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?

Sometimes I just ignore these long threads thinking that with so much advise there can't be much to offer. Well this one sure ruins that idea. What a bad bunch of advise. Wrong numbers for OVP relays. Characterised auto diagnosis. Repair advise based upon the "blue car theory". That theory states that if it was a blue car the ...... must be the problem (fill in the blank).

Parts that go bad either do or don't do something. As a diagnostic technician I cringe at any diagnosis that isn't based upon testing. AND testing is based upon hypothesis. A bad OVP relay can do one thing only. It can reduce or eliminate power to control units it suppies. Do any of these control units have anything to do with hot starting. Actually NO!. Idle will be raised when hot by not powering the idle valve through the KE controller. The fuel enrichment starting will be gone but plays ever so much less of a part than it does cold.

The OVP can also just reduce voltage (acting like a resistance to the circuit). When doing this the idle can drop and cause stalling and poor idle. If this was the hypothesis, I would disconnect the idle valve and see if it made a difference to the problem. In other words draw an hypothesis based upon knowing the system and pick the easiest way to evaluate the hypothesis.

The NUMBER ONE, all time answer, for hard hot starting on any K, K-Jet w/Lambda, or KE system is fuel pressure leak down caused vapor locking. This leakdown doesn't matter to cold starting as pressure builds almost instantly. On a hot motor if you release the pressure the fuel boils in the injectors and lines. This messes with fuel delivery.

Here is where diagnostics gets complicated. I can give you many simple ways to evaluate whether OVP could be the problem (other than parts replacement; which is one way), but I can not tell you any way to view the simple problem of fuel pressure leak down except to put a gauge to it.

I would never use the "blue car" diagnostics to suggest that fuel pressure could be the problem. But, I will always promote diagnostic concept.

Sounds to me like the fuel pressure accumulator is the culprit. In my experience 9 times out of 10 when it starts okay when cold and then refuses to start for about 20 seconds of continuous cranking after shutting it off for 15 minutes or so the accumulator is the fault.

As Steve, emphasized TESTING IS THE KEY! I found out the hard way.

I, too, had a 300E warm start problem. I researched posts here and concluded (without testing) that I could solve the problem simply by replacing parts. Well, after a new OVP, a new fuel pump relay, new fuel accumlator and new fuel pump check valves ($200 plus and a couple hours!), the problem was still there.

I was convinced it was fuel pressure leakdown but had no way to measure it. I took it to the dealer and they measured 2.9 bar for 60 min after shut off, which is well within the spec. Still, they could not diagnosis the problem.

I then took it to an independant shop (Brooklyn Motoren Werke in Brooklyn, Wisc). They finally solved the problem.

You know what it was -- fuel injection nozzles. Now the car had only 70K miles, but the tech concluded the age and the type of driving by it's previous owner probably contributed to flow and pattern restrictions with the nozzles. (I had run a pile of Techron through it earlier to no avail). They discovered this by examining some of the nozzles and they appeared to have inadequate spray patterns. He then replaced one with a known good nozzle and experienced some improvement in starting. So, nozzle replacement seemed the best bet and it worked. If not, examing the fuel distributor in detail would have been the next step.

So, while you might get lucky from time to time by replacing components by guessing without testing, it will more than likely cost you more time and money than a procedure that includes proper diagnosis.

Hope this helps....

Steve hit the head on the nail when he said that "testing is the key".

Most of us who have occupations that require troubleshooting skills of some sort know that we go through a series of testing approaches (in IT, it's called "debugging").

While I can't apologize for all of us for some of the relatively poor advice that we may have dispensed, I am sorry that the OVP did not solve your problem. The serial numbers I quoted were drawn from a previous thread I had read. Given that you are struggling with a problem that was similar in nature to what I had experienced with the same engine and model, I sought to give you a bit of insight from my experience that solved my problem.

The OVP took care of that in a heartbeat, and from the threads I've read, many others gained from swapping it out.

Of course, many of us cured our own nagging headaches simply by taking aspirin, while others may have headaches that are caused by some other events that aspirin may not solve.

Consider that while there are a few of us that are bonafide MB experts, the rest of us Joe's are simply MB owners that are looking for alternatives to running to the dealership everytime our cars hiccup! And if someone can recall a similar experience to our own with a solution that worked, then we should probably take it with a grain of salt. Alternatively, my lack of knowledge of MBs in general would never have led me to the OVP, no matter how much diagnostic skill I possessed.

Again, I apologize for the expense you have incurred from the "parts-throwing" advise I dispensed. I hope you find the solution to your "hot-start" problem soon...with proper diagnosis.

I hereby from this day forward, refrain from shooting OVPs at threads entitled "W124 hot-start problems"!!

G-Benz,

No need to apologize. The decision to replace the OVP was mine, not yours. I took your advice, true, but only after doing a search and reading numerous posts by others whose problems were solved in a similar fashion. I'm happy to have replaced it even though it did not resolve the hot-restart problem. I consider it a bit of insurance considering it seems to be a known weakness anyway. I'd probably feel worse about it if it was a $400 part.

I could not agree more that proper testing and diagnosis is the right way to go. Unfortunately, I'm only a shade-tree mechanic at best. I enjoy tinkering and don't mind trying a few inexpensive part swaps in hopes of a quick-fix. Like the stock market, its a gamble (actually, the stock market is worse than a gamble). I replaced all my ignition stuff (wires, plugs, dist cap, rotor) hopeful that I'd resolve the problem. The car runs noticeably smoother and I was due for service anyway (120K).

The other problem is that I've not had good success with the mechanics that I've used. Generally I go home feeling ripped off. I'm looking forward to the new shop opening in Duluth, GA by our kind host. For now, I'll just stumble along for the first few minutes after re-starting.

Thanks to all.