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Cranky when not used daily


My car is causing a whining noise. Sadly, the whining is coming from me. The whining started when I talked to the dealer about solving a reliably intermittent problem the car has. It went something like this:

The dealer: “We are not sure what is causing the problem, but our tests show that the wiring harness is showing a lot of faults. We recommend that you replace the upper and maybe lower wiring harnesses.”

Me upon hearing what they are asking for this: “Cough. Uh…do you have any ideas that won’t cost me over $5,000?

My car has had this reliably intermittent problem for several years, and I have not been able to find an answer on how to remedy the problem, and thereby eliminate my whining.

My car is a ’92 Mercedes 400e. It has about 135 thousand miles on the odometer. I’ve had the car for about 13 years.

Here is the reason for my whining: The car is reliable as long as I drive it daily. If I let it sit for more than a day or 2, the car will start right up, and run and idle great for about 15 minutes. After that it starts to sputter a lot at idle and when pulling away from a stop. Sometimes it stalls while at idle or pulling away from a stop. It does this even if I turn off all the electrical things that I can. If I run the tach up to about 1500 rpm, no sputtering…until I come to a stop.

Once the car starts to behave all sputtery, I usually let it rest for about 30 minutes or so, and after that the car starts right up and usually runs fine.

If the car sits for longer than 2 days, it takes more trips before it starts to behave again.

I’ve found the easiest way to avoid this problem is to use the car daily. Of course, I don’t drive every day and every time I let it sit for a couple of days it’s back to being meister misfire.

This has been an intermittent problem for several years. I’ve had a complete tune up by the dealer, which actually helped for a couple of months, but the problem came back. Most recently the dealer replaced the rotor caps (it had been about 35 thousand miles since they were last replaced) and again this helped, but only for a few weeks.

The dealer can’t provide an answer. Their latest guess the wiring harnesses - and that’s where my whining noises started.

The car is garaged when not in use, but I’ve found that keeping inside or out does not affect this problem, nor does it being warm or cool, nor does a lot of rain or a little rain (I live in Seattle, where the only thing that changes about rain is the temperature).

Any suggestions on what might be causing the car’s cranky behavior, other than the part where the car is over 20 years old?

Many thanks


The problem could be the wiring connectors, the plugs that connect various components to the harnesses.
The wires themselves don’t fail unless they get pinched, chaffed or repeatedly twisted.
That kind of problem can be spotted visually and they should be able to point it out to you.

Back to connectors: I would disconnect each connector, inspect visually for corrosion, spray contact cleaner and apply dielectric grease.
Do this on every connector on ignition parts, fuel injectors, sensors and engine computer.
Not on the oxygen sensors, they get very hot and can be contaminated.
This will be time consuming but not thousands of $$.

Distributor cap helped? Maybe you need plug wires and/or ignition coil.
How old are the spark plugs?

Look for a good independent shop; try the “mechanics files” button at the top of this page.
Also, there’s
There’s generally no need to go to the dealer for non-warranty work.

Justan I have absolutely no opinion one way or the other about the shop’s diagnosis or competence.

Because I’m not there.

But I worked at a Benz dealer for many years.

I replaced many wiring harnesses on cars just like yours, because the wiring would get extremely brittle. Intermittent misfires and other strange things would occur, due to the very poor electrical contact caused by frayed wiring.
Some guys would actually try to fix the wiring harness, but unfortunately, it very often wasn’t effective. The harness was often so brittle that simply flexing the wiring would cause a break in another location. The broken wires were often the ones going to the injectors and coils.
I will say that many engine problems were indeed fixed by replacing harnesses. This is not a case of simply guessing. Parts that were visibly defective were being replaced.
I can only hope that whoever’s working on the car has been there awhile and halfway knows what he’s doing.

There was one particular Benz that comes to mind. It would stall intermittently. Impossible to reliably duplicate. I painstakingly ohmed out every single wire on the main engine wiring harness, while flexing the harness. To check for breaks. Well, guess what, the very last wire I checked showed open circuit when I flexed it. It was connected to the ignition control module. After replacing the harness, the stalling was fixed.

Like @db4690 said above, these cars are very prone to engine wiring harness failures. We call them “biodegradeable wiring” because the insulation on the wires literally disintegrates and falls apart with age, leaving you with a bunch of shorting out wires in your engine. All it takes is one wire to go open circuit or short circuit and cause all kinds of trouble. I too have replaced several upper engine wiring harnesses.

You won’t be able to find a good used one. Even the auto salvage yard in Seattle that specializes in Mercedes parts won’t sell you one even if it looks good, because it won’t be good for long. There are sources for aftermarket wiring. There are a number of Euro auto shops in Seattle that can help you, from getting a second opinion on the diagnosis to repairing it at a little more affordable cost.

So much for German Engineering…

I have no doubt whatsoever that what the others are saying is true, but to me this sounds like a typical fuel delivery problem. Did the shop mentioned check the line pressure?

This is another reason American cars are better than imports. I have owned many foreign cars, Saab, Triumph, Jaguar, Alfa Romeo, Fiat, MG, and all of them have presented a problem with the factory wiring. Usually a problem with weak, brittle wiring insulation and electrical motor problems. I can’t recall ever having any major electrical problems with an American branded car except for a heater blower switch on a Corvette that I once owned.

“I have owned many foreign cars, Saab, Triumph, Jaguar, Alfa Romeo, Fiat, MG”

Would have been more reasonable to say:

“This is another reason American cars are better than European imports”

+1 to circuitsmith’s comment.

I got hooked on “riceburners” back in '76. After numerous quality problems with my '72 Vega (bought new) I traded it for a '76 Corolla when the rear axle slid out of the housing while driving. This was a known problem with these cars. With the Corolla, I just kept driving it and driving it and driving it. Nothing ever broke. I finally traded it in '82 when my needs changed.

I tried “American” again in '93, at the urging of my (then) wife. Got a Saturn. It had to go back a few times for minor warranty work, and ultimately at 100,000 miles the headgasket (or head, I’m not sure…my ex had the car by then) went out. Nothing like that had ever happened to any of my riceburners.

The difference in reliability and quality between the “American” cars and the riceburners was, in my personal experience, dramatic.

The Triumphs, Jaguars, Alphas, Fiats, and MGs were all known for needing constant TLC…but oh, the passion! I love those old ragtops. I’ve almost bought one on more than one occasion, but the need for reliability is just too ingrained in me. The Miata became famous for being a replica of the MGB without the reliability issues. Right now, if I had the spare change, I’d see if I can still fot into a Lotus Exige.