Rough idle but RPM's are steady @ 1100

First off, my hat is off to Ernie for helping me with my process of bringing this wonderful car to an eventual top running 348. So far it has been hella fun to drive but still has some sorting out.

I've got a 91 348 that had a few codes when I bought it. Replaced both temp sensors under the plenum, replaced both 02 sensors, and replaced all the bits in the flywheel as well as the tripple seals and the o ring that caused the flywheel to chew up everything inside of itself.

So, I did the relearn procedure and after driving it all the CEL codes have remained gone!! Even better, the car doen't idle at nearly 2k RPM like it did before. It will start cold and idle at about 1500-1600 rpm and once it warms up it will drop right to about1100 rpm and stay there. The rpms stay nice and steady but the idle isn't exactly what I would call perfectly smooth. There is a slight shaking/rumbling when sitting at idle and when I accelerate it feel like it gets a little bit worse and then seams get better as the RPM climbs above 3500+. I was wondering if it could be a cam sensor or crank sensor but wouldn't that throw a code? The cats are gutted (test pipes on the way) and the CAT ecu's are disconnected. The exhaust is straight piped althought that will change in the future.

The engine is leaking a lot of oil from what I assume to be either the cam seal or the cam cover seal? I will probably drop it over winter to check out. It had a major service in april of 2016 but sat for a while after and I suspect the seals dried up in that time. I did a leakdown and compression test on it with the motor warm (unfortunately I took too long to setup and the engine was not a driving temperature) but the cylinders were around 195 psi with no more than 9-10 psi difference across all 8. The leakdown was something like 6-8% across all cylinders so the engine checks out and it has good acceleration on the street for being an underpowered and heavy car. Side note: I don't care how many cars will beat a 348, the experience is worth all the eccentricities this car has to offer. What a fun driving car!

For full disclosure, I will say that there is a little gear whine which is why I'm dropping the tranny to check if the ring nuts are loose and if the bearings are okay. There were no bits of teeth in the fluid when I changed it to Redline 75-90ns and the inspection plate had no debris of any klind in it. The visible gears looked okay except for a slight bit of wear.

I don't see how the gear whine would cause any idle issues but perhaps may have something to do with the very slight vibration on acceleration.

 

I replaced all 8 spark plugs with the NGK plugs recommended on the forum. It is possible the Magnecore wires need replaced but they "appear" ok. Anyway to check the ignition coils?

 

When I get the gearbox back in the car I might try spraying a fine mist of water over the wires and see if any arcing occurs.

 

Yup, I might as well replace the coils then. Heck, thats cheaper than my miata!

I will look into the plug wires you mentioned. I agree on the magnecore attention to detail not being superb. The length of the plug ends were not very precise and left gaps in the round seal that rests on the valve covers. I had to repair one of the wires due to the end pulling off so that could be part of the issue as well...