911sc engine hesitating after an hour of highway driving

[Mattblacksmith]

Hey Guys,

I'm having an intermittent issue with a '78 911sc. What happens is the car runs really well for the first 45 minutes to an hour then it will start to hesitate slightly around 3K revs when under load (loss of power, jerking delivery of power). If I continue to drive it gets progressively worse, over a wider rev range (2.5k and above) until it is so under power it is undriveable. It may also backfire through the airbox, though this doesn't always happen.

If I pull over and let it sit for an hour or more it will go away and the fault wont appear for another hour or so as though it was starting fresh.

It only seems to affect the engine under load, if I stop on the roadside when the fault is happening, it will idle perfectly and I can rev the engine freely through its range without any hesitation or misfiring but when I set off again it will be right back to the fault condition again.

I have had the fuel system checked over and the fuel distributor rebuilt. Injectors cleaned and warm up reg checked. It had new spark plugs put in only a few months ago.

I'm thinking it may be related to the ignition controller, coil or spark in general. It could always be fuel but after having the rebuild I'd hope thats less likely. I don't think its the warm up reg because the fault starts long after the oil temp is at working range. I also don't think its vacuum related because if I take off the oil cap the revs drop and then restores to a good idle after replacing it.
All gauges read in good working ranges.

Has anyone seen this kind of fault before?

Thanks and all the best,
M

[Bob Ashlock]

Possibly a problem with the CD Ignition box. Long term degradation of its internal discharge capacitor is a common problem. As the part heats up, the internal foil layers of the capacitor start losing contact with the terminals at each end of the part. As the part cools, contact is re-established. The result, initially, is lower spark energy and eventually complete failure.

[theiceman]

i would say CDI box also ... and if you have a silver bosch coil , that is always on the suspect list.

[917_Langheck]

What are the status of the fuel pump and pressure regulators? Is it possible that you are getting heat saturation of the fuel lines in the engine bay, and while cold the pressure is strong enough to supply fuel, but when hot not enough to overcome vaporization, leading to the lean mis-fire ("backfire through the airbox"), and when allowed to cool, all works again?

[Bill Douglas]

Matt, only I'm silly enough to make this mistake, but it's easy to check.

I had cleaned the inside of the distributor car and when I refitted it the plastic bit was in the wrong spot resulting in the cap not seating properly. My car ran fine for an hour then missed under load. Once cooled down it went well again. Strange symptoms for a distributor cap indeed.

[icarp]

simple but effective if it is the cause
please remove the gas cap when the car stumbles
restart if you can
just an idea
had a customer car with that same problem this week, all vent and vapor hoses got replaced
ian

[T77911S]

start with the coil, its cheaper.

[Schulisco]

Also from my side...either the coil or the CDI Box. The coil is more likely. I would cross check it with a heat gun and ice spray to warm up the coil and cool it down. When you're warm the cold coil in the garage with the heat gun and the engine then shows the same symptoms as during the test drive, cool the coil with the ice spray and test ist again. If the coil is not the cause then more likely the CDI Box...

The MSD8222 ist a good replacement. Not all coils are capable for overhead mounting, especially the oil filled ones.

Thomas

[david.avrahami@]

check if the ignition wires tight over the spark plugs.

[Bill Douglas]

As David said. Also you could remove the distributor cap and leads and check the resistance with a multimeter from inside the cap to the ends of the leads. Don't worry to much about the ohm settings, just make sure they are all much the same.

[smadsen]

Exact symptoms I had with my '82 911 about 10-12 years ago.

It's the CD box. The proper diagnosis is to change out for a known good CD box & drive the car for a few hours. If that solves the problem, then send your box out to one of the many places that repairs them. Usually involves touching any faulty (from age) solder joints and replacing the odd flux capacitor.

It's the gradual heat soak in the engine compartment that's causing your problem and it only gets worse.

A competent shop will have a working spare CD box. They will also send your box out for repair. The repair on your box should be under $250.