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 the car to the deal where they'll charge me more than the car is worth.

Thanks!!

 

Hi there,
just a check suggestion. this sounds like miss fire, check all 6 cyl. compression, see if you are all even with all of them.
thanks
kenz

 

Check your fuel pump relay and or fuel pressure. My relay was functioning intermittently and gave me the fluctuating idle. I noticed it when coming to a stop. The engine would speed up unexplicably.

 

Check your fuel filter as well.

 

Update: Followingsome advice, I changed the fuel pump relay ($25 @ autohausaz.com). 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 heavyacceleration(usuallyfrom a complete stop) the engine will hesitate (maybe rumblea 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 @ autohausaz), 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).

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

 

Remedy:[/u][/b] 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![/align]