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Old 11-30-2020, 05:34 PM
ajschank's Avatar
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Location: Grand Rapids, Michigan
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Post W126 420sel running issues

Hello! I've got an 88 420SEL with 240k miles that I am working on that is misbehaving in the running department. When it was yanked into my barn, it did not run. Previous owner had swapped so many parts it was hard telling where to start, 3 fuel distributors, a normal distributor, 3 EHA valves, 3 FPRs, 2 Kilma relays, cold start injector, Jetronic computer, aav and more parts to list roamed the trunk. With some help from here I was able to figure out the fuel feed/return lines were swapped around causing no fuel to get to the injectors. Got that fixed around and car still wouldn't start, instead flood to no end. Found the adjustable mixture allen screw and leaned that out a few turns and it fired up.

Now comes the part I can't seem to nail down. It'll idle(high), but only with the EHA unplugged, around 1500rpms. If I plug the EHA in, it floods it out and slowly stalls the engine in about 3-4 seconds. I have to take out the FPR (fuel pump replay) and crank till it comes to life and burns out the fuel leftover. Then I can put the FPR back in, unplug EHA, and it'll come to life. Another symptom it has is when I put it into any gear, it'll slowly bog and die flooded out yet again, and I have to repeat the process above. Timing chain has some stretch but not enough to cause a timing issue, I've been checking that with my light and it seems okay. Spark is sufficient as well, also did a compression testing and values were all acceptable per what I found on the forums. Wondering if there still could be something internally wrong with the Fuel distributor that is just going rich when the EHA is plugged in or not. Any thoughts are greatly appreciated


Last edited by ajschank; 11-30-2020 at 05:48 PM.
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Old 11-30-2020, 08:57 PM
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Quote:
Originally Posted by ajschank View Post
Got that fixed around and car still wouldn't start, instead flood to no end. Found the adjustable mixture allen screw and leaned that out a few turns and it fired up.

Wondering if there still could be something internally wrong with the Fuel distributor that is just going rich when the EHA is plugged in or not. Any thoughts are greatly appreciated

If, in fact, the main mixture screw was mis-adjusted by multiple whole turns, continue to adjust in the lean direction (CCW) with the EHA connected until it is not possible to start the engine. Then come back rich until it will start.

Consider also the possibility that the Cold Start Valve is continuing to supply fuel even after start. This can be caused by either a mechanical leak in the valve, or by a continuing electrical signal.
The potentiometer that is operated by the air flow sensor could also be supplying a continuous max air flow signal (5V).

Last edited by Frank Reiner; 12-01-2020 at 09:58 AM.
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Old 12-01-2020, 09:19 AM
ajschank's Avatar
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Originally Posted by Frank Reiner View Post
If, in fact, the main mixture screw was mis-adjusted by multiple whole turns, continue to adjust in the lean direction (CCW) with the EHA connected until it it is not possible to start the engine. Then come back rich until it will start.

Consider also the possibility that the Cold Start Valve is continuing to supply fuel even after start. This can be caused by either a mechanical leak in the valve, or by a continuing electrical signal.
The potentiometer that is operated by the air flow sensor could also be supplying a continuous max air flow signal (5V).
Thanks Frank! Following your suggestions I did get it to cooperate and start up, not nearly as lean as before. Seems to be a lacking in power when in gear, but it doesn't stall out now, so we're making progress.

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