Odd jerk on startup - PeachParts Mercedes-Benz Forum

JBG · #1 06-13-2008, 06:30 PM

posted about this a bit ago, then the problem disappeared, and now is back. I have a 85 TDT wagon, running grease. However, this issue is upon startup on diesel...basically, the after starting the car, maybe 10 seconds in, the car starts to act fuel starved or like air is getting in--it jerks and idles hard. Each time it jerks, some white smoke shoots out the tailpipe. After giving it some fuel it evens out, but after putting it in drive and running at low speed (5 mph to a stop, and upon starting from a stop) it runs rough, like at low rpms it's not getting enough fuel or air is getting in. Once the car is fully warm, it's fairly unnoticable, maybe just a slightly hard idle at stoplights.

Is there anywhere obvious to check? And what should I be looking for? I was idly speculating that maybe the air filter is clogged...will check this soon. Any other thoughts?

Thanks, wise ones.

JBG

jt20 · #2 06-13-2008, 06:35 PM

are you losing coolant at any rate?

whats your timing and chain stretch at?

its definitely WHITE smoke?

JBG · #3 06-14-2008, 01:49 PM

Coolant level appears good. I just did a flush a month ago and haven't lost any significant amount. The outside of the expansion tank has some white buildup on it though....strange looking stuff. Like salt from the winter. Don't know what that's about.

Timing Chain....have no idea. I'm paranoid about it since the car's at 220K and still on the original chain (so I suppose).

Are these likely culprits????

Smoke was NOT black; either white or bluish, not that good at telling the difference so I assume it's white.

JBG

Diesel911 · #4 06-14-2008, 02:33 PM

You did not say if the problem went away after your car was compeltly warmed up or not.
Since the problem was posted before you might already have tried the suggestion. Hook up your fuel inlet hose to a container of regular diesel fuel and drive it around and see if the problem changes; better or worse. If the problem goes away you have some problem from the fuel tank up to where your removed the fuel inlet hose or possibly with your fuel.
An air leak in your fuel system could cause some of the problems.

JBG · #5 06-15-2008, 11:35 AM

D911,

"Once the car is fully warm, it's fairly unnoticable, maybe just a slightly hard idle at stoplights."

Thanks for the suggestion, I will see what I can do. I just don't want it to be the timing chain....

dieseldiehard · #6 06-15-2008, 11:55 AM

sounds like air in the fuel line to me. On second thought, a head gasket leak also has these symptoms, ie goes away when hot. So watch out for any loss of coolant. Thats your clue. Or do a coolant system pressure check.

You will forever be thankful you replaced or rekeyed the timing chain if that hasn't been done. Its like getting a complete new engine for just a $100 so

That and opening the ALDA a tad (free) are the way to get a turbodiesel to run properly. '85 was a good year for the wagons!

captainmonk · #7 06-15-2008, 11:57 AM

a fisherman is a jerk at one end waiting for a jerk at the other

careful jimmy!!!!!

Alastair · #8 06-15-2008, 12:12 PM

Nah, Injection timing retarded due to general wear and tear of chain and sprockets, especially if it hasn't been checked/re-set before....

Check chain-stretch, and re-set I.P timing.....

It just could be very close valve-clearances....Had that recently on a 300D I recently aquired. It would only run on 4 pots until warm, then all 5....(Valve clearances are set up to increase just slightly on the 615-617 engines as they get hot, by up to 2-3 thou....Ally cam-bearing towers expand more than the steel of the valves/followers etc...)

Blue/white smoke is unburned fuel and a cold engine will be much more prone to mis-fire when the timing is not right....

Chains are generally good for around 300K they say......

Cheaper than a whole top-end re-build, (If it should break!) and reasonably easy to change--If you are worried, roll in a new chain, and check pump timing after.......

Diesel911 · #9 06-15-2008, 01:21 PM

Quote:

Originally Posted by Alastair

Nah, Injection timing retarded due to general wear and tear of chain and sprockets, especially if it hasn't been checked/re-set before....

Check chain-stretch, and re-set I.P timing.....

It just could be very close valve-clearances....Had that recently on a 300D I recently aquired. It would only run on 4 pots until warm, then all 5....(Valve clearances are set up to increase just slightly on the 615-617 engines as they get hot, by up to 2-3 thou....Ally cam-bearing towers expand more than the steel of the valves/followers etc...)

Blue/white smoke is unburned fuel and a cold engine will be much more prone to mis-fire when the timing is not right....

Chains are generally good for around 300K they say......

Cheaper than a whole top-end re-build, (If it should break!) and reasonably easy to change--If you are worried, roll in a new chain, and check pump timing after.......

I believe if his valve clearances were too tight cold they would only get tighter when the engine is hot and would cause more noticiable trouble when hot.

Diesel911 · #10 06-15-2008, 01:30 PM

To get some idea of your timing chain stretch remove your valve cover and rotate the engine just as you would do for a valve adjustment until the timing lines on the back of the Camshaft Grear and the line on the Chamshaft tower are perfactly lined up. See split image Pic; Left side is the Camshaft/Chamshaft tower lined up on the righ side is the degrees on the Crankshaft pulley. When liked up at the factory it was at TDC on mine I am aproximately 2 degrees past tdc.

TMAllison · #11 06-15-2008, 01:39 PM

I'd check your grease vs d2 plumbing for leaks. Clear fuel lines makes it so much easier to track down pesky bubbles.

Bypassing that mess as was suggested with a temp tank would prob help narrow down where a leak was comming.

JBG · #12 06-18-2008, 03:19 PM

Haven't gotten a chance to look at this too carefully yet--I did replace the air filter just for kicks, seemed to mitigate the effects. Wife has been driving the car since Sunday so I don't know how the symptoms are progressing or regressing.

Last valve adjustment was about 20k miles ago.

The air in diesel lines issue is still a possibility; I will get a can of diesel purge and run it on that, see if it jerks.

Thanks for the thoughts!

OH YEAH: and if i can get my timing chain replaced for $100, sign me up....

rrgrassi · #13 06-18-2008, 03:39 PM

You are supposed to adjust the valves every 15K miles.

sethyboy85 · #14 06-18-2008, 03:51 PM

im going to say worn nozzles, try running it on ATF.. if it start and idles fine/better you got a bad spray pattern.

Alastair · #15 06-18-2008, 05:13 PM

Quote:

Originally Posted by Diesel911

I believe if his valve clearances were too tight cold they would only get tighter when the engine is hot and would cause more noticiable trouble when hot.

This may be true of an engine that was made entirely of the same metals....

BUT the cam-towers are Ally....

Ally expands More than steels/irons....

M.B. Do quote 6 thou on a Hot Engine for Inlet, and 4 thou on a Cold engine....(OM617.912 N/A)

Rather goes to support the theory.

I can definately confirm that the valve clearances do increase just a little between a cold and a hot engine, having measured both hot and cold clearances.....