420 SEL idles rough only when cold
 

1988 420 SEL, idles rough during warmup.
Runs fine at operating temp.
I checked fuel pressures with coolant temp 30deg C and found system pressure of 95 psi and lower chamber pressure 88 psi. Seems ok.
I checked the coolant temp sensor and it seems to be working correctly. 1.4k ohms at 27deg C; 280 ohms at 85deg C.
I unplugged the EHA and the idle dropped slightly and did not improve.
Checked the resistance across the terminals of the EHA and found 19.7ohms, within specs.
I read in another thread about adjusting the EHA to change the amount of enrichment during warmup. Anyone know something about that?
Any thoughts on what I should try next?

Thanks,
Jim

A couple of thoughts.

1. I chased a similar problem on my 560. My advisor, Peter Fuchs of Silver Star Motors, advised being sure the plugs and wires were good. His theory is that everything needs to be perfect during warm-up, but a marginal part will still function at operating temp. I blew this off and checked everything you have. Finally, the miss got worse and I was able to isolate it as - guess what - a plug wire.

2. That fixed about 90% of the problem, but I still had an occasional rough idle on warm-up. The remainder turned out to be a very small crack in the rubber connector for the cabin vacuum line on the back of the intake manifold.

3. Have you run a check of the on/off ratio. Maybe you are really lean so it runs crappy until it gets into closed loop and the lambda system starts to enrich.

For what it´s worth. Those temprature sensors can test ok at some ranges and fail completely at others.

bbu

update:
checked lambda and found it running very lean at idle. checked for vacuum leaks and found the intake boot leaking at the clamp that holds it to the throttle body. waiting for parts right now, hopefully that will clear it up.

Good old "false air". I'm surprised that you did not have a high idle as well.