81 SC dying at speed

[mzavada]

Sorry in advance for a longish question, however I'm working on a head scratcher of an issue on this one, and would appreciate any thoughts/help you all could lend while I try to chase this one down:

What it is doing (approx 3 times so far):
At highway speed, vehicle shuts off completely, with no warning. Meaning: cruising at 60-80 mph under power, then...nothing. No hesitation, no sputtering, no shuttering, no surging idle, not a thing...just off. Completely off. The only initial indicator is the sudden lack of engine noise, and slowing of vehicle. Note: the 3-4 cases of this doing it are days a part with NO driving on the days between.

What didn't work:
2 times popping the clutch while coasting restarted the vehicle and I continued on...not the 3rd time. Vehicle came to a rest, and turning the key resulted in starter turning over engine, no audible indication of engine trying to fire.

Roadside repair attempts and then started again:
After 5+ minutes and a few failed attempts to start it, I replaced the red relay and one random fuse for the blower motor that looked a bit worn. (doubt the blower was part of the issue, just addressing while in there) The car fired up on the first try and got me to work/home that evening with no further issues. Somewhat makes me think of vapor lock symptoms, but would that happen before it is at temp on a CIS vehicle?

Other thoughts:
This is now the 3rd red relay I've had in the car since fall. Replaced the first one after the vehicle wouldn't start after warm. Second one was replaced the other day when I started having these issues, now on the 3rd relay.

Conclusions/questions:
Seems to be either fuel or electrical, but where do I start with that wide range of variables. Concerned that it isnt likely that 3 relays all going bad...that there is an underlying issue that manifests (coincidentally) with the relay replacement.

Long story short: what do you all think I should start by further investigating/replacing/etc. Thanks so much for your help on this one!

[walt]

Faulty ignition coil maybe. Checking the primary and secondary winding resistance may not help as you may have an intermittent short. If it's silver and made in Brazil just replace it but I don't think these were installed on SC's.

[mzavada]

Thats a good point, I can check the coil, however I replaced it about 3 years ago with a Flamethrower. (Doesnt mean its not faulty, of course...)

[Bob Kontak]

Perma-Tune CDI box?

[Mark Salvetti]

How to test rev limiter on SC (CIS)

Mark

[mzavada]

Originally Posted by Mark Salvetti

How to test rev limiter on SC (CIS)

Mark

This is interesting...I hadnt considered, and of course I probably dont recall what RPM I would have been at.

Ignorant question. Suppose the limiter DID cut off ignition...would it have trouble restarting for some reason? Logic tells me that it should restart without issue, but you know what they say about logic and reason...

[pmax]

The airflow safety switch at the back of the CIS metering assembly also cuts off the fuel pump by grounding the circuit (See pic in Mark's thread above How to test rev limiter on SC (CIS)).
Try disconnecting it temporarily to see if it is the culprit.

[LIRS6]

Have you checked fuse contacts? .. Maybe a critical fuse has poor contact. Happened to me, although I was not at speed; was about to park my car at a restaurant when engine just shut down. Nothing got it started. A few people helped me push it into a spot. I went inside, enjoyed a nice steak and a cab ... went out to the car and fiddled with the fuses, rotating some of them. Started up, no subsequent issues.

[Bob Ashlock]

Verify the plug is secure into the CDI unit. Verify the green coax cable coming out of the distributor is not the culprit. With engine running, gently manipulate that little cable to see if the engine stutters or dies. Finally, what you describe could be the CDI unit itself. A common symptom of near failure is that it dies with some driving, but can restart immediately or after a short rest. Substitute with a known good CDI box to at least eliminate that as a potential problem. I have a loaner that I routinely provide to people for the cost of shipping to try/troubleshoot. --Bob

[Mark Salvetti]

Originally Posted by mzavada

This is interesting...I hadnt considered, and of course I probably dont recall what RPM I would have been at.

Ignorant question. Suppose the limiter DID cut off ignition...would it have trouble restarting for some reason? Logic tells me that it should restart without issue, but you know what they say about logic and reason...

