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  #1  
Old 12-07-2011, 03:03 PM
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Intro and First Question: Engine Starts Smooth, then Runs Rough

Hello,

Longtime Benz owner, and for better or worse I find myself a Diesel owner again. I started out with a '79 240D 4 speed. While it was a low mileage car, it hadn't had it's oil changed as frequently as it should have, and had a bit too much blowby. In spite of this, I ran the wheels off of it until I didn't need it anymore. After storing it for a couple of years, I sold it to kid who bought it sight unseen, taking a bus here from 3 states away. He arrived with a plate, and left directly to Colorado. He made it, too!

A 300E, E430 and 560SL have all been mine to enjoy since then. Times change and now I have a 1991 350SD. I know this is noone's favorite engine, but it's what I have, and hope to get some use out of!

Here's the situation. The car has 205k miles, and is in outstanding condition. The PO "rebuilt" the engine 40k miles ago. The rebuild consisted of a new cylinder head, a reground crank, 2 new connecting rods and all new rings. He claimed at that time the cylinders still showed crosshatch and checked out fine. Oil changes always at 3k miles.

Here is what I am experiencing. The engine (reportedly) consumes no lubricating oil. Without getting into it, I trust the owner's testimony on that. The engine starts easily and instantly from dead cold, and runs perfectly smooth and smoke free for 10 seconds or so. After this time, it begins to skip and shake, idling quite rough, and puts out a pretty fair amount of whiteish bluish smoke. Car has low power, but is driveable and the engine gets smoother under load- I can do 70 on the highway without any trouble. Occasionally power seems to come up, engine smooths out even more, but then will lose power again.

My indie has "cracked" the injectors loose one at a time and run the car. Only 3 cylinders significantly affect the running during this test; the other 3 didn't make any difference. He deemed this inconclusive.

These symptoms are pretty confusing to me. Anyone with a similar experience and cure?

Thanks much!
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Last edited by ChiefRider; 12-07-2011 at 03:17 PM.
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  #2  
Old 12-07-2011, 03:07 PM
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Welcome to the forum.

Although it doesn't sound like a compression issue, you really should do a compression check before going further to make sure that you're not kicking a dead horse. If the compression test is good, then you can move forward with confidence that you are working with a good foundation.
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  #3  
Old 12-07-2011, 03:18 PM
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Quote:
Originally Posted by ChiefRider View Post
My indie has "cracked" the injectors loose one at a time and run the car. Only 3 cylinders significantly affect the running during this test; the other 3 didn't make any difference. He deemed this inconclusive.
He's correct about "inconclusive".

But, that test is the starting point. The problem is either low compression in those cylinders or very low or non-existent fuel to the cylinders.

I'm far more inclined to believe the former, as the IP's rarely give the trouble that you've described. Additionally, I'd make a wager that the blue/white smoke is fuel smoke (you can smell it). This would confirm that you've got the fuel, but do not have proper combustion in the cylinder(s).

At that point, it is definitely time for a compression check.
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  #4  
Old 12-07-2011, 03:35 PM
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Answer

Before borrowing trouble, try pouring a heavy dose of Cetane boost in the fuel tank. It may be bad fuel magnifying other issues.
http://www.peachparts.com/shopforum/diesel-discussion/309367-warning-all-diesel-owners.html
Once that potential issue is removed:


A compression test with the engine at operating temperature is required as a starting point.

Remove the injectors and have them tested for spray pattern and pop pressure.

depending on those results.

* The timing may be seriously off, or the timing device may be damaged.
Diesel Injection Pump Timing Device threads

* The injection pump may be damaged.


.
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  #5  
Old 12-07-2011, 04:18 PM
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Thanks for the fast responses! I know the indie is going to be doing a comp check, but I've been led to believe that this test can be somewhat vague for a diesel engine, although I would assume a finding of "no compression" would be quite clear!

Obviously this test and its verified result will dictate the course of action. With reference to my gasoline engine experience, what doesn't make sense to me is why the engine would start so well, only taking on its problems after a brief period of running. Perhaps a crack in the head opens after warming? I shouldn't think a valve would do this, or perhaps a cylinder might? And the periods where it slips back into running well. Confusing.

At any rate, I'll post on the progress, and always welcome everyones input!
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  #6  
Old 12-07-2011, 04:52 PM
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Originally Posted by ChiefRider View Post
With reference to my gasoline engine experience, what doesn't make sense to me is why the engine would start so well, only taking on its problems after a brief period of running.
That is a perfectly valid question. If the compression was NG, it would not start well and there would be continuous smoke right from the start.

If it does perform a clean start, without smoke or miss, the compression at start is good.

The possibility of marginal injectors that deteriorate with temperature certainly starts to look more interesting..............

Let's see what the compression shows...............
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  #7  
Old 12-07-2011, 11:07 PM
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Quote:
Originally Posted by ChiefRider View Post
Hello,

Longtime Benz owner, and for better or worse I find myself a Diesel owner again. I started out with a '79 240D 4 speed. While it was a low mileage car, it hadn't had it's oil changed as frequently as it should have, and had a bit too much blowby. In spite of this, I ran the wheels off of it until I didn't need it anymore. After storing it for a couple of years, I sold it to kid who bought it sight unseen, taking a bus here from 3 states away. He arrived with a plate, and left directly to Colorado. He made it, too!

