Rough idle and hard start when warm 78 450 SEL

Just wondered if you had any idea on what to look at for a rough idle? When you first start the car when cold it seems fine, but after it warms up usually need to pump the excelerator peddle to get it started?

[Aaron]

This is a hot start problem. Somewhat unusual with the CIS systems, but in this case, you are probably looking at a new enrichment compensator. They are usually about $375. While you're at it, you might as well replace the injectors as well. I think they're about $28/each from MB. Hope this helps. Oh, by the way, make absolutely sure you are running the highest octane fuel available, preferably Shell 93 or Sunoco Ultra 94.


------------------
Rgds,
Aaron Greenberg
MB technician
'67 250SE Cabriolet
'77 450SL
'79 6.9
'79 6.9
'80 300SD
'81 240D
'85 380SE
'89 420SEL
'93 300E 2.8

I had this same problem with a 1974 450SE. There is a cold start enrichment valve which is probably remaining open after the motor heats up - thereby continuing with an overly rich mixture which fouls the plugs. Check the plugs - if they are gas fouled it may be your problem. The valve was cheap and easy to replace.

------------------
Abe
1970 280SL
1974 450SE
1998 E430
2000 E55

[Benzmac]

The 74 and the 78 have different fuel managment systems!! I agree with Aaron on the warm up compensator.
After you check it we can get you one.
We can ship them anywhere via partsshop.

------------------
Benzmac:
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

[4JUSTIN]

Sometimes I get the older cars mixed up but isnt the 78 450 SEL ( 116 chassis), an Electronic injection system, therefore it does not have an enrichment compensator. The enrichment comp. is only used on CIS cars.
I believe CIS was introduced in 84 on the 201 chassis with a 102 or 103 engine.
Now on the 78, if it is electronic injection, enrichment is done through Injector open duration via the engine control computer.
The first thing I would check is to check fuel pressure at rest and make sure its not leaking ( either an injector or the regulator).
hope this helps!!!

[Aaron]

CIS was used on '76 and later V8s, including the M-100 (450SEL 6.9). Cars pre-'76 suffer from vapor lock problems. Not fun to deal with and a major pain in the butt! This injection on the early cars was known as D-Jetronic, and on '76 and later was K-Jet. And in 1980, the final year, came K-Jet with Lambda. Finally, the continuous injection systems (CIS) were as follows: K-Jet, K-Jet with Lambda, KE-Jet, and KE-Motronic. So Justin, I believe you are wrong in your thinking. Hope I have clarified this.

------------------
Rgds,
Aaron Greenberg
MB technician
'67 250SE Cabriolet
'77 450SL
'79 6.9
'79 6.9
'80 300SD
'81 240D
'85 380SE
'89 420SEL
'93 300E 2.8