I recently did a compression test on my friend's 1985 300TDT with 266K miles on the engine. His engine idles VERY roughly, smokes more than a diesel should, and is VERY difficult to start, even in warm temperatures. Oil analysis showed 0.0% coolant in the oil (nothing else alarming in the oil), valves were adjusted, rack damper screw was adjusted etc. Nothing helped. I finally decided to do a compression test. It was done with a hot engine, each piston's value was repeated three or four times, each time the engine was cranked (with the injection pump disabled) for about six or seven-seconds. Piston #1 was 140 p.s.i.; piston #2 was 380 p.s.i.; piston #3 was 380 p.s.i.; piston #4 was 340 p.s.i.; piston #5 was 280 p.s.i. Obviously these numbers aren't the best. Upon removal of the fuel injector #1, I noticed that from the threads to the pintle, it was as clean as it should be. Injectors # 2-5 were dripping with motor oil in the same location. It was not coked, it was definitely wet oil that smelled like oil too. My questions are simply:
1. Why were injectors 2-5 covered in oil?
2. Why does this car idle roughly at around 800 r.p.m. or so but anything over 1,000 r.p.m. it drives beautifully? Even on the highway. With low compression, you'd think it would hesitate constantly. I now understand why it starts so poorly.
3. What causes these pistons to lose compression in general? He's religious about oil changes and preventive maintenance etc.
4. I suppose the next step apart from rebuild is to have him drive it into the ground. Due to rust, it's not worth rebuilding.

1) Have to think about that one. Doesn't make immediate sense.

2) Cylinder #1 is not firing. So, at low rpm's you feel the shake due to four cylinders firing and one not firing. You can't feel this at higher rpm's, although the vibration is still there.

3) It may not be a piston. It's very possible that a valve is bad and allowing gases past the seat. 250K is a lot of miles for the valves and seats.

4)The next step is a leakdown test, which will tell you if the problem is with a valve or with the lower end. If it's the former, you can easily have the valves done for $500. or so. Once you are in there, the prechambers, guides, and seals will add another $500. to the job. You'll be amazed at the performance of the engine with a rebuilt head.........provided that the lower end is good.

Another option would be to swap a another engine in. Everyone says you can get decent 617's for a couple hundred bucks from junk yards. If your labor is free it may be a decent way to get a couple more years out of it.

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1969 280SE
2023 Ram 1500
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as for the oil you found under the injector.this could be caused by a poor sealing valve cover gasket.a cheap fix

With the transmission in drive and engine warm rev the engine to the point where it stops shaking. Crack each injector line loose to cause a missfire and notice the rpm drop. Each cyl should cause a noticable drop in rpm. Do this and report back and we may be able to help you make this engine run smoother.

Good ideas, thanks everyone, I'll pursue this further...

A leak down test will tell you exactly what you need to know.
1.Put #1 on tdc and apply air pressure (150 psi should work).
2.Now, take off the oil fill cap and listen for sounds of air leaking into the crankcase.
3.If you don't hear anything do the same for the intake and exhaust.
4. Where you hear air escaping, will point you to the culprit

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Valve adjustment?

.......post #1......

A quick test to determine whether it is valves or rings. Squart a couple of shots of engine oil into a cyl. Do a compression test on that cyl. If the compression comes up then the chances are that the rings are bad. If it doesn't come up then the chances are that it is valves.

You guys must not be running this engine hard enough

User Addicted and I worked on his 300D. He got the head back on and such and it just WOULDN't start (had been sitting without its head for quite some time).

I popped the oil cap off and sweet jeebus...we had a steamy fuel gyser coming out of there! haha.

The rings were basically stuck on a couple cylinders.

We tow started it...with a already toasted transmission (found out later) and it finally started.

The more its being driven, the easier it starts.

Cylinder 1 seems to prevelantly loose compression first on the 617's and I think it is related to the fact that it is the one that gets the blunt of the heavy EGR/oil material.

My 300SD probably has low compression on 1 too...but that cylinder still fires and it works okay The shake at idle was corrected with an IP adjustment and such. You need to get that compression up first before you go furthur

cyl may well fire at higher rpm cause it is actually giving more compression at higher rpm. at crankover it has more time to leak out. so at idle it is much more noticable since it doesnt fire at all. and at higher rpm it may well fire weakly.

this doesnt alter the above offered advice.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Thanks, it was hiding in there somewhere.