My 1991 Loyale FWD 5 spd SPFI has the CEL for EGR code #34 coming on. Wiring is ok from ECU box to blu/red wire at EGR solenoid. Both wires ok. Solenoid reads 45 ohms. With ignition ON/engine OFF the reference voltage at EGR solenoid reads 12 volts. Ground is ok at ECU case and at the wire harness to engine block.

Some history - Two years ago I changed out the EGR solenoid when code #34 showed up. Still CEL on. After wiring path checked ok I changed out the UPA1478H transistor array chip inside the ECU box and all was ok for a few months until the code #34 showed up again and I again swapped out that same chip. I even used a 35 ohm dummy load resistor in case that the EGR solenoid winding inductance was causing voltage spikes to fry the chip. All good once again. But later CEL w code #34 showed up again.

CEL comes on and then goes off by itself after some driving. Always code #34 . All wires ok. So when CEL on I disconnected the battery for a minute or so and then restarted the car. ECU then gave no code for EGR even after raising RPM to activate the EGR. ECU acts like nothing was ever wrong. I even tested by unplugging EGR- and CEL went on. Then when plugging back in CEL went off. Am aware that revving the engine was needed to trigger any EGR code #34. For now no CEL.

* So, could this be a possible interaction or contra-indication from the RPM sensing circuit? Maybe just a design flaw?

Out of my league Ricky, as soon as you start work inside ecu !!

What I can offer is by memory of the first EA82 MPFi turbo ECU . According to manuals the EGR is to only operate at operating temperature and below 45 kph controlled by ECU

Third parameter of control is manifold vacuum .Not sure if it is specified as X value inHg or not but from tinkering think it needed about minimum of 10 to activate the diaphragm.

Sounds to me you are capable of making a dedicated EGR controller in stand-alone form and surgically remove the troublesome circuit from the ECU

I ran a T onto my EGR vac line, ran extension into the cabin to run a second working EGR valve . When safe to do so, I held this in my hand, little pinky (finger !) poked into and on the diaphragm to feel when engine mounted EGR unit would work in relation to all other parameters, including vacuum gauge.

I found the theory of ECU control was not the case

Temperature had no say - it worked at all temps

Speed had no say - it worked above 45 kph right through to highway speeds, so long as the vacuum was there.

This was my first own EFI car to tinker with and when rescued nearly raped at the wreckers yard, it needed a new heart. Once running, I checked the codes to find "EGR solenoid is sticked to open or sticked to close"

Yup. Solenoid no worky, swapped in one from a carb model - all fixed. Only ecu problem ever experienced, fixed but was like ECU just left it powered up all the while ignition was on

ECU is 30 year old "Japanese Electronics" (hot garbage). Could be a cracked solder joint (thank you lead free solder!), bad component, or cracked PCB. Doesn't really matter it's junk. Either replace it or wire up a LINK ECU and Dyno tune it. 

GD