6.9 Lumpy idle

[TJ 450]

Hi all,

So, now that my 6.9 is actually running (albeit without exhaust system ATM!), I'm trying to figure out why the engine runs roughly. It seems to smooth out when I rev it over 2k rpm, but the whole car shakes when at idle (a bit less than 1k when cold). It doesn't sound like it's missfiring through the exhaust and I have a new set of ignition leads too. When driving, it has little power at low rpm, but picks up over 2k as well. I have tested compression to be within 5% across all cylinders.
I'm thinking it is vacuum related, like there's a massive leak somewhere, possibly the EGR valve... any ideas? I might add that the car had been sitting in someones front yard for over three years, and has done 13km since resurrected.

Regards,
Tim

 

[SELfor50]

Sounds similar to when i had the vac advance disconnected on my 450. Also possibly check the air / fuel mixture, might be struggling either way on that.
But vac leak is most likely. These few things worked for me...

That's where the extent of my knowledge / assistance ends.

Cam.

 

[Andrew280SEL]

Same here, I can only think vacuum leak.

My 280SEL has a vacuum leak, somewhere between the inlet manifold though I can't trace it yet, and it idles like a dog to be blunt. However, rev it to/over a 1000RPM and it's smooth as anything and power is fine.

So if it's a vacuum problem, I'm tipping it's a lot worse than mine.

 

[Michel]

Did you say running without an exhaust?

It could be the lack of back pressure

 

[TJ 450]

Thanks for the replies

Perhaps I should get an exhaust system fitted before jumping to conclusions. Athough, it was running like it is now even with the exhaust installed (having said that, both welds were broken at the centre muffler creating a massive exhaust leak that was quite loud)..

Tim

 

[Michel]

Thanks for the replies

Perhaps I should get an exhaust system fitted before jumping to conclusions. Athough, it was running like it is now even with the exhaust installed (having said that, both welds were broken at the centre muffler creating a massive exhaust leak that was quite loud)..

Tim

My thoughts precisely.
Have an exhaust fitted, before you worry about it..

 

[Styria]

Hi Tim, you haven't told us what it is exactly that you have done to your car by way of 'qualified maintenance' since you bought the car. I am ignoring new oil, oil filter, spark plugs, fuel filter etc. You say you have new leads - are they genuine or after market carbon leads ? Also, did you fit new suppressors?

Unfortunately, there are any number of components that could be out of adjustment, unserviceable, faulty etc. I think you will need to fit an exhaust system to start off with, and then perhaps (if registered) try and take it for a 10 kilometre drive. Then call in at a mechanic with a tune-up machine, and a mechanic that is familiar with the K-Jet system and the right testing equipment, to check the operation of the WUR (warm-up regulator), and the fuel pressure system as well as injectors etc.

You have to ask yourself why the car was laid-up for three years - it obviously must have suffered from some problems and you have bought the car with the problem still in evidence, whatever it is. Unfortunately, you will just have to work your way through it. Good Luck, Styria

 

[koan]

Any black smoke, unburnt fuel smell after warmed up?

koan

 

[TJ 450]

Hi Styria,
So far this is what I've done regarding maintenence:

All the basic items have been attended too, rocker cover gaskets replaced, new fuel pump, filter and hoses (old pump was seized, hoses were rotten). Upper control arm bushes have been renewed as the old ones were gone completeley. The car was basically driven until it broke by the P.O., they inherited the car presumably in excellent condition, but could not afford to maintain it. The oil filter had not been changed since 1988!

The timing chain is original @ 223,811km, I intend to replace the chain and all asociated components when the car is registered and on the road.

Regarding the suspension, I am awaiting a set of new spheres, as they are on order. I will PM you on friday RE strut mounts too.

Hi Koan, there's no obvious smoke, not even at startup after sitting for a month. No unburn't fuel smell either... if anything it's running lean.

Tim

 

[WGB]

If there are no vacuum leaks, plugs and timing are OK I would trya a bottle of injector cleaner.

