88 300E low idle problems after fuel pump fix

[benzborg]

1988 300E (124), 200K
recent low idle(to stall) related to fuel pump repair problem so far:

Car breakdown leads to re and re of fuel pump, no gas being pumped through line to engine, we assumed FP was to blame from square one.

Cleaned contacts and test. Still no fuel thru line, can't hear pump.
We then checked the FPR (fuel pump relay) and find a bad cold solder connection on board. Repair solder connection as per this being a common problem for these FPRs according to Google. Fire it up - car starts fine, good pressure. Shut it off. Change fuel filter and put it all back together.

While putting it back together, I noticed and replaced a burnt fuse in the OVP. This was likely burned out for the 2 yrs I've owned the car. It looked very old, not a new burn. ABS lite never came on so who knows...not I.

Anyway, Fired up fine again, go for test drive - notice low idle and car will stall at a stop w/o a little foot on the pedal or putting car in neutral. Worse with load like headlights and heater. Checked alternator and VR - 13.6 volts. So that is rulled out.

Replaced FPR,(yesterday) no improvement in idle speed..
Replaced OVP (today), still no improvement to idle still.

I'm almost thinking that removing the good fuse from the OVP might bring my idle back up to 700 like before. There can not be *anything* else that caused that RPM to drop from 7/800 to 4/500 other than that fuse being replaced. (AFAIK) I read about a "mechanical" rpm level kicking in around 500 if the electronic end is not working correctly, and suspect that is what is going on here.

Removing the fuse defeats the OVP however, and still no idea why that fuse was burnt out from the onset of all this. The car was likely driven with that burned fuse over the last two years since there was never a reason to check it. Fuel pump was always fine. New day tomorrow, I'll pull the replaced fuse in the OVP to get a final ruling. If this turns out to be the case, this car runs better w/o a fuse in the OVP which would tell someone who really knows their Benz(s), what is really going on behind this low RPM deal.

Any ideas what the heck would be causing the change in RPMs from 700 down to 4 or 500 hundred now, since we did not touch anything but fuel pump, fuel filter, FPR and OVP? The low idle came immediately after replacing the burnt fuse in the OVP.

Believe I've used up all my "get out the dealership" free cards and may have to hand it over for the high dollar diagnosis.

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why me?