My SC Backfired and Died

[mca]

Thank goodness this happened within 10 miles of my home because I was on the tail end of a 2 1/2 hour Sunday drive. Could have been much worse b/c the wife had to bail me out.

Anyhow, I was doing some easy cruising and the car backfired and died. This was exactly the same thing that happened when I was testing a CDI for a buddy to see if it was good - backfired, died, swapped back in my original CDI, kept going.

My wife picked me up and I went home to grab my tools / parts (I used to carry them in the car but everyone made fun of me and I caved to their teasing ways - you know, too much extra weight to carry all that stuff). Got back to the car and did the following:

1) Swapped in a spare CDI (previously tested and in good working order). Still no start. Pop off valve was closed and seated properly.

2) Swapped in a brand new Bosch coil. Still no start.

3) Swapped in a new fuel pump relay. Still no start.

4) Swapped in a new 02 sensor relay. Still no start.

5) Pulled dizzy cap. Looked fine. Still no start.

6) Turned on ignition and lifted sensor plate. Heard fuel spray. Still no start.

The car turns over just fine. Seems to be plenty of electrical juice. Battery is an Optima red top and is only a year old. Alternator was tested last year and is working fine.

I had to leave the car in a parking lot and I'd like to do some further testing tomorrow morning before having it towed to a shop. I can't work in my current garage b/c I am temporarily living in an apartment complex. Garage is TINY.

I am a bit scared of testing for spark. Never done it before and would hate to try it the first time without someone there to guide me.

I do have CIS guages so I could test the fuel pressure - see if pump is bad. The pump is only 20k miles old. Other than that I am at a loss. I pretty much covered all of the possible roadside fixes.

Thoughts? Ideas?

[Hugh R]

Sorry to hear. Sorry, don't know zip about SC's. Do you have that blow off valve thingie on your air box.

Spark, pull a plug wire at distributor hold end 1/2" or so from terminal and have someone crank the car, you should see a fat blue spark. If you get zapped, it's not going to hurt, just a jolt, no pain, really!.

[billybek]

I will contribute the standard "green wire of death" comment....

[ossiblue]

I'm not familiar with later cars, but from reading other's posts there may be a sensor on your car--I think it's a "flywheel positioning sensor" or something like that--that may be bad. Someone will chime in and clarify.

As far as testing for spark, get a spare plug and pull the connector from one of your cylinders and connect it to the spare. Ground the base of the plug to something in the engine and have an assistant crank the engine. You should see a strong blue/white spark. You can also connect a spare plug wire to the spare plug and plug the wire directly to the center of the coil. Either way, it is a safe way to test for spark as long as no fuel is present in the immediate area.

[VaSteve]

Originally Posted by ossiblue

I'm not familiar with later cars, but from reading other's posts there may be a sensor on your car--I think it's a "flywheel positioning sensor" or something like that--that may be bad. Someone will chime in and clarify.
.

No. Not in an SC.

Seems like it "jumped timing". Not sure how that would happen with a timing chain and not have messed with the wires, and especially while moving.

mca, pull the cap again and really look at the rotor. See if it moves independent of the distributor as if I might have bound up and the the center shaft broke allowing spark to go to unintended cylinders.

Did you do any work to the car recently? Maybe disturb the green wire as billy thinks?

[vitalstat]

Does your car have a popoff valve installed in the air cleaner box, if it does it may not have sealed itself properly, check it this happened to me once. If not a backfire can crack the air box and it will have to be replaced.

[mca]

Originally Posted by VaSteve

mca, pull the cap again and really look at the rotor. See if it moves independent of the distributor as if I might have bound up and the the center shaft broke allowing spark to go to unintended cylinders.

Did you do any work to the car recently? Maybe disturb the green wire as billy thinks?

Good idea to check the rotor more closely. I simply pulled the cap and glanced inside. Didn't spend much time here b/c cap and rotor are relatively new.

I did find that the rear clip of the dizzy cap came off and the cap was loose. But that probably happend when I pulled the coil wire during the coil swap. If it happened while driving, then the rotor could have bound. I'll take a look at this first.

No, haven't done any work in that area recently. I rebuilt the engine about 16 months ago. Went through everything during this time. So all CIS seals are new too.

[mca]

Originally Posted by vitalstat

Does your car have a popoff valve installed in the air cleaner box, if it does it may not have sealed itself properly, check it this happened to me once. If not a backfire can crack the air box and it will have to be replaced.

Yes, forgot to mention that. The pop off valve was in place and closed. I verified while checking the sensor plate.

This backfire was not major. Not mind blowing or anything. But sufficient.

[theiceman]

could be your fuel distributor if your air box isn't cracked .. sounds like it went lean. if your seals are all good and you don't have unmetered air entering ( cracked air box ) could be not enough fuel . My 78 has no O2 sensor or lambda circuit but could this cause you to lean out also ? CIS is usually ROCK solid once set up.

