Hi Everybody,

I hate to add to the list, but my 197,000 mile 1993 300CE has developed a bad surge at idle to the point it want’s to stall in gear. I’ve owned it for about 22 years now and have done almost all the wrenching.

In the past (10/18) it developed a stall and just shutting off condition, which turned out to be the stoplight switch:

details http://www.peachparts.com/shopforum/tech-help/395441-w124-codes-stumbling-shutting-off.html

June, 2023 it started dropping cylinders randomly at idle, but otherwise ran great.

Details: http://www.peachparts.com/shopforum/tech-help/420154-1993-300ce-dropping-cylinders-idle.html[/URL]

That turned out to be coil, plugs, wires, OVP and O2 sensor. Not sure what actually cured the problem

It has the updated harness, I’ve opened up the harness near the throttle body and injectors and everything looks fine and all the insulation colors are bright and flexible.

It sat most of the winter inside. First couple of days it drove fine. Just a slight unevenness at idle. Last week it started surging badly. Idles about 600-700 rpm but past that it runs fine. Cruise control work,

No CE light, 14 volts at idle. 80C temp but took a long time to warm up when it was colder. Oil pressure fluctuates at idle. It did start almost instantly cold or hot but now takes some cranking and acts like a flooded carburetor

CODES are with the button, non. With homemade code reader, socket #8 = 13 (02 sensor control system operating at rich or lean limit}
and socket #14 is 1.

I’ve lubricated the throttle linkage as that’s been a problem before,
Checked for vacuum leaks,
Used MAF cleaner which seamed to help a little

MAF was replaced at about 120K. EGR holds vacuum.

Vacuum at the manifold end of the EGR hose is only 10.5 HG.

Fuel Pressure regulator doesn’t small or weep any gas.

I wiggled the wires around O2 sensor which was replaced last year.

Any help would be appreciated. I’m stumped and discouraged. I had open heart surgery a couple f years ago and am afraid I’m not as sharp as I use to be figuring this stuff out.

I read through about 18 posts on surging and didn’t really find an applicable answer.

I bought a claimed updated Throttle body for $285 and insulation is just dust. I haven’t found anything like that on my car yet.

Thanks in advance

My guess/gut reaction is a bad MAF, the symptoms match and cleaning changed it a little. Hopefully you can find a known good one for test purposes.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I ordered a new MAF to try. The tamper proof screws on mine have 5 points instead of six.

The code #13 on socket 8 Sounds like it means that the O2 sensor is out of range but working?? I've thought about switching back to the old O2 sensor just to see what happens.

I replaced the MAF today and no change. Starts instantly cold races for a second and then idles down. Runs smoothly for a minute and then starts to run a little rough and then after a couple of minutes starts to surge badly.

The idle slows down to 600 rpm.

I used MAF sensor cleaner on the air temp sensor. Checked that the boot connections were tight. Wiggled all the harnesses and connections and vacuum lines again.

I sprayed Starter Fluid all around and it did seem to smooth out and pickup speed slightly when sprayed around the throttle body. I wiggled everything I could find and I can't figure it out where or why it would affect the throttle body.

I did buy a used throttle body off ebay for $286 from a supposedly reputable Mercedes parts guy. It looks beautiful but the harness has been cut open and there is no insulation visible. just powder. I was thinking of keeping it and trying to rewire it but my hands shake so bad I don't know if I'm able to solder it.

The MAF is new. It's not a Bosch but Western. I've had good luck with them in the past with mostly made in USA parts, although this MAF is Mexican. Better than China?

Just thinking,
Could it be the throttle body gasket? I don't know why it would start leaking all of a sudden?

And in my experience with Holley and Edelbrock carbs, A vacuum leak usually causes a higher lean idle and back firing. This idle seems low and rich. I'm lost

Thanks again

Sorry the MAF didn't bring joy. I would try the old O2 sensor if you still have it. It could be the gasket under the throttle body but I'd think that could be detected with brake cleaner or carb cleaner sniffed at it.
If you have a shop nearby that would smoke test it for vacuum leaks it may be worth getting it done. Any chance of a leaking brake booster?

