Help Request: '78SC Smoke out intake, Quit running. It's not the CDI!

[b. hilliard]

Hi all-

I'd loves some help diagnosing. Apologies for the novel, but unfortunately the car broke down on a road trip and is currently 200 miles away, so repair work attempts have been intermittent. I'm trying to get it running again for the trip to Watkins Glen Historic weekend soon!
Here's the details:
1978 911SC, 170k miles, owned it for 24 years. Lots of seat time.
Prior to road trip:
-New Beru cap and rotor. Basic maintenance always up to date. Historically has been rock solid and reliable for long and short trips. Within last few driving seasons preventative fuel pump, filter, accumulator, injector seals, cleaning all WUR connections, a few things I'm forgetting, etc. Pop-off valve installed. Older battery but always good and strong.
During Road Trip:
-Warm day, approx. 175 miles into trip. Tach needle bouncing some but has been for at least this driving season maybe longer. Oil temps a few degrees above what is normally 9 o'clock on the gauge (assumed hot/ humid responsible but hindsight maybe not), steady 75 mph. Felt a hesitation/ cutout that I've never felt, seemed to catch and continue. Then a colossal backfire out the exhaust (SSI's, Bursch muffler) that was like a shotgun. More hesitation. Pulled off highway, found nothing, cooled off, started again and continued. 10 miles later, more hesitation at cruising speed, backfire, pulled off and let cool. Continued on secondary roads, temperature slowly rising a bit above 9 o'clock, started to climb a hill in 2nd gear, hesitation then a colossal explosion of white smoke out the intake, engine audibly dieseled for split second and quit. Properly scared at this point I towed it to a safe storage location and left it there.
Return and Repair/ Diagnosis
-Pulled all plugs, turned engine by hand, all felt fine. Installed new plugs
-Pulled distributor and replaced green wire. Distributor pickup coil Ohms out within spec
-Procured known good CDI, installed and got a fat spark while cranking over.
-Pop off valve and airbox seem intact and healthy
-Brought an old school oscilloscope and probed pickup coil while cranking. Showed approx 5 volts peak to peak.
-Coil resistance also within spec. Old school black Bosch coil.
-Older shielded/ grounded Beru plug wires, Ohm'd out fine.
Started Car:
-Idled a bit rough and seemed out of sorts compared to normal but thought perhaps it was cleaning out old fuel from original failure.
-Tach needle bouncing like crazy.
-Drove in first/ second gear, never above 2200 rpm for about 1/2 mile uphill and could tell it wasn't quite right, turned around and it hesitated/ lost power while coasting, more white smoke out intake, dieseled for a split second and quit. Properly scared again, it's back in storage.
So, I am returning this weekend for round 2. Can the experts chime in on what I should do next? Thank you in advance!!

[kmcteer]

Hmm, my first thought was your timing shifted. I assume you set it when you RRd the distributor? Is your distributor mechanical and vacuum advance acting right? Definitely take a close look at the Green Wire (routing, pins in connections)

Maybe do a sanity check on your CIS system. Is your fuel distributor Needle dropping after you lift and drop the air flap? Pull the injectors and do a fuel flow check at various flap levels. I personally wouldn't check absolute volumes, just that it is acting right.

Other idea, engine/transmission grounds?

[kmcteer]

duplicate

[trials935]

Look carefully at the new distributor cap orientation and locating tab, my customer was sold a cap that when installed properly was clocked wrong. Also rotor button was bubbled on top of the epoxy fill between the center and end brass piece, the internal resistor had overheated

[b. hilliard]

Thank you @kmcteer, in my rush I did not check the timing after reinstalling the distributor. I did mark everything and put it back as such, but it doesn't mean it's quite right. I can check the advance, but I believe it's also working correctly. I feel like even if the timing/ advance was off slightly, it may not run perfectly but likely wouldn't just poop it's pants after 20 mins of running?
The new Green wire checked out from distributor to the first connection (old one not great, male connection had deteriorated) but will have to double check from that connection back to the CDI.
I'll look back into the CIS as you suggest as well.
I didn't look at the eng/ trans ground but will. I know there's one by the transmission, any other major ones that might cause issues with running?
@trials935 great point. I'll look, and unfortunately I think I threw my old one away! I had done a few hours of driving on the new cap/ rotor before the road trip, so it was working for a while....

