87 300E - intermittent power loss

[JoeR]

I'm having a real problem with my daughter's '87 300E. The fact that she is 19 and not car savvy doesn't help troubleshooting here!

The car will run fine, but occasionally will experience a "bog" where the car doesn't want to respond to the gas, and sometimes stall. In my driving, it only happened once to me in heavy traffic, but it was only momentary.

The other day, while driving it again, it happened while turning into the drive. Pressing the gas, it would bog. Playing with it, if you could get the RPM's up, it would catch. At idle, the vacuum (economy) gage is normally high. When this happened, the gage was considerably further towards the red, about 1/3 over. Pressing the gas part way, it would go all the way into the red. I also smelled gas. Then I noticed black smoke (over rich!) coming from the exhaust.

This condition is intermittent, making troubleshooting very difficult. It will clear up just as mysteriously as it happened.

Other notes: Plugs, wires, cap & rotor are all good. O2 sensor was just replaced, as was fuel filter (both were on hand and due).

I've spent hours searching here and reading threads without hitting that majic "BINGO!!", that's the problem.

Any insight, solutions, tips or pointers to likely items would be greatly appreciated at this point.

[Mike Murrell]

Did this problem exist BEFORE the O2 sensor and other parts were replaced?

[JoeR]

Yes, and thanks for responding Mike. I had both and was going to do them, but hadn't yet.

The problem started before replacement of both and didn't change. I just thought that I'd get these two due items out of the way before troubleshooting.

Intermittent problems are a PITA to chase!!

[Mike Murrell]

One item that takes a lot of blame is the OVP - overvoltage protection relay. There was once a photo of it at the parts site here, but I see it's no longer available - the photo that is.

On my car(same motor as yours), it's located back around the false fire wall, midway across. A couple of small bolts hold it in. There's one or maybe 2 blade fuses in the top of it.

With engine off, remove it and shake it, being sure to hold the small flap down. If you hear rattling, replace it. I had an intermittent stalling issue that was resolved with the replacement of this part after it flunked the shake test.

It's also quite possible the duty cycle needs to be adjusted on your car. Steve Bortherton has an article in the DIY section and if you search on things like "duty cycle" and "EHA", you'll come up with enough stuff to keep you busy for quite awhile.

Old, hardened vacuum hoses could also be an issue.

There's no silver bullet for this issue - too many things that can cause intermittent stalling.

[MacGyver]

I had the same problem on my 87 300E. After a lot of troubleshooting, I finally found the problem. It was the E.H.A. valve, and I beliebe that's a fancy name for a fuel pressure regulator. It's a Bosch part and a bit pricey for such a small thing($200-250) (Dealer may charge $450), but very easy to replace. The "bog" on mine occurred only when the engine was warm, and only when starting out in second gear. (Shifting to "2" made it start in 1st, and it didn't bog then.)

[haasman]

My favorite elusive "bog" problem was moisture in the gas tank.

Haasman