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  #1  
Old 08-28-2002, 03:35 PM
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Question Rough idle and smoke.

Well, since my '84 300D is new to me and I don't know the last time the valves were adjusted, I decided to adjust the valve lash this past weekend. The car ran great before, but after doing the lash adjustment the car has a really rough idle and smokes a lot. So, I unbuttoned her again and checked the lash. It was right on the MB manual specs of .10mm for intake and .35mm for the exhaust.
Once the car gets up to operating temp, it smoothes out some and smokes less, but is not runnning as smooth as it did before. Does anyone have an idea of what is causing the rough idle and smoke? Like I said, all I did was disconnect some of the throttle linkage and remove the valve cover and adjust the valves. Thanks.

Tim

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Old 08-29-2002, 09:49 PM
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Greetings Tim,

Just wondering if you checked the valve gap before changing it to spec? If so what was it? It is very possible that there was valve leakage before as the valves never really closed completely and the engine sure seemed to run smoother, but now the valves are fully closed when they are suppose to be and the compression is actually where it needs to be to burn efficently. I'd look at the fuel filters and replace if need be as well as the air filter as well as the engine mounts as internally I don't think you have a problem but need to look at the increased air flow as well as the additional power at idle the engine is producing.

Charles
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"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
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  #3  
Old 08-30-2002, 07:49 AM
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Interesting that this one came up

I rolled in a new timing chain on the 300TD last night and figured I would check/adj the valves while in there. Noticed that I have to use a .203 mm gauge to get through the exhaust, the intakes were within spec. I will open the exh up a little but something that is interesting is that the 300TD is alot smoother than the 300D of which I have just finished the valve adj to spec.
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Old 08-30-2002, 10:53 AM
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Thanks for the replies.

can-do,
Prior to adjusting, I only checked to see if the gap was within spec using a .10mm for intake and .35mm for exhaust. On all the valves, I couldn't get the spec feeler gauge in. I also filled up last night and noticed that my mpg went down from an average of 27.2mpg on the prior two tanks to 24.9 mpg on this tank. I burned about half the tank after I did the valve adjustment. As for the fuel filters and air filters, they all have less than 1200 miles on them. When I bought the car about a 6 weeks ago, I changed all the fluids and filters.
Could the problem be injection timing?
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Old 08-30-2002, 12:23 PM
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Greetings Tim,

Initially I'd say that the injection timing has not been affected by the the valve adjustment you did. You can check it if you like but there are a few other items I'd look at first. Have the injectors been cleaned or checked for opening pressure? Also when you mention that there is more smoke and rough idle on start up it indicates that perhaps there is something amiss in the glow plug circuit. I'd remove and check out this first as it's possible you have lost a glow plug or two or the wires going to them have a break in them or are loose now.
I've heard several folks mention the same problem as you are having so there has to be some common ground as to where the problem lies. Maybe it's in the feeler gage we are using and the specs we are setting aren't calibrated because the gage is not properly made to that thickness. I'd be interested in others ideas if they have encountered the same problem.

Charles
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'84 300SD 256,000 Gold on Brown (Mileage Award)
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  #6  
Old 08-30-2002, 01:14 PM
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Let's ask the question another way. Does anyone set their valves at something different than .35 mm and .10 mm? I kinda hate to make any change because it (300TD) does run so smooth/good.
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  #7  
Old 08-30-2002, 03:26 PM
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I don't want to bring in a red herring here, but did you follow the book on the intake and exhaust valves carefully? More than one good mechanic has set an exhaust valve to the intake specs.
The irregular pattern on the MB does cause some problems.
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  #8  
Old 08-30-2002, 06:32 PM
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Do you have the EGR vacuum mechanism on top of the valve cover? If you do, possibly some of the vacuum lines may have been disconnected when you replaced the valve cover.

One thing it isn't is the fuel filters. If the fuel filters are plugged you will get less not smoke more because of a lean mixture caused by lack of fuel.

The only other thing you removed was the throttle linkages. Could they have been changed somehow when you replaced them.

Were the cam lobes perpendicular to the valves when you adjusted the adjustment nuts?

As mentioned above, are you sure you set all the intake valves to 0.1 mm and the exhaust to 0.35 mm and not visa versa?

I just can't think of anything else that would change the running characteristics of the engine by adjusting the valve clearance.

