6.2 compresion test results

[richingalveston]

I tested the compresion on an engine that I am selling. I have never tested a 6.2 before but have done gas motors. I have a brand new guage from Mac tools and propper tips. I first warmed up the motor to opperating temp (190 degrees with temp gun) and then began testing. The first cylinder I tested showed 500psi. I nearly burned my self, the glow plugs were to hot to handle. I also had a buisness call so I took a 30 minute break to let things cool down. When returning the engine was warm but not hot.
I retested the same cylinder and got around 440psi. All other cylinders tested between 430 and 440.
I know a fresh motor cold should be around 430 from what I have read.
The motor starts real easy, has no smoke out the exhaust, It runs and starts better than the one in my 1009.
Just checking if the hot motor should read higher.
It was the first time the gauge was used so maybe first test was not accurate. It is a very nice gauge set ($275 from my local mac tool vendor) gauge is silicone filled. I had another tester that was bought on line but it did not work, it did not have a check valve and was very poorly made. I guess I learned my lesson not to buy tools online, fortunatley I only wasted about $50 bucks to learn this lesson.

just looking for some feedback on what the readings should be hot and cold.
thanks

 

[doghead]

What does the TM say?

 

[richingalveston]

i read the tm and it did not say anything about engine temp for test. I removed air cleaner. My directions on the tester said to warm engine to opperating temp. So I did a combination of both directions.
else where in the forum I found references to fresh motors having around 430psi compresion. I searched the forum also.
Thats why I was asking on the forum if there should be difference between cold and hot compression.

Thanks

 

[doghead]

Normal compression should be 380-400 psi

 

[richingalveston]

I am going to retest again tomorrow, yes tm says 380 to 400, will test with engine cold so that I see if there is a difference since i gather no one here knows if it should be different on a hot motor vs. cold motor, that was my question.
I have read when researching turbo set up and other items, that upto 430 is considered good and anything less than 380 is bad. and if you turbo try to get it down to 380 or even 360 with thick double head gaskets.
New tester so maybe it needs some working in also.
I will post my findings tomorrow.
Thanks for the input.

 

[richingalveston]

I retested today per the tm and with the engine cold, with temp gun the block was 60 degrees. It was cold last night, down to about 38F. I tested it mid afternoon when it was about 70F. All of the cylinders tested similar to each other the lowest reading i got was 420 psi and the highest was just shy of the 430 psi mark. The tm says 380 to 400 is good and that less than 380 is bad it says nothing with regards to being over 400. It also says nothing about engine temp or atmospheric conditions when testing. Per my previous test I can conclude that a hot engine will test higher. When the engine was at 180 degrees per the temp gun, I had readings as high as 500 psi. when the engine was around 120 degrees I got high readings of almost 450.
My conclusion thus is the higher the engine temp, the higher the reading on the gauge. I can only make a few assumptions, the first being that hot air expands thus causing the gauge to read higher than what actual compression would be.
Also with a hot engine, the metal is expanded to its maximum designed size so compression could be higher when hot vs. cold.

I could not find any information on how atmospheric conditions effect the test.
I am at see level where the air is denser. Where I am located we also have much higher humidity especially the last two days I was testing. Even though yesterday and today were very nice, sunny and a high close to 70, it has been raining for a week and everything is water logged.
I would think humidity would be the biggest factor in the test since water does not compress.
And finally temperature. If it were 0 degrees F vs. 100 degrees F. I can imagine some variation in the test. Especially since I saw a variation when the engine was at different temps.
Since the engine is a mechanical pump, I would also think that max compression is max compression no matter what gas/air was put into the mechanical pump, thus none of the above would matter, however the above atmospheric conditions could possibly fool the gauge.
The TM says nothing about any of the above factors and says nothing about testing the engine hot or cold. Since the test procedure is in the engine tear down section, I can assume that you would not tear down a hot engine thus test should be done when cold. Just how cold it does not say either.

I beleive the TM lacks a lot of information about the test and possible outcomes. And it appears that not many people here know much more about the test than what the tm says. I can also assume my gauge could use some calibration for which I plan to discuss this with the Mac tool guy that I bought the test set from. With only a 20 psi range to determine a good cylinder from a bad, I would think there should be more information on how to perform the test with accuracy. I only had one test gauge, If I had two then at least I could get some comparisons and feedback on how accurate each gage was. The only variation that I could do was change the engine temp for which I did and got the results listed above.

One big question I do have with the TM that someone here may be able to answer is that it requires you to remove the air cleaner or serious engine damage could occur. I cannot see any reason why the air cleaner could effect the test or the engine in any mannor.
I am not an expert in physics even though I took many classes on it in school, nor am I a mechanic, So I do not have the answers and neither does the TM.

