I replaced the connector entirely for the cam position sensor to eliminate the possibility that my repair was causing the latest misfire issue. No help. I also saw that the vacuum tube had come detached from the heater hose valve below the cam position sensor connection. I re-attached it, but again, it didn't help.

Now, I'm starting to wonder if this is actually a different issue.*At the start of this, I was having misfires tied to low-voltage conditions. They were worsened by running lots of accessories where I had "less headroom" for low voltage conditions.

If this had just started now, I would describe it differently. The end result is the same (misfires making the car nearly undriveable), but the voltage is not coupled to the issue at all. I used to get the engine to run better by stopping accessories like the headlights, fog lights, HVAC fans, and AC, but that doesn't help anymore. It seems more related to engine load. I can idle for seemingly forever without a misfire, but I used to be able to reproduce the misfires by idling on the lift and standing under the car on a warm day. Even if it's actively misfiring, I can usually push in the clutch and rev the motor. I couldn't do that before. Now, the problem is worse if I'm going up a hill than down, even if I'm on the gas going downhill. It still usually gets me a mile or so before it starts acting up, and that's the case whether it's been just started or its been warming up at idle for 20 minutes.

1. I have a new ignition coil coming, just to eliminate ignition component failure entirely.
2. I have a fuel pressure gauge for the fuel rail coming, though I'm unsure if I'll be able to use it to troubleshoot much since it wont' be visible while driving. I'll cross that bridge when I get to it.
3. I will go through all my other sensor connections looking for degrading or loose wires and connectors. Maybe something is "shaking" loose after a bit of driving, though I suspect it would probably not "fix" itself by sitting overnight for a bit.

According to clarks, hesitation/stumbling can also be caused by the following:

-Dead spot in AFM (easily fixed as outlined on Clarks)
-Vacuum leak
-Spark plugs (you said you replaced)

Some things (surprisingly not mentioned on clarks which can cause hesitation/stumbling):
-Wires (you said you replaced)
-Cap and Rotor (you said you replaced)
-Reference sensor (can be tested if you have an oscilliscope)
-Cam position sensor (you said you replaced the connector, but I would replace or test the sensor as well)
-Fuel related issues (FPR, injectors)

I haven't inspected my AFM at all yet. I know that connector has been disconnected/reconnected a million times too, so I should probably give that some love to make sure it's healthy. I'll put that on the short list.

Vacuum leak... I did buy all new vacuum lines recently. They aren't here yet, but I think I will begin a wholesale replacement of any that I haven't replaced yet and cross my fingers.

I haven't tested my reference sensor yet. I don't have an oscilloscope, but this isn't the first time I've thought I should have one. I expect that to have just as much impact while idling and revving as it has when driving, so I'm not too suspicious, unless it's somehow loose.

FPR - I actually replaced that with the fuel rail because the used one I bought came with the a stock FPR attached to it. So if it's the FPR, then it's both FPRs.

Injectors have been reinstalled with new seals and inline filters. I haven't flow-tested them. I couldn't actually remove them from their wire harnesses without destroying the connectors. I know I can buy all new engine harness connectors for them, and that's tempting to be able to get them cleaned and flow-tested. I don't intend to replace the injectors without knowing they are a problem. That's just too much to spend without being sure it will help.

-Neil

AFM - I am not referring to the connector, its the pickup points on the actuator.

You should read through Clarks Garage and follow all the testing procedures for everything I mentioned. I wish more people would go here as their first step to troubleshooting

Here is the testing and repair procedure for the AFM, you will find many other procedures for other sensors I have mentioned here as well. The engine troubleshooting section is also a good place to start. Vacuum leak can be tested by pressurizing the intake with 10 or so psi (listen for leaks, make sure engine is at TDC so valves are closed)
http://www.clarks-garage.com/shop-manual/elect-22.htm

Yes, I know you weren't referring to the connector, but I also know that the connector there feels flaky, so I mentioned it.

I've spent more than a year restoring this car with over 211k miles from having sat in a friend's driveway for ~7 years without moving. It's not my first vintage car, first vintage Porsche, nor even the highest mileage vintage Porsche I've worked on. I'm not new to this, and this problem is not as obvious to troubleshoot as you seem to think.

And yes, I'm familiar with Clark's Garage. You don't need to suggest it so rudely. Be nice.
-N

I opened up the AFM tonight. It was operating fine. I did clean the tips of the contacts a tiny bit anyway, while I was in there, but it all looked really clean to me, and the voltage was fine across the whole range of motion.

Really crossing my fingers that new ignition coil comes soon and magically fixes everything.
-N

Sorry wasnt trying to be rude! It sounded like you might have not been familiar with the procedures to test these sensors.

You may also want to check the Throttle Position Sensor making sure it clicks as soon as the throttle valve is opened and getting correct voltage. Its a binary sensor I believe. I was getting some part throttle hesitation and discovered my TPS wasn't clicking and was an easy fix once I opened it up. Solved my issue.

I've found an electrical connector that looks in rough shape, definitely could behave badly if wet or hot, and looks like it might be shorting at least occasionally. I don't know what it is, but I posted another thread to see if anyone can identify it:
https://forums.pelicanparts.com/porsche-924-944-968-technical-forum/1123581-sensor-idenfication.html

-Neil

How hard would it be to disconnect or ground out the signal from the knock sensors?

Also I'm sure the DME coolant temp sensor has been considered but I have had a problem with one (on a Mustang) where it would go open when warm and the ECU assumed it was very cold and flooded the car. You could shunt it with a resistor as a test. Feels like this has been suggested before.

