Fixing misfires/flaming exhaust

Topic: Fixing misfires/flaming exhaust (Read 4175 times) previous topic - next topic

Fixing misfires/flaming exhaust

August 23, 2016, 12:47:09 AM

So my car hasn't moved much in a few years. I had it out of the garage this last weekend and heard that when I fired it up to pull back into the garage, idling for only a few seconds before moving forward, flames were shooting out the exhaust. I saw this years ago when running open headers during transmission work and started tracking it down. Months later I paused the troubleshooting, time passed, moved houses/drove car to new garage, and now I'd like to start tackling it again.

I've had a mild but existing misfire issue for many years and spent a lot of time and money narrowing it down.

Applicable details:
Cam: '89 GT stock at 0-degrees
Heads: TFS TW 170cc 54cc with 1.7 RRs
Headers: 1 5/8" unequal length FMS stainless
Exhaust: 2.5" mandrel with cats, two straight-through glass-pack lers
Distributor: stock with old TFI module
Coil: strong aftermarket unit with spark gap tester - works better than Accel supercoil unit I tried years back
Wires: firecore50
Grounds: 4awg and 1/0awg

I seem to be getting great spark and have never seen it miss a beat with timing light pulses on any wire. Issue seems to exist at all rpms, and whether engine is cold or hot - there's always a miss you can hear, feel, and see on the tach. Distributor is stabbed in there perfectly - I've spent much time over the years messing with it, and have seen how it behaves when one tooth off. Set at stock 10-degrees without EEC plug.

With a COLD motor only running for seconds, what would cause flames out the exhaust when revving forward at 2-3k rpms (higher stall converter)? Yes it is richer when cold, but the exhaust piping isn't hot enough to ignite gases so soon. I wouldn't think any part of this situation would send a flame all the way back through the cats, and out the tail pipes.

Where would you guys start checking on a car shooting flames on a cold start? Could a bad TFI module cause extremely erratic ignition?

Fixing misfires/flaming exhaust

Reply #1 – August 23, 2016, 02:53:43 AM

When I was 16 me and my buddy did plug wires on my 86 cougar. I did my side one at a time, my buddy didnt. He messed up All four. It would should flames through the stock exhaust past the bumper if I reved it at all.

That is the only time I've seen it happen.

Fixing misfires/flaming exhaust

Reply #2 – August 23, 2016, 01:50:47 PM

Quote from: Haystack;456852

When I was 16 me and my buddy did plug wires on my 86 cougar. I did my side one at a time, my buddy didnt. He messed up All four. It would should flames through the stock exhaust past the bumper if I reved it at all.

That is the only time I've seen it happen.

Yeah, the plugs are on right. Car generally runs and drives fine.

I don't see how I could get spark when the exhaust valve open. Makes me think the valves aren't closing. I don't have much preload on the hydraulic lifters...

Fixing misfires/flaming exhaust

Reply #3 – August 24, 2016, 08:50:41 AM

Run a leak down test on the cylinders and that will tell you if the valves are closing.

Fixing misfires/flaming exhaust

Reply #4 – August 24, 2016, 01:48:56 PM

Quote from: Aerocoupe;456878

Run a leak down test on the cylinders and that will tell you if the valves are closing.

Yeah, I dropped off the car yesterday for a trans R&R. Also having them perform a leakdown test while it's there.

If I've had a leakdown tester for 2-years and never got around to doing it, it's better to just get it taken care of while at a shop.

Fixing misfires/flaming exhaust

Reply #5 – August 28, 2016, 03:48:47 PM

I'm sure this is related to the same reason my tach and speedo do random things. Opening new thread to troubleshoot that.

Leakdown showed no issues. <10% on all cylinders. Would be nice to try a second gauge since I know these can all be inconsistent unless spending $100+.

Fixing misfires/flaming exhaust

Reply #6 – August 28, 2016, 05:48:32 PM

I don't know what pistons you're running, but assuming stock type slugs with flycuts, your 54cc heads are putting you in the neighborhood of 11.5:1 compression OR MORE. What are you running for fuel? I've got 62cc heads with domes working out to 10.75:1, my base timing is 14 degrees, and I have to run 93 octane to avoid huge spark knock. Sliding up the scale a little will get you into detonation, and detonation can cause your flames.