[nickelplated5s]

Oil leak into the intake.

[b. hilliard]

Hi nickelplated5s, interesting thought. Where would I look for that to see if it's a problem? Could that cause it to shut down/ backfire/ etc?

[nickelplated5s]

Tow it home and try a compression/leak down test. You stated plugs are cleanish so my guess is rings, valve stems or seats i.e. something internal. Not going to be fun.

[Bobboloo]

Appears to be an electrical problem. Losing power and then getting it back followed by a backfire in the muffler is a tale tale sign. Un-burned fuel got ignited after the fact in the exhaust = boom.

The Beru wires you have are notorious. Find a replacement set you can temporarily use to replace them for troubleshooting. Also, there is a good possibility your timing is off so check the timing with a timing light.

[rwest]

A bouncing tach is not okay. I think if you figure out why it is bouncing, you will solve your problem.

Maybe do a search and see if you can find another person who had a bouncing tach and how they solved it.

[b. hilliard]

Thanks @Bobboloo, I'm leaning towards electrical issue as well. It seems like an ignition/ spark cutting out (I'm assuming unburnt fuel is continually dumping and accumulating either in the hot exhaust = backfire, or in the intake = pop off valve venting ignited fuel and lots of white smoke out the intake) is what is happening.
Can you expand on the Beru wires being notorious? I've always assumed they were a good product, although they have been in for probably 40-50k miles.... but I'd need to check my log book which is with the car. I don't have access to another set to swap in, perhaps I should buy new ones to eliminate the issue? I see the prices have gone, well, UP! Is there a manufacturer that's trustworthy, but less than the $850 Porsche is charging for their OE?
Will reset the timing with my light this weekend too.

[b. hilliard]

Thanks rwest, I feel like I've heard that too regarding the bouncing tach but cannot figure out what the various symptoms/ solutions are. Even with the known good CDI that I bought and swapped in the tach bounced, although it produced a nice spark where the the original CDI did not. So the car ran, but not for long!!

[rwest]

Have you tested your voltages at the battery at rest and when the engine is running? Also check what the voltage is going to the CDI.

[b. hilliard]

rwest I have not done that since this issue reared it's head, so I will add that to the list. Thank you!
Can a malfunctioning alternator or weak battery cause major issues similar to what is happening?

[Bobboloo]

The last thing you did before this problem was change the cap and rotor. That's the first place to look. Make sure your plug wires are securely connected.
You should also inspect the ends of the sparkplug wires where they connect to the sparkplug for any that have discoloration and corrosion to pinpoint a problem wire.
If you need new wires, Clewitt Engineering sold by Pelican are a great choice.

[b. hilliard]

Excellent, thank you Bobboloo. All great thoughts. I'm really starting to think the cap/ rotor could be an issue as it is responsible for firing ALL the cylinders which seem to cut out at once. It's not an intermittent misfire, it's a full shut down and then igniting all the unburnt fuel in the wrong place.

[Bobboloo]

Good datapoint on all cylinders cutting out.
Check the wiring from CDI all the way to the distributor to make sure everything is seated properly.
6-pin connector
Green trigger wire
Coil to distributor
12 volts and ground wires on coil
12-volts to CDI
Ground to CDI
(You can turn ignition to run position and listen to hear if the high pitched CDI whine cuts in and out as you giggle wires if that helps you to troubleshoot)

[Bobboloo]

Maybe switch back to original cap and rotor if nothing catches your attention.

[b. hilliard]

Unfortunately I chucked the Bosch cap/ rotor that was in there previously, which is rare for me. Typically I keep functioning old parts around!

[rwest]

Look at this current thread- seems like one poster had bouncing tach from a bad voltage regulator.

http://forums.pelicanparts.com/porsche-911-technical-forum/1182699-voltage-regulator-alternator.html