Hard acceleration and now 3.0 misfires seemingly randomly.
 

Thanks for responses - checking Search for similar symptoms for other '78 SC owners...

Car has been running great. However, hard acceleration on highway, passed some vehicles, and now a hesitation/misfire seemingly at random. Any rev range, and also at constant rev/speed.

I have:
= Checked for oil in sensor plate/airbox (blowby?) = none; clean

I will:
= Replace fuel filter (~2.2K on it now)
= Replace all plugs & check wire connections

Could I have jumped a tooth / bad timing? Would the car even run? Other than swinging the car across a lane (stir up fuel tank detris?) I cannot think what was any different from any other hard acceleration I've done before.

Thanks in advance for your thoughts on this.

Injector seal?
 

You may have ruptured an injector seal. Similar thing happened to me this time last year during a spirited drive early in the driving season. Gave me a piffing or weird put-put out the exhaust at random intervals. Wasn't too complicated or costly. If that is issue or similar, a good porsche wrench should be able to find issue pretty quickly. I wouldn't throw parts at it.

Did a quick check of the injectors in their seats and they seem OK - would I expect any visual clue?

No recent changes BTW - just the acceleration.

Do you have that protective cap on the coil? Spark can jump through the coil boot and ground to one of the little primary lugs.

Also, are you running a fat gap on your plugs? That can make the spark find an easier ground before the plug.

Wire condition?

Do the late night light show trick. Dark as can be. Let your eyes adjust. Look in the engine compartment while idling. Bet you see some arcing.

I'll second the darkness test. Its free and many years ago it helped me determine my wires were shot after a misfire surfaced that the shop couldn't track down. New set of wires fixed that one.

But on injector seal failure, there was no visual clue or looseness. To have a good shop diagnose and fix was something like $200 but it was the injector next to the ac compressor (very easy to access). Pretty cheap fix and car was back in a day.

Some free trial and error/ investigation sounds fine. My point would be don't throw parts at what might not be a huge issue in the end that either yourself or a shop can fix pretty easily.

Okay, time to close this thread.

The problem has not returned - it simply "disappeared". My hunch is that I had fuel starvation of some sort - it was a hot day, 1/2 tank, and plenty of hard acceleration. I need to find out why so will pursue that while also minding the advice above about arcing wires.

On a sidenote, this caused me to finally remove the A/C. Wow, so glad I did - engine cleaned up remarkably. Next step is the heater backdate.

Many thanks to all who read and replied.

random miss does not = timing change. by random do you mean it runs fine then all of a sudden it has a miss?

as far as injector seals go they are VERY easy to change and cheap. if they are old just replace them. takes about 30 minutes.

i saw this thread when you posted and tought stay away, just TOO many things it could be and not a lot to go on.

fuel starvation is something you will notice with hard acceleration, when it needs a lot of fuel.
you can always do a flow test on your fuel pump. also, try blowing thru the old filter, see how clogged it is.

from what little we ahve to go on i think i would try to make it do it again. run it hard for a bit and if it does not do it, i would shut the engine down after a good hard acceleration, no idle, and pull the plugs. i might even jsut go ahead and change them and do it again. after all, who does not like to go out and run their car.
just thinking maybe you fouled a plug.

i also thought maybe you knocked some carbon lose and maybe stuck in a valve. but that would be more of a consistant miss.