What's happening?? 103 runs rough after prolonged hi-speeds
 

I could really use help from you experts on a problem with my 124. The car normally runs beautifully under all conditions; idles fine, accelerates without hesitation or stumbling, pulls strong, shifts cleanly, runs all day long at 80 mph. Cranks a bit longer sometimes on cold starts, but that's about the only weakness. 194K miles on the clock.

Now the problem: last weekend made a 6-hour high-speed trip on Interstates, cruising for several hours at a time at sustained 75-80 mph. Pulled off for gas, and as soon as I'm on surface streets at low speed, particularly stopped for traffic signals, the engine idles very rough (RPMs go below 500), to the point that I have to keep light pressure on accelerator to keep it running, and it stumbles badly trying to accelerate from a stop. Accelerating, car is very sluggish, totally under-powered. Shut the engine off, even just long enough to fuel-up, and starts and runs fine again in every respect. Run along at 75 mph for a few more hours, slow down in town and the rough idle/sluggish acceleration returns. All it seems to take to get rid of the problem is to turn the engine off for even just a few minutes. Problem never exists if I just drive around town, only after prolonged high-speed driving.

Recent (within 2,500 miles) maintenance/service performed: new OVP, new battery, all new filters, all fresh fluids, new correct NGK non-resistor plugs. Routine maintenance always performed. No obvious evidence of vacuum leaks ("economy" gauge usually pegged left at idle, except with A/C running, when it rides about quarter way). No CEL. What do you make of this situation? Thanks guys.

What it is doing almost seems like the idle control system is shutting down for some reason and its idling on mechanical injection (hence the rough/low idle)...but is then "rebooting" or re-initializing once you start the car back up...

I'd suspect the engine EZL temperature sensor....at least as a first thing to check....

Thanks
 

How many temp sensors are there? I've been under the impression that there was only a single temp sensor in the M103, the 2-pin CTS that (among other things) controls the aux fan. Am I looking for a different sensor that supplies signal to the EZL or CIS? Where is that component?

I believe there is another temp sensor, on the cylinder head, which signals the injection computer. But I'm not positive, I've not worked on a 1988-vintage M103 engine. It could be the same sensor for both on 1988 (I know on 86/87, there were separate sensors, but something changed in '88, the 3-prong, dual-temp switch went away).

Otherwise, I really am short on ideas, if the car "fixed itself" after turning it off & restarting. Unlikely to be anything mechanial (injectors, etc) for the reasons pawoSD mentioned. Some wild guesses would include fuel pump, EHA, or a bad electrical connection somewhere. Sorry I can't be of more assistance!

:confused1: :wacko2:

I'm pretty sure that the engine temp sensor for fuel management is a 4 wire sensor at the back of the head. Two wires are for fuel and two for ignition.

But, I doubt that is the problem. I think that system would substitute if a huge change took place. The substitute value would simulate a warm motor value. A more likely prospect would be a bad OVP relay as it specifically runs the idle valve and would also leave a default mixture when lambda control went inactive. it could easily work witha recycled key.

After thinking about it I'd have to agree that the OVP relay (back by the battery) would be a better first thing to replace.

The M103 temp sensor for the injection system is on the top rear of the engine, with a big 4 pin connector attached to it. Later models also have a temperature sensor for the intake air....but yours will not have that.

Thanks to all for your ideas. I'll hit the FSM diagnostic section and haul out the multimeter for some component testing. Assuming it's not something obvious like a blown OVP fuse, is there a procedure for testing the OVP? Thanks again, I'll report back when I know something and/or have it fixed.

ABS lamp on when condition presents would indicate OVP power failure.

Right, I understand that. No ABS light though. Arthur, have you ever come across the situation I described with rough running after prolonged high-speeds?

Possible EGR vac leak... Plug the egr vac feed line and try it w/o egr operation.

I'm planning on doing this to our 300E in general....will it harm anything if I pull the line off and plug it? :D I know it doesn't matter on a diesel.....

No EGR in MY1988.

<No EGR in MY1988.
____________>

I thought for sure Cal version was EGR...does it have a Purge regeneration system??

The production history is a bit confusing, but generally speaking there was no EGR or air pump though MY1988, even on CA version cars. Exactly what date those were added I don't know, but they started with engine serial number 070068, which may have been in a 1988 vehicle, but more likely in MY1989 or even MY1990. There was a major production breakpoint in 9/89. Maybe that's when this stuff was added.

To add to the confusion, my CA version 1988 car has the 8-pin on-board diagnostic setup, and DTC #12 says "Exhaust gas recirculation temperature sensor". I'm guessing that that code applied only to MY1989. Anyway, my understanding is that the EGR valve on M103s was fitted to the exhaust manifold at cylinder 3, and I definitely do not have anything fitted there. Could someone with a later 103 post a photo of that area; I'd like to confirm its location on the exh. man.

The 070068 breakpoint was for the M103.940 engine. Your 300E has a 103.983 engine, and for that series, EGR appeared around engine #181462. You can look up your VIN in the EPC, check the datacard, and see what engine number you have.

:scholar: