Help needed - Compression test (a bit long)

[JGK]

I’ve been trying to chase down a slight lope or chug to the engine lately and the next test I’d like to do is a compression test (for more info on the problem, see the end of this message). I searched the archives back for the last 6 months and I found 4 things.



First, someone said compression should be around 400 to 460 lbs. I’ll have to see if that’s in the service manual. Anyone know for sure?



Second, DRAWSON said that Snap-on has an adapter for the injector hole to fit a compression gauge. Dee, are you there? Do you know the part number? Does Miller or someone else also have it, maybe at a cheaper price?



Third, BPINE and DRAWSON mentioned a “leak down” test. What is this and how is it done? Is it something I can do?



Finally, LSMITH mentioned pressurizing the cylinder with an air compressor with both valves closed and listening for a leak through the exhaust or intake valves. Anyone have anything to add to that?



Here’s my symptoms: The engine has a lope or chug, like I said, sort of like a gasser that has one weak cylinder. I hear it most in the 1200 to 2000 RPM range in 4th cruising around town, but I can hear it through the full RPM range. Idle is smooth, though. And it runs strong when you get on it.



My remote temperature gun shows #3 is running about 50* cooler than #2 and 80 – 100* cooler than 4, after a hard WOT run up a 6% grade about 3/4 mile long (I charge up the hill, then pull over, set the brake, pop the hood and start checking temperatures right where the exhaust manifold mates up to the head). So it appears #3 is not as strong as the others.



I suspected a bad injector, but I swapped 3 and 4 and there was no change. I’ve been suspecting the pump, but then I got to thinking maybe #3 has a gasket, valve, or (worse) ring leak. There is no obvious indication from the outside of a gasket leak. What are the symptoms of a these types of leaks?



Any suggestions?



Thanks in advance,

Jay

 

[Bigramguy]

By the sounds of it you have probably checked the basics first (I hope),like lift pump,filters,lines,overflow valve,timing,valve adjustment,etc etc ?



I have a MAC compression tester and the adapter part number is CT760CD. The matching tester is CT700D. They also sell a kit (CT900D),with both of the above and a few more adapters. I don't have my Snap-On catalog handy at the moment. They aren't really that cheap,compared to a regular compression tester. MAC stuff is usually cheaper than Snap-on,and just as good. A lot of times it is made by the same manufacturer,just marked up more. Another option is to borrow,or rent one,if you just need it for this purpose. Some jobbers will also loan out special tools at no charge.



Pressurizing the cylinder with air and listening for leaks will work,but may be hard to hear if the leak is very slow or small. A leakdown test is essentially the same thing but using precise gauges which show the air pressure being applied and the actual pressure in the cylinder. The difference (or pressure drop) is the amount of leakdown.



If you have a valve leaking it may not show any other symptoms,but a head gasket,cracked head,or broken\worn ring most likely would.



For all the work involved and tools\adapters and gauges needed it may be better to take it to a competent diesel shop and have them do it.



If you have already swapped injectors,maybe try the same thing with the #3 and #4 delivery valves ?



Hope this helps

 

[Don M]

Hey J,



I would lean toward checking your delivery valves first and see what happens with that before getting too nervous over ring seal or valve job if you have been good with maintenance and not flogged on it too hard over the years.



Isnt the number three and four exhaust runner kinda tied together? Were did ya take the reading at?



I was wondering about ring seal on mine for awile. I know it may not be optimum, but it is not too bad either.



Don~

 

[JGK]

Bigramguy,



Yes, I've done most of those other things (valve adjust, overflow valve replaced, timing). I'm working on getting a gauge to test fuel pressure. The trick is finding a competent shop around here to do the test. I'll have to start with the Cummins shop, I guess. I'd prefer to have it done by a professional if I can do that quickly and cheaply.



Don,



If you look down at the motor you can see an area about the size of a half dollar on the head itself just upstream from where the exhaust manifold bolts to the head. It is actually sort of the side of the exhaust port on the head. It's a pretty clear, clutter-free shot for the gun to get a reading from about a foot away.



The truck has not been run hard - not compared to what I read here! I don't tow much. Mostly just my daily drive.



As for the delivery valves, that was my intention. I thought I swap 3 and 4 and do another run up the hill and take the temperatures. However, in talking with a couple of guys at the Bosch shop, I got all kinds of warnings about not to fiddle with the valves. Big heap-o trouble if you mess with them. I'm thinkin' that was just a bunch of hooey. Too many folks on this site changing theirs out (another reason I don't trust the local shops).

Odd to hear from the Bosch shop owner since he's an aquaintance of Piers!



Anyway, if I do want to check or swap the delivery valves, what is the procedure? Any special do's and/or don'ts?



Thanks, guys.

