Random misfiring again!

[Turbofiat124]

Well looks like I jinxed my Spider. I was thinking to myself when coming home this morning, "What a smooth running car". Then I stopped to get gas. The card reader did not work so I went inside to see if I could pay at the counter but the guy said it would be easier if I just moved to another pump. I started my engine and noticed it was idling kind of rough. Pulled into the other pump and filled up. Drove home with the car misfiring. So I can rule out bad gas.

I initially thought it was the plug wires but this makes the 2nd set of plug wires and I'm beginning to think it never was the plug wire issue to begin with. It's just a coincidence that when I would test a cylinder using my timing light and see an intermittent flash and use a spare plug wire on that cylinder, the problem would go away.

I replaced the spark plugs last summer. Used but low mileage (<1000 miles) distributor cap and rotor button from a spare engine.

The problem comes and goes. There does not seem to be no rhyme nor reason to it.

I pulled the distributor cap and wiped some graphite or grease off the center of the rotor button and rerouted one of the wires on the pickup. Restarted the engine and it seemed fine.

When I got up today, I was going to go on a test drive. I started the engine and it idled smooth at first. I let it warm up idling and after a few minutes, the car started to idle rough.

So I tried a known good, tested control module I keep as a spare in my trunk. The engine still idled rough. So I am confident it's not the control module, plug wires or spark plugs.

I'm not seeing any black smoke from the exhaust.

I shut the engine off (had to mow today), then when I got done mowing started the engine and let it warm up to normal operating temperature. This time it still idles smooth. So I missed my golden opportunity this morning to see if it was any particular cylinder that was misfiring. Now I just think it's random.

This misfiring comes and goes. The car ran fine for about 50 miles between when I changed the Bosch plug wires out for Magnecors the other day. Then for no apparent reason, I stop to get gas, starts idling rough again.

Unlike the Bosch wires which only misfired under boost, this time the misfiring occurs at idle, cruise and boost. It does not seem like it's any particular cylinder but just random.

The pickup wires look fine but the pickup has been on the car for 17 years. But when the last pickup failed, this time I disconnected the vacuum advance hose and plugged off the nipple on the throttle body thinking this might extend the life of the pickup which it seemed to have done. I've been running the vacuum advance hooked up now for about 2 years since I went away from Jacobs Boostmaster and started using the 5 pin HEI module.

I checked both pickups with an ohms meter both read 750 ohms. I don't know if that test means anything. I know a bad pickup will show an open circuit.

Question: Every Time I've had the pickup fail (about three times), the tachometer needle goes mad and usually the car just dies. I'm not seeing a jumping tachometer needle when the engine is misfiring. So can I rule out the pickup? Is this a bad assumption?

My next item on the list is to replace the pickup with my virgin spare.

The only other thing I can think of left to replace is the coil. It's a 17 year old Bosch coil. But normally when a coil is bad, when it get's hot, the engine just shuts off and won't restart until it cools down.

I'm betting on the pickup simply because that is the only component I've replaced numerous times. Once a vacuum advance. I've never had a coil, distributor cap, rotor button or control module go bad on a Spider.

I have had 2 HEI modules go bad on my father's 76 Chevy truck (same one used on spiders) but the engine just dies. And I have bought HEI modules that were bad straight out of the box.

If the coil and pickup does not fix the problem, what's next? Assume it's something to do with the fuel injection system?

[Yadkin]

The high voltage portion of the ignition is the least dependable;e system so let's start there. Check the plugs for proper gap and undamaged insulators. Check each wire for resistance; they should all be the same. Check the connections at each end for tightness. Use dielectric grease on the rubber seals at each end.

