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Old 08-26-2001, 10:39 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
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.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 08-26-2001, 11:44 PM
Senior Member
Join Date: Apr 2001
Location: Beech Island, S.C.
Posts: 468
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.
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Old 08-27-2001, 11:41 AM
Dan Dan is offline
Registered User
Join Date: Jun 2001
Posts: 29
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....
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Old 08-27-2001, 03:16 PM
G-Benz's Avatar
Razorback Soccer Dad
Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
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"!!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car
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Old 08-27-2001, 04:42 PM
Posts: n/a

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.
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