P E H
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  #9  
Old 08-30-2002, 07:49 PM
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I just checked my craftsman feeler gauges (well .102mm and .356mm feelers) with my Starrett laser micrometer (which I luckily have access to) and found them to be within +.002mm of their rating, systematic error but negligible. A dial caliper or a clamp micrometer should work too; if you want to check your own.

If your concern is about your feeler gage, then buy a craftsman, they seem to be OK. Otherwise, It is tricky to remember which valves are intake and exhaust, so mark it out clearly on paper first and check off each valve on that paper as you do it; so you don't get it mixed up. I always wonderd why mercedes did it this way. All my other cars I have, BMW etc, have the same clearances for intake as well as exhaust. My car always seems to work better after valve adjustments, but it's an 85' non-turbo and it's exhaust clearances are set to .300mm rather than .35; less exhaust I guess.

Anyway; I think the only way that adjusting valve lash could cause the engine to smoke and idle rough would be if the exhaust valves were too close (perhaps mixed up intake and exhaust). That would cause the exhaust valves to open slightly prematurely, before combustion was completed, yielding smoke from unburnt fuel and also roughness. Of course this is considering any of the valves weren't so tight before you did the valva job that they never really closed. I guess that would make it run smooth but also really poorly (no power and tough starting) but unlikely I think.

When I do valve lash, I check the clearance between the cam lobes and the carbide rocker arms (I guess that's what they are called, though I have never seen them done that way in another car) with success. Also, I double and tripple check the adjsutment after evey time I tighten the lock-nut. The feeler should go through with some slight resistance to start it, not easily or forced. Perhaps if your not sure of the right feel, adjsut a valve too close and too tight, and feel the differnce. Properly set valves should feel inbetween.


Hope this helps.

ATLD

Last edited by ATLD; 08-30-2002 at 07:55 PM.
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  #10  
Old 08-30-2002, 08:02 PM
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Oh, I thought I should mention, that my BMW 745i runs much better with a 20-50% looser valve lash and slightly opened spark-plugh gap (not applicable to diesels), which will eventually wear to that if you let them, so time makes them runn better between tune-ups. But my BMW valve lash wears in the oposite way that the Mercedes does. Bimmers lash gets bigger faster; as the MB's lash gets smaller slower. Though BMW makes it reasonable with a really easy way to adjust the valves, takes 1/10 the time it takes to do a 300D! Less stuff to remove first too.

But I still think the 300D has the Bimmer beat in almost every way; except power.

ATLD
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  #11  
Old 08-30-2002, 08:12 PM
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Tight exhaust valves will cause rough idle and smoke when hot. The exhaust "grows" quite a bit since it is sodium filled, and if you set one as an intake, it will surely be too tight hot.

Easy to keep track, the manifold is right there for reference. Also easy to set the valves wrong, I've done it.

The only other thing could be the EGR valve stuck open. Pull the vac line off of it and see what happens.

Peter
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1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #12  
Old 08-30-2002, 08:22 PM
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Really, I never knew that the MB exhaust valves were sodium filled too. Maybe it's just with the turbos. Damn, wish I had a turbo then. My bimmer's ehxaust valves are nimonic alloy 90 and sodium filled, but I thought that was for better heat transfer because of the added heat caused by the turbo. Thought diesel's ran cooler for some reason? Cost me $90 to replace each, ugh.

Humm. Maybe it is, it costs $51 from fastlane for exhaust vs. $26 for intake. A lot like the $20 and $90 cost differnce between intake and exhaust in my bimmer.

P E H; do you know if the NA diesels have sodium filled valves too?

ATLD
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  #13  
Old 08-31-2002, 02:03 AM
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ALTD:

As far as I know, all MB diesels have sodium filled valves, and I don't know of any diesel engine, in fact, that doesn't. Way too much heat at high load, has to get transfered to the head or the valve burns.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #14  
Old 09-01-2002, 01:33 PM
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Wow;

Now that makes me feel all warm and fuzzy inside!

I'm so happy. I am always amazed at how many neat things I can learn about my cars form this forum that otherwise I'd never know.

Is it intake and exhaust, or just the exhaust valves.

ATLD

Last edited by ATLD; 09-01-2002 at 02:03 PM.
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  #15  
Old 09-02-2002, 02:24 PM
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Just exhaust, ALTD, the intakes don't get all that much heat when they are open.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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