When they first brought out unleaded (only 91 was available)in the late 80's I had a 126 - 300SE which every month would stop idling after initially becoming rough.

2 bottles of injector cleaner (brand seemed unimportant) into the tank each month at the start of the rough idling and it settled.

Latterly they brought out 95 which had a detergent in it and then even later they added detergent to the 91.

Bill

 

[Styria]

Hi Tim, I am in no way critical of what you have done. I understand that you have obviously attended to various matters as per your post, but I am afraid that there could still be a number of items that could be responsible for some of the problems you have with the car, and these will need to be checked one by one.

Just to give you an instance - I switched a fuel distributor which was running perfectly onto another 6.9 engine. The motor would hardly run - in fact, it was terrible. You see, fuel distributor and the warm up regulator should be tuned as a pair. The WUR affects the running of the engine right through its entire range, and unless you have the test equipment, you're up against it. You may also find that your ignition control unit could be at fault-I could go on ad infinitum. Not a lot of solace for you, but you need to check everything. As a matter of interest, did you check timing chain slack when you replaced the rocker cover gaskets ? As I said, any number of things could be causing you problems. Regards, Styria

 

[TJ 450]

Hi Styria, That's OK, I took it as constructive advice .

I suspected that the fuel injectors were a likely issue, so I ordered some seals a while back so as to remove them for inspection. Tonight I removed the injectors #5 and #6 for inspection and found the appearance of #5 okay, but #6 looks very suspect. #6 also has a reasonably fouled spark plug as well.

I will not be running the car until the exhaust is installed, so I will be ending this exercise at the injectors (for now). Is it a wise move to purchase a set of new injectors at this stage, or get the old ones cleaned? I will be posting some pics shortly.

EDIT: About the timing chain, I didn't check the slack, but it is obviously worn... I am expecting a worst case scenario here. After I have done the required work on the suspension, steering and exhaust, I will be taking the car to a Mercedes specialist for a thorough service with the emphasis on the K-Jet system.

Tim

 

[Styria]

Hi Tim, because of their design, it is not possible to clean these injectors. You can have them tested for their spray pattern, but that's about the limit. Regards, Styria

 

[BAR]

Not sure if your problem is fixed, but I've had a similar problem for a long time with my 450.

It's been serviced and still had a problem, many items were suggested but I FIXED the problem myself.

What I 'saw'. I lifted the bonnet whilst the car was in the garage at night, without any lights on. Looked around the distributor and spark plug leads and noticed that there was an occasional electrical spark from the leads to the dizzy cap!

So I pulled off the leads, one at a time and examined them. On one connector the screw on plug at the distributor point [call it the connector for lack of better technical knowledge] for the lead was corroded: also noticed that the lead was relatively easy to lift off.

I cleaned up the connector and cut down the insulating rubber - this was long enough to prevent the clip inside of the plug lead from making secure contact with the connector. Once cut down [but not so much as to expose shank] it went back on nice and tight - with a click - and I knew it was on right.

I did this with the 8 other leads [including the one to the coil] and fired up the beast. Well since then it's been running nice and smooth.

Another possibility could be the vacuum line for the ignition advance, that is the one at the bottom of the diaphragm attached to the distributor, may not actually be attached to a vacuum take off point on the manifold. The other one at the top is for ignition retard, but doesn't really make too much difference to the running of the motor from what I've discovered.

 

[TJ 450]

I fixed the problem... it was due to incorrect firing order, cylinders 1 and 2 were swapped. I installed a full set of brand new injectors and seals as well as lubricating the airflow meter. The idle was much smoother than even my 450, however it's now in bits again as I'm in the process of replacing the water pump, front crankshaft seal and timing chain.

Tim

 

[BAR]

That'll get you every time. Engines work better when sequenced correctly. Glad it was a 'simple' fix. Probably took hours to track down.

Enjoy the rest of your work schedule.