My 85 240 has developed a stumble/hesitation. When it happens, if I turn the car off and on again quickly(yes, while I'm driving)it seems to go away for awhile.

The following has already been changed:
Spark plugs
rotor
fuel filter
amm
fuel filter relay
cleaned 25V fuse on fire wall and spun fuses on panel

Also, noticed now that sometimes the idle goes way up when I am stopped

Help!!!!!!!!!!!!

 

could be almost anything from an intermittent electrical problem (85 was in the middle of the bad-wiring years) to a fouled idle air controller and/or throttle body. It would be smart to remove the throttle body and idle air controller and clean them out with throttle cleaner spray (avoid saturating the electric parts with this), and when you're reassembling it, inspect all the rubber hoses associated with these parts.

you mentioned plugs and rotors, what about the spark plug wires and the distributor cap? usually the cap needs replacing at the same time as the rotor. and use Bougicord wires, accept no substitutes...

if your fuses aren't the proper euro copper/brass ones, I'd get a set of those, clean the contacts on the fuse bay with a pencil eraser, and replace ALL of them.. the silver-tin ones are quite problematic. You can find the copper/brass ceramic fuses on ebay sold as Mercedes W123/W124 fuses (the w123 and w124 are early and later versions of the 300D/300E families, they used the same fuses).

another candidate in weird running problems is the engine coolant temp sensor (ECT), this, or its associated wiring, can give bad readings, confusing the computers and resulting in sub-optimal mixture or timing.

 

I am going to try the wires from my 92 on my 85 to avoid needlessly buying new wires if that's not the issue.

Any chance this could be a problem caused by either the main pump or the in tank pump?

 

I can't see how a fuel pump would cause an idle surge.

a bad in-tank pump can cause stumbling under heavy throttle at higher RPMs and loads (eg, climbing a steep hill at speed with nearly full throttle in 2nd or 3rd gear), and it often is much worse when the gas tank is below about 1/3 full.

a bad main pump usually causes total no-run conditions.

 

Is there any clue in the fact that turning it off and on very quickly while I am driving cures the problem for a bit?

 

that makes me think its more likely to be electrical, a flakey connection under the hood somewhere.

 

Took bougicord spark plug wires from my 83 and installed them on the 85, car still "stumbling"!

 

Change the Power Stage!

 

What does the power stage look like? And where would it be located on my 85 wagon?
Thanks!

 

on the cars that have them, they are between the ICU and the coil.

your 85 uses the chrysler ignition system, I believe, with the ICU (ignition control unit) on the right fender just behind the headlight, and this ICU has a vacuum baffle thing on the bottom? that drives the coil directly without a power stage.

the main electrical connector on those ICU's can be problematic. it has 10 pins in a 2x5 block connector, and it can get pretty corroded inside if someone didn't pack it full of dielectric grease... further, 1985 is in the middle of the biodegradable harness era. absolutely, if you unplug it, DO NOT ROCK IT, pull it straight out. I had to tighten the socket 'tubes' a bit with a small probe on my daughter's 87 240, which has the same ICU.

 

Yes- I have the vacuum thing on the bottom of the ICU, so no power stage

 

Resolution to the stumbling/hesitation on acceleration and idle surge-
Had to replace both alternator belts as they broke when alternator came loose.
Replaced alternator bushings and re-tightened everything. Also replaced distributor cap.

Both the hesitation and idle surge are gone!!!!!!!!!!!!!!!

If anyone has insight into this let me know?