M103 300E hot drooping idle; cold OK
 

OK, I have read many of the M103 posts on 'rough idle'. And done many tests or inspected components. Problem: Cold start OK. Idle at 800 rpm. Then engine heats up to 80C. Then I drive, back off accelerator and engine idle drops to 600, and sometimes dies. Like when cornering, or after a high speed (50+ MPH) run, to a stop light. And on a hot day in July, difficulty cranking a hot engine, until the temp drops (or I call AAA, and by the time they arrive it has cooled and will crank. This is my 1989 300E car, w/ only 105K miles.

Have replaced FPR 2 months ago. Plugs, dist cap & rotor 2 months ago (non resist plugs). Checked kick-down micro-switch OK. New O2 sensor 12 years ago (but only 25K miles ago), Checked/cleaned the PCV pipe (thin one.) But the nipple into the fuel pressure reg is cracked. Carb/throttle body cleaner. Also cleaner onto the air flow meter (inside throttle body). Removed & cleaned the IACV, and tested OK. Tested the action of the throttle plate position sensor. OK, and switches just as plate moves a few degrees. Tested air flow meter (low-high-low) as air plate moves.

The only anomolie is the rear 2-wire temp sender to the FPR. The analog code readout blinks 3 times. The thing ohms out at 3,830 on a cold day (9-deg-C). And after running at 259 & 264 ohms at 79C. According to the book/temp curve, the low end is a few percent below-scale. (Book says 290-370 @80c).

This problem is driving me nuts! But I want to win this one, real bad.

Suggestions? Change the temp sender (so close.. but maybe the computer does not like it?) Anything else to check (short of every tube, hose and wire/harness in the engine compartment? What about the fuel evaporator canister & it's temp sender/switch in the left front fender?

For my money, I'd change the OVP relay. They are wear and tear items.

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bnObob remove the electric plug on the rear of the IACV if it is working as it should the reves will go over the top. And any hose thats are split will need replacing , or put a little tape over it to seal it .

Since you know the temperature sensor is out of spec (and by quite a lot) I'd change that to eliminate it as a possible problem.

When you changed your plugs, did you properly gap them? I had an instance helping a friend recently where .002 made the difference of the car stalling when hot and running perfectly. .040 was too wide, .038 was perfect (this car wasn't a Mercedes, just an example).

It's worth checking your fuel pump relay as well. They seem to have a reputation for failure due to bad solder joints. It's possible that the temperature is coincidental to the relay packing up or flicking on and off as you drive. If the fuel pressure goes south due to the pump not running properly, you can expect problems. The same goes for the OVP mentioned above, however you'll usually have signs it has problems in the cabin (warning lights on the dash for example)

Clean the throttle plate of carbon, check lambda specs hot and adjust if neccesary

Just so there aren't gray areas. The OVP relay can be pulled out and there won't be any lights that pop on. The relay's sole purpose is to protect the electronics under the hood that deal with engine management.

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The OVP also feeds the ABS system. If the OVP fails, or is pulled out, the ABS light will come on at a minimum since the ABS system is fed off the protected OVP circuit. On the diesels, the tach also dies, not sure if the gas engines are the same.

Very good points. Gassers get the tach feed from the EZL.

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Another thing to check that fools people is you have 2 fuel pumps if one dies it will cause hard starting, and stalling after slowing down, go under and carefully remove the ground wire while running , or supply voltage and ground and make 100% sure both pumps are running, they both have separate power and grounds, fuel accumulator can also cause hard hot starting issues,with a connected fuel pump test gauge check stopped fuel pressure and see if pressure drops quickly if it does pinch fuel return line and see if that stops pressure from dropping quicky ,if it does replace fuel accumulator

Thanks everyone! I had considered the 2 fuel pumps. Did change the check valves about 5 years ago. So, pull on wire to one pump. If nothing changes, then the other pump OK. Then reconnect & try same to other pump. If 2nd OK, then I'll look elsewhere. Or if engine dies, I found IT.
Yes, gaped plugs correctly. Rechecked them again. All in spec. As is dist & rotor.
I may return to the old FPR. Just for kicks.
The cracked hose is small leak, but into the fuel regulator. So, the regulator is not getting sufficient high vacuum. But then, high vac is when I'm accelerating, which is no problem in my car. Only at low vac is there a problem.
I have also read that topping the gas tank could cause fuel accumulator to head south. Are there other parts of that emission system that can influence RPMs/operation?

High vacuum is at idle (throttle plate closed). High RPM/high load is low vacuum (throttle plate open).

The line that runs from the FPR isn't a vacuum line. It is a safety line to have raw fuel drawn into the air intake stream should the FPR rupture. This is much better than raw fuel on a hot motor. If fuel, any amount, is leaking from the FPR, it is done. Period. It does not operate off vacuum.

Jayare

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Oops my bad. I used the word "vacuum"; meant crankcase/valve cover vacuum as it is pulled by the throttle body into the air intake/filter. And that is pulled from the FPR via that tiny 'picollo' PCV tube. Yep, high throttle vac is at idle. FPR is fine, but the rubber nipple is not. Need to fix that.

OK, about vacuum: The ECONOMY IP needle points to what for which vacuum condition?

Also: When at idle & throttle plate closed, then the only air comes through the IACV?

When idling and throttle closed, the IACV maintains RPM bypassing the main throttle plate.

The "economy" gauge should be pulled fully black at idle, and drop to red as the throttle is opened.

I replaced the 2-wire T-stat today. Will test run Wednesday.
I did check the air flow meter. Pins 1-2 read 5K....15K...10k (at max depression of plate.
But the ohmmeter needle jumped around, like the air flow inside was "skritchy."
From pins 2-3, I read (plate closed) 9K...15K...5K (at max depression.) Again, the ohmmeter needle jumped around as I depressed the plate.
May need to replace? Or clean the contacts inside the air flow meter...?
But is there and adjustment for it when reinstalling?