Hi, this is my first time posting to this forum. Seems like there are lots of people here who are very knowlegdible when it comes to fixing MBs.

I ran into a odd problem with my 420SEL (255K miles). Last week while driving on the highway (for 15 mins), the car suddenly lost power (the engine did not die) and pulled over to the side of the road. The engine continued to idle rough and finally died. I could start it up and it would rev up and die within 2-4 secs. I can keep doing this indefinitely.

I left it for a couple of hours and it ran for another 10-15 mins and did the same thing. Seems like when the engine is cold ( i live in minnesota - brrrr - its -5 today) it seems to run till the engine warms up and then it does this.

Any ideas guys? Thanks in advance for your help!

Praba
praba@newequities.com

First warm car up, shut car off, disconnect CIS computer, and restart car. If car runs fine, you probably have a sensor problem in the Lambda control circuit. If car does same thing, remove plugs and do compression check starting with left bank.

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What is the CIS computer and where is its location?
Is it available in the 560SEL 1989?

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Master Computer Engineer, A+, Network+, MCP/MCSA/MCSE 4/2k/2k3 - Messaging, MCTS, MCITP, MCT, Brainbench, ICDL and starting Ph.D

380SE 1984 <------- Totaled by flipping accident
300SE 1988 <------- Sold
Volvo S70 2000 <---- 4 family
Opel Omega 2002 <-- 4 family
560SEC 1989 <------ Sold
560SEL 1990 <------ Sold
560SEL 1991 <------ 4 my mother
560SEL 1989 <------ Sold
600SEL 1992 <------ Current

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autozen, thanks for the suggestion. can you please tell me where the CIS computer on the 420SEL?

thank you

On boyh the 420 ang 560 it is located in the right side footwell. You will need to remove the carpets and slide the right kick panel out towards the back of the car to release the clips.

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autozen,
thank you. I will try this out asap.

btw, is it possible for a old catalytic converter to essentially block the exhaust and thus cause it to behave this way too? i.e. when it gets hot, it blocks the exhaust and when it is cold it lets the exhaust thru.

does this have anything to do with the idle control valve? <--- saw another post that talked about this.

thanks again for your help!

Anything is possible. I doubt the cat is a problem. You can check it with the economy gauge on the dash. When you are experiencing the problem hold the accel down and hold the engine at 2500 or so. Once a steady RPM is established, look at the econ gauge. It should be steady and well out of the red.

I don't think you have an idle circuit, because you said the problem occured at cruise speed.

After you've down the checks I suggested, I would next suspect a problem with intermittent fuel dlivery. One of the two pumps may be crapping out. The fuel pump relay may or the OVP may be failing.

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