Compression Test
 

Who has done a compression test or had it done?? Had a dealer tell me "we cant do a compression test on a Cummins" - I laughed and walked away.

I would gather you would have to pull the injectors to do the test right? Also I think you would need a adapter to fit the hole??

If so I know the numbers should be quite high middle 200's ??? (Not sure???)

I know most gassers are about 140-150 psi on good dayz... And about 110-120 on the bad dayz...

Curious... :)

Re: Compression Test
 

Probably because they don't have the tools.
There are adaptors for every different engine's injector style. Remove all the injectors and turn the engine over at least five revs for each cylinder. I used to have a chart from Cummins that showed the compression for a new engine and the minimum compression. I'd have to guess at the exact numbers but normal is the low 200s, worn out below 180. Most times what you're looking for is the difference between cylinders rather than the individual number. 10% difference is about the limit.

Thanks for the info Bill (yet again) - I was quite sure that its possible.

I've actually read that a compression test on a CTD will put you over 300psi. I would say 200 psi is a bum cylinder...

The trick is most diesel testing apparatus leak a little, giving you low readings. Don M has a proper setup and has tested over 300 psi..

justin

when I took my truck in to Cummins BC to have the engine tested (dealer installed incorrect air filter) they didn't do a compression test, they did a blowby test. they hooked a bunch of tubes up to the truck, stick it on a dyno, chain the rear down, and run the truck up to the governor (sp??) in gear. They check the ammount of gas coming out of the crank case at wot 3Lbs H2o is what it should be around for a good engine, I think 7 lbs is max they also checked boost, oil pressure, at what temp the fan came on, and the hp rating at specific rpm's.
To me, it sounds reasonable they can't do a compression test, I am looking into wv diesels now and 425psi per cyl is considered reasonable I think upwards of 500 psi is considered new (I think)
I would suggest if you want to check an engine out, find a heavy truck shop that has the dyno and equipment to run the truck like they ran mine.

I have the factory manual, and they talk about doing a compression test and it says if compression pressure is below specification to do the leakage test.

I've looked everyplace I can think of, and I can't find any mention of what that "specification" is.

well I did have a very experienced diesel mechanic (35+ years) and Cummins expert tell me to get a compression test.....so I dunno.

I think that either the compression test or cylinder leakage test would be about the best way to diagnose the health of the engine when buying a used Cummins.

You can make your own blowby measurement tool for under $10. A blowby orifice tool is just a tee with one .221" (15/64-in) outlet (pipe plug with a hole drilled in it). Connect one end of the tee to the end of the blowby tube. Put a manometer on last tee outlet. That is your blowby tool. A simple manometer can be made by looping 6' of clear tubing with water in it half way. Measure how high the water level rises with a tape measure, multiply it by 2, convert it to LPMs

Rough conversion is:1"= 27 lpm, add 3 lpm for each one inch (1/2'' of rise in the tube) of water

New engines numbers are;
63 liters per minute(2.5" water rise = 5" of water) @ 2200rpm,
76 L/Min (3.5" rise) @ 2500rpm
85 L/Min (4.5" rise) @ 2800rpm.

Worn engine that needs rebuilding are roughly double i.e.
126 L/Min(10.5"rise) @ 2200rpm
152 L/Min(14.5"rise) @ 2500rpm
170 L/Min(17"rise) @ 2800 rpm

The valves could also be out of adjustment.

The way I do it, same idea, is to block the blowby tube with a 1/2'' pipe nipple with a cap that has a 15/64 hole drilled in it. Use 3/8'' id looped clear tubing with water in it slipped over the oil dipstick tube. Use sharp tipped felt marker to mark the water level with the engine off, have someone start an already warmed up engine and run the rpms up to 2.2, 2.5 & 2.8k rpms. Mark each water level with the pen, measure then multiply each by 2.

This is all very simple to do, just hard to explain with words.

thats interesting Bill..........especially since I have access to high quality pressure measurement equipment.

Ya lost me where you had the clear tube slipped over the dipstick tube?? The engine must be running before putting it on? Or how do you keep the H20 out of the engine??

If you want to check the health of a used engine use a Dyno its the best way to get a truer picture of the engine condition and drive train......a complete dyno run costs about $100.00 with a printout sheet

Dr., the tube is only filled halfway with water then draped to form a U with the water only coming part way up the sides of the U. Hopefully no water will get sucked in as it would indicate your engine is sucking air up the blowby. Don't think that situation is even possible.

I like connecting the tube to the dipstick tube since it makes access much easier. Also the 15/64 orifice restriction will sometimes blow the dip stick out anyway. Cummins sells a setup that does the same thing but you have to fool around under the truck more.

I first did this test on my truck at around 50k, it was at the levels Cummins says are for a new engine. When I did it again at 200k the water level had only increased by 5/8'' at 2200 rpm. At this rate the engine should easily go 750k.

Here's a U-tube manometer that even though very simple is more complex than you need for diesel blowby measurements.

http://www.rverscorner.com/pics/manometr.jpg

I have had compression tests done on both the 89 and 93 cummins I had.

As new numbers are around 400 psi.. My 89 with 300,000 Km had compression of around 380..

Any decent diesel shop can do the test, Snap-on makes a compression kit. S&W Diesel in Red Deer would be one of my picks for a shop...


Crankcase pressure measurements are fine as long as you have a clean breather system that is equal from the documented values and what you are testing. In my stationary engine analysis we measure the blow-by flow from the crankcase breather for engine health comparison. I have an annubar flow tube that is similar to a pitot tube to measure flow without imposing significant back pressure like a rotameter or other variable oriface flow meter. I have not tested my truck. We do this test under load and depends on engine configuration.

J-eh

Good info Jay,

Needless o say I passed on that truck (man did that hurt)....they wouldnt budge on the price, said they couldnt get a compression test done, and the front end was bagged. Decided it just wasnt worth it - even though its such a rare truck - looking at another 98 lifted LB up north this weeked. Hopfully itll be a bit better shape.

That sux...:mad: I have found they don't like to do much, if anything other than a quick spray bomb from CanTire on the trucks to sell em... I didn't find them very forth coming. ITR was good to deal with, have a shop etc to help out in the sale.

Good luck up north...