Bad Misfiring

[Matt 4.3 TBI]

Hey guys, been quite a while, but I'm back with yet another fun problem.

Earlier this week I changed out my spark plugs since they were getting worn. Just used stock AC Delco CR43TS's, and noticed a bit of misfiring in the high RPM, over 4200 or so. Figured it was just the computer re-adjusting the fuel trim since I had just changed the air cleaner as well.

Anyhow, was out today and got some bad misfires and a couple backfires. I disconnected my MSD-6AL, thinking it might be overheating (it's been 110 degrees lately). Hook the factory wiring back up to the ignition coil, and it won't start. Try a couple times, still no luck. Figure it might be flooded, put the pedal to the floor to put it in 'clear flood' mode, it fired up, ran fine. Drove it back home with no issues.

Left a few hours later, and when making a right turn onto an onramp it misfired bad, and I had to let off the gas to keep from stalling. After that I didn't have any problems for the remainder of that trip.

I was thinking the ignition coil was shot, I've been through a few of them in the past 4 years, so I went to get a replacement Wells from AutoZone. Misfired terribly on the way there, couldn't even get through first gear without it stumbling and backfiring. Limped up to the store and put in the new coil. It wouldn't start.

Checked all the connections, everything looks good. Put it in clear flood again, it fired right up. Ran fine, no misfiring in the low and mid range. Gave it some heavy throttle to test it, and again got some missing in the high end, over 4000 rpm. This is really getting old, so any thoughts you guys might be able to offer?

Next I'm thinking either ignition module (the distributor module), ESC module (since I've had bad timing cause misfires like this...just usually not randomly), or fuel pressure regulator. If it was a fuel issue I'd expect it to be fairly constant, since TBI's run at a constant pressure, not vacuum-decay.

[Sparky2263]

Good to have you again Matt!

Was this happening before the plug change? If not, I'd be taking the plugs out and giving them a good look. Also, how old and what type wires? Definitely sounds like a secondary ignition problem, not primary. Unless, of course, the plugs are sooty black when you pull them out.. ;)

[Matt 4.3 TBI]

I don't recall any misfiring at all before I changed the plugs. I'll check out the plugs this mornings. The wires are AC Delco's. I bought them a few years ago and never installed them until a few months ago. Starting to wonder if a contact or two came loose while I pulled off the wires. Time to get out the multimeter.

[Matt 4.3 TBI]

Alright, I tested all my ignition wires, these are the results

(using 20k Ohm setting on multimeter)
#1 - 6.21
#3 - 5.40
#5 - 4.14
#2 - 10.12 (I had a spare wire that read 2.21, so I swapped that in)
#4 - 4.53
#6 - 4.09
Coil - 1.97

I pulled all the plugs as well, I can find no noticeable deposits, they all look brand new (I just installed them Tuesday evening).

I checked the base timing, it's still at 4 BTDC where I set it over two years ago. The timing seems to advance normally (timing connector connected, watching the timing mark while opening and closing the throttle).

There were some deposits on the distributor contacts, but less than 1mm worth.

So, any thoughts from here? I'm still wondering about the ignition module and the ESC module.

[nomaboy]

if it fires up when you put the pedal down wouldn't that indicate that it is flooded??

[Matt 4.3 TBI]

Yes, it would indicate there is excess fuel in the cylinder because the ignition is not firing.The question is WHY the ignition wasn't firing properly.

I got a ride back to my garage to pick up several spare parts, so I'll be doing some diagnostics later. I have my fuel pressure gauge, an old distributor cap, old IAC valve, new O2 sensor, the old plugs I removed earlier this week, old ignition module and my old ignition coil. If anyone has thoughts where to start first, I'm happy to hear them.

[nomaboy]

i was just saying i would make sure the fuel system is working fine cause there must be quite a bit of fuel in the cylinder if it won't start. i know you said that it wasnt doing this before you changed the plugs but you never know. sometimes when i do somthing to my truck somthing else, completely unrelated, goes wrong.

[Matt 4.3 TBI]

Well I went out today and put in the old plugs, old Conrad distributor cap (cleaned the poles and button with some sandpaper), cleaned the rotor tip, and fired it up. I ran it around the parking lot, hammered on it a few times, didn't feel or hear any misfires...it's running well enough to be laying down rubber. I didn't reconnect the MSD yet, because I've had it keep dead components alive by cramming extra voltage through them, so I wanted to be sure the factory parts are working properly. I'll give it a few more days before I have a final verdict, have to run some errands today, hopefully my gut feeling is right.

Notice a difference between these two plugs, even though they are both part number CR43TS? (top one is the old plug, bottom is the new, and apparently problematic plug)

http://i80.photobucket.com/albums/j185/exoframe2/Random/get-attachment.jpg

[Sparky2263]

I think you got some counterfeit plugs maybe. Never seen a CR43 with that kind of shell.

[Matt 4.3 TBI]

Well after driving home, I can safely say the problem lies in something else. Driving back on the highway, about 15 minutes, no problems. Got into the gas, ran it up around 90, no misfires. Pulled off the highway to stop for fast food in town before heading home, no problems driving around town. Cut the it off at Sonic, started back up and ran fine until I got on the road, gave it some gas and it sputtered and popped, but kept running. So, I'm seeing a pattern, at least I think so... runs fine cold, runs bad hot. What do you think, ignition module?

[Matt 4.3 TBI]

Found the problem...this wire:

http://www.hooverae.com/upload/pics/DSCN0398.jpg

Connects the ignition module to the ignition coil. Any idea where the hell I can find a replacement? I'm going to cut and solder in new wire in the meantime to see if it corrects the issue. Anyone that could give me a GM part number would be immensely appreciated.

[Sparky2263]

My burned one I went and stole off the parts van I got. A trip to the U-Pull-It yard oughtta solve the problem.