[Odoyle]

I had a similar issue like this with my 83 and it ended up being a bad ignition control module. I initially tested my pickup and it read 740 ohms, you said both of your pickups read 750 ohms which is right on spec because my factory manual says it should be between 700-800 ohms. I would look behind your ignition coil and check out your ICM.
Side Note: My ICM decided to die in the middle of a 4 way intersection, engine was running fine then cut out and wouldn't start back up. I had to get out and push the car through the intersection to the side of road. Talk about timming, luckily everything was fine after i towed her home and fixed it

[76was124]

I had a similar problem when I revved. I found the cause one night in the dark where I could see an intermittent arc across the coil. It turned out to be a combination of moisture and conductive (enough) grime. Wiped down the coil surface and the problem went away.

[Turbofiat124]

OK today's progress:

I installed new magnetic pickup. I allowed the engine to idle for about 15 minutes. Refueled my Trabbi in the meantime. Engine ideled smooth. Revved the engine up to 4000 rpms. Still no misfire. Shut hood, engine started to misfire. Went on a test drive. Engine runs rough.

I could be wrong but this misfiring is somewhat consistent. Meaning it seems like it's just one particular cylinder that is misfiring, not multiple at the same time. But it's random. It could run smooth for a few seconds, then start misfiring again.

BUT, I made two (well three) very strange correlations when this misfiring happens.

I removed my Spectre plug wire separators to check each cylinder with my timing light.

http://www.jegs.com/images/photos/800/865/865-4245.jpg

While using my timing light to see if it's any particular cylinder that is misfiring, I could not get the engine to misfire.

So I went on a test drive. Over a course of 2 miles, no misfiring at all!

So did the plug wire separators have anything to do with it? I'm not so convinced.

Started piddling around with something else before calling it a day. But I was not so sure the car was fixed.

Started the engine about 15 minutes later. Misfiring it back.

Popped the hood, misfiring seems to be gone. Even revving the engine up.

Slam the hood, immediately the misfire returns! WTF!?!?

Repeat.

Just to make sure it was not my imagination. I held the hood open, applied my foot on the accelerator and revved the engine. Slammed the hood again and the misfiring returns.

So what would cause the engine to misfire with the hood down but not up?

I do not have a hood insulator. Is that important?

My guess is the hood is touching one of the plug wires? Could it be shorting out through the paint?

Could it be pulling the plug wire just enough out of the distributor to cause a weak or no spark? I cannot see if any paint that has been rubbed off.

Really these plug wires need 90 degree boots instead of straight boots considering the distributor cap is so close to the hood.

Am I on the right track or is all of this in my head?

[Turbofiat124]

I had a similar problem when I revved. I found the cause one night in the dark where I could see an intermittent arc across the coil. It turned out to be a combination of moisture and conductive (enough) grime. Wiped down the coil surface and the problem went away.

Yeah I had a set of Accel wires that did that. But it's worth checking tonight.

[narfire]

I had a plug lead arc to the hood years ago. I didn't notice until I picked up on it when the garage was darker.
Same thing, the engine ran rough on one cylinder but when I started pulling leads to figure what cylinder was acting up, nothing.

[Turbofiat124]

So I pulled and rotated the distributor today. After I adjusted the timing, it seemed I didn't get it rotated clockwise as much as I had hoped.

I did notice where the #1 plug wire had been rubbing against the hood. But not real sure if that was the problem.



But when I cranked the engine to set the timing, it idled unusually rough. This time I noticed black smoke coming from the exhaust pipe. Which I did not notice when the engine was misfiring.

As everyone knows, if black smoke is belching from the tail pipe, check the coolant temp sensor. Then I discovered this:



I had replaced this sensor a few years ago for some reason (green connectors maybe).

I'm not sure why the wire broke. If it's because the connector is in such a vulnerable location or the fact new connectors don't come with boots which protects the wires from breaking.

My guess is I had allot of wires and cables running across the engine bay like my remote starter, timing light.

I soldered a new connector in and so far so good. But it's still too early to tell whether the coolant sensor connector was the problem, #1 plug wire was rubbing the hood shorting out or crossfiring or a combination of both.

Maybe finding the broken wire was just a coincidence.

At least one known problem has been solved.