tsharples

Hello, our 1990 300TE (190K miles) has started to exhibit some odd problems at idle. It tends to hunt after starting, however after warming up to 80c and with the engine holding reasonably steady at around 800 RPM, the RPM gauge wildly and erratically jumps around indicating between zero and 2000 RPM. If I push up the actual RPMs the gauge stabilizes and shows the actual RPM. This phenomenon only happens when the engine is warm.

I've looked at the feed wires to the RPM gauge with a scope, and it shows very erratic square wave duty-cycles with occasional flat-lines and runt pulses, so the gauge itself I think is fine, it's just showing what it gets from whatever feeds it.

In addition to the above, if left in the hot sun for a few hours when the outside temp is over 100, the car will occasionally start but dies immediately after running for just a couple of seconds. This repeats over and over until things cool down. It hasn't been very hot lately so hard to know if it's still happening after I did the things below.

I've done obvious and inexpensive stuff like replace the OVP, check the pump relay, the air-flow potentiometer, and look for vacuum leaks. Haven't tried shotgunning expensive items or hard to reach items like oxygen sensor, crankshaft position sensor, etc.

Codes don't mean much to me - they are "Current to EHA Illogical", "Oxy sensor voltage illogical" and (once) "intake air temp illogical". I checked the intake air sensor and it's fine. Also pulled out the dash looking for a short, found nothing obvious.

Does any of this ring any bells? Thanks in advance.