3.2 misfiring after engine drop

[Bigtoe32067]

Keepp in mind that the a/f sensor doesn't measure raw fuel so your lean reading may in fact not be actually a fuel cut issue.
Ive had similar issues and the last time it was the distributor rotor had cracked around the shaft and would wobble at higher rpms. A new rotor solved this.
One of the other times it was a loose flywheel causing it to wobble and misfire.

Could there be an issue with the chassis grounds since these would have have been disconnected to pull the engine and trans?

We'll figure it out eventually. Hopefully sooner than later.
Good luck
Tony

[bdonally]

Good point about interpreting the AFM reading.
Possible that the flywheel bolts have loosened but I was pretty careful about that.
Thanks for the support.

[Steve W]

Originally Posted by bdonally

I'll check that.
How would that cause the problem , though, especially what seems like a fuel cutoff?

When your rotor is not phased correctly to the cylinder that the ignition is to fire, the spark won't be delivered and the engine will cut out. Depending on how badly it's phased, it can fire a different cylinder. The ignition timing is variable from the Motronic, depending on engine load, the timing is more advanced under light load vs full throttle by as much as 15 degrees, so if the rotor is barely touching the correct cylinder in the cap at full throttle when say it wants to fire at 25 degrees BTDC, when it's under light load such as in neutral, and is supposed to fire at 40 degrees BTDC, the rotor hasn't even approached the cylinder close enough yet.

[bdonally]

After checking the rotor timing, taking another look at the wires and connections, and checking the coil resistance, I decided to dive in deeper and attached an oscilloscope to various spots.
I thought perhaps the sensor signals might be marginal and fade at high speeds, or that the coil was faulting at high speeds.
What I found was this:


The engine speed calculated from the scope frequency works out to 6800 RPM, which is the rev limit set point.
I verified this with similar data from the coil feed.
Then removed the tach, and scoped the signal from the DME, and it also correlated with 6800 RPM.

The *&^$#(&* tach reads incorrectly.

I changed it ( everyone has a spare tach, right) and all is right with the world.

I never suspected such an issue, because the tach was reworked a few years ago, and was working correctly 2 years ago when I took the car off the road for the rebuild. Possibly one should have noticed road speed versus RPM but as part of the transmission rebuild I changed 2nd, 4th, and 5th gears, so more or lees everything was new to me.

What a incredible waste of energy. Thanks to everyone who provided suggestions - following them up developed a great deal of confidence that the things mentioned, any one of which could have been done sloppily during the rebuild and subsequent rework of the transmission.

BTW, I have scope traces for the coil input and the reference sensor, if anyone is interested. The reference sensor out put looks messy, and when I get a minute I think I will replace it, just in case.
I did notice that once the engine is running, the reference sensor can be disconnected without the engine stopping. I did notice a little hiccup in the engine speed when reconnecting it.