It doesn't matter what RPM you were at. When the module fails, it stops the fuel pump, regardless of the RPM you were running. It could happen at idle.

When mine failed, sometimes the car would immediately restart, but most of the time I had to wait a bit for things to cool for some reason.

Just unplug the white connector and see if the problem goes away. If it does, then you got off easy. If the problem continues, then you have more work to do.

Mark

[Bob Kontak]

Originally Posted by Mark Salvetti

It doesn't matter what RPM you were at. When the module fails, it stops the fuel pump, regardless of the RPM you were running. It could happen at idle.

And it will sputter and die. OP is not experiencing this. Instantaneous engine cut out.

[Brian Fuller]

Mark is right. It's been a while but as I recall on restoration of a 78' SC, my rev limiter was defective. I disconnected it from the harness and the car ran like a top. My issue as a little different than mzavada; once you plugged the rev limiter back on, the fuel pump got no ground, hence no electric charge and the car would not even start.

[Mark Salvetti]

Originally Posted by Bob Kontak

And it will sputter and die. OP is not experiencing this. Instantaneous engine cut out.

Sure, at idle. But at speed, it's pretty abrupt.

Mark

[mzavada]

Thanks everyone! You've given me a great list to start combing through!

I got myself organized tonight, and will start figuring these out tomorrow night!

[Bob Kontak]

Originally Posted by Mark Salvetti

Sure, at idle. But at speed, it's pretty abrupt.

Mark

I have experienced it under acceleration. It's brutal.

Assuming just cruising and fuel pump cuts out it would be similar to the red relay breaking connection.

[Nditiz1]

I maybe experiencing the same issue with my 82 engine. I don't believe I have a rev limiter since my car is a 77, but not sure about airflow safety switch. I am leaning towards the coil/CDI. Car starts up and runs great, idles smooth. I haven't driven it yet since the engine swap. About 20 mins when temp is up around 180-190, sputters, backfire, stalls.

Run it in your driveway for about 20 mins to see if it happens then. I have a feeling it will.

[boyt911sc]

Originally Posted by Brian Fuller

Mark is right. It's been a while but as I recall on restoration of a 78' SC, my rev limiter was defective. I disconnected it from the harness and the car ran like a top. My issue as a little different than mzavada; once you plugged the rev limiter back on, the fuel pump got no ground, hence no electric charge and the car would not even start.

Brian,

The presence of a ground contact (terminal #85) in the FP circuit shuts off a running FP. Installing back the Rev limiter with an open ground to the car will have no effect to the pump (normal). The three (3) sources of ground for terminal #85 are:
1). AFS switch.
2). Rev limiter.
3). Car alarm/immobilizer.

You got it backward. Putting back a defective Rev limiter (grounded) will energize the NC (normally closed) 87a~30 to 87~30 (normally open) with the ignition switch @ RUN. The problem with your Rev limiter was it was grounded causing the coil inside the FD relay to switch off the FP.

Tony

[mhackney]

@mzavada - that "to do" list is over the top!

[Nditiz1]

Timed it today. Idle in the garage. Everything smooth with warm up and settled around ~950. 21 mins in sputter, loss of idle, backfire, dies.

So I would say very similar to your issue. temp was up around 180 - 190

[boyt911sc]

Originally Posted by Nditiz1

Timed it today. Idle in the garage. Everything smooth with warm up and settled around ~950. 21 mins in sputter, loss of idle, backfire, dies.

So I would say very similar to your issue. temp was up around 180 - 190

Mzavada,

Nditiz1’s CDI was sent to Bob Ashlock for evaluation. And Bob found a cracked solder @31/1 terminal. As the CDI warms up from the engine heat, the cracked solder opens up and lose ignition signal causing the motor to stop running. When the motor is allowed to cool, the cracked solder regains its contact back and allowing the CDI to operate again until the next cycle to shut off. Have your CDI tested.

Tony