Jay

 

[Deezul1]

It seems to me that someone posted a while back about checking temps with a heat gun and after checking several engines decide that 50 deg diff was fairly normal

 

[A Johnson]

I would go for the compression test as it is the easiest / cheapest thing to start with. I have never seen a published figure in a Cummins manual, but a rule of thumb is that they should all be within 10% of each other. Make sure you pull all the injectors so you cranking speed is consistent. I must add that if you have a big problem with a piston, rings or otherwise causing a miss, I would think you would notice the increase in blowby. A Johnson

 

[A Johnson]

Forgot to add in my last post the sympyoms of the different leaks.

Bad rings/ pistons= Lots of blowby out the draft tube

Bad valve = Pop, hiss noise in the intake or exhaust that correlates with the bad cylinder firing

Bad Head gasket = Normally ends up pressurizing the coolant system first, or I have seen it blow compression to the outside perimeter of the cylinder head. A Johnson

 

[JGK]

Thanks, A Johnson.



So far, none of the symptoms you mentioned are evident.



I stopped by the local Cummins shop after work yesterday to see if they'd do a compression check. The chief mechanic really downplayed the need and showed no desire to help. He indicated the chance that it was either the gasket or the valves was very remote. Also, too busy to talk much So I guess I'll have to look elsewhere for getting this done. He suggested it was more likely fuel supply to the pump, like a cracked fuel line letting in air or a dirty filter. I would think that would affect more than just 1 cylinder and show different symptoms



I Still can't help but think it's all about a reduced fuel charge into #3. I wish I could just measure the output of each pump barrel with the pump still on the engine. You know, pop the lines off the injectors and let each drain into 6 separate cups, then crank the engine over for a few seconds and compare the fuel squirted out by each. Sounds easy enough, but I have a hunch, due to the pressures, I'd have fuel all over the engine bay!



Still looking or directions/advise on R & R -ing the delivery valves.



-Jay

 

[Bigramguy]

Try swapping the delivery valves and see if it makes a difference.



Here is a link for setting timing that shows how to remove and install them.



http://www.dodgeram.org/tech/tsb/1994/18-10-94/18-10-94a.htm

 

[Don M]

Jay,



You can check the amount of fuel each line/barrel is making. The fuel does not come flying out like you might expect. It comes out under pressure but the pressure is not very great with the injectors outta the way and not connected.



I tend to agree with the guy from Cummins. He may be a butthead, but he is probably right about the valves and rings.



Pull your number three jug and check your valve lash too if you have not yet. It just takes a second to do.



You could try and get 6 graduated cylinders and measure the output of each line/barrel. It would not tell you the amount of fuel you are delivering at high RPM though because you could not turn the engine over at the faster speeds. Prolly a waste of time since your troubles are further up the rpm band.



Have you checked your fuel pressure yet? Its very important.

Also, your fuel shut-off solenoid may be weak and not lifting your fuel shut-off lever all the way up. They do get weak and can act fine on the driveway and begin to sag while driving down the road. There is an updated solenoid from the Cummins/Dodge folks to replace the older design. I had to it on mine already.

Turn the ingnition on and go to the solenoid and lift up on the fuel shut-off lever and see if it holds the lever up and if you are getting it to hold it all the way up.



Jay, i gotta ask... what is the last thing you worked on? I almost always track things wrong back to the last thing I touched on my truck. Not to make fun of you or me, but it frequently adds up to this.



Don~

 

[JGK]

Don,



Excellent thoughts and suggestions.



I checked/adjusted all valves last Sunday. A few were a little tight, but none were way out. #3 was not particularly any different than the others.



I'm working on a fuel pressure gauge set up (permanent) to keep tabs on that, but I haven't checked it yet (no easy way I know of).



I'll check the shut-off solenoid this weekend. Thanks for that suggestion and how-to.



The last thing I worked on (other than having the clutch replaced, and the truck was exhibiting the problem before that) was to replace the turbo with one of Pier's modified 35's. Stock exhaust housing. It was this fact that got me to thinking of the compression thing. Maybe a little too much boost. However, boost has only gone up from about 34 or 35 to about 38 or so.



I have noticed it tends to be more noticeable after then engine warms up.



Bigramguy, thanks for the link to the delivery valve thing. I was looking for that last night but did not consider looking for it under timing. Just one question. Do you have to be at TDC for the barrel/cylinder being R & R'd? If so, how can you take 3 and 4 out at the same time to swap them? Do you really need new copper delivery washers as indicated?



Thanks one more time!

-Jay

 

[mblair]

I have a compression gauge..

you can borrow if you want. It's one I saw on e-bay. They were selling them for about $35, so I called the company and I believe I got mine for about $28. It's an inexpensive one, but it has about 5 adapters and it worked okay on my son's Ford diesel. It supposedly has a Cummins adapter. If you or anybody else is interested email me. I'll check if it really works on our trucks.

max blair -- email address removed --