87 Euro 560SEC irratic missfire
 

When I bought this car, I got it to run like a scalded ape. It was fast and smooth.

Recently, i've had a bad miss which I've been trying to chase down. I initially thought it was a bad fuel distributor or injectors caused by bad gas so I pulled the injectors and checked them. Some of them were a little weak so I replaced all of the injectors and I got the car to run very well at idle with a silky smooth idle. This ran for a while then it would develop a miss on and off.

After a long drive, the miss would get worse.

I've been able to disconnect the electrical connection to the EHA and get the car to idle better.

I've checked for vacuum leaks but could not find any. The vacuum gauge on the dash is pinned to the left until the car develops a miss and then it will move towards the center (about 1/2 way between far left and center).

I swapped out the EHA with another that I had on shelf but this did not change the symptoms.

What else?

02 sensor ? if so , how to test?
Blocked cats?
Computer?

Plugs okay?

Ohmed out the wire set?

Inspected the cap and rotor?

Coolant temp sender to computer?

Most miss-fires are ignition related, how are the plugs, wires, cap & rotor??

Also have you checked the basic distributor alignment?

Plugs, coil, and plug wires are all pretty new. Plugs are NGK non resistor plugs. I don't think that it's ignition related because the engine will idle smoothly on occasions and then start running foul. This in some cases can be cured by disconnecting the EHA.

If it was ignition, the poor running will usually be constant.