True....but adding boicide is a lot easier than a compression test!

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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.

True............but as you know, "easy" doesn't usually get you to the desired end result.

You know that saying..........."do it once.........do it right".

Doh........I guess I'm the one who cannot read this morning...........

I'd want to do two compression tests on it. A cold compression test and a warm compression test.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

You of course have a problem of some sort. I would do the nut loosening sequence over again yourself. Three cylinders making no difference at all I would not think the engine could continue to run.

I could see three stronger cylinders and three just less so but still a noticeable drop. Do you know what number those three cylinders showing some issue are? For example cylinders number 4, 5, and 6.

Also make sure to do this after adding a fuel celane booster and getting it mixed in the fuel as Mr Hunter mentioned. An engine with no power contribution from three cylinders mind not being able to run normally except perhaps like a cement mixer should not start easily either.

The chances of three cylinders going bad all of a sudden compression wise while not impossible is not that great.

So I've been trying to educate myself a bit more as to how my Diesel engine operates, and how this relates to the experience I am having. The operating temp. compression test should be done in a day or two, giving me the first opportunity to go down one path or another. If, as I hope, the engine is sound, I will replace fuel filters, add biocide and do a purge. I don't expect a wholesale change in engine operation at this point, but am willing to be pleasantly surprised.

Given this is the way the scenario plays, I am inclined to think that timing may be the primary issue. The injector "test" was somewhat vague, and I keep going back to cold start. For me, it is a question of what changes after the initial 10 seconds of "perfect" running. Someone opined that residual glow plug heat might account for this. I haven't discounted that, but in a compression ignition environment, assuming the compression test is good, I would tend to look elsewhere.

As I understand it (please correct me if I am wrong!) there is a starting position for pump timing, and a running position. If the timing is accurate for starting, but shifts to run position and encounters problems, could this not be a good fit to the symptoms?

Anyway, I'm continuing to research, and will pass along what I find. Thanks for helping me along here!

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Chief

1991 350SD

What is a Leak Down Test?
What is a Leak Down Test? - PeachParts Mercedes ShopForum

help with compression test
help with compression test - PeachParts Mercedes ShopForum


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So there's been a bit of down time, and a bit of "progress."

It seems that the skipping cylinder is #2. A second cracking of the injector lines revealed consistently that when number 2 was loosened there was no change in idle, and the smoke was greatly reduced. Tighten back up, idle stays the same, smoke comes back. All other injectors caused the idle to worsen when loosened.

Driving at higway speed, the heavy grey smoke disappears, but anything other than that the car is a public nuisance.

I tried to beg, borrow or steal a compression adapter with no luck- noone seems to have one for the 603, so I ordered one from Baum tool, should be here tomorrow. Based on the fact the car starts perfectly each time, and occasionaly seems to regain power on all 6, I don't think (and hope I won't!) I will find a compression issue, so that puts me back to fuel. I'll know more tomorrow.

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Chief

1991 350SD

When you do the compression check. Especially if you pull the injectors to do it. Number them. Then when putting them all back in switch the number two injector with one other.

Thats if the compression test does not indicate something. See if the problem has moved. In your case I am not suspecting an injector or injection pump as there is smoke with the injector feed on and no smoke with it off.

Also have a look at the head casting numbers as it was a replacement during the rebuild. Is it a number 22? You will also want to look and inspect the valve train for the number two cylinder. But do the compression check first.

If the compression check shows that cylinder low especially because it is a 3.5. You will have to do a leak down test. Low rate leakdown could be a bent connecting rod. They apparently already changed out two rods in this engine. The reason why may be unknown or the last owner may know.

Not potentially the happiest of situations. At least it is fairly easily diagnosed. Personally I would take this car out of service until it is totally diagnosed. Driving on a bent rod may do additional damage.

Strange the guy that repaired the engine 40k ago did not change all the rods if he found two of them comprimised. There is also of course a chance my line of thinking is wrong. I hope so in this case.

For example if air was escaping around the glow plug. Or head gasket. Or a rocker arm was not doing it's job.

Where are the head casting numbers? I'll take a look and see which head it is. The PO bought the head from Adsit around 2008 for $1800. I have the VC off, and it looks like a new engine, it's incredibly clean.

The PO apparently didn't know anything about the 3.5's reputation when he rebuilt the engine. He bought the car not running, with the engine "seized." Apparently a couple screws (windage tray?) found their way into the oil pump and that was that.

He replaced the pump, rods 3 and 4, rings and the head. As stated before, the engine doesn't use any lubricating oil, so I'm hoping it's not a compression issue, but I am confounded if it isn't.

__________________
Chief

1991 350SD

Its about here on the other side of the injection pump.

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1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

Got it- Thanks! What are the "good" numbers, and what's bad about the bad? I have read some relevant info here and there, but nothing comprehensive.

__________________
Chief

1991 350SD

14 is bad
17+ is good
22+ is the best

Got the compression test adapter and here are some early returns.

Since cylinder #2 seems to be consistently the weak cylinder, we checked comp so far on #1 and #2. Cold, both are around 400. Hot, #2 drops to 300. Swapping injectors makes no change- #2 continues to be the problem.

__________________
Chief

1991 350SD

With those numbers; The LEAST you have is a BAD head gasket.


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