1988 420SEL idle speed starts racing after engine gets hot - PeachParts Mercedes-Benz Forum

william.wood · #1 10-24-2005, 06:19 PM

idle speed too high ~1800RPM [excerpted from MB DIAG. CD-ROM]
check overprotection relay, fuse on relay may be blown or relay defective, air/fuel mixture too lean. oxygen sensor shorted to ground, check wiring between CIS-E control unit and EHA/Electro-Hydrauilic Actuator; EHA current should be ~75mA

turned out to be that the overprotection relay fuse was blown
everything else so far checks out to be ok.
started up like a champ after replacing fuse.
idles cold at about 850RPM manual says should be around 650 when warmed up.
but after the engine got hot was back up tp 1500 RPM what gives ?? if i disconnect coolant temp sensor or oil temp sensor or idle speed control valve, idle shoots back up to 1800 RPM.
adjusting fuel/air mixture made no difference either. and using a dwell meter didn't work either, the dwell meter needle just sat there at 45 no matter which way i turned the air/fuel mixture adjustment.
if i jumped on accelerator rpms jumped down to 900 then worked its way back up again , in gear drops down closer to where it should be manual says idle is supposed to be 650 but is around 800 in gear.

any ideas?

mbdoc · #2 10-25-2005, 08:24 AM

DID this problem just happen OR has there been other work done to the CAR?

STARTING point is the disconnect the idle "motor" & connect ground to one of the male pins(start engine) & then power to the other one . IF everything is in order the engine will QUIT the second that is done!

jimnewt · #3 02-03-2008, 05:37 PM

Quote:

Originally Posted by william.wood

idle speed too high ~1800RPM [excerpted from MB DIAG. CD-ROM]
check overprotection relay, fuse on relay may be blown or relay defective, air/fuel mixture too lean. oxygen sensor shorted to ground, check wiring between CIS-E control unit and EHA/Electro-Hydrauilic Actuator; EHA current should be ~75mA

turned out to be that the overprotection relay fuse was blown
everything else so far checks out to be ok.
started up like a champ after replacing fuse.
idles cold at about 850RPM manual says should be around 650 when warmed up.
but after the engine got hot was back up tp 1500 RPM what gives ?? if i disconnect coolant temp sensor or oil temp sensor or idle speed control valve, idle shoots back up to 1800 RPM.
adjusting fuel/air mixture made no difference either. and using a dwell meter didn't work either, the dwell meter needle just sat there at 45 no matter which way i turned the air/fuel mixture adjustment.
if i jumped on accelerator rpms jumped down to 900 then worked its way back up again , in gear drops down closer to where it should be manual says idle is supposed to be 650 but is around 800 in gear.

any ideas?

Where is the overprotection relay? I am having an idling problem as well.

jimnewt · #4 02-03-2008, 06:51 PM

Disregard, found it and it was the problem, Thanks for the post!

Now to solve the leak above three cylinders....could it be valves or head gasket? hmmm, only $$$ may find out.....