No Spark After Rebuild…Distributor Cap Issue?
 

First attempt to start my ‘87NA after a complete engine rebuild (plus lots of other stuff), while the car was on my lift for 20 months. Result? Good oil pressure, good tach bounce, fuel being delivered as evidenced by spark plugs smelling gassy when removed. But apparently no spark. Ugh! (Diagnosing electrical issues is not my strong suit).

I did replace the foam gasket between the distributor cap and cam tower…and have since noticed that I can rotate the cap slightly from side to side, whereas before replacing the foam gasket, the cap seemed more or less rock-solid in its position. I also suspect that this new gasket, in its not-yet-compressed state…is causing the distributor cap itself to be held at a slightly greater (than specified) distance from the cam tower. At any rate…I’m thinking that the resulting “out of spec” positioning of the distributor cap may have something to do with the current “no spark” (no start) condition. Does this make sense?

Another question: Is there a correct orientation for the distributor cap? Seems like I can install it either way up…but maybe there is a “rightside up” orientation?

Why not check for spark at the coil wire? You should get four times as many as you would from a plug wire. You can also look for the negative pulses at the coil.

The small issues with the cap and rotor IMO would not prevent spark from getting past the cap, that spark will jump 10mm easily if the ignition system is up to snuff. To be able to jump 0.7mm in the head under all that compression it needs to be able to jump a lot farther in free air.

Yes...I will test the coil wire.

Hopeful that this will tell me something...because if not then I'll need to get up to speed with a multimeter - which does not fit my brain nearly as well as something mechanical, or at least something physical I can see actually happening...like a spark.

The PO had replaced the plugs/wires/distributor/rotor shortly before I purchased the car from him - so I think the issue lies further back. Coil is original...but my understanding is that these don't fail that often. But the coil test makes sense.

So...I'll attach the coil wire to a spark plug and, with insulated pliers, hold the threaded portion of the plug against something metal on the car (like the chassis? Any other good places?), have someone (my understanding/supportive wife) turn the key...and I should see a good spark. Sound good?

a bit later: no spark at the coil...so at least I know something!

Funny thing happened...as my dear wife turned the key, a loud "pop" was heard - flipped my wife out just a bit! But what I saw...was that the naked spark plug boot (belonging to the plug with which I was testing), which I'd pressed back into the empty spark plug bore...popped out spectacularly with compression!

That can happen if air pressure builds up when the boot is installed, you can put a WD-40 straw or something in there to prevent it. Or maybe you have a loose plug and are leaking compression. More typically it's air pressure from the spark plug boot going down the bore, and heat from the compression.

Don't confuse any brown staining on the plug insulator with a compression leak. That's corona stain.

Maybe a crank trigger sensor is going out, I had an 84 that the sensor went out and left the car sparkless..

Speed and reference sensors are both new and the tach bounces nicely. But is there a crank sensor additional to these someplace?

I’m away from home this weekend but when I return I will try to isolate the coil to test it. Would be amazing if I could simply replace a bad coil…one can always hope!

If there is no spark at the distributor main lead, proceed with ignition troubleshooting, especially testing the coil as described here...
Ignition System Troubleshooting

If the coil passes, the next step would be to Check engine cranking / running signals (scroll down)
Troubleshooting Fuel Supply Problems

If your sensors are new, other possible causes could be the solenoid wire, also one of the sensor plugs is not fully seated, or even a bad DME...

If your tach is bouncing, you have eliminated the flywheel sensors and the DME as the causes for no spark (since the tach is directly wired to the coil and the DME).

If you are still getting no spark at the end of the cable between the coil and the distributor cap,it is either the coil of the cable.

Chk coil and see if it is in spec (ohms test of the low and high sides)

https://www.pelicanparts.com/techarticles/Porsche_944_Turbo/31-ENGINE-Coil_Testing_and_Replacement/31-ENGINE-Coil_Testing_and_Replacement.htm

You should also chk ground and the continuity and then resistance of the wire back to the DME.

I would post up a link to Clarks as I have a pin out for the DME connector posted over there but the forum is down at the moment due to the sessions folder being full and I am away on vacation and don't have access to it at the moment

Here you go looks like pin #1

DME Pin Item
1 Ignition Coil
2 Closed Throttle Switch
3 WOT Switch
4 Starter/Key Switch
5 Ground, Shield for Pin 25 & 26
6 AFM Return
7 AFM Out (UV)
8 Speed Transmitter
9 AFM +5V (UP) [+8V – 12v (UP)]
10 Performance Curve
11 Tachometer
12 Test Socket A
13 Temperature Sensor
14 Fuel Injector, Cyl 3&4
15 Fuel Injector, Cyl 1&2
16 Ground
17 Ground
18 12V - DME Relay
19 Ground Brown –
20 DME Relay – Return (fuel pump)
21 Tachometer
22 AFM Air Temperature
23 Ground Shielding For Pin 8, 24 & 27
24 Oxygen Sensor
25 Reference Mark Transmitter
26 Reference Mark Transmitter
27 Speed Transmitter
28 Ground Altitude Compensation Switch
29 A/C Compressor (Pos +)
30 Altitude Correction Box
31
32
33 Idle Positioner – 3
34 Idle Position – 5
35 12V - DME Relay

Oh and I remoted in to my home PC and reset the sessions so the forum on Clarks is back up

Thanks George, good to know!

Still away from home posting with my iPhone, but truly appreciate everyone’s responses. Will get back into it on Tuesday and start tracing wires and testing for continuity and resistance. Thanks again!

This can happen if the spark plug isn't tight enough, as pressure from the chamber bleeds into the area where the spark plug boot is and causes it to pop out.

No...that spark plug was out of its socket - being used to test spark - but I'd pressed the spark boot back into its empty bore, and it popped out with engine compression when the key was turned.

Update: Primary coil resistance - .3 Ohms (should be .4-.6). Secondary: 5.2 Kohms (within range).

Maybe OK...but the low primary resistance could indicate a developing short in the winding, so I've sprung for a new coil (will come in tomorrow). Also a bit of corrosion present on coil terminal #15 (positive terminal), but I have a sense that the new coil, with a clean terminal, will still not fix this (no spark) issue...but I'll find out tomorrow!