Fixing misfires/flaming exhaust

Reply #7 – August 28, 2016, 07:42:22 PM

Quote from: TheFoeYouKnow;456933

I don't know what pistons you're running, but assuming stock type slugs with flycuts, your 54cc heads are putting you in the neighborhood of 11.5:1 compression OR MORE. What are you running for fuel? I've got 62cc heads with domes working out to 10.75:1, my base timing is 14 degrees, and I have to run 93 octane to avoid huge spark knock. Sliding up the scale a little will get you into detonation, and detonation can cause your flames.

I haven't thought about the pistons and compression numbers in years, but iirc, I went up to 9.8:1 compression with the head change, with the headgaskets I'm using. Not sure I can still find the pistons I used (something like -8 or -12cc), but they were cut and dished, but nothing extreme for boost.

I was closer to 9:1 with the gt40p heads. Had the TW170 heads (and lower intake to match) milled from 62cc to 54cc just to get back some of the thermal losses by going to aluminum heads, but nothing that would prevent me from running 87 octane with aluminum. I put 92 or 93 octane in it while troubleshooting. Fresh 15-gallons yesterday.

Fixing misfires/flaming exhaust

Reply #8 – August 29, 2016, 06:40:23 AM

Get the cc numbers for the dish on the pistons and feed the whole thing into summit racing's compression ratio calculator. You also need to know that usually the compressed thickness of the headgasket is .038". The 8cc drop in chamber volume will have more of an impact than you probably think. I've been wrong before, but it seemed like you were overlooking possible combo related issues.

Fixing misfires/flaming exhaust

Reply #9 – August 29, 2016, 05:06:10 PM

Quote from: TheFoeYouKnow;456940

Get the cc numbers for the dish on the pistons and feed the whole thing into summit racing's compression ratio calculator. You also need to know that usually the compressed thickness of the headgasket is .038". The 8cc drop in chamber volume will have more of an impact than you probably think. I've been wrong before, but it seemed like you were overlooking possible combo related issues.

Dug up scanned receipt from Summit - 9 years ago. +7.40 pistons, 9333PT gaskets, 0-deck, 9.8:1 compression

Fixing misfires/flaming exhaust

Reply #10 – August 30, 2016, 08:46:35 AM

What does your exhaust smell like?

Fixing misfires/flaming exhaust

Reply #11 – August 30, 2016, 05:50:19 PM

Quote from: Aerocoupe;456957

What does your exhaust smell like?

It smells like exhaust?

From cold start, rich of course. Datalogs show standard rich mixture of 0.81-0.87 lambda for a bit depending on ambient temperature, leaning out slowly as it warms up over 60-seconds, then holds 0.9 lambda for another 30-seconds before going into closed loop. The car never smells as clean as our newer cars, even in closed loop, but nothing different than the way it has always smelled.

I'm going to be installing my lightning mass air sensor next week after larger tubing parts come in, and will swap the distributor this weekend. Just these two items may drastically improve how the engine runs.

Fixing misfires/flaming exhaust

Reply #12 – August 31, 2016, 06:42:09 AM

What are your cat temps? Strictly speaking, you shouldn't be able to blow flames through a cat, but when they're being fed that rich, they'll light off quick and get really hot. How hot? Do you have an IR thermometer to check them with at 30 second intervals? With that much fuel, the cats could be getting hotter than normal, and with more fuel than they can process, it might be the cats lighting the unburnt fuel. Compare to the same intervals post-start on your other cars.

Fixing misfires/flaming exhaust

Reply #13 – September 01, 2016, 01:01:56 PM

Quote from: TheFoeYouKnow;456970

What are your cat temps? Strictly speaking, you shouldn't be able to blow flames through a cat, but when they're being fed that rich, they'll light off quick and get really hot. How hot? Do you have an IR thermometer to check them with at 30 second intervals? With that much fuel, the cats could be getting hotter than normal, and with more fuel than they can process, it might be the cats lighting the unburnt fuel. Compare to the same intervals post-start on your other cars.

I can check it. I'll just throw the thermal imaging camera on it next time I start the car, and record a video. With rain today and tomorrow, may not do much until Saturday.

Fixing misfires/flaming exhaust

Reply #14 – September 01, 2016, 05:37:44 PM

All I read was "shooting flames" and thought to myself "I wish MY car did that!!!" Hahaha.

Seriously though, that is a LOT taken off those heads. As stated, stacking the HG (literally and figuratively) on top (well, under) may push things over the edge.
I have those on my one car with TFS 10:1 pistons, everything seemed fine. But up over 11:1...should that be the case...would be potentially problematic.

I'm pretty sure E7's / GT40s are ~58cc. So it's definitely significant.