compression test 1997 12 v - Dodge Cummins Diesel Forum
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post #1 of 8 (permalink) Old 02-15-2008, 01:34 AM Thread Starter
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compression test 1997 12 v

i posted - 12v w/300k i am looking to buy. figure i should do a compression test.
i have a set from harbor frieght. i will look for blow by. if possible, i'd like to do a compression test to be safe.
i guess also listen for any type of bearing knock.
biggest concern i'd have on this truck is the engine bottom end. top end and transmission are relatively "affordable". engine bottom end is expensive.
can someone explain how to do a compression test?
it seems to be well maintained and looks real good.
jon


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post #2 of 8 (permalink) Old 02-15-2008, 06:09 PM
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Much easier to do a blowby test. This is how Cummins checks for a worn engine.
You need a blowby tool.
The Cummins blowby orifice tool is simply a tee with one .221" (15/64-in) outlet. Connect one end of the tee to the end of the blowby tube. Put a manometer on the last tee outlet. That is your blowby tool. They sell them at Cummins, but I have made my own plenty of times, less than $10. A simple manometer can be made by looping into a 'U' 6 feet of clear tubing, fill it 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

The reason for multiplying by 2 is that inches of water equals the water rise in the open end of the tube plus the inches the water is pushed down on the engine side of the water tube. For simplicity my numbers below are the measurement of rise only.


Cummins new 5.9 engine numbers are:
63 liters per minute(2.5" water rise) @ 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

Beside indicating a compression problem the valves could also be out of adjustment.

Another way (mine), 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. Other tubing end remains open. Use a sharp tipped felt marker to mark the water level with the engine off, have a helper 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 the distance from engine off mark then multiply each by 2.

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

Bill
'95 2500 4wd auto
'95 3500 5 speed heavy hauler
Stock for one day
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post #3 of 8 (permalink) Old 02-15-2008, 08:47 PM Thread Starter
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replies are much appreciated. i have been looking at this truck for over a week. it looks real good in the pictures (clean paint), not a dent or any scratches that i can see, interior looks real clean also. definitely the appearance of a well maintained truck. asking $7995 w/305k. 1997, 4x4, auto, CC, 8' (north carolina, no rust). he says it runs/drives well. problems - fuel sender doesn't work (i'll fix that someday) and hard start, has to add a little throttle. i expect either a air leak in a line or out of adjustment idle screw. w/that many miles i really would feel best w/a compression test, costs $220 at the dodge dealer, i figure worth the money. also i'll have them check fuel pressure (and a full presale inspection). my biggest concern is that it will get me home 300 miles w/o having a fuel problem. he says it runs fine after it starts. i have a list of fuel system components to inspect and service, with a little time, i'll go thru it all.
as long as it runs/drives well, pass inspection, no immediate issues w/transmission or differentials or other big ticket items, my biggest concern is the bottom end of the engine. the top end is not quite as expensive.
i just wanted to ask around to see if i look nuts buying a truck w/300k. if the compression is good, the bottom end should be fine and have plenty of power left in it. that's determined by oil changes and how it was treated. i would hope that they weren't giving it too much pedal when starting it and running high above idle speed.
anything else should be concerned about? i was thinking of offering $7,000.

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post #4 of 8 (permalink) Old 02-15-2008, 10:17 PM
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Cummins does not recommend compression testing on the 5.9 and publishes no numbers.........what will you be getting for your $220???
Neither compression or leak down will tell you anything about the bottom end.

Bob

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post #5 of 8 (permalink) Old 02-15-2008, 10:37 PM Thread Starter
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i have a '04.5 that is using a bit of oil. i have had discussions w/the tech at cummins in Laurel, MD. he said the first thing to do is a compression test (unless i'm mistaken, i spoke to lot's of people about it, i know that amsoil told me to check compression, almost positive that cummins said the same). i am just concerned about the condition of the bottom end - oil consumption, bad bearings will show up somewhat as low oil pressure (i'm told). do we just check for blow by? i spoke to two dodge dealers and was given a similar price for compression test. i'm just trying to be thorough. i can't afford another truck w/major oil consumption. the bottom end is the most expensive part of the truck. truck looks real good, seems to be well maintained. i bought a compression tester from harbor freight to check if i need to, should i just return it?
any suggestions?
jon

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post #6 of 8 (permalink) Old 02-15-2008, 11:09 PM
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I doubt a hf compression gauge would read high enough nor would it have the correct adapter for the Cummins.The blowby test and a dyno will give you a better indicator of the condition
Bob

05 2500 4x4 Carli Dominator 3.0 Suspension Baja proven
94 2500 4x4 Carli performance 2.0 with full leafs
90 wrangler injected locked and caged crawler
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post #7 of 8 (permalink) Old 02-16-2008, 06:25 AM
 
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yes the hf tool reads up to 1000 psi and does have the correct adapters, cheap, but gets the job done i guess.
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post #8 of 8 (permalink) Old 06-04-2008, 12:53 AM
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I know, old thread. But I just wanted to thank this forum for this info. I just did a blowby test using illflem's method and it worked great. I picked up a very healthy '98
12v for a swap into my '75 Ford 4x4. The motor has 180k kms(108k miles) and the blowby #'s were close to factory spec.
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