'79 SC develops miss/stumble as engine temp rises
 

I must admit, this problem has been plaguing my car for some time now, but it has recently gotten bad enough to affect the drivability on my 37 mile commute home from work on warm days. The car runs great for maybe 10 miles, then starts stumbling/missing when running up thru the gears. It starts/idles fine regardless of the temp, but it seems that as run time accumulates, something is getting heat soaked and causing the miss to start showing up. I have checked my injector flow rates and hot/cold control pressures and they are OK. This morning (50 degree temps), after driving 37 miles to work, the car ran perfect for about 20 miles, then began it's stumbling under load. I had been thinking it was simply a bad coil issue that showed up when the coil itself got hot over time, but touched the coil this morning when I got to work and it was cold to the touch (50 degrees this am... engine compartment was cool).

What else could be a cause for my worsening problem that develops only after 10 miles or so of use and is worse when ambient temps are high?

CDI.

Get one of those inexpensive non-contact IR thermometers from Harbor Freight and use it to measure the temperature of the CDI box when it starts acting up. Then let it cool down and try it. A heat gun on the fins would replicate the heat soak in the engine compartment. That is my guess as to what the trouble is.

Good call on the heat gun John!!! I in fact have a cheapo IR thermometer from HF already. I have no excuse for not thinking of that on my own :o I was so frustrated about the intermittent nature of this problem, I failed to think about replicating it with artificially induced heat. This might be an easy fix after all! Thanks

Have you looked at your head studs lately?

You are scaring me Bill! :) No I have not, but I rebuilt this engine a couple of years ago (15,000 miles ago roughly) and replaced the divilar studs with steel.
IIRC, the steel ones pretty much eliminate the old head stud problem (at least in a stock NA 3.0).

I quit driving it to work a week or so ago as on a HOT day (low 90's), it got to the point in the last five miles, that it was begining to miss at a steady 60 mph. I had myself convinced it was probably the coil and decided to park it until I got a new one. Well I got sidetracked with other projects and just for the heck of it this morning, with the cool weather we are having, I decided to drive it again. The problem almost has to be heat soak related as the problem was barely noticable in the cool weather this morning (although it did start acting up a little towards the end of my commute). My guess is that it will be fairly well pronounced again on my way home today in mid 70's temps.

I was just sure it was the coil, but as I stated earlier, it was COLD to the touch after the 37 mile commute in 50 degree weather.

I mentioned it becuase the first sign I had of broken head studs was a stumble/ miss when hot.

Another problem could be your fuel pump. I had a similar problem with my 73.5 When the pump got hot, it didn't maintain pressure (also made a metallic whirring sound)

Smart guys like you, Tim, carry a spare CD box in the trunk. Packed in ice. Kidding. They are internally grounded, so there is nothing wrong with temporarily just plugging it in whether or not it is fastened to the fuse panel.

Various connections and especially resistors in the high voltage part of the ignition system can go open, or fail to pass the signals properly. Those Beru spark plug connectors, for example, can do this. As heat rises and those resistances increase, at the same time the heat and pressure in the combustion chamber is also making it more challenging for the spark to be bright and snappy.

You know all this. Better than I.

Hmmm, old spark plug connectors can be affected by heat....my plug wires pass the arcing at night test and the whole heat soak thing made me not too concerned with the plug wires, but I have never replaced them since I have owned the car. Maybe I should think about changing them if the CDI does not prove to be the problem. With my kind of luck, they will ohm out fine and only screw up when hot and bent just the right way.

Thanks Supe and while I do know SOME things better than you, I do not know ALL things better than you;):D

Last time I checked, and it was starting to exhibit these problems, fuel system, hot and cold pressures were good with good even flow from all six injectors, so I am thinking this is electrical and not fuel related.

I'll take your word on that Tim but in the meantime, whenever you and I disagree, I'll just accept your beliefs.

I had a '73 Datsun 620 pickup that I loved, but it was a POS. On one of the days when it wouldn't start, I learned something. When I removed a spark plug connector from the plug and grounded it, I got a nice spark. Blue and fairly snappy. But with the connector plugged back onto an engine spark plug, it would not trigger the timing light. Long story short, a spark under compression is more difficult than a spark at atmospheric pressure.

And yes, I have seen Beru spark plug connectors test fine, but not work fine. And yes, they can change at different temperatures. Warren found one that reliably went 'open' at an ambient temperature of about 90 degrees F. Those high voltage signals are so finicky, it's as if they're trying to find a ground alternative besides the spark plug. I don't know where you'll find the cause of your current problem but regardless, it probably is a good idea to replace those ignition wires. Carrying from twenty to a hundred 30,000-volt signals per second over a multi-decade period has perhaps taken its toll on them.

I had a similar problem on my SC. Check out your fuses (don't ask why), and check your wiring to power the coil. If they are worn replace them. It has made a huge difference on my car.

For the fuse part: my car had a lot of lag after a very recent rebuild, and it took 15 minutes to warm it up properly, it ran fine, until under load. Changed the fuse for the interior lights (thats just weird) and the car ran much better. Changing the wiring made it run like new. Good luck Tim.

This is worrisome. The world is going to stop rotating if Macgyver Hancock encounters a problem that can't be solved. ;)

The Bentley manual has the resistance values for go/no-go checking of the plug wires.

Swapping in a known, good CDI box is an easy test as well, and many of us do travel with a spare.

Brian

Lots of smart people have recommended this. Me, for instance.:D

How do all the "smart people" end up with a "spare CDI"? Is it from guessing wrong when the car had a previous problem? :D:D:D

Thanks for the tips thus far guys.

Brian, other than the fuel pressure test a couple months back, and a few brief searches here and there, I have not begun to really do anything about this issue. I have just been too busy with TOO many other projects to devote some quality time with the 911. :(

I recently got my 944 turbo out of moth balls again and it required an AC compressor re-seal and a power steering rack rebuild. I have a customer's airplane engine tore apart in my shop and another one coming after that. On top of that I picked up two Bridgeport milling machines and new old toys are hard to avoid tinkering with whether one has the spare time or not. ;):D

I am going to do some playing with a heat gun and an ohm meter sometime this weekend and see if I can shed some light on this thing. (If I can find the time;):D)

No guessing about it. We smart people apply rules of logic. Why let a critical item potentially ruin a good road trip? Veho plures parco secui. :)

Brian

Stop at Autozone-get a MSD Blaster coil. Cost is about $35. If it is not the coil you will have a good spare. Sounds like coil or CDI.
Check fuel volume/pressure cold and hot.
Other areas to check if you rule out fuel and ignition.
Elec ignition switch.
Engine harness/conections.
Vacuum leaks.

It comes from long experience with German electrical systems. Cars before the 964 were prone to corrosion and weird problems mostly derived from having the car's prime mover from an electrical standpoint located 2" above a hot vibrating engine.

Look closely at a 964 electrical system sometime and you'll see evidence of a LOT of engineering time spent excluding moisture.

That said, it's still a better design than most airplanes. Case in point, Porsche sends current from the alternator to the starter and then uses the big cable for the starter to carry current to the battery. If it were an airplane, there would be a separate cable from the battery to the electrical bus and then another heavy cable from the bus to the starter.

Heat, moisture and vibration are the enemy of electrical components and pretty soon they fail.

My money's on one or more fading plug wires. Ohm up to the problem.

Brian