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Topic: Troubleshooting persistent misfire (Read 1394 times) previous topic - next topic

Re: Troubleshooting persistent misfire

Reply #15
If I attach a stethoscope hose perpendicular to the side of my tail pipes, but blocking out open air, the exhaust drone is cut down, but I hear an occasional faint sound like this from the passenger side: https://youtu.be/mW0q_M_sMrw?t=121

Higher tone, sort of a 'tapping on a shot-glass' sound when using the stethoscope.

Nothing on the driver side pipe.

My mid pipe does need to be re-created though (BBK pre-made). It's right up against the transmission pan lip.

Edit:
Best recording I can get - pushed one of my calibration mics into a hose: LINKED MP3

The recording (1KHz 6dB high pass filter applied, gain applied) is different than to the ear, but this sure sounds like pinging.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #16
It sounds like metal hitting something. The exhaust isn't touching anything is it?


If it's not that you could try disabling one cylinder at a time on the bank to see if the noise goes away. At least you would be able to pin point the noise that way.
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #17
It sounds like metal hitting something. The exhaust isn't touching anything is it?


If it's not that you could try disabling one cylinder at a time on the bank to see if the noise goes away. At least you would be able to pin point the noise that way.

Yeah, this side is the one where the exhaust is right up against the transmission pan's lip. The 4r70w bellhousing is larger than the aod, so clearances on pre-fabbed exhaust pipes are lacking.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #18
Yeah, this side is the one where the exhaust is right up against the transmission pan's lip. The 4r70w bellhousing is larger than the aod, so clearances on pre-fabbed exhaust pipes are lacking.

I bet that's what it is. To my ears it sounds like something hitting a pipe and the sound traveling down the pipe.

Have you been able to scope the ignition coil?

88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #19
I should be able to oscope some things this weekend, after trying to install some sn95 mustang seat frames with new 03/04 cobra foam and upholstery. Goodbye leaning driver seat!

Also have my new, longer 1/0 power wire here to re-do my current alternator cable for a cleaner install, and using non-set-screw terminals. Lots of time sinks planned for this weekend! Will also look into where I'd like to weld in some thick grounding bars. Been planning on cleaning up all the electrical connections, wire looming, and techflex sheathing for a long time. Been sitting on parts since summer 2012...
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #20
Make sure you don't have the LH and RH oxygen sensor connectors swapped. I did this inadvertently when installing the 5.0L engine in my car and I fought a rough idle and random cylinder lean condition for a few months before I figured it out.  The car would run great in open loop, but when it warmed up and went into closed loop it would run like crap, because it was leaning out the wrong side of cylinder banks.  It's an easy mistake because LH and RH connectors are keyed the same, yay Ford  :beatyoass:
1988 Thunderbird
306 HO w/ A9P processor
AOD w/ Transgo Reprogrammer
Full Digital Dash w/ twin Cyberdyne A-pillar gauges 
245/50/16 Tires on Snowflakes
Engine swap - CA smog compliant

 

Re: Troubleshooting persistent misfire

Reply #21
Make sure you don't have the LH and RH oxygen sensor connectors swapped. I did this inadvertently when installing the 5.0L engine in my car and I fought a rough idle and random cylinder lean condition for a few months before I figured it out.  The car would run great in open loop, but when it warmed up and went into closed loop it would run like crap, because it was leaning out the wrong side of cylinder banks.  It's an easy mistake because LH and RH connectors are keyed the same, yay Ford  :beatyoass:

I don't. I did some decade ago and found that the ecu would start to richen up one bank (thinking it was lean), and lean out the other (thinking it was rich). Eventually it'd fail out of closed loop and go open again. These computers really aren't the smartest in terms of things like this, or having functions that don't always have increasing values (computer flat out fails and stalls the engine), but they work.

The o2 sensor thing is really obvious in tuning software! I wish my issue was a simple as that  :frown:
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #22
A friend came over today to put two heads together on this.

With the stock ECU/tune, 19lb injectors, and stock MAF. We did some tests and eventually got to revving the engine. Above 4-4.5k, getting some heavy popping out the exhaust after a few seconds, and tach dropping down. Happens with both SPOUT in and out, but seems to be more drastic with the computer advancing the timing (occasional 1k rpm drops instead of 300-400rpm).

Purchased 2 timing lights and chopped them up to test coil wire and individual spark wires. Not seeing missing spark, but the inductive signal being read by the oscope shows some signals being picked up almost half the strength of others, out of the coil itself. Difficult to properly test this though, with the inductive readings. A recording showing some of the signals (ch1 coil, ch2 wire):

At 0:26, it shows the signal zoomed in. 50μs scale has some common 25μs discrepancies, but I have no idea what's normal.
https://www.youtube.com/watch?v=7AjDoco1fQc

We moved to installing new front seat foam and upholstery, and got stalled there also due to needing to get 2 more 12-inch metal rods for hog rings...


Will mess with more and get some datalogs once I have a Quarterhorse back.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #23
Hmm. Not sure what a normal scope should look like.

Just had a thought. How are the caps on the computer?
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #24
Hmm. Not sure what a normal scope should look like.

Just had a thought. How are the caps on the computer?

"Replaced capacitors in A9P"

Just replaced these weeks ago. A bit of a pain to scrape off the coating over the board, but it's relatively easy to replace the caps. Didn't change a thing in how the car runs though.

My old caps didn't leak and corrode the board, but I had a couple weak legs.

Would still like to try a completely different EEC.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #25
Hmm. Missed the new caps part.

I hate to parts swap but trying a different coil or different computer are kind of on the test list. No light show from the plug wires when it's dark correct?
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #26
Technically with a camera and long exposure, light will always show through a spark plug wire. No way around that besides coil-on-plug (or much thicker wires than even my Firecores).

But no, nothing obvious by eye.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #27
How's the car's wiring harness? There's nothing odd going on there? No repaired wires? It's got to be some weird electrical thing.

I still wish that you could compare with normal scope readings but I have no idea where you can find those readings to compare.
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #28
This started to look to be a different issue than what I initially set out to tackle years ago. Installed new coil and my high rpm breakup is gone, and 2k rpm revving is more steady, but I still hear and feel what I believe to be light misfiring,. Really curious if a different distributor/pip/tfi now would help.

Now I need to revise my testing and see what I should re-test now that a failing coil has been replaced.

Will be nice to have m y Quarterhorse back so I can test things via tune changes, and dropping out of closed loop.

I will re-test with oscope at some point to see if the wave forms look different, and more importantly, more consistent. As for oscope itself, using an induction test still means much less than sticking a $2,000+ high-Voltage probe directly to the coil...I won't purchase such a thing.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #29
Wiring harness looks good, but potential issues with that ( no idea yet why my idle doesn't purr) is one of the reasons why, in the past, I was close to dropping the $10k to do a coyote conversion...all because of what I see as a poorly running motor. Most others wouldn't care, and for a long time I just did my best to ignore it. "It's an early computer-controlled v8 with now old sensors, harness, etc, and who remembers now how smooth they ran in the 80's"

...but I still want it to idle and rev rock solid. It has a stock '89 HO cam after all - nothing aggressive.
1988 Thunderbird Sport