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Old 08-04-2003, 12:43 AM
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Join Date: Aug 2001
Posts: 32
rough running when running A/C on hot day

91 300SEL 150K. New copper plugs (.035); plug wires & cap are recent; fuel filter replaced a little while back; OVP just replaced too! Air filter is clean, radiator was flushed this spring (by myself) and new thermostate. Car always starts fine and idles smooth at start up.

OK, when I drive the car without A/C and its hot outside the car temp will get between 85 and 95C. The car seems to do fine, a very slight rough idle (not much) at a stop light after some stop and go, but nothing to cause me concern.

Here is my problem. When I drive with the A/C on, like a 30 min. highway drive to work with a little traffic (if any) along the way the car temp runs 85, maybe 90. When I get close to work I hit a traffic light or two and here comes Mr. Rough Idle, sometimes really rough to the point of not being to accellerate through the last light. The car temp hits 110 plus....the ABS light comes on...even after I relaced the OVP last week!

Do I simply need a professional radiator flush? It seems as if the car just can't handle the A/C being on when its hot. Does a car that gets too hot trigger the ABS? Is 110C too hot (I read a post that suggest not)?

PS, After I recently had my upper timing chain replaced my mechanic told me I should replace the valve seals too. I did not do the valves. I still loose a little oil over time, but the plugs don't seem to be fouled too bad. So could poor valve seals cause overheating and rough idle at high engine temps...and cause the ABS light to come on with a new OVP?

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Old 08-04-2003, 01:02 AM
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Join Date: May 2003
Posts: 545
Going through your post here chronologically I hope. Just my 2 cents.

- Radiator flush and new t-stat. Car has 150k. Is this the orig. radiator? T-stat was likely installed correctly - air-bleed valve @ highest point and a quality German t-stat?

- ABS light on - they have been known to be bad out of the box. Not saying this is the case. Had you doubled checked the fuse or maybe fuses. Some OVP's have one; others have 2. Pull them and make sure they're not blown.

- Upper timing chain or upper timing chain cover reseal? Which was it? If the timing chain was replaced, I would hope the timing is correct.

- Were the valve stem seals replaced? Not sure I follow this part? You mention not too oily? If you have oil on the plugs, this isn't helping things in general - oil is finding it's way to the catalytic converter as well if you have oil on the plugs.
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Old 08-09-2003, 01:43 PM
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Join Date: Aug 2001
Posts: 32
On a long business trip this week sorry for the late reply...thank you for taking the time to respond.

It is the original radiator as best I can tell.
I replaced OVP to deal with the rough idle after plug replacment showed no improvement, the fuses on the old one were good by the way. Still have rough idle, more of a rough idle the higher the engine temp.
Replaced just the timing chain cover seal, timing is good it seems but have not measured it...maybe the timing was knocked off somehow during this repair?

The valve seals have not been replaced. I was told that when I had the timing cover seal repaired that since they had the valve cover off I should do the valve seals too. It would have turned a $500 repair into $700 plus so I did not.

The point I was trying to make about not too oily was that I do not think that the valve seals were that bad yet. I owned a 300E once and before I had the valve seals replaced it seemed as I was fouling spark plugs every other fill up. Right now the plugs do not seem fouled (no burnt oil caked on the end of the plug) but I do see a little oil next to the treads.

I have since experienced more problematic engine stall even when I do not run the A/C in order to keep the engine temp below 90. When I turn on the A/C eng temps run around 100 or 105 in stop and go traffic. Then it idles even rougher to point of not wanting to accelerate at times.

Even when the engine temp is down to 90 the idle will skip a beat every now and then.
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Old 08-09-2003, 06:27 PM
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Join Date: May 2003
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Regarding the rough running with the a/c on.

I found an old post(1999) - 300SD. Benzmac suggested that the aux. fan operation be verified. If the 2 fans in front of the condensor are not coming on, refrigerant presure will build up and cause rough running.
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Old 08-11-2003, 11:06 PM
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Join Date: Aug 2001
Posts: 32
My shop fixed my car today...

The fan clutch was the cause of the over heating problem.

The rought idle, stall etc. was the coil wire. Somehow it was damaged, looks like the fan blade hit it. Not sure how it got that way but the core was slightly exposed. The coil wiring is now running through a rubber hose that is just under the air cleaner.

Besides that the air mixture was adjusted too.

Oh and the thermostate was replaced, even though I just put in a new one...I don't think that had anything to do with it but when you are trying to find a problem I guess even the smart guys start with the easy stuff first.

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