Mercedes-Benz Forum banner

Infrequent Idle Problem

2K views 15 replies 4 participants last post by  Captain Scott 
#1 ·
72 280SEL 4.5. At certain times, upon start up the engine has a rough idle. If I do not gas it, the vehicle dies. Even while depressing the gas pedal, its an uneven acceleration. However, after driving it for a few blocks, everything is fine, idle and acceleration smooth.
Any suggestions?
Thx all!
 
#2 ·
Time for an injection system overhaul,cold running idle problems can be; incorrect cold control pressure
Auxillary air valve not opening
airflow sensor maladjusted or sticking
or the injectors may be due for a clean,and how long since the fuel filter was changed?[:I]
 
#3 ·
I have a similar problem. '73 450 SL in Seattle (it can pretty cold in the AMs) where it seems to infrequently run rough on starting, idling and then pushes smoke out until it hits 2000 RPM. It is always smooth and clean by the time I get to work. Chevron gas helped and injector cleaner seemed to improve things. But it still happens. It even happens in the afternoon on the way home - sometimes.... So far, I have given up on trying to solve the problem and just accept that it is an old car and just won't run perfect all the time.....

Scott
 
#8 ·
No,if you discount worn rings from too much fuel washing down the bores...[:(]
It should take any mechanic worthy of the name,about 20 minutes to warm up your car,put the analyser in the tail pipe and adjust the knob to get the enigine within specs.
But it sounds like you have sorted it out OK[:)][:D][:)]
You should buty the Mercedes workshop manual for your car,on ebay there are manuals listed sometimes,but get the paper vewrsion,rather those CD's you see listed.They are genrally cheap copies of the proper thing,which you can access right here.Check out the top of the forum for online manuals.Lots of good stuff there.[8D]
 
#9 ·
Shes running pretty well now that I think I have the FI controller adjusted to "smooth." But I have a couple of things now that I am trying to learn about. The tach needle is now jumping around a little at all RPMs, like about 100-400 above and below the actual. It does it both while accellerating and decellerating. I think it is worse when the car is cold. Could this be a by-product of it not being set perfect? It was fairly smooth over the past month or so since I acquired the car.

Also there is a bit of water moisture spray coming out of the exhaust pipe when it idles. I mean it actually leaves a damp mark on the driveway if I let it idle for a few minutes or more. Of course, it is taking carbon with it so it leaves a messy dark mark. I know the back bottom edge of the muffler can was rubbed thru from the cars prior life of driving up a steep driveway into my pops garage. It dragged if 2 people were in it. So when I got the car, I found a loose clamp on the equillizer crossover pipe. I took some liquid exhaust pipe repair goop and fixed it. I also sealed up that rubbed-thru spot on the muffler can. It never dripped any water, but could the can still have some residual water in it? Or is this also an indication that I have not "tuned" the FI properly?

Scott
 
#10 ·
How far do you drive your car?.You should ALWAYS run the engine until the temp is reading at least a third up the scale and a minimum of 20 minutes.The water should only be condensation from when you shut down previously,Cold moist air condensees in the exhaust,when you start up it is blown out the back.
A typical mercedes problem is the (short running caused) deterioration of the rear the rear muffler.
If you are only running a short distance to work,the rear muffler fills with condensate and eventually rusts through the bottom.that takes about 6 months.
I often hear of people complaining about poor quality Merc mufflers when it it is the fualt of the driver,not allowing enough time to warm up.
When repacement time comes,buy a stainless system from timevalve.
The tacho sounds like a bad earth connection from the body to the engine,Down by the stater is a heavy braided earth strap that is connected to the body.ensure that it is tight.
Also there is a terminal block,where the cables run from the starter,It has several cables on it,a larger one from the starter and several that power the car.Ensure that it is tight,no loose or dirty connections.
Finally,Make absolutely sure that the coolant is Genuine Mercedes coolant NOT cheap supermarket green water.It is vital that the correct fluid is used to prevent coorosion in the water jackets.[:)][:)]
 
#11 ·
I only run about 15 minutes to work, so I suppose water may be gathering in the can. If it continues, I may drill a hold in the back bottom corner to try to drain it then seal it back up... It is far from a new muffler! Since I was messing with the cluster, I wonder if just reseating the wires on the back there will clear the tach jumps? Since I use the car so little (in terms of time) I don't suspect the ground strap, but i'll crank it down just the same. The clock has been dead for a while, too. Could this be a related problem? I pulled the tach/clock unit out of the cluster housing to clean it and see if it was a power problem, but it seemed to be getting 12v... Hummmm. The cooling system was completely reworked last summer by a certified mercmach, including a new radiator and hoses.

About that rough idle, I have been trying slightly different positions on the FI computer knob, and it seems to make no real diff. Even at idle when it is not warm yet, it doesn't seem to make much diff.

On the bright side! it has been running great, morning and afternoon!! Almost no roughness at all. Although, I suppose when I start her today, I may be in for a big surprise...[:eek:]

Thanx for all the advice. I am really good with mechanical stuff, but this german engineering is a new thought process!

Scott [B)]
 
#13 ·
RE: Infrequent Idle Problem - SOLVED

After taking her to a shop I found and trusted, it took 2 hours and (shop rate was $80US/hour) but it was worth it since I figured the trigger points were fine, and the mech identified them as intermittently BAD. One of the micro switches was not switching all the time. I replaced then and I am golden (for now.) With all of the things I checked and hopefully improved, I am much more in tune with this car and how she operates.

Now I can get on to some of the more fun issues like my leaking trunk gasket and noisy fuel pump.

Thanks again all for the ideas, guidance and assistance.

Scott
 
#16 ·
verdejor - 1/14/2006 3:20 AM

I do have one jeopardy question:

2- What are micro switches?
Those are the two pairs of tiny open/close single pole switches inside the trigger point assembly that open and close as the cam lobes spin past. Yes, that means that your distributor has (or at least mine does) 3 sets of cam lobes that transmit timing information around under the hood. 1 cam for the ignition breaker points and 2 cams for the TPs.

I pulled the distributor and then the trigger point assembly and used a multi meter to test them. They looked clean, with no oil residue or other stuff and reacted properly to the continuity tests so I put it back together and wondered some more about why my girl was a fickle as she was...

Not always, but often enough after warming up, they started to misbehave and the car ran like junk.

I'm so glad that I/we finally found a smoking gun. I had been chasing it down for months and was ready to give up. Now she hums!

S
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top