Very rough idle problem: 75 W114 280C with M110 carbbed engine - PeachParts Mercedes-Benz Forum

lunchbox · #1 04-06-2009, 06:12 PM

Well, I always like a good challenge! I'm happy to say that my newest free car has given me plenty of unique challenges, and I am hoping that I can get a little input from you all on the forum as to this rough Idle problem. It's the US spec M110 engine, straight six, DOHC, original style Solex 4A1 carb that looks to have been rebuilt or replaced recently. The car starts up perfectly every morning with one pump of the accelerator, idles like an angel, oil pressure is always above 45 while the car is warming up. The car runs like it came out of the factory UP UNTIL I hit about 2/5ths of the way up the coolant temp gauge, about 2-3mm above the second tick mark. Then the car abruptly has a worse idle and stumbles on acceleration from stoplight to stoplight. After this change in idle, the RPM's fluctuate up and down 50 or so RPMs. I believe there must be a warm up valve or something that abruptly shifts how the car runs at exactly that temperature.

Can any of you tell me if there is a switch or a control that would stop operating when the car warms up and may be the cause of this hesitation? I've checked all of the accelerator linkages and they move freely. The solex carb has little to no leaks, I tried setting the idle speed higher than normal, but that did not solve the issue. Spark Plugs are all new, distributor rotor is new. All of the factory smog equipment is in place and functional. The car has no other major problems besides this idle issue and an exhaust leak that I am in the middle of fixing. Thank you in advance for all of your help!

lunchbox · #2 04-06-2009, 06:16 PM

Just thought that I would include some brand new pictures of the car after I buffed the paint out on the chocolate colored roof!

wolf_walker · #3 04-06-2009, 10:26 PM

Can't tell you which, but you need to look toward the vacuum relays on the drivers inner fender, after a certain temp, which you have discovered, they start to open and close on a program based on rpm and temp and engine vacuum. Timing is one of the controls, I'd disable it and see what it does. It's a **** system, frankly. 70's smog was bad for everyone.

250 Coupe · #4 04-07-2009, 12:07 AM

There is a change over valve that does something with the vacuum advance and retard diaphragms on the distributor.

Sounds like the hoses may be swapped at the distributor or there is a hose off after the valve so that you have a vacuum leak after the valve switches over.

Could also be something with the EGR valve. It should be closed at idle, maybe the switchover valve for the EGR is not right or the advance valve has been misplumbed to the EGR

The Chilton's manuals do a good job of reprinting the vacuum diagrams or you can get the CD from MBUSA

Michael

meltedpanda · #5 04-07-2009, 10:14 AM

I think I have some of those relays that Wolf is talking about and I vaguely remember seeing cut out and cut in temps on them, I will try to did them out and see what they say tonight

wolf_walker · #6 04-07-2009, 12:01 PM

If you have the emmesions section of the owners manual, mine had a pretty decent description and vacuum diagram in it for my 73. Between that and the chiltons manual I had around I managed to figure it out, there was better info for the 75 and up models I noticed, since it's a little more involved.

meltedpanda · #7 04-07-2009, 09:16 PM

ok on the relay I found, it says on at 3400RPM, out at 1800RPM and then HYST400 with a down arrow and HYST 200 RPM with an up arrow.
Probably useless info , but I told you I'd look

this is on VDO relay 0005454432

ctaylor738 · #8 04-08-2009, 02:47 PM

I would bet on the EGR valve opening when it shouldn't - I don't think the ignition retard could make the car run as badly as you describe. Try disconnecting and plugging the vacuum line from the valve and see if the car behaves better.

lunchbox · #9 04-08-2009, 11:55 PM

Thanks Chuck, I'm gonna try that friday when I have the day off, but that was my initial thought as well. Granted I didn't trust the thermal reactor from the second I picked the car up, that's why I'm trying to swap it out as we speak, it gets everything in the engine bay ultra-heated in my BMW, so I figured it would be similar in this mercedes. I'll keep everyone posted on if it works.

And Ron, that definitely was not useless information! I do appreciate it, anything I can learn about my car makes me feel that much more comfortable when I'm driving and something happens!

lunchbox · #10 04-11-2009, 09:39 PM

Thank you to everyone for the advice, the EGR valve was most definitely stuck wide open when it opened at approximately 140 degrees F. I removed the rubber line that connected the actual valve to the controller on the driver's side fender, and the car has never run better! I don't think it could have run much better coming out of the dealership!

If anyone else has this problem or a similar problem, PM me or let me know, because I have all of the factory booklets including the smog diagrams and explanations for my specific year.

winecountryone · #11 04-11-2009, 10:24 PM

That's a really nice looking coupe!
Did you get it here in the bay area?
I've only seen these older models in the pick-n-pull yards.

ctaylor738 · #12 04-12-2009, 10:02 PM

Nice wrench finding the problem!

It would be interesting to further diagnose this and find the real cause. Is the transducer (control unit on fender), or one of its inputs?

lunchbox · #13 04-13-2009, 02:45 PM

Winecountryone, this was most definitely a California car all it's life, and I appreciate the compliments! In fact, I may need to ask you what the paint code is on your SL, because when I eventually respray everything but the roof of this car, I would like it to be the original color which happens to be the same as yours I believe. Also, let me know if you see one of these models in the pick n pull yards, cuz I was just there 2 days ago and couldn't find anything of the sort! I'm looking for anything with an M110 engine in the junkyard so that I can grab the standard exhaust manifolds to replace the thermal reactor in this 75'.

Chuck, I'll further investigate the problem this week (I'm on spring break!) and report back to this post so that the information is there for anyone who might need it.

meltedpanda · #14 04-13-2009, 04:15 PM

I may have one manifold, do you have a part number?

ctaylor738 · #15 04-13-2009, 04:22 PM

Are you sure you can remove the TR's without bringing the wrath of the CARB down on you? I thought all stock emissions stuff had to be in place to pass in CA.