hesitation, running rough when hot and accelerating

[toddetch]

[Note - I realize that much of this may have no bearing on the actual issue - but I'm including for completeness. Sorry for the long post.]

A few weeks ago I took my 1980 SC Targa on the longest drive I have since I bought it last summer. About an hour in I noticed it seemed to be running rough. I had recently changed the fuel accumulator to resolve my hot start issue. While doing that, I found a disconnected weatherpak connector lying in the engine bay underneath the accumulator. Turns out the connector was inline with the oxygen sensor, so the sensor was disconnected. I reconnected it and since the sensor was 33+ years and 33k+ miles old I installed a new one.

If anything the rough running seemed to get worse - or maybe I was now just more attuned to it. It seemed to only be when the engine was hot and on acceleration. At highway speed if I pushed on the gas to speed up to pass someone it would hesitate and stutter. Going at slow speed up a hill in the neighborhood it was really unhappy.

Not having all the tools and wanting to get it fixed so I could enjoy the nice spring weather I took it to my local air-cooled mechanic. He was able to reproduce the issue and inspected and adjusted a number of things, including:

• Checked fuel pressures - cold and hot are both okay and in spec.
• Checked fuel filter - okay (fuel pump and filter were replaced last year).
• Reset timing, idle speed, and fuel mixture.
• Tested throttle position switch with ohm meter - okay.
• Checked idle and enrichment circuits - function normally.
• Tested frequency valve function - okay.
• Tested oxy sensor function and duty cycle - okay.

He also noted that the ignition distributor is not original - thinks it may be from a 78 or 79 or a European unit. It only has a single vacuum advance. I believe the vacuum retard had been connected to the ignition distributor and George switched it to the vacuum advance. He gave it back to me to drive and see what I found.

After getting it back I also replaced what looked like a cheap set of spark plug wires with the Beru braided steel wires with ground. During the first week of having it I drove it around town and autocrossed it last week and didn't notice the issue. I did noticed that it ran better in general, better idle and better throttle response. However, yesterday I took it on a spirited drive and after about 20 miles finally saw the issue again - when accelerating it is really unhappy. It can be at faster speeds and throttling up to pass someone, or even just starting from a stop light. I never see it when accelerating from a complete stop, but it will happen when shifting into second or third.

Any suggestions of what this might be?

I'm sure some will suggest a vacuum leak. But would that only show up when the engine is really warm? I would think it would show up when under load, like accelerating uphill, whether the car is cold or warm. How do you even check for a vacuum leak under such specific conditions (really warm, under acceleration, etc.)?

The car sat for about 12 years, until a few years ago, and has only be driven maybe 3000 miles since taken out of storage. The gas tank looks fine. One thought George had was maybe the fuel distributor has some debris in it and that is intermittently interfering. However, I don't see this as intermittent - it is very specific - when the car is hot and under load, specifically accelerating.

Any suggestions are welcome! Again, sorry for the novel here.

[steely]

Not an SC guru, but just from reading: could it be an issue with the advance on your dizzy. Just a guess before you let George know, but see what someone else comes up with.

[toddetch]

Thanks @steely. Anyone else with some thoughts?

[T77911S]

check the fuel flow out of the FD and the flow to the WUR.
this will also check the fuel filter

could be the coil or CD unit is going bad. (heat related).
same with the O2 system,. bad solder joint on either the O2 relay or other boxes.
if you can borrow an enrichment control unit I would try that

[pmax]

Originally Posted by toddetch

If anything the rough running seemed to get worse - or maybe I was now just more attuned to it. It seemed to only be when the engine was hot and on acceleration. At highway speed if I pushed on the gas to speed up to pass someone it would hesitate and stutter. Going at slow speed up a hill in the neighborhood it was really unhappy.

Sorry if this is a dumb question but what gear and speed are you at when it stutters ?

[toddetch]

@pmax - here's a bit of the note that I sent to the mechanic last night, trying to summarize my observations:

• It only ever happens once I've been driving for a while, then continues through the end of the drive.
• It seems to happen on acceleration. It seems worse at slow speeds, lower gears, especially going uphill - maybe because it is under more load. It is also noticeable at highway speeds in fourth or fifth gear when accelerating to pass someone.
• It doesn't seem related to elevation.
• I don't believe it happens when driving at a constant speed, though I need to really think about that the next time the issue shows itself. As I think back to the first time I noticed it, on that drive on 101, I don't think I was necessarily trying to pass someone, but I do think I was increasing throttle to speed up a bit and noticed it.

[pmax]

If the rpm is below 3000ish, then you are simply in the wrong gear. Just checking if that's the case.

[toddetch]

Originally Posted by pmax

If the rpm is below 3000ish, then you are simply in the wrong gear. Just checking if that's the case.

That’s not the case at all. Definitely in the right gear.

[75 911s]

I've noticed these symptoms in connection with heat soaked coil problems. Is your coil one of the lesser quality blue ones?

[toddetch]

Originally Posted by 75 911s

I've noticed these symptoms in connection with heat soaked coil problems. Is your coil one of the lesser quality blue ones?

In my internet sleuthing I’ve seen the coil as a possible culprit. I’ll check tonight if I can.

[toddetch]

Originally Posted by 75 911s

I've noticed these symptoms in connection with heat soaked coil problems. Is your coil one of the lesser quality blue ones?

@75 911s not home yet, but I checked a pic on my phone if the engine and it looks like it still has the original black coil. I’ll check the documentation from the POnwhen I get home.

I saw the Pelican technical article in testing ignition, including the coil. But is it possible to check for this specific condition? Is it simply getting the issue to occur and then checking the coil?

[Bob Kontak]

Originally Posted by toddetch

In my internet sleuthing I’ve seen the coil as a possible culprit. I’ll check tonight if I can.

I had a problem with my 81.

Cruising no problem. Under load the coil secondary wire would short to one of the little +/- primary terminals on the top of the coil. Bad things.

I know you changed your wires which makes this a slim chance but do the dark test of checking your secondary ignition in the garage. Garage door open.

Wait for five mins for your eyes to adjust. Find the throttle plate flipper. No neck ties unless a break-away.

Blip throttle. Stress it. See if there is arching anywhere.

Kill this variable.

[SCadaddle]

When my 82SC exhibited these same symptoms, I narrowed it down to the distributor by lightly grabbing the distributor body while the engine was running and could actually feel a crunchy vibration. Give it a few revs and see if the feeling in your hand on the distributor changes.

I removed the distributor cap and the rotor on the shaft could be lifted about 3/16" of an inch or so. The rotor was tight, so this upwards motion was in the shaft itself. Set the engine to TDC #1 and removed the distributor. At that point it was clear that the fiber washer inside the distributor body on the shaft was toast. I can only guess that the distributor rotor shaft was "jumping" up and down in its meshing with the intermediate shaft in the engine. Fitted a known good distributor and all was well. I could feel no vibration in the newly fitted distributor with the engine running.

[toddetch]

Okay...

@75 911s looks like it has the original coil. I will at least test it as per normal testing. I'm intrigued by @Bob Kontak's by "dark test" but fearful at the same time.

SCaddle I tried your test and didn't really feel anything didn't have a chance to take the distributor cap back off and feel around. Will do so this weekend.

Still corresponding with my mechanic as well to see if they have any further ideas.

Thanks all for the input!