3.2 stumbles only when turning right
 

I have an '82 SC with a 3.2 swap. I did a head stud replacement over the summer that included splitting the case but didn't include any piston, cylinder, or head work. The car runs very well, pulls hard up to 6k (farthest I've gone so far), and runs well at idle, under acceleration, under braking, and in left turns. However, in right turns the engine stumbles and is seriously down on power until I straighten out.

I have less than 200 miles on the engine since the rebuild. It was not doing this before the rebuild, and it wasn't noticeable at first afterwards. In driving it now it is pretty consistent. I'm making this new thread because I can't see how this would be a mechanical issue from the rebuild, it has to be a sensor or something stupid I did on an electrical or other connector during assembly, right? But so far I've checked the obvious stuff:

To be clear, the car stumbles but does not completely cut out. It feels for all the world like a carb when the fuel has sloshed away from the one of the jets in the bowl, but this is the stock Motronic setup, so I don't think that's it. :)

CHT sensor is new and connector seems tight.
Reference sensor connectors seem tight.
Ignition switch (searching revealed some issues here) is new at the same time as rebuild. Harness seems OK.
No oil smoke or anything visibly evident during stumbling

Thoughts? I'm really scratching my head here.

so if you just drove in a circle clockwise the car would stumble constantly ? and if you drove it in a circle CCW it wouldnt ?

Basically yes, though haven't actually done that since the roundabouts here run CCW. :) It feels for all the world like fuel starvation. I tried it a few more times today and if I'm hard on the gas it is pretty severe. If I'm almost coasting it is almost imperceptable.

To test the CW stumble, find a large school parking lot (after hours ) and see what happens after doing both CW and CCW.

I had a loose spark plug wire that caused a similar issue. when I turned, in this case left, the connector would move away from the spark plug just enough to start missing. Plugged it back on properly the fault went away.

loose plugs
 

+1 on what STU said - I've heard plugs can do those symptoms.

Thanks for the suggestions, I hadn't considered plug wires.

This morning quickly before work I wriggled and shoved all the plug wires in to ensure they were tight (seemed to be) and did a full disconnect/reconnect on the CHT sensor because upon closer inspection I wasn't 100 confident that it was fully 'clicked' in.

Well, I did something because it is much worse now. :) Now even in a straight line it stumbles and bucks, right turns are worse. The only time it ran smoothly this morning was during left turns.

With the wheels blocked (hand brake applied) engine running and a fuel pressure gauge connected , jack up the RH side and see what happens.

Since the symptoms changed after you disturbed the CHT connector, it would seem that the CHT or the connector is suspect. Has the CHT or either of the pulse sensors ever been changed? What kind of condition are they in?

I had an intermittent bucking/stalling problem that turned out to be a partially connected CHT - once I snapped the connector into the fully locked position, problem solved.

Copy and paste this into Google, lots of good reading:
CHT test site:forums.pelicanparts.com/porsche-911-technical-forum/

Hopefully this helps to solve your problem.

Good luck
GK

The CHT is a brand new OEM part from our host. I have not tested the resistance (since the car ran well most of the time) but will do so as a double-check.

The pulse sensors are of unknown age. My experience with BMWs has been that these sensors either fail totally or exhibit RPM-based maladies, but anything is possible, I suppose.

I would love to have a fuel pressure reading and/or an A/F ratio display in the cockpit while this is going on! I don't have a fuel pressure gauge with the right fittings to attach to this car, so I guess I'll see if I can scare something up in that regard.

I still assume that you may have a g-force based ignition fault.

Pull each of the plugs and check them.

I don't recall off hand if these wires use the plug with the screw on round cap or not, but many times that cylindrical barrel nut works loose. I usually tighten them with large slip groove pliers. Also check for bad/shorting spark plug wires (burn holes). Also check for shorting/cross fire between wires near the distributor. (best done in the dark as you can see the light-show) Another item to potentially check is the cap and rotor. Check for play in the distributor shaft and check the cap and rotor for carbon tracing.

You might also want to check all/most of the wiring plugs around the engine. Many of these older engines no longer have the metal spring/lock clip that keep the connectors together and can wiggle loose. It's best if the metal spring clips remain in place, but sometimes they are broken/lost and replacements are difficult to obtain.

Good luck with it.

Thanks, John. I should mention that the plugs, wires, cap, and rotor are all new as well. The wires are not OEM, I think it is a Bosch set(?). Seems of reasonable quality but does not have screw-on caps.

I will pull the wires, check the tightness of the plug connections, look for arcs, and re-re-check the CHT tonight at least.

You may want to take a look at the flywheel sensors. I came across one that would be ok sitting, but would short itself out when wiggled. You can check these with an OHM meter at the plug side. The manual gives you a range of OHMS that these should be. If the car starts, I doubt it would be the reference sensor (senses the single set screw), but rather the one that reads the many notches on the flywheel ring.

By wiggling the temp sensor plug, you may have bumped one of the two flywheel sensor plugs/wires. The insulation on these get brittle over time and can cause these to short. There is a BMW 535 sensor that can be cross referenced through our host (pelican) that is much more economical. I have used them and they work well.

On the upside, you are getting closer.

Do one thing at a time to maintain sanity. e.g, disconnect/reconnect the flywheel sensor things (I have a 3.0 so don't know pacifics) Drive it. Then etc. with CHT sensor and wires.

battery positive clamp shorting to body?

Check gas tank. I have seen the pickup line in the tank crack or develop pinhole leaks. When cornering gas flows to one side exposing line to the air. Fuel pump then sucks in air.
Not uncommon for high mileage track cars.

My 3.2 had an intermittent stumble on hard acceleration.

I eventually traced it to the fuel injector harness. The insulation had begun to fail and it shorted to ground when the engine tugged a tiny bit.

Of course I didn't figure it out until I had replaced every sensor, rebuilt the DME and the AFM!

+1 to this....

I had a recent stumble that progressively got worse also turned out to be poor connection at the fuel injector harness....thru a lot of parts at it looking for it.

regards,

Having owned 4 BMWs I'm only too familiar with these sensors. I did go back in and take a closer look. The insulation at the base of the connectors turned to powder in my fingers and fell apart.

http://i1269.photobucket.com/albums/...ps3f4cf231.jpg

These obviously need to be replaced. However, I'm not convinced this is my problem. I taped them up and then wriggled, yanked, etc with no change in idle quality while running, and a subsequent drive revealed the same issue as before. (yes, I do have the infuriating metal clips for these, just not in the picture)

So, to recap today:

0 - After work I poked around again, wiggling things like the CHT, reference sensors, and plug wires. After this it was back to the original issue of stumbling while turning right, straight ahead was OK.

1 - Pulled all plug wires and double-checked that they are seated well. No change.

2 - Pulled, found broken insulation on reference sensors. Taped up, wriggled, etc. No change. Hard to believe that they are causing issues in right turns when I can mess with them at idle with no change.

3 - Jacked up the right side of the car while idling (no fuel pressure gauge yet). No change.

4 - Checked injector insulation jacket and harness - all looked very good. Wriggled and pulled on various injector harness pieces at idle. No change.

5 - Checked positive terminal/cable - battery is secured well, cable is secured, insulation and cable jacket near battery is good. No change.

Vacuum leak? Check all including the rubber sleeve that connects the two sides of the intake. I had a situation that was exposed only in slow right turns at the track. The body twist was just enough to allow the sleeve to leak. Tightened it down (large hose clamp) and all better.