300E rough starting when warm. . .
 

My 89 300E starts on the first try and runs smooth when the engine is cold. Once it is warmed up, restarting is rough. It never restarts on the first try. . .it'll start, stumble, and quit. On the second try, it will start and idle very rough for the first minute or two. Gradually it smooths out and everything is fine.

Any thoughts?

FYI, I've recently replaced the fuel filter, dist cap and rotor, and plugs. I have a new set of wires on their way as well. I also ran a can of Techron through the fuel sytem and changed the oil a couple of weeks later. The car now idles smooth as glass and runs great. But I cannot seem to cure the warm restart roughness.

Thanks for your help.

Have you replaced the Overvolt Protection Relay? Or atleast checked the fuse? :eek:

Yes, I checked the 10A fuse and it is OK. Any other ideas?

Thanks.

The OVP may still warrant replacement, as the part is defective, even if the fuses are intact.

To determine if you have the OEM part, remove the OVP and look along the side for "Siemens Austria". If you see that, it is the OEM part and needs to be replaced. The OEM unit has been deemed defective due to poor solder joints. This was coded as part number 000-540-52-45 (which may also be embossed on the relay).

The corrected retrofit can be ordered and is now issued as part number 000-540-67-45. It's about $65 and FastLane has got 'em.

GBenz,

Thanks for the OVP advice!!! I removed it and it says "Siemens" then "5WK1763", no mention of Austria. The part number is 201 540 38 45 (could be 36 45. . .the 8/6 is partially rubbed off).

Do you know if this is the "bad" version? If so, I'll go ahead and order a replacement. Also, would a relay cause rough starting only when warm? Also. . .I assume the wire harness just unplugs from the bottom of it?

Thanks again for your help. BTW. . .I sent you an e-mail and later decided to post it here so others could see your comments. . .sorry for the duplication.

Yes, it simply unplugs from the harness. It takes only a minute to swap out. I would go ahead and order the new one. It solved all hard starting (when warm) and stalls for my C280. If you do a search, you'll find the function of it... basically it protects the system from being overloaded with too many volts or whatever.

You have the OEM version. Go ahead and get a new one. The OVP causes hot starting problems only. Cold starts are fine. About $65 at FastLane.

Make sure you disconnect negative lead on battery before attempting any replacement of electrical components (make sure you know your radio security code, though). It may seem a little overcautious, but many of us have gone in to replace a 10-cent light bulb or fuse only to wind up frying out a $700 MAS or instrument cluster, or $400 climate control unit, etc.

Thanks again!!! I ordered one from Fastlane this morning. I would not have disconnected the battery on my own. . .such a hassle to reset radio stations! I sure will now.

You may have just saved me a small fortune.

With appreciation,

I'm having the same problem. Where is the OVP located so I can check mine as well?

Thanks,

Geoff

Consider what Benzmac had to say in the thread below. His words in this thread have been echoed over and over here.

http://www.peachparts.com/shopforum/tech-help/3938-rough-idle-300e.html?highlight=300E

goliver:

OVP is behind the battery. A black plastic shroud resides behind the battery that needs to be removed. It is held on simply by being wedged in the firewall lip. Push down firmly and remove.

You will see three components with large harnesses attached. The silver box with the clear top is the OVP. There is a 10 or 20 amp fuse in it. It is held on by a bolt on the side, so you have to remove the bolt before pulling it off.

By the way, you shouldn't have to reprogram the stations after putting in the codes in the radio, unless the battery is disconnected for a very long time...

Thanks for the info. I'll check it today when I get home from work.

Geoff Oliver

OK. . .replaced the OVP on my 89 300E as recommended. . .no change. Car drives exactly as it did before. . .idle may even be a touch rougher than before. On warm restart, it'll stall after the first try. On the second try it starts, idles roughly for a few minutes and gradually smooths out. Plan B?

Should I replace the fuel pump check valve? If so. . .where is it located?

Thanks.

Leaking EHA,maybe?
Just a thought.

What is EHA? Could the problem be a leaky fuel pressure check valve?