the results were the same. So I don't think the wet test results can be completely discounted. The wet test did not cause a spike in pressure from the initial dry test. I think these wet test results are valid since the same (incorrect) testing procedure was used for both the wet and dry test. The variable of 'throttle closed' remained the same, so in both tests, the same amount of air was being delivered to the cylinders on the intake stroke. My understanding of the wet compression test is that you allow oil to fill the gap between the cylinder walls and piston rings, creating a wet seal. If this creates a spike in pressure, you know the rings are worn since the oil created a better seal than without. If the pressure doesn't spike fairly dramatically, the piston rings are already creating a good seal without the oil. So, I think my wet test results (though tested with throttle closed) should still reflect worn piston rings. There was no dramatic increase in pressure that would indicate oil creating a better seal with the rings. Again, the same amount of air should have been entering the cylinders between the wet and dry test with the throttle closed in both cases. I think I understand what you're saying about the cylinders being an air path themselves. The cylinder on the intake stroke is pulling air in due to creating a vacuum. This air comes via the intake valves which are supplied with air from the throttle body when throttle is open. When throttle is closed, the air comes from PCV and idle air bypass and maybe the block via the piston rings. So if air is being sucked into the cylinder on the intake stroke through the piston rings, in theory, more air enters the piston and compression will be greater. I agree, this makes sense. But when I did the test with throttle wide open, the compression readings were the same. So maybe my PCV and idle air control valve are already letting in enough air to generate the same compression as if the throttle were wide open?

"But you're still not holding the throttle open! This forces the pump to have to try to suck all its air through a small straw. Imagine trying to go snorkeling with a soda straw vs a proper snorkle. You understand this essentially strangled the pump, regardless of the pumps health it has a finite amount of time to breath in set by the valve duration."

I realized after the fact that I had made that mistake and then conducted another compression test, this time with throttle wide open. The results in my earlier post demonstrated that there was very little to no increase in pressure with all other variables constant. I tested again by cranking for 10 compression strokes with throttle wide open. Here's the data from the earlier post:

"Now, I forgot to mention: when doing these tests initially, I forgot to open the throttle by depressing the accelerator pedal...smh...I felt like an idiot. So I tested again, with throttle wide open this time:

#1: 105
#2: 105
#3: 105
#4: 103"

So again, no increase in pressure between throttle wide open and throttle closed in my case.

Again, thank you for the reply. I'm certainly learning.

 

Haha, the interest in compression was driven by a hunger for more power! There are some hills I'm not able to climb without downshifting into fourth or sometimes even third...

I didn't know if this was normal, but I have heard and know from experience that these old 22res are no power wagons