3.2 misfiring after engine drop
 

Four weeks ago I installed my rebuilt engine and transmission, and the engine ran well.
Pulled hard, ran right up to the rev limiter, etc.
Two weeks ago I dropped the engine and gearbox to attend to an issue in the gearbox.
Didn't do anything to the engine other than change the oil.
Now, it starts fine, and pulls well up to about 6000 RPM then starts misfiring, and won't go past that speed.
I thought about the flywheel speed sensor, so checked the gap and it was fine.
Also checked over all the wiring for a loose connection.
The symptoms:

• in neutral, the misfiring happens at about 4500 RPM - engine won't go past that speed unless its given full throttle from a lower speed, in which case it will go up to about 5000
• in 2nd gear, accelerating gently, the misfire happens at about 5500, but if I accelerate hard it will go to approx 6200
• In 3rd gear,accelerating gently, it misfires at about 5500, but accelerating hard it runs up to the rev limiter.
• Up to the point of misfire, it runs as it did before

When it misfires, it feels a bit like the rev limiter, so it leads me to think electrical rather than fuel.

BTW, it has an Autothority MAF and a SW chip; both have been in place for years.

I can't think of what I might have disturbed during the drop that would cause such an effect.

I have run out of ideas, and sure would appreciate some help.

I'd check and clean the Hotwire in the MAF if you haven't done so recently using MAF Hotwire cleaner. When dirty, the Hotwire can't accurately sense airflow and can provide the DME/SW chip with invalid readings. Not sure why this would not have manifest itself before engine drop, but is similar to a situation I had with my (non-autothority)MAF some years ago

I have had similar experiences. The culprit a few times is the way the distributer cap sits. Sometimes it can rock a few degrees off center causing the symptoms you’ve described. I would check it and see if maybe you knocked it while moving the engine.

I'll double check the MAF, and the distributor.
I did notice when poking around that I got a shock from one of the plug wires at the distributor cap.

Checked the distributor, it looked fine but I reseated it anyway.
MAF looked clean but I gave it a thorough cleaning.
Neither helped; it still misfires
4500 with no load, 6000 or so with a load.
It feels much like hitting the rev limiter.

longshot, but broken idle position microswitch (or loss of continuity in circuit between microswitch and DME after second engine drop) would cause fuel starvation/cut-off at higher rpm (if it always reported closed throttle/idle status to DME) -- see http://forums.pelicanparts.com/porsche-911-technical-forum/906493-idle-position-switch-question.html

You would have removed the coil to distributor lead to drop the engine, so I would check the continuity of the wire with a multimeter and make sure it is seated properly on both ends. I would also check every electrical connection that you had to unplug and plug when you dropped the engine. When I did a major service on my 3.2 a while back, I didn't get one injector plug and the plug to the CHT sensor properly seated, so shortly after the service the car started running like crap. I just checked every connector that I had touched and I found the problems.

It was the CHT connector. It was almost seated, enough that I didn't notice.
Also I didn't think of the CHT because it didn't seem to me it would cause such a problem.
And, of course, I went looking for zebras instead of horses :-)

Thanks to all for the great advice.
Bob

great news - thx for posting the solution!

I thought the problem was solved, but ....

The CHT connection got rid of the lower speed stuttering.
However, at higher speeds the feeling that it is hitting the rev limiter is still there;
-in neutral, or in 1st gear, whether increasing revs slowly or quickly, occurs at about 6100
-in second, occurs at 62-6300
-in third, at about 6500
-in 4th, was OK at 6600 then I ran out of road.

I have replaced the DME relay and the fuel filter, and cleaned the MAF and all the connectors on the engine.
Also verified the idle and full throttle switches.

The 14 pin connector is cracked, and Ihave a new one coming, but it isn't clear how that connector would cause such a problem.

The car runs great up to the stuttering; at a track event I just shifted a bit early.
With the mild state of tune of my engine, I don't need the higher rev limit but not being able to find the problem is frustrating.

Is there any chance that the flywheel speed sensor would cause this if it was on the edge of the adjustment?

An ideas will be appreciated.

Just curious... what did you do to your transmission that you needed to drop the engine?

I'd look at that areas first.

Hi Bob- FWIW (and also after engine drop) my misfires occurred at around 4500 rpm. Adjusting both the Speed & Ref Sensor clearances solved my problem!

Dropped the engine/gearbox to replace a bad bearing on the input shaft.

I adjusted the speed sensor but it is hard to do with the engine in, maybe I'll redo those.
Any tips for doing them without dropping the engine?

I had a similar experience with a 3.2 a while ago.. turned out to be one of the connectors from the sensors, was not 100% plugged in at that bracket on the lhs of the engine bay.. it 'looked' plugged in, but when I went around pushing on everything, it clicked, and I got lucky.

Hi Bob- I still think this is one of the best references on how to replace/adjust the sensors without an engine drop:
http://forums.pelicanparts.com/porsche-911-technical-forum/552276-no-spark-problem-finally-resolved.html

Aren't these rpms just about at redline?

The chip provides a 6800 rev limit, not that I need that high with my engine.
The issue I am chasing is that something has changed.
Next step is to reset the flywheel sensors, maybe one or the other is providing a weak signal at high rpm

The early rev limit problem persists.
The sensors have been re-checked, and I swapped in a new one just to see if it made a difference - no change.
It acts and feels exactly like hitting the rev limiter.
Data from a newly installed A/F ratio gauge shows that the mixture is fine up to the point the "stuttering" starts, then it goes lean. I believe the rev limiter cuts fuel so going lean makes sense.
But it happens at 5900 in neutral and first, 6200 or so in 2nd, and 65-6600 in 3rd.

I found a bad wire in the chassis side of the connector for the speed sensor - the signal wires had adjacent cracks in the insulation, and if the cable was moved into certain positions, the shielding wires would short the signal. Took a while to find that, but once fixed, it didn't solve the problem.

I have contacted S Wong asking if the ECU could be doing this, given that it seems like it is hitting the rev limiter, just at different engine speeds.

Since the A/F ratio is OK up to the point this occurs, I don't think there is any danger to the engine, and will attend a couple of DE's, but this is starting to be a bit annoying. Getting good at changing and setting sensors, though.

Since you rebuilt your engine a few weeks ago, my bet is that you clocked the distributor into the case one tooth off. Take the distributor cap off, turn the crank pulley by hand watching the TDC mark as it approaches the mark on the case, and also watch where the rotor points to in relation to the TDC mark on the distributor housing. When the crank pulley comes to TDC, so should the rotor, or within 2 to 3 degrees. If it's off like 10 degrees or more that is your problem.


-Steve

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

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

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.

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.

Feeling a bit stupid
 

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:
http://forums.pelicanparts.com/uploa...1568579684.jpg

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.