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thanks. I do still have the other O2 sensor and have thought of switching it back.

I did find a Mercedes repair shop locally, soif i don't come up with something this week I'm going to try him I just hate giving up.

Do they work on cars of the '93 vintage? Are cars like that around the shop?

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yes. I. talked to him today and he has the correct scanner. He's been in business since the 60s. He suspects a vacuum leak. He said he doesn't believe in throwing parts at it. We'll see.

He reminds me of my late dad who owned his transmission repair shop for 40 years. He "lost" (but gained a reputation) a lot of work because he'd diagnose a leaking vacuum modulator on Ford automatics and repair it for $20.

I reinstalled the last O2 sensor I had switched out. The old one is a Bosch sensor. I don't remember what brand the new one was. Anyway it didn't make any difference.

other than the bad surge at idle, it runs great. I love the way that little six pulls.

New (or old) problem is that I discovered 3 wires between the firewall and valve cover/oil filter. It gets moved around whenever replacing the filter and now the insulation is crumbling if you touch it. It looks like one goes to the oil level sender and that wire is frayed by the plug. The other two I can't look like they go behind the firewall insulation behind the valve cover.

Wires are brown/green stripe
brown with white stripe
black with brown/red stripe.

I thought I had a wiring diagram on the old 2CD set somebody was selling years ago, but I can't find it now.

I don't see a wiring diagram on the forum so far. Does anybody have a lead?
There are some bad scans doing an image surch,but hard to read.

The indy shop I was going to bring it to won't touch the wiring. I was reelived to find that every loom I checked was in great shape. I guess I didn't escape the curse.

It might be the wiring. I believe the coupes were spared, until 1993 from the biodegradable harness. I know my 1992 300CE was.

__________________
1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

IIRC that was referred to as the lower engine harness and it included wiring to the alternator.

Agree love the pull of the M104.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Just to update, I got pulled away from my coupe when our daily driver 97 Jeep Cherokee (bought new) ate the distributor gear. Replaced the gear and then found cam gear chewed up too. It's turned into a nightmare with parts availability and poor quality offshore crap. I'm stumped because these AMC 4.0 sixes are pretty much bullet proof. It's been maintained since new with Mobile 1 and looks brand new inside with 175K.

I've been trying to drive the Benz as little as possible. It runs terrible. It smells and drives like a real rich carburetor does. It pops through the intake when starting.

The Mercedes shop I was going to take it too until I found the harness problem suggested trying a used throttle body. I didn't want to just through more parts at it, but as I mentioned earlier, I bought one that had a bad harness and was refunded.

So I ordered another one from AutoHubShop in Calif from ebay.
That came today and harness is bad too.

Pretty discouraging.

P.S. how do I add a signature? can't find it anywhere.

Thanks!

Well, I finally got back to my 300CE.

I removed the throttle body to change the base gasket because spraying starting fluid around the area made a slight difference.

I found a small crack in the boot around a clamp indentation. It almost seems like somebody's been in it before because everything came apart too easily, but I've had for 22 years and nobody has touched it.

What I did find was the EGR tube flange bolts were loose and one was missing. It wasn't tight to the manifold so I doubt the EGR sytem is working.

There also was a vacuum hose that had fallen off on the throttle body. I don't know how it ran as good as it did?

Also there is oil puddled in the intake manifold and a slight oil film on the throttle plate and resonance valve. Would that be the PCV system not working? I can't find any mention of a PCV valve.

I also found that my throttle body harness insulation has crumbled. The engine harness has been replaced.

I removed the cover and somebody's initials are on the plastic gear. I'm trying to work up the nerve to rewire it.

I bought another used unit on ebay that was supposed to be tested (how?),
but that harness is bad too. This is the second one. Somebody has been in this one and sprayed some green coating on the wiring/circuit board. I was going to try rewiring this one so I didn't screw up my original, but I don't know if these internals are any good. The date on this is 5/95.
I wonder if I should rewire this one?

The original is 8/92.

What do you think?

Thanks