Bad Ignition or What?

Well, my car is up to it's old tricks again. Frequenters of this bbs have probably read and responded to my previous emails about my backfiring, 1973 911 with Webers. So far, what has worked for the longest is doing a full cleaning of the carbs. That lasted for only 100 miles!

I noticed a few different things tonight, as I've learned to be much more observant about the running (or not) of my engine :

1)The backfiring came out the right side only of my sport exhaust (nice 9 inch flame!), not back through the carbs.

2) The fuel pressure guage flutters a bit where it was rock solid at ~2.5 psi (single Facet fuel pump - small gold box shaped one mounted in the engine bay). Vapor lock?

3) The CDI box was giving its usual "all OK" whine when I heard a electrical like snap from somewhere near the coil or distributor. The whine changed pitch during the snap, which occurred 3 times. I cleaned the cap and rotor and changed the coil with no better running.

This is a new one for me. Possibly unrelated, but to I'll mention these anyway, my cockpit blower fan and high beams no longer work. Could this be related? I haven't checked fuses but I figured the more info I throw out there the better. I'm trying desparately to get this car running perfect for the Watkins Glen vintage weekend, which is a mere week away. Anyone else going? Hope I am!

Thanks,


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Brian Scotti
'73 S Coupe

Brian,

I think you've got more than one problem going on there!

What coil were you using before the POP, and what did you replace it with. OEM Bosch 0.221.121.001 is the only coil that should be used with that CDI-unit! The primary resistance is approximately 0.65 Ohms, and the secondary is approximately 676 Ohms, just in case you want to check old and new.

That small, 3" square Facet pump is not intended for 6-cylinder applications, and it should have been mounted in the original location down on the left side of the transaxle, close to the supply hose! Just for reference, Performance Products has always shown a much larger Facet pump (more than twice as large) for many years for the 911 application ... the little model has never been shown for anything but 4-cylinder applications ... 356,912, and 914!!!

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Warren Hall
1973 911S Targa

Cool. A '73 S Coupe.

Anyway, big explosions out the tailpipe suggest to me that the ignition system for one or more cylinders is turning off briefly, and then turning back on. In the meantime, gasoline gets pumped into the exhaust creating quite a bomb. then it goes off. BOOM. You can do this by turning off the key while driving in gear, and turning it back on. Don't do this though.

Clicking or snapping noises in the engine compartment combined with 'missing' is a dead give-away. A high-voltage ignition (spark plug) wire is grounding the spark against something. These lightning bolts can ground themselves on a metal object, right through the insulation. In the dark you'll see it, a pretty blue.

It could be your cap and rotor. Cleaning them is not very useful, they just go bad again quicker. Replace them.

Anyway, this is my thought and i hope it helps. It all seems consistent. A high-voltage spark, meant for a spark plug, is getting grounded. Snap.

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'83 SC

I knew I forgot some info ... the snap was heard with the engine not running, ignition on. Because the exhaust is so loud, I can't hear what the ignition system is doing at idle or hear any snaps. I wish I had turned out the lights last night and looked for a lightining storm in my engine bay! Also, it seems like the engine hesitates at low to moderate engine speeds, however when I go full throttle, it seems OK. I say seems because when I back off at 5000 RPM, I get a barage of backfires. Cool for the racetrack, but not the street. That's why I've always concentrated on the carbs, but since they're clean, something else is out of whack.

Warren, My intentions have always been to move the fuel pump to that location! I have a larger capacity fuel pump from another car that I'll give a try. The coil I replaced the original with was another original from another problem solving exercise. I'll check both for resistance.

Superman, Your diagnosis sounds good to me. I'll replace the cap and rotor and try to make sure the plug wires are not resting against any metal surfaces.

Unfortunately, I have to go to work now, so I'll check in with you guys later!

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Brian Scotti
'73 S Coupe

NEW Information!!

After reading the two posts, I went out and just looked around the engine bay. I didn't see anything obviously wrong, so I turned on the ignition. It sounded fine, however, I noticed that the fuel pump would change its speed momentarily, and at the same time, the ignition whine would change pitch. Could I have a bad battery that's shorting out and sending inconsistent power? If it's a new battery I need, where do you guys suggest buying one. I went to the local Battery Warehouse (that's really what it's called!) and I got some really stange looks when I plunked down my battery and asked for 2 new ones. Some warehouse ... should be called wherehouse.

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Brian Scotti
'73 S Coupe

Just letting you know,
Checking coil resistance alone can tell you if you have the correct coil, but no guarantee that it's a good coil without other tests (ie. hypot test). Coils often fail under load only. When winding insulation begins breaking down, internal shorts only happen at the higher working voltages, and not the low voltages of a typical ohm meter (1.5 volts).

