hello everyone.... my 420SEL has developed a problem. i have read the recent thread relating to an unfortunate fellow's 420SEL hot running problems.

my problem is very similar except it exists when cold. idle is good under all conditions, and it starts quite easily either cold or warm. after it has been running for a few minutes, it accelerates cleanly and smoothly as expected (perhaps a little low on power, though). when cold, however, if one jabs the throttle, it will gasp for air, backfire through the intake, and stall. if one depresses the accelerator in a smooth motion, it will bog down (it seems like a lean condition) and backire/die unless one quickly removes one's foot from the accelerator, at which point it usually idles.

when cold, the car is undriveable, but after a few minutes of running, the car runs perfectly.

i have played with a few things... taken a few measurements with a multimetre... but i'm not that sure what i'm doing. when i rotate the throttle pot, nothing happens (nor does it matter whether or not its connected). the running of the car remains the same whether or not the pressure solenoid is connected - but resistance at the loom plug changes as temperature increases.

really the car seems to respond as a plain jane CIS car would respond... could it be that the pressure solenoid is faulty? or does the throttle pot also control the idle solenoid?


thank you, kaydee

Sounds like the coolant temp sensor isn't sending the correct signal to the CIS control unit. If that was corking correctly then the current flow to the EHA would allow for a higher differential pressure & the engine would accelerate.

Measuring current flow to the EHA durning warm-up would show that problem.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
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how do i measure current flow to the EHA, exactly? i can see that resistance here changes as the car warms up.... but there is no difference in the car's operation whether or not the pressure solenoid is connected.

To measure current flow you must use an ammeter between the wiring connector & the EHA.. MB makes a special adaptor..but if you use a jumper wire from 1 female pin of the connector back to the corresponding male pin of the EHA & then connect the ammeter wires between the other female back to the other male pin you can watch current flow.
Should be 75ma with key on & engine not running. When starting that will change & can go as high as 200ma while cranking. will(should) slowly drop to a cycling positive/negative flow as the engine warms up.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

thank you Doc.... can you tell me, what is the pressure solenoid's base mode of operation... i mean to say, if the E in CISE isn't functioning, will the car have a tendency to run rich or lean?

Doc, i think i may have sorted the problem, but until tomorrow morning when the car is stone cold, i wont know for sure.

first thing of note is that no matter what i disconnect (pressure regulator, pot, AAV), the idle never changes, nor does the driveability of the car.... i know the AAV actually works (tested manually), but the injection system doesn't cause it to... nor does the cold start valve work. i swapped the over voltage relay from another car - no difference....

however, i played with the throttle pot and found was going open loop near the top of the flap's travel... i thought perhaps i'd adjust the pot until it passes the damaged section.... and the car seemed to behave normally from here on in.... disconnecting the plug would recreate the symptoms... but like i said, i won't know for sure until tomorrow.