[VaSteve]

Originally Posted by mca

I did find that the rear clip of the dizzy cap came off and the cap was loose. But that probably happend when I pulled the coil wire during the coil swap. If it happened while driving, then the rotor could have bound. I'll take a look at this first.

Start there. If the cap was loose, something made it loose (pressure from the plug wires on the air ducting...poor seating after the coil swap). If the rotor bound up, that would/could knock it off. The bakelite doesn't stand up to significant revs and cracks. The fact it won't start now makes me think the dizzy is turning and the rotor is not.

When you say it "doesn't start" what does it do? Crank? More backfires? Nothing?

[mca]

Originally Posted by VaSteve

Start there. If the cap was loose, something made it loose (pressure from the plug wires on the air ducting...poor seating after the coil swap). If the rotor bound up, that would/could knock it off. The bakelite doesn't stand up to significant revs and cracks. The fact it won't start now makes me think the dizzy is turning and the rotor is not.

When you say it "doesn't start" what does it do? Crank? More backfires? Nothing?

It turns over and sounds like it wants to start. No backfires.

The cap could have come off by simply not being tight enough. Those holding clips are adjustable. Mine weren't incredibly snug. And these engines do vibrate quite a bit. Very possible scenario.

Tomorrow morning I will check the rotor again and then perform a fuel pressure test. My system pressure was at the bottom end of spec - 64 psi I believe on a 70ish degree day.

Also, I believe my WUR is original. Maybe it failed.

[haycait911]

FWIW - I would make absolutely certain it's not an ignition isuue ( which, in my experience, most things are ) before diving into the fuel system. an inductive timing light is handy for checking for spark and timing as well. Don.

[al lkosmal]

+1 on the popoff valve. Check it 1st...it's easy to detect and fix.

Regards,
Al

[aadrew10]

I would be sure to test for spark. That's a good indicator of which way to look. I've been shocked through thick mig welding gloves while testing for spark before. Make sure you get the edge of the plug firmly on a ground somewhere. If you get shocked, it won't kill you, but you'll know something got you.

- Andrew

[-84 Carrera]

I am to be considered a beginner in these matters so please do not laugh to much. I am not sure how it works on an SC, but I used to have an old VW typ1 transporter. On that car the bottom of the distributor shaft had a nasty habbit of jumping out of its groove resulting in 180 degree jump of the rotor and a massive bang in the exhaust due to non ignited fuel being pushed out. As I said, maybe won't happen on a Porsche, but the thought did occur to me.

/C

[T77911S]

hey craig sorry to hear that.
you can put a spare plug on a wire and rest it on the motor and have someone turn it over as said, or you can put a timing light on the plug wires to test for spark. try another rotor if you have one. what about the dizzy itself?

check the rubber boot that goes from the throttle body to the FD, make sure it did not come off, especially in the rear, hard to see. or any other hoses that might cause a big air leak.

[ossiblue]

I'm with VaSteve on this one--start with the ignition (no pun intended.) Fast and simple to check for spark, followed by timing.

[1990C4S]

Originally Posted by ossiblue

I'm with VaSteve on this one--start with the ignition (no pun intended.) Fast and simple to check for spark, followed by timing.

Exactly, spark and timing. If you have a spark get the engine to TDC and see if the rotor is somewhere close to where it should be. This should be step #1 IMO.

Fuel would be next. Pressure gages will help if you get past the spark/timing issue.

[mca]

Well, I ran out to the car this morning and did some further inspection.

I found that the rotor was cracked. It did some damage to the cap as well.

I replaced the rotor and cap then fired her up. She started immediately but ran at low rpms. I shut it down and then restarted and got a backfire. Saw some smoke in the intake - didn't smell like oil or gas, but more like burning electrical.

Upon further inspection I discovered that I had crossed my wires on the cap. #5 and #6 were accidentaly swapped.

Tried to start her up again and no luck. Basically the same symptoms that I saw last night. My guess is that the battery is too weak or the plugs are fouled. Didn't have my volmeter with me and didn't have time to run back home and grab more tools.

So I had it towed to a very trustworthy local shop.

Here is a pic of the rotor.



Given that this rotor is only 16 months old / 8,000ish miles, I am surprised to see this happen. There are two possibilities: 1) the cap came loose and destroyed the rotor 2) the rotor broke and knocked the cap loose.

I'll keep everyone posted as I get info from the shop. Needless to say I was pretty pumped when I found the busted rotor - ez fix afterall. But now I am stumped. Hopefully it is just a battery issue at this stage.

[sailchef]

Same thing happened to me. I swore I checked and rechecked the pop off valve.

It had developed a small hairline crack in the sealer I used to hold it in place.

Help-Violent backfire- now engine runs rough

Look at the screws holding the air box to the manifold, they may be loose if the box blew.