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Old 01-23-2003, 09:06 AM
kennysin's Avatar
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Join Date: Jan 2003
Location: Singapore
Posts: 59
Question w126 280SE decel dump valve problem


I have a 1983 w123 200 which has given me great satisfaction for the last 5 years. I have recently acquired a 1983 w126 280SE with 150k on the clock.

I have noticed some difficulty in warm starts since running the car requiring a few tries before the engine runs. The starter cranks fine, the battery is new, the car just serviced. Once started, it runs fine.

I have gone through the postings and noted a similar problem by Mervyn, who in addition also found an increased idle speed (jumps from 800 to 1300) when in "P" mode as well as slightly harsh shifts (mainly 1-2-3, less 4), both which I also noticed in my car. One posted suggestion as a source to all these problems was that of a variable vacuum leak, possibly from the decel dump valve.

The decel dump valve was indeed found to be faulty. However, on replacing the valve, the engine starts, and promptly stalls when the valve engages, presumably from an excessive lean air mix. I am unable to compensate by manipulating the CO adjustment, and have had to put the faulty valve back!

I presumed that by correcting the vacuum leak, my initial problems should be solved. However, I don't understand the design role of the valve, and why it has created a new and unexpected problem. Please help!!



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Old 01-24-2003, 08:50 AM
kennysin's Avatar
Registered User
Join Date: Jan 2003
Location: Singapore
Posts: 59
Thumbs up Decel dump valve: problem solved

Thanks steve and mervyn!

My workshop neutered the valve and the car runs fine. No more warm start difficulties, and normal idle throughout the gear ranges.

Steve - spot on on diagnosis, and on the obscure function of the valve......Thanks again!!

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