Just wanted to share my findings. If anyone here as info to add, I am always willing to learn.

 

[scottladdy]

Compression Test Analysis and Commentary

Hi Rich,

I noted some items in your write up that I may be able to assist with.

There are some semantics at play here. Please bear with me on this. It is important to understand exactly what the TM is stating.

All of the cylinders tested similar to each other the lowest reading i got was 420 psi and the highest was just shy of the 430 psi mark. The tm says 380 to 400 is good and that less than 380 is bad it says nothing with regards to being over 400.

The TM specifically states "Normal compression should be 380-400 psi (2620-2758 kPa)." Not "good" as you stated.

But, the key is in the next paragraph. No cylinder should be less than 380 (that is a problem). And lowest cylinder pressure should be no lower than 80% of the highest. For your consideration, 80% of 475 psi is 380 psi. So, there is a pretty big range there.

It also says nothing about engine temp or atmospheric conditions when testing. Per my previous test I can conclude that a hot engine will test higher. When the engine was at 180 degrees per the temp gun, I had readings as high as 500 psi. when the engine was around 120 degrees I got high readings of almost 450.
My conclusion thus is the higher the engine temp, the higher the reading on the gauge. I can only make a few assumptions, the first being that hot air expands thus causing the gauge to read higher than what actual compression would be.
Also with a hot engine, the metal is expanded to its maximum designed size so compression could be higher when hot vs. cold.

My understanding is that you want to test a warm engine (approximately 90 deg. F) as a general rule. But, I believe the TM would state a specific pre-condition OR procedure to pre-warm the engine before the test if it would significantly affect the results.

I could not find any information on how atmospheric conditions effect the test.

Correct. The test procedures and results analysis take the normal range of atmospheric conditions into consideration. Unlike an air conditioning system which is far more sensitive to ambient temp. and humidity. Cylinder pressure at starter crank speed is not that affected by these conditions.

I would think humidity would be the biggest factor in the test since water does not compress.

Please, a correction. Water as a liquid resists compression. Water as a vapor compresses. Simple thermodynamics and phase state. Increased water vapor does tend to reduce the percentage of oxygen in the air charge, but will have a negligible affect on compression readings at starter speeds.

Now, if you really did have liquid in your cylinders you would find out really really fast. It would also likely have ruined your day.

One big question I do have with the TM that someone here may be able to answer is that it requires you to remove the air cleaner or serious engine damage could occur. I cannot see any reason why the air cleaner could effect the test or the engine in any mannor.

The exact quote from the TM: "Covering should be removed from air Intake opening before cranking engine. Failure to follow this caution may result in damage to engine."

The air cleaner is removed as a pre-condition of this test (See the "INITIAL SETUP" section at the beginning of the test). SOP is to keep the intake covered whenever the air cleaner is removed to prevent "stuff" from falling into the intake. This is a reminder to the overworked technician that a really big boo boo will be made if they don't take the coffee can lid off the intake before the test. Not the air cleaner which must be removed for accurate compression readings.

So, let's analyze your test results. Your lowest cylinder is 420, good. Your readings indicate less than 3% difference, so your engine is tight, good. You pass!

Your readings are right in the ballpark of my CUCV's BTW.

 

[richingalveston]

thanks for the analysis, it makes sense to me now, I did not find any thing about pre warming in the tm but it could have been in another section. The section I read was in the engine breakdown section 3-2 of the 289-34 manual. If it was in there then I missed it. I was not real clear about the %80 comments in the manual.
Thanks for the reply. I hope this helps others in testing their engines.

 

[scottladdy]

thanks for the analysis, it makes sense to me now, I did not find any thing about pre warming in the tm but it could have been in another section. The section I read was in the engine breakdown section 3-2 of the 289-34 manual. If it was in there then I missed it. I was not real clear about the %80 comments in the manual.
Thanks for the reply. I hope this helps others in testing their engines.

Excellent. Glad this helped! Very happy you are taking the time to validate the condition of an engine you plan on selling. I hope you are well rewarded for the extra time you invested.

And now that we know you have a good engine (compression wise), will you be listing it in the classifieds?

 

[richingalveston]

the engine is already sold, I offered to test it for the seller and was just trying to make sense of my findings.
Thanks for your help.

 

[ltm]

Excellent post, good logical questions and good logical answers. For someone to take the time to analytically, and patiently answer a question that has been posted is a real help to all of us. Armed with this information I think I'll check my compression.

SM Ps1