'86 420SEL Rough initial idle when Hot - PeachParts Mercedes-Benz Forum

Randy · #1 01-23-2013, 10:26 PM

Hi, my '86 has developed a different kind of rough idle problem. When starting cold, it starts instantly and idles perfectly. On a hot restart after driving, it will start quickly but shake for about 5 to 5 seconbds, then smooth out and no more problem.
It has 162000 miles, and I have owned it since December 2003 with few problems. I changed the timing chain several years ago; new plugs, injectors and seals; typical periodic maintenance such as oil changes, coolant, et al. What do you think?

S-Class Guru · #2 01-24-2013, 07:12 PM

Randy, my 300 did just that for 15 years, and I tried everything.
The exact symptom: when hot, the engine would fire right off, than chug-chug on what felt like 3-4 cylinders for about 5 seconds. Then one cylinder would cut in, then another, and she smoothed right out.

New injectors (2-3 sets over 10 years) seemed to help some for awhile, but not completely and not for long.
Awhile back I tried another injector set, and it totally healed it.
I think aftermarket Bosch injectors may be hit/miss; I don't know if paying 5x the price at the dealer nets any better quality. But that was certainly the root of my problem.

DG

pawoSD · #3 01-26-2013, 11:33 PM

That can be caused by the fuel pressure accumulator back by the fuel pumps going bad and leaking down when the system is hot, causing temporary lower pressure/poor engine running.

Randy · #4 01-28-2013, 09:37 PM

Good points; thanks. After 26 years of faithful service, anything is possible.
I replaced the original injectors with OEM circa 2004, as I was bringing her maintenance up to scratch just after purchase from the original owner. I could not tell any difference in operation then, it was good before and after. I had no reason to suspect injectors (and seals) other than the age of the car. I also replaced both fuel pumps and the filter. I was thinking somethign with the fuel distributor, maybe, as I have never done anythign to it, and I understand rotten o-rings are a major suspect there, but I don't know what the symptoms of a faulty fuel distributor are. Also, I have never done the rubber seals in the manifold.