Doesn't sound like a battery to me. Usually when cells short out in a battery it's not intermittent like that. Sounds like possibly something in the wiring is intermittently shorting to ground or a component is shorting to ground. Possibly only a partial short. Could also be an open. Try turning on the ignition and banging around the car, or pulling on the wiring harness in various places to see if you can find the area. Looking at my schematic these two items share wiring from fuse box-1 at the power side of fuses 7, and 8, (they're tied together here and get power from the ignition switch), back thru a 6 terminal connector to the ignition switch then thru another 6 terminal connector to the battery. These two parts get power from this tie point at the fuse box from the ignition switch directly, and from here it splits off to each unit. It doesn't look like these two items have fuses on your car. I could email you scanned copies of the schematic if you want, (2 sheets).


Leon 78SC

[This message has been edited by leon (edited 09-01-2000).]

I guess it's time to show my electrical ignorance ... how would one check the resistance of the primary and secondary windings?

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Brian Scotti
'73 S Coupe

Get a cheap digital multimeter from radio shack. Set it to read ohms, Disconnect the two small wires from the coil, and the big coil wire going to the distributor cap. To check the primary side of the coil, (CDI side), connect the multimeter to the two small terminals on the coil. You should get a low resistance, (mine was about two ohms I think). To check the secondary side, (spark side) connect one multimeter lead to either of the two small terminals, and stick the other into the center tower where the big coil wire plugs in you should get a much higher resistance, (mine is about 630 ohms). Remember even if these values checkout it's usually no guarantee your coil is good under normal running conditions.

Leon,

Please email me those scaned diagrams.
Thanks

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Brian Scotti
'73 S Coupe

[This message has been edited by bscotti (edited 09-01-2000).]

I think you're closing in on it. While I still would suggest a cap and rotor change if they're old, these other guys know what they're talking about. Leon says intermittent power is not likely the batterys' fault and I agree. Voltage regulator maybe. Short or bad connection probably.

Your multimeter could be hooked to your wire harness for your to view as you drive. Mine is right now because I'm monitoring voltage. Might be interesting. But Leon's procedures make sense in case you have a short or bad connection. If your volt meter shows you have 12.5 volts in the morning before starting the car, your batteries are probably fine.

There's a gremlin in there, and you likely have a combination problem. One seems electrical. Don't go swapping more parts, except the cap and rotor if they're old. But do fix the fuel pump thing.

What's mysterious is that the problem does not appear at high rpms under load. Keep in touch. Warren alone can fix this for you, using yoru hands, eyes, etc., but I'm curious too.

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'83 SC

Sent you two emails
Schematic_page_one.zip, and Schematic_page_two.zip.
Unzip them they will be in jpeg format. I also marked up two copies showing what I believe is current path to these two devices. Sorry only page two is in color. Let me know if you got them OK, and if they are correct as there are a couple of different ones for that year.

Hope this helps,
Leon 78SC

[This message has been edited by leon (edited 09-01-2000).]

Glad you got them,
I replied to your email with a description of the schematics I marked up detailing power flow to these devices. It's a little tricky to follow as it jumps back and forth from page to page.

Leon
78SC

[This message has been edited by leon (edited 09-01-2000).]

Superman,

I just replaced the cap, rotor and points this spring and all the contacts still look great. I saved the old cap, rotor and points (actually have a few used of each) and will try swapping them tonight. Could the ponts gap have opened up? The vacuum advance on the distributor is OK. Is there a mechnical in the distributor guts as well that could be binding?

I guess the worst part of all this is that I was teased with a week of perfect driving and now it's been snatched away. I suppose its all part of owning a older car, but how long before you go nuts by not driving your Porsche?? I'm sure most would agree that it's not very long!! It's been less than a day and I miss it already!

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Brian Scotti
'73 S Coupe

[This message has been edited by bscotti (edited 09-01-2000).]

[This message has been edited by bscotti (edited 09-01-2000).]

You recent joy should be ongoing. Don't give up, in can run like that all the time.

If your cap and rotor are fresh and looking good, then they may just be alright. Again, if they are bad, they usually give problems at higher rpms. I believe they may be expensive too, for your car. I guess I was reacting to your comment that you cleaned them. Scraping them can relieve their 'tiredness' briefly, but I don't like doing that. If I have to scrape the contacts I will, but I'll replace them shortly after.

Mechanical (centrifugal) advance mechanisms, which your car has, usually go to full advance instantly at startup and stay there. They are just there to retard your ignition to about zero degrees TDC, so the engine does not try to run backwards. The advance "curve" is performed by the vacuum advance alone (unless Porsches are different). You can probably test whether they are 'stuck' by comparing a static timing test with the dynamic one. Warren, do you suspect a ticky centrifugal advance? Is this worth checking to find the current problem?

