Missing cylinders during idle

[David T]

Since your motor is no longer stock you should start at the beginning. I would do a compression test and leak-down test to make sure each cylinder is working well mechanically. Also worth checking valve adjustments. Then you can go on to checking spark and fuel to each cylinder. A vacuum leak can also cause a misfire to the cylinder closest to the vacuum leak so you must check for vacuum leaks too.

[jmettee]

Our cars sound similar... I can't really tell from the video, do you have stock exhaust? My exhaust pipes come directly from each set of three cylinders so I can get a clean sound signal from each bank. If you have a similar setup, I'd love to hear how each side sounds.

OK, knowing you have a non-combined exhaust helps. From the PC, it does sort of sound like your left side may have a miss, but I can tell exactly in the video.

My exhaust is like stock with a 1 in/2 out muffler. That video I posted was without the muffler, but still had the exhaust manifolds & crossover pipe installed, to both sides of the engine were still tied together in that video.

Like others have mentioned, try pulling the plugs or perhaps try the infrared thermometer to try to locate the bad cylinder. With a long extension ,you should be able to get all 6 plugs out without removing anything other than the air filter housing. The fuel injection & idle speed motor always stay in place on my engine when I remove the plugs.

[spikeygg]

Concerning Cylinder #4: the only reason the idle speed motor is attached to the intake manifold is to facilitate assembly line manufacture (so the engine can easily be dropped into a moving car). When I was K-Jet I relocated my Aux Air Valve to the driver side pontoon and connected it with long hoses (my Aux Air Valve was an electric solenoid, not thermal). Makes engine access much easier. especially Cylinder #4. Also makes fuel/air mixture unit removal/installation much easier.

What is the pontoon? This sounds like a good mod to make things more accessible. I don't know enough about it but your statement about the electric vs. thermal solenoid makes me wonder if this isn't something that I can do...

[spikeygg]

Since your motor is no longer stock you should start at the beginning. I would do a compression test and leak-down test to make sure each cylinder is working well mechanically. Also worth checking valve adjustments. Then you can go on to checking spark and fuel to each cylinder. A vacuum leak can also cause a misfire to the cylinder closest to the vacuum leak so you must check for vacuum leaks too.

I have a compression tester, since the plugs are accessible without removing stuff I will attempt that. I don't have a leak-down tester, unfortunately. I found a vacuum leak in the fuel-mixture adjustment hole which I plugged with an extra vacuum plug I had in my garage but how do you propose I detect other vacuum leaks that may exist? On my turbo 3000GT I can plug the intake and pressure test it, is the process similar here?

[David T]

I have a compression tester, since the plugs are accessible without removing stuff I will attempt that. I don't have a leak-down tester, unfortunately. I found a vacuum leak in the fuel-mixture adjustment hole which I plugged with an extra vacuum plug I had in my garage but how do you propose I detect other vacuum leaks that may exist? On my turbo 3000GT I can plug the intake and pressure test it, is the process similar here?

The best way I have found is with a smoke machine. It pressurizes the induction system and makes any leaks visible.

[spikeygg]

Haha, I used the temp gun method of troubleshooting. It works really well, especially on that side since it's very accessible. My measurements look like this after about three minutes of runtime:

Cylinder 6 - 330'F
Cylinder 5 - 180'F
Cylinder 4 - 128'F
Cylinder 3 - 325'F
Cylinder 1&2 - ~315'F (tough to separate these since the header is gnarled in this spot and the starter is in the way)

At this point I think 5 and 4 are out of the loop so I pull cylinder 5's plug, the easy one. It's black:
Cylinder5Plug.jpg

I use my wire brush on it and a few files to try to revive it a bit, then I have my wife turn the engine over while I watch for sparks to come out the cylinder 5 HT wire. Looks good so I throw the plug back in there and fire it up. Still sounds uneven, probably due to cylinder 4 being dead but I use the temp gun one last time to check the temps on cylinder 5 exhaust and it's HOT, ~325'F. Of course, cylinder 4 was still dead at about 180'F.

Anyway, I tried to figure out how to get to that cylinder 4 HT wire but man it's tight! Any pointers?

Thanks for the help so far crew, we're makin' a dent!

[spikeygg]

The best way I have found is with a smoke machine. It pressurizes the induction system and makes any leaks visible.

Smoke machine? Can you post a picture of this thing? I'm not familiar with that, is it like for Halloween? How do you use it to pressurize the induction system? Do you somehow install it just in front of the Air Flow Sensor Plate?

[spikeygg]

One thing I found when updating my ignition ECU was I needed to add a capacitor at the 12 volt power input to the ECU to stop a miss fire.

Use a 1000 uf 16 volt or higher voltage as close to the ECU as possible. The negative side of the cap should got to the ECU ground wire.

Do you happen to remember if the misfire was on a specific cylinder? I'm wondering why my cylinder 4 and 5 have fouled in the first place...

[Kherns]

Very interesting read, as my Stage 2 always had a strange iddle like ta ta da ta ta da during for or five cycles then tadatadatdatdatdatdta and this goes on and repeat itself...

[Bitsyncmaster]

Do you happen to remember if the misfire was on a specific cylinder? I'm wondering why my cylinder 4 and 5 have fouled in the first place...

Don't know which cylinders were causing the miss fire. It happened when I replaced the ECU with the chevy ignition module and when the cooling fans were running. I could see the miss fire with the timing light it would fire way advanced just looking at #1 cylinder. I did see large voltage drops with my oscilloscope looking at the voltage at the ECU.