I may still replace the DME coolant temp sensor just to eliminate an option, but I'm less suspicious of it right now.

I didn't have a means to troubleshoot fuel pressure before, but I replaced my ignition coil with an MSD unit last night, and _also_ put the Lindsey Racing fuel pressure gauge on my new fuel rail. After a short drive, the misfiring was happening, and when I came back, I took this video of the fuel pressure gauge:*https://www.youtube.com/watch?v=k6nBCmzI5Wg

You can pretty clearly hear the fans kicking in and watch the fuel pressure go down, down, down until the fans kick off and the fuel pressure goes up again. So, now for the brainstorming...

Reasons for the fuel pump to stop pressurizing the fuel rail:
1. Fuel pump failing.
2. Fuel filter clogged.
3. Fuel pressure regulators adjusting their target pressure _really low_.
4. ECU is turning off the fuel pump.
5. Fuel pump isn't getting adequate power.

Logic right now:
1. Because of the clear correlation with the radiator fans kicking on, I suspect it's not a fuel pump failing. It was also a new fuel pump just over a year ago.
2. Because of the same reason, I doubt a fuel filter is clogged (new at same time as pump). I do have a filter though to replace the in-tank basket-style filter, so I'll replace that while my tank is really low anyway.
3. The fuel pressure regulators can adjust fuel pressure via the vacuum lines, but it seems unlikely to me that it can bring ~45psi down to ~20psi. I guess it's plausible that there's a vacuum issue at play in those lines, but I can't fathom why it would correlate with the radiator fans.
4. ECU is turning off the fuel pump - this seems somewhat likely to me. I have the F9 solid-state fuel relay, but unfortunately it's the one without the LEDs that tell you when the various circuits are on. I'm tempted to buy that one because it's so much more awesome.
5. This is similar to #4, but suggests more of a low-voltage situation to the fuel pump. This is supported by the fact that this problem worsens if all the accessories are on. But it's _not_ supported by the fact that I can usually drive fine for a bit until the engine warms up. And then the misfiring worsens quickly.

Next steps:
I'm going to attach a meter to the fuel pump wiring to see what's happening to the input voltage at the fuel pump when this is happening. I want to know if it's straight-up stopping or if it's just a low-voltage scenario. I'd venture that's the difference between #4 and #5 above.
-N

Compare the voltage drop at the battery terminals against the voltage drop at the fuel pump when the fans kick on, too.

Compare the voltage drop at the battery terminals against the voltage drop at the fuel pump when the fans kick on, too.

No need to be judgmental. With the exception of the ignition coil, I haven't replaced anything that didn't need to be replaced. And considering how rusty the one I removed was, I don't regret that a bit.

Anyway, family vacation coming up, so it'll be a bit before I get time to collect more information about this. :-(
-N

I think I've found it. I still need to test drive to be sure, but this seems pretty cut and dry to me.

Here's the voltage I was seeing at the fuel pump while the misfiring was coming every few seconds: https://youtube.com/shorts/jkaiFhyS4cE?feature=share

And then, I swapped out my Focus 9 Technologies solid state fuel relay for a mechanical one I repaired: https://youtube.com/shorts/uc7xKjz7oms?feature=share

Night and day difference. Zero stuttering with the mechanical relay, solid 12V to the fuel pump, even with all the accessories on.
-N

The Troubleshooting guide in the Factory Service Manual says this about the female contacts in the Fuel Pump Relay socket:

http://forums.pelicanparts.com/uploa...1660012874.png

Try putting the F9T relay back in and see if you can re-duplicate the intermittent issue. If the intermittent issue stays eliminated, you might have a loose catch one one of the female contacts in the relay socket.

I realized that I uploaded the same video twice above before. I fixed the second video link now to show the steady voltage at the fuel pump now.

That's a good idea to check/confirm that the relay socket isn't the actual culprit. I did go for a drive last night and 10 miles went by without a misfire. I'll swap the F9 relay back in today to check.
-N

Last night, after posting, I went for a drive, about 10 miles, engine loaded, high and low rpms, every accessory on, etc. No misfires. During all of these issues, I've only once had issues starting the car when in the pouring rain that one time.

This morning and evening, the car cranks and has plenty of fuel pressure, but it won't start. I tried both relays, but since there is fuel pressure, I doubt that would make a difference. I did get a tiny stutter once, but otherwise, I couldn't get it to run. At all.

This seems like an entirely different issue now, but I had one good drive last night! WTF...
-N

Related note: F9 was super responsive, and they are sending me a new relay.

I read that you had done something with the injector wires at the connectors? Have you checked there is not an intermittent short circuit in there? My car had that, would work fine but one day didn't start at all. Wiggling one injector wire caused the car to fire up.

There was no spark at all. I swapped out the brand new MSD ignition coil for the old stock one. That was among the parts I replaced, but didn't technically need to. The stock one just looked bad, and I liked the idea of having all new components in the ignition system (and couldn't find a stock replacement).

Started right up like it never wondered if it should as soon as the stock coil was hooked up again. I cannot fathom what could have caused a total failure of that so soon. I will definitely have to have a look around the electrical system to see if I can find anything else misbehaving. I certainly haven't seen any evidence of it yet.
​​​​​​​-Neil

Hi Neil, just re-reading the last couple of pages, I’m wondering if you’ve had a look at the condition of the rear of the fuse panel, where all the wires come in & out, to see if it’s every suffered any thermal events or bodgery by someone less skilled and experienced than yourself? And - don’t yell me if you’ve already said this - but have you already replaced the electrical side of the ignition switch? They do seem to crack.

Best, John