I own a '96 C280 with 120k miles and have recently been having a problem with the engine. Intermittently, the engine:

1) revs the RPM's while at a complete stop; or
2) cuts out during acceleration or at speed.

I've been searching the forum posts for the last several weeks and the general diagnosis for this type of problem is that it's the Engine Wiring Harness, Coil Packs, or MAF.

Since it appears to be the most common issue, I started with the Engine Wiring Harness. But, after checking the exposed sections of my harness it appears to be in pretty good shape (unlike the rotting/cracking pictures in other posts). I bought the car from a MB dealer in '98 with 30k miles on it, so the dealer may have already replaced the faulty harness.

Second, I replaced the spark plugs and cleaned the coils and MAF. Although this gave the car more pep, it doesn't appear to have cured the problem. Plus, I don't think it's a shorted coil pack since the car has a pretty smooth idle generally (other than problem #1).

I haven't been able to run a OBDII diagnostic test yet (not that those codes ever do me much good), but the only visually obvious engine issue is a leaky head gasket (leaking in the very front of the engine). Is it possible that the oil from the leaky head gasket is interfering with an electrical sensor (injectors or air)? Before changing the spark plugs I cleaned and degreased the engine so its pretty clean now but still has the same issues.

Please help. I'm fairly new to the DIY game, but can't fathom taking it to the dealer where they'll charge me more than the car is worth.

Thanks!!

Replace your OVP relay.

Forgive my ignorance but what is the OVP relay?

Just type in OVP in the search box or Over Voltage Protection Relay.

You'll get more info to read than your brain can process.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Update: Following some advice, I changed the fuel pump relay ($25 @ ***************). This appears to have solved my initial two problems mentioned above (i.e. revs the RPM's while at a complete stop; or cuts out during acceleration or at speed), but now the car is suffering from two new issues:

1) Immediately after starting car RPM's fluctuate up and down (is "hunting" the correct term?). No roughness to it, just constant fluctuation between 500 and 1,500 RPM. Upon shifting out of "Park" this goes away, but returns upon next ignition start.
2) Upon heavy acceleration (usually from a complete stop) the engine will hesitate (maybe rumble a little) then kick in. No hesitation at normal throttle while moving.

I researched these issues and thought it may be the Overload Protection Relay so I replaced it ($58 @ ********az), which doesn't appear to have helped. I finally pulled the codes which revealed P0301 (misfire cyl. 1) and P0101 (MAF discrepancy). I'm hesitant to include the codes on this posting because I don't know how old they may be and I'd prefer advice based primarily off of the symptoms rather than codes (FYI... per previous post I did clean the MAF, but only want to replace as a last resort $$$).

Thanks again for your help! All of your advice is very much appreciated and I feel that I've already learned a ton from y'all about DYI car repair. Please let me know if you have any ideas on the new issues.

Thanks,
Sideshow76

Change the plug connectors under the coils..........
and make sure you don't have Plats

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A Dalton

Remedy: After changing the fuel pump relay, overload protection relay, and fuel filter, to no avail, I finally got a new MAF. Replacing my MAF fixed the "hunting" idle and delayed acceleration issues. Work Time: 5 minutes; Cost: $184.37... Yippee!!

Special Thanks to all who contributed to help me diagnose the problem!

I have a 95 C-280 with 170k and the same problems as described on your 96---idle "hunting" and cutting out and misfiring during acceleration. I have had the engine wiring harness, EGR valve, plugs (not platinum) and ignition wires replaced. Still suffering with the same symptoms. Most recently a tech found and repaired 2 vacuum leaks...did not solve problem. I had the MAF replaced but when the techs found it did not cure the problem they removed the new MAF and re-installed the old one at my request. We have spent a bunch of $$ trying to fix this and am running out of patience (and money). Any other suggestions?

p.s. there was evidence of mice chewing wires and vacuum hoses. The tech said to put whole cloves under the hood, in the trunk and under floor mats. He says the smell of the cloves irritates the rodents and will repel them.

Where did you obtain your MAF?

Cheapest I could find is at www.*************** for $185.

If your idle is "rough" (i.e. heavy vibration/shaking) rather than just "hunting" (i.e. bouncing RPM levels), then your problem may be an ignition coil rather than an MAF. They can cause similar symptoms (stuttering idle and delayed throttle response), but the ignition coil failure symptoms are much more pronounced and will cause a rough idle and possibly stalling.

You probably need a new MAF anyways, but if that doesn't fix the problem then check the ignition coil packs, which are attached to the top of 3 of the spark plugs (6 cyl). The ignition coils are very easily switched out (15 minute job) and cost about $65/each. While you're changing your coils you may also want to swap in a new set of spark plug wires ($60) and possibly spark plugs if they're old ($1-$3/each). My theory is that its best to swap out everything while you're in there to minimize the work later.

Hope this helps.

We have replaced the plugs and wires, but not the coils or the connectors that go from the coil to the plug below it.

I will probably change those first since I have taken the engine cover off and am comfortable with working in this area.


2 different techs had switched the MAF but this did not solve the problem so the old one was re-installed.

By the way, should the MAF have 2 quick release clamps on it or one? Mine has one clamp on the top and none on the bottom.

Mine has two quick release clips, one on top and one on bottom of the side of the MAF connecting to the air intake filter box. The other side of the MAF is connected with a large worm-gear clamp.

I am missing a clamp

Could be a problem, as any unmetered air will result in wrong A/F mixture.The one that usually allows air is the one right at the throttle body end of the cross-over air pipe.

It is hard to get to , so guys forget about it...a real long screwdriver works well from the front...I check that one first as it is alway getting taken on/off [ like for plug changes and stuff]..so it is an automatic check.

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A Dalton