Hesitation and intermiittent rough idle in my 400E
 

I've reviewed the archives and can't seem to find an answer...Any advice will be appreciated !

92 400E 105K...just serviced...
pre service - rough cold idle and hesitation upon acceleration...If accel is moderate to hard, will experience a "backfire" sound up through the throttle intake.."a thud" Also experiencing significant sulfur odors....this odor is more frequent than ever notice prior.

post service - same thing !..no improvement...
The 15K service did not call for replacing rotors and dist caps. I checked mine and found significant oxidation. Removed, Cleaned and replaced.."try that before I replace them with new"...Idle seems to have improved but hesitation is still there.

What could this be?
Need new Dist caps and rotors?
02 sensor?
Something more exciting?

Again, thanks for the input ..
Jeff

Did you put in new plugs?

Replace the caps and rotors!!!!!

I had a cold start problem in my 400E, so I took it in for a service. Here is the transcript:

CODE 17 - Crankshaft position sensor out of range. Run system check on vehicle cold, Locate no crank sensor reading generated at lab scope. Vehicle needs cranksensor. $130

- Attempt to remove cranksensor from housing, sensor broke in housing. $62.50

- R/R Transmission assembly, gain access to housing that holds crank sensor, remove broken set screw/sensor, reinstall all and recheck all fluids. $500

Parts:
Crank Sensor $230.75
Shop Supplies $8.52

Total $943.73


Cold start problem was fixed, but I had the thud when car would shift and a terrible idle. Brought the car back, they adjusted something and its perfect now. Hope this helps.

:)

I had the same problem in my C280... when I accelerated harder than normal, POP, and then the individual cylinder's fuel injection would be cut as a protective measure. Anyway, I needed new spark plugs, plug wires, and coil ends... problem is GONE now!
I think our ignition systems are a little different... but I would go with benzmac's suggestion and do the cap and rotor... and the plugs.
My car had 112k miles on it... I bet this is your problem.
Good luck!

BenzMac is right on with the first step towards fixing this problem. If this doewsn't fix it 100% the next step may be to replaced the Mass air flow sensor, but I wouldn't suggest just throwing one of these on the car without first testing it. They are quite expensive.

Pardon previous typos. Still early, fingers not warmed up yet.

Well, I changed the rotors and caps..no prob and I still have a significant rough idle while the engine is cold. Hesitation is obvious from dead stop.

My shop put my car through the mill and said they found a failing Fuel Pressure Regulator...Said the fuel PSI is slightly above normal and combined with the 400E's original fuel injectors (107K miles), excess fuel (running rich) is the most probable cause.

Is changing the PSI regulator the next link on the chain?

I priced it and new injectors...Man ! the injectors will certainly set me back a coin or two if I replace all 8 !

Additional diagnostic help will be appreciated.

hey Jeff. I'm think something might be grounding out. From My past experince especially for the early 124 V8 cars. The engine wiring harness gets really hot and brittle and falls apart. Its the main engine harness that wires all sensors,injectors what have ya. If you remove the aircleaner and inspect. Its possible some of the injector wires my be grounding internally under the insulation causing a lean idle misfire. Causing a indivdual cyl or cyl's to starve for fuel causing hesitation,backfiring, missing.

Its worth a try. I've seen alot of these. First hand experince on my 500E also. But I had the aux fan constantly running due to wires touching them selves. If you going to keep the car. I rec doing this. Also with that Mileage I rec new timing chain,Rails,tensioner.


Hope This helps Guido.

Thx for the input. Chain and Rails are on my radar....As far as the harness:

what type of job are we talking about if repalcementis in order? time, cost and degree of difficulty..

At first glance, this sounds like a major undertaking.

Thx again.

Same problem be very careful and take care of immediately
 

Just bought a 1992 400E and the wiring harness failed exhibiting the same symptoms. Wiring Harness replaced. Still malfunctioning. Although I don't have car back yet, the short supposedly fried the brainbox for that cylinder causing constant injection and is now being replaced.

I hope the brainbox on your car isn't fried yet, but I would be very careful as it is quite expensive.