You can and should monitor your point gap if you still have points. Just put a timing light on it occasionally. If the gap changes, so will the timing. If the timing changes, reset the gap, and (probably) the timing will also get back to spec. There is some significance here because I believe that wear will close, not open, the gap and that this advances the timing. Over-advanced timing is a bad thing.

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'83 SC

Warren and others,

Just to make sure I get the right fuel pump, the one that I think I was supposed to order is cylindrical in shape (Performance calls it a Facet/Bendix pump)? Is there a better one out there I should consider?

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Brian Scotti
'73 S Coupe

[This message has been edited by bscotti (edited 09-01-2000).]

Yes, the round Facet/Bendix is the pump I was referring to. An American Holley or Carter might be cheaper, and available locally, but an external regulator would surely be needed to get the pressure down to the 3.5 lb level Webers prefer.

I hope you have been using genuine Bosch points, any generic replacement from a local NAPA store (or others) is junk!!!

I strongly recommend pulling out your distibutor for thorough disassembly, cleaning, and re-lube job ... every screw and part HAS to come off! Be careful of the ground strap between the moveable point plate and fixed base ... check for fraying and cracks in the uninsulated wire right at the spot welds, as they can break, giving intermittent or no spark at all! Bosch grease is the preferred lubricant for the distributor, but it is a little late to get it in time for your track event!

Do a search for CDI/ignition problem posts in the past and there are some thorough discussions about distributor cleaning/rebuilding.

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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 09-01-2000).]

Thanks Warren! I happen to have access to a Carter fuel pump so I'll get that installed as soon as I get this car running. I only use Bosch distributor components. I just happen to be lucky enough to have a tube of the Bosch grease, so if the electrical wires check out OK, I'll be attacking the distributor next.

A big thanks to everyone who has given advice today. I've got a lot of work to do tonight and I'll update everyone with my progress and success (I'm trying real hard to be positive).



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Brian Scotti
'73 S Coupe

sorry you are having such a bummer time getting to the bottom of this. you mentioned some interesting stuff about other electical problems elsewhere in the car. clean the fuse box!!! you may be getting an intermitant power outage to the ignition caused by dirty contacts in your fuse box. pull the fuses out and scrub the contacts. this might cure some of that other stuff too. you may also check your ground strap on the motor to the body. next you stated that you were getting more flames and back fires out of one side of the motor. if it back fires on deceleration you may have a lean miss and you can try a richer idle mixture on those cylinders. check for vacume leaks!! with a fire extinguisher nearby, spray chemtool around the bases of the carbs, if the motor changes speed, it's sucking in unauthorized air. i hope this helps. if you get any more frustrated i'll buy the car off you. Alex.

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Any luck yet?

I have been at it for 6 hours now and here's my progress report ...

1) Replaced cap and rotor with good used ones with no improvement.
2) Checked the resistance across the primary and secondary windings of the coil:0.7 ohms pri, 706 ohms sec. Seems OK
3) Battery voltage 12.7 V (both were the same)
4) While I took a break from the misfire to track the cabin blower fan problem, found the fuse blown. Blew another when I replaced it and tried the fan. It worked for 2 sec then blew the fuse. Tracked down the blower motor switch and found it severly corroded, with one terminal barely attached. I disconnected and taped all connectors to the blower and will order a new switch. One problem down.
5) I was under the dash and found a 6 pin connector not really connected. Plugged that in and the high beams work again. 2 down.
6) Removed the distributor and took apart the guts. Didn't see anything broken or any frayed wires. The mechanical advance was smooth and the vacuum advance was OK as well. Lubed everything with Bosch distrib grease, set the points gap to .012" and reinstalled it to the same timed position as before (I had marked the distrib body and the case to make sure it was the same). Also disconnected the vacuum advance with the engine running and the rpm did change and the engine backfired worse under load.
7) Snugged dowm the intake manifold-head bolts and the carb-intake bolts. Didn't see any evidence of a gasket leak.

Unfortunately, nothing so far has fixed the miss. Under no load, there is a hesitation in the motor, but no backfiring. Under load, it backfires pretty bad from ~3000 RPM and up and if I let off and engine brake from 4000rpm+. I haven't given up hope yet. I've made some good progress and found lots of little items that needed fixing. I did break one thing, though. On the aluminum cover that has the oil breather and return line, there was some electricval connection that was barely held together. I had a rubber boot over the wires (2) and they attached to a brass screw in fitting. The oil pressure and temp gauges are still working, so what was it?

Where do I go from here? Voltage regulator? How do I check that? Or??

Thanks for all the help so far!

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Brian Scotti
'73 S Coupe

[This message has been edited by bscotti (edited 09-03-2000).]