1980 SC Suddenly Won't Start

[troutmaskreplca]

1980 911 SC that ran perfectly fine, and suddenly stopped.

I am absolutely going to go through the CIS troubleshooting, figure out if there's a spark, etc. And I was already planning to upgrade to the Modern Retrofit CDI+ just for fun, and I have a new fuel pump relay on the way (both from our host).

But something about this is just bugging me, and I'm curious if it can be solved with a little knowledge or thinking instead of banging my head against the thing.

I got into the driver's side door a few days ago to clean and re-grease the lock mechanism and lube the window frame rails. I had the battery disconnected the whole time. It took a few days because I'm slow and because I have been sneaking time for this during busy work days.

I got the lock working nicely, put the window frame back in, and reconnected the power window switches on the driver's side door, and was happy with myself. I figured hm, I should start it up and maybe go get coffee down the street.

But she won't start. Odd, right?

What are the chances of something just crapping out while it's sitting there, battery disconnected, and nothing else going on for a couple of days? I have to think that something really simple is happening. I'm trickle charging the battery just because I want it to be something so simple...

With the key in the 'on' position, I hear a slight whine from the old Bosch CDI unit. I *think* I smell something vaguely burning, like fried electronics, after giving it a couple start attempts, but I'm also thinking that I'm just driving myself crazy and inventing that.

Here's a video of the start action. I'm just really curious if there's something that jumps out at you. Sure, the CDI might have died. Sure, there might be a bad fuel pump relay. Okay, the fuel pump itself might have crapped out. Yeah, there may be a brand new air leak. But just while messing around inside the door for a couple of days? What are the chances?

[troutmaskreplca]

The one thing that comes to mind: I needed to lower the window just a bit during this process, and I miswired the driver's side window switch so that when I turned the key, it started rolling down on its own. I turned the key off and continued working. I wonder... did I fry something?

[GH85Carrera]

The bullet type fuses often have been replaced with the crappy plastic core fuses over the years.

Get all ceramic cores fuses, and a set of the various sizes, and replace them all if they are old. Clean the contact surfaces on the fuse box, and replace one fuse at a time. That often cures a lot of weird electrical issues. As a bonus, you have a set of fuses you can trust, for not much money.

[troutmaskreplca]

Originally Posted by GH85Carrera

Get all ceramic cores fuses, and a set of the various sizes, and replace them all if they are old. Clean the contact surfaces on the fuse box, and replace one fuse at a time. That often cures a lot of weird electrical issues. As a bonus, you have a set of fuses you can trust, for not much money.

This is great advice! The odd thing is that I've been running with the Modern Retrofit fuse panel for a couple of months now, with no issues. They're all modern blade fuses now. I examined the fuses and they seem fine.

When I try to start without the fuel pump fuse, I don't get that big oomph, just basic turning over... so my (probably dumb) assumption is that the fuel pump is running just fine when the fuse is in.

[theiceman]

sounds like you have spark .. almost looks like a blown air box .. it kicks but wont keep running ..

if you lift the air plate do you hear the fuel pump run ? ( dont do it for long. )

[1Bad86]

Start from the beginning, you had the switches off and tinkered around. Car was running when parked, I'd disconnect the window switches and try to start just to see if something is shorting. You did say you smelled something burning. If it was a blown air box you probably would have heard a backfire when you were trying to start it. Hopefully a previous owner installed a pop off valve if that is the case and it's just stuck open.

[troutmaskreplca]

Originally Posted by 1Bad86

If it was a blown air box you probably would have heard a backfire when you were trying to start it. Hopefully a previous owner installed a pop off valve if that is the case and it's just stuck open.

theiceman and 1Bad86 were right: the pop off valve popped out. 30 second look inside the box and there it was, just sitting there, inside the box, about an inch away from the hole. Obviously will need to figure out how to get it fastened back in there, but a gentle nudge of it back into the hole and she started right up.

Is there a sweeter sound than a 911 roaring back to life after you've been contemplated removing the whole damn CIS system? No, no I don't think there is.

Thank you all. Really.

[GH85Carrera]

Be extra happy some previous owner put in a pop off valve. Instant fix is way better than replacing the expensive airbox.

[troutmaskreplca]

Originally Posted by GH85Carrera

Be extra happy some previous owner put in a pop off valve. Instant fix is way better than replacing the expensive airbox.

Count me as a believer now.

[targa80]

If you had a backfire strong enough to blow the pop off valve out of the hole it was epoxied into then you need to look at the screws holding the two haves of the air box together. If they are lifted then you have stripped the plastic threads on the bottom half of the air box and you will certainly have vacuum leaks that will screwup the air/fuel mixture on the CIS system. Below is the thread I posted on my blown air box repair and fault isolating vacuum leaks using a home made smoke device.

https://forums.pelicanparts.com/porsche-911-technical-forum/1081238-blown-air-box-repair-upgrade-spider.html

[troutmaskreplca]

Originally Posted by targa80

If you had a backfire strong enough to blow the pop off valve out of the hole it was epoxied into then you need to look at the screws holding the two haves of the air box together.

…

https://forums.pelicanparts.com/porsche-911-technical-forum/1081238-blown-air-box-repair-upgrade-spider.html

Thank you, and a great thread pointer. I will definitely take a look.

[pmax]

Originally Posted by troutmaskreplca

Count me as a believer now.

Be aware of the possibility that the backfire was caused by the toilet seat cover aka "pop off valve" itself not sealing properly to the airbox.

[troutmaskreplca]

Originally Posted by pmax

Be aware of the possibility that the backfire was caused by the toilet seat cover aka "pop off valve" itself not sealing properly to the airbox.

I suspect (well, and also hope) that this is right. I checked the screw threads on the airbox and did a close visual examination (not a smoke test, so...) and I don't see anything odd.

The epoxy for the valve had hardened and flaked. It was just scratching off. JB Weld and a hundred miles later it seems to be fine. I also don't recall hearing a pop at any point.

What I'm not clear on is when it would have happened. If the valve had popped out on my way home, would I really have been able to drive in without noticing anything? Or might it have popped out during startup?

[targa80]

During startup would be the likely scenario.

[Guardsred911]

there's a multi-plug on the driver's side firewall behind the blower motor if you still have that in the car that can become loose/detached while driving, or parked and causes erratic idle, failure to start and a fair bit of profanity.... just check that it's in firmly...