A 300E, E430 and 560SL have all been mine to enjoy since then. Times change and now I have a 1991 350SD. I know this is noone's favorite engine, but it's what I have, and hope to get some use out of!

Here's the situation. The car has 205k miles, and is in outstanding condition. The PO "rebuilt" the engine 40k miles ago. The rebuild consisted of a new cylinder head, a reground crank, 2 new connecting rods and all new rings. He claimed at that time the cylinders still showed crosshatch and checked out fine. Oil changes always at 3k miles.

Here is what I am experiencing. The engine (reportedly) consumes no lubricating oil. Without getting into it, I trust the owner's testimony on that. The engine starts easily and instantly from dead cold, and runs perfectly smooth and smoke free for 10 seconds or so. After this time, it begins to skip and shake, idling quite rough, and puts out a pretty fair amount of whiteish bluish smoke. Car has low power, but is driveable and the engine gets smoother under load- I can do 70 on the highway without any trouble. Occasionally power seems to come up, engine smooths out even more, but then will lose power again.

My indie has "cracked" the injectors loose one at a time and run the car. Only 3 cylinders significantly affect the running during this test; the other 3 didn't make any difference. He deemed this inconclusive.

These symptoms are pretty confusing to me. Anyone with a similar experience and cure?

Thanks much!
When the Injector Lines were Cracked on the 3 Cylinders that it made a difference on those were the 3 cylinders that were working like they were supposed to.

The other part of the test is to remove one of the 3 Injectors from one of the Cylinders showed an effect when the Injector Line was cracked. And, swap it with an Injector from one of the Cylinders that had no effect on the Engine when the Injector Line was cracked.
If the no effect problem follows the Injector it means you have a problem with that Injector.
Do the same with the remaining 2 Injectors.

Also Air Mixing with the Fuel Supply can cause rough running; and would mess with the Fuel Injection.

EGR issue?

Also you did not mention if the Injectors were rebuilt? At 205K I would think they were past due.

Did your Mechanic note if the on the Injectors the line was cracked on that had no effect on the Engine; if the Cylinders were adjacent to each other?
If so it could be the compression test may show a Head Gasket issue.
 
Trouble shooting this problem is going to be expensive if you are not doing the work on the Car. More so if your Mechanice does not know too much about your Benz.
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  #8  
Old 12-07-2011, 11:13 PM
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Just a thought but it seems to me that it still could be compression related because the glow/afterglow cyle can allow for smooth starting/running for a bit and then you get the rough idle when you no longer are getting the benefit of the plugs helping the combustion.
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  #9  
Old 12-08-2011, 06:09 AM
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My first thought after reading the title was fuel issues. Could be a plugged screen in the tank. I would suggest biocide to verify good fuel....also fuel and air filters if you have not done that yet.

As far as a compression test not being decisive I disagree.

I also predict your compression test will show this engine is mechanically sound.

Good luck!
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #10  
Old 12-08-2011, 07:47 AM
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Quote:
Originally Posted by Brian Carlton View Post
That is a perfectly valid question. If the compression was NG, it would not start well and there would be continuous smoke right from the start.

If it does perform a clean start, without smoke or miss, the compression at start is good.

The possibility of marginal injectors that deteriorate with temperature certainly starts to look more interesting..............

Let's see what the compression shows...............
I tend to agree. Compression issues would make it hard to start. Since this one starts well, I would lean toward fuel or injection issues. But a compression test would help to put that issue to rest. The unknown, or at least uncertain history requires caution. Assume NOTHING.

Trouble-shooting becomes difficult when we fail to isolate the problem, and just use the shotgun approach. Eliminate one source of concern and then move on.
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Old 12-08-2011, 07:49 AM
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Chiefrider,
Oh, and welcome to the forum. Great guys here who have a lot of experience and are willing to share. Listen carefully to what they say.
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  #12  
Old 12-08-2011, 07:51 AM
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Quote:
Originally Posted by t walgamuth View Post
My first thought after reading the title was fuel issues. Could be a plugged screen in the tank. I would suggest biocide to verify good fuel....also fuel and air filters if you have not done that yet.
The symptom of three cylinders operating correctly and three cylinders operating weakly will tend to discount fuel issues. Fuel usually doesn't care about specific cylinders.
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Old 12-08-2011, 07:54 AM
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Originally Posted by t walgamuth View Post
As far as a compression test not being decisive I disagree.
The mechanic concluded that his results from cracking the individual injector lines were "inconclusive". Nobody suggested that the compression test would be "indecisive". It was generally concluded by all that this step is necessary to proceed.

............are you still sleeping, this morning.........??
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  #14  
Old 12-08-2011, 07:59 AM
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Thanks everyone! Great thoughtful responses to my first post- I appreciate that, and will keep you all informed about my progress with this issue.

I have been a member over at another forum on their 107 discussion group. A great board, with lots of great members. Some of them have referred to the PeachParts group very favorably, and I can see why! I'll contribute where I can, and learn from your experience. Thanks again!
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  #15  
Old 12-08-2011, 08:20 AM
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Quote:
Originally Posted by Brian Carlton View Post
The mechanic concluded that his results from cracking the individual injector lines were "inconclusive". Nobody suggested that the compression test would be "indecisive". It was generally concluded by all that this step is necessary to proceed.

............are you still sleeping, this morning.........??
Please read post number 5 first paragraph, then consider another cup of coffee.
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..I also have a 427 Cobra replica with an aluminum chassis.
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