Pull the plugs from the cylinders that have the coils. Replace any showing carbon tracking on the insulator along with the mating coil boot. You can check the plugs on the cylinders with the plug wires but those are never the problem.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

Sorry for the delay. Busy with work and other projects.

Figuring everything was at least 15 years old (but not many miles) I replaced the plug with standard copper core Bosch plugs (made in Russia), Bremi coils (German) and Bremi wires and boots.

I found a little corrosion on the secondary wiring plugs on the coils and number four plug was oily on body, but not electrode.

Also lightly sanded all the fuse contacts. Some were pretty corroded. Will order the copper ones

Put every thing back together and couldn't believe how great it ran. Dead smooth idle, more responsive. I'm assuming vacuum was up because transmission shifted better(modulator?)

I was afraid to say anything for fear of jinxing something.

Then it sat for a week and when started it had a dead skip on one cylinder. Much more violent than before. It shook so bad in gear I was afraid of breaking something.

Got it in the garage and started it with out the valve cover top plate and skip was gone ?!. All I could think of was maybe one of the coils wasn't grounded which I don't get because of the copper inserts in the coils and small stud on the head. But I took it apart and found a couple of wire connections weren't snapped completely tight. One PO issue is that most of the valve cover plate retaining screws are stripped. Most are replaced with studs and nuts, But I'm afraid to go more than wrist tight on the fasteners.

I also did a compression test and got 145-150-152 on all the cylinders and used a borascope and found the piston dome on #4 has some oil. My scope can't look backwards so I couldn't see the valves. It has 193K miles

So as of yesterday it was running great. Who knows about today.

Latest odd thing is the oil pressure is now dancing around at 1.5 bar at idle. It's always been rock steady. It jumps right to 3 bar on start up and when driving.

Our Jeep does it and it's the sending unit plug gets loose.

Thanks for your thoughts

We all hate throwing parts at things.
Odd that it ran worse after changing the 02 sensor.

1. I would start with vacuum leaks. Om my 93, 300Ce, the intake manifold has one stub that needs a rubber cap/plug.
Those clear vac lines tend to crack too.
2. Check fuel pressure regulator.
3. If it ever stalls on you after car is very warm/hot. Replace the Crank position sensor.

My guess is you have a faulty ETA. I experienced similar symptoms. I then moved the ETA cable around a bit and would get varying symptoms and sometimes would run fine.
When I took the ETA apart to thoroughly inspect the wiring, it was shot.

Very easy for me to spend your money but I have never looked back since replacing the entire ignition system, from coil (only one on mine) to plugs with genuine parts sourced from MB. They all should be available, if not cheap.

Good hunting.

RayH