Rough Idle on warm start 1984 500sec - PeachParts Mercedes-Benz Forum

chitown · #1 10-10-2006, 01:50 PM

Been having a rough idle problem and have not been able to diagnose what is wrong.

On a cold start the engine turns over quickly and idles very smoothly.
When the car has been off and sitting for 30 minutes or longer and still warm it is very difficult to crank and it runs very roughly when it finally catches.
Sometimes if I shut off the engine and then restart after 5 minutes it will turn over quickly and idle smoothly.

My local mechanic thought it was cylinder #2 that was not firing. He replaced the fuel injector with no change. I replaced the spark plug on that cylnder with no change. The mechanic determined the faulty cylinder by removing the spark plug wires on each cylinder - if the engine ran rougher then that was not the problem - Cylinder #2 seemed to not make the idle worse when the spark plug wire was removed.
Looks like spark is being delivered to the plug since you can see the plug arc as you bring the spark plug wire to it.

I have tried to eliminate the fuel pump as the problem by jumping the contact so that the fuel system remains pressurized - that didn't seem to prove anything.

Often when the engine is idling roughly I can smell fuel that leads me to believe that a fuel charge is being delivered to the engine but not being burned. I was going to try a new plug wire to eliminate that as the problem.

The idle speed is good, idles faster when warming up and then decreases when at temp.

Any ideas?

Thanks in advance.

Mike

chitown · #2 10-15-2006, 10:46 PM

An update

I changed the sparkplug wires and spark plugs with no changes.

The plugs looked good - all of them showed a light tan coloration
and no abnormal erosion.

I did notice that when I pulled the plugs on cylinder #1 and #2
there was a lot of oil on the threads. I didn't appear to me that
oil was leaking from above from the outside of the head.

So could I have oil seeping into the cylinders? I have not noticed
the engine burning any oil - no blue smore when it is idling roughly.
Seems to me that if there was oil in the cylinder that starting cold
would be more difficult since it has had more time to pool in the
cylinder. After the engine has warmed up I would think that
the oil would have been burned off so the problem should go away.
When running rough the problem will continue no matter how long
you run the engine - it does not go away.

How can I verify oil in the cylinder? Would a compression test
prove anything?

t walgamuth · #3 10-15-2006, 11:09 PM

this is a classic case of warm start difficulties. it is very typical of this era of fuel injection. the v8s seem to be worse than the inline engines. when i first got my 500sec i did all the stuff you mentioned and more but the problem still persists. it is lessened if you always remember to push the pedal down some before cranking it over.

mine has become pretty tolerable as long as i remember to do that.

tom w

chitown · #4 10-16-2006, 10:45 AM

Thanks for the reply. It is more than just a warm start problem though.
It will idle roughly until I shut down the engine. I have read on this
forum the warm start issues but many seem to clear after the
engine starts running. Once the engine starts in this mode it will
continue running like that.

I can shut the engine down and then restart right after and sometimes
the rough idle will go away.

When the engine starts are there parameters that are set, like
engine temp, outside temp, that may not be set correctly
and do not get updated while the engine is running?

david s poole · #5 10-16-2006, 11:10 AM

see if you can find someone to do a smoke test on your intake manifold.many rubber parts,very old lots of little vac leaks that add up to large leak in effect.have worked on many of these cars symptoms you describe all point to the same problem------many vac leaks.

A264172 · #6 10-16-2006, 02:04 PM

Oily plugs means your O² sensor is likley taking a beating... try disconnecting it, when you have rough running, to see if it clears it up.

david s poole · #7 10-16-2006, 03:12 PM

i think he said that it was not burning oil just some oil on the threads of plugs.

chitown · #8 10-16-2006, 07:03 PM

the plugs from #1 and #2 cyl look good -had a light tan color with no erosion. When I pulled them out I noticed a lot of oil on the threads but none on the plug tips - which I thought was odd. I didn't see that on any of the other plugs.

I suspect vacuum leaks as well and have been all over the engine looking for broken lines - found a couple and replaced with no change. A smoke test sounds like a good idea though.

What has me stumped is if I am leaking oil into the cyl why am I not seeing any fouling or smoke? and why would it run so smooth on a cold start?

chitown · #9 10-16-2006, 07:06 PM

Don't think the O2 sensor is working. The warning light went on long ago and apparently the thing to do is remove the bulb from the indicator light. I asked my mechanic if I should change it out and he said I would not noticed any improvement in the engine. I bought one anyways and just haven't gotten around to installing it yet.

A264172 · #10 10-16-2006, 08:50 PM

Quote:

Originally Posted by chitown

Don't think the O2 sensor is working. The warning light went on long ago and apparently the thing to do is remove the bulb from the indicator light. I asked my mechanic if I should change it out and he said I would not noticed any improvement in the engine. I bought one anyways and just haven't gotten around to installing it yet.

...well since you already threw the $ ... why not see if it worked?

P.S. I believe you also have a 'Warm Up Regulator' that costs $1,000 and can cause such trouble... think it can be rebuilt cheaper... best not go there till you look for cheaper solutions first...

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