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Topic: Troubleshooting persistent misfire (Read 2162 times) previous topic - next topic

Troubleshooting persistent misfire

Cross post from corral.net, where I've received no ideas from anyone.

This will be a long post so that I can lay out what I can think of in terms of powertrain parts, and what I've tested already. I have made followup replies in this thread with powertrain specifications, and what I've tested.

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I recently picked up my decade old project again on "restoration"/re-engineering entire chassis as an updated take on an old fox platform (including CAD/CAM work). This is a 1988 vehicle running mass air, with newer 306 and 4R70W transmission.

Anyway, short disclaimer is that I don't remember exactly how the car ran a decade ago, or even 5 years ago when I drove it to a new house, but I believe I've always had a misfire. This includes iron heads I ran 15 years ago, but memory is extremely fuzzy from that far back, and I knew far less about cars back then. For awhile I also had transmission issues, and spent my time troubleshooting that more than the motor. With a properly working 4R70w in there today, and a 3000 stall Precision Industries converter, all of which runs generally great, I need to sort out some motor drive-ability issues.

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The problem:

I have what sounds like a slight misfire (lean misfire?) - the rpms don't hold steady at idle, or when rev'd to 2k.  This is very noticeable after the engine warms up/AFR comes down, but it's also there when cold, or if I force open-loop via tuning software. The engine isn't as smooth as it should be. Years back I was told that I had flames popping out of open down-pipes, but also later out the tail pipes (with cats) when backing out of the garage, with cold engine. Wideband numbers always look good, and don't report lean conditions when I feel/hear the "misfire".

Video/audio of engine in closed loop, with stock-ish tune and me having EV1 19lb injectors, stock MAF installed:
https://www.youtube.com/watch?v=3b8iIwVc5e8


Any ideas of what else I can look at? I need this fixed for my own sanity - a v8 with a stock cam should run smoother than this. I also believe I had the same issue on my previous gt40p heads, different sets of injectors, maf, intake, etc, but it has been way too many years. I'm documenting my troubleshooting steps this time.


Car runs great otherwise - no WOT performance issues from street driving. Closed loop runs clean. 3.55 gears brought me down to 25mpg highway. 3.08 gears returned 30.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #1
My powertrain specifications:

Stock-type 306 rebuild from 15 years ago, 20k miles on the rotating assembly:
TRW-L2305F30 pistons
SLP-E-251K030 rings
SCA-35090P i-beams
Stock 1989 Mustang camshaft
Stock pan, pump, crank machines and balanced with the rotating assembly
Powerbond 50oz SFI balancer
Timing setup dot to dot, and seems correct for TDC marks on balancer for each cylinder when I tried leakdown tests

Rest of engine (newer than shortblock):
TW 170cc heads cut down to 54cc chambers (lots of clearance with 1.7 rockers, clayed)
Comp-cams high energy hydraulic roller lifters set to 0.025 preload
Whatever pushrod length was needed when the heads were initially installed
GT40 tubular intake with lower porrted by tmoss
19lb stock injectors, and 30lb FMS injectors
Stock MAF, and PMAS 75mm "24lb" MAF
Accufab 70mm throttle body
FMS stainless "gt40p compatible" shorty headers with 1-5/8" primaries
Autolite 3924 plugs gaps tested at .040 and.050
MSD spark wires, <100 miles
Stock distributor/TFI, and Richporter FD04 replacement
Standard "195" thermostat
New Motorcraft TPS
New Motorcraft air charge and engine temp sensors
Various gaskets, 9.8:1 static compression ratio.
EGR and IAC still installed
Currently have fresh 92 octane ethanol-free fuel

Drivetrain:
4R70w transmission/stock VB/clutches from 2003 3.8L
Larger capacity trains oil pan with cooling fins
PI Stallion 3000 stall converter
Stock driveshaft
8.8" rear differential with 3.55 gears, 26.6" wheel diameter

Other:
1989 A9P ECU with new capacitors
Quarterhorse tuning board
New Motorcraft fuel regulator
2.5" exhaust, catalytic converters intact (few hundred miles)
Larger, soldered engine grounds with contact areas using NO-OX grease
Dual Innovate LC-2 wideband sensors mounted horizontally near bottom of exhaust down-pipes
DataQ datalogger  for widebands
Young narrowband o2s that sweep well
1/0 gauge alternator power wire, 4awg timing cover to battery ground, 7awg dual braided head to firewall grounds, crimped and soldered terminals

Much of the above hasn't seen more than around 100 miles in over 5 years. A week ago it was driven to a glass shop, and likely doubled the mileage it has seen since being driven to a new house years ago.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #2
What I've done/re-tested lately:

- Smoke machine intake test, no leaks with oil filler cap taped down
- Distributor swapped out with a new unit, new TFI, as a test. No change. Caps/rotors still both look new.
- New BAP, TPS, air charge, engine temp sensors
- New IAC
- IAC block off
- Rocker arms checked to ensure preload
- EGR valve tested
- Cleaning of TFI plug
- Cleaned and increased contact tension of the salt and pepper connectors at engine harness
- New battery
- Shaken/wiggling harness connections when running to look for change in idle quality (none)
- 30lb EV6 Injectors flow tested - good.
- Reinstalled some original refurb/flow tested 19lb EV1 injectors to test stock tune.
- MAF seems to sweep fine, but reinstalled stock 1989 MAF to test stock tune.
- Leakdown test shows 8-10% across all cylinders. No sound from intake or exhaust - crankcase only. PHOTO PHOTO
- 165-170psi compression test across all cylinders
- plugged off vacuum lines an pcv, no vacuum at oil filler tube (lower intake gaskets should be good). Slight vacuum at oil filler tube with pcv re-attached.
- no exhaust smoke
- no coolant or oil issues
- stock tune (minus increasing CID to 306, and disabling thermactor) with stock injectors and MAF
- new engine bay grounds with conductive grease
- cleaned and re-mounted eec-iv computer grounds
- timing at 10 degrees base. According to tuning software, issue still exists at all timing so not a timing advance issue
- timing should be correct on balancer - markings seem to match TDC of various cylinders perfectly (for leakdown test). Timing chain doesn't appear to be off.
- no engine running codes. Cylinder balance test comes back as passing
- engine is idling fine at 650-750rpms, around the 672 being commanded. Should be more steady.
- 16in/Hg vacuum at warm idle with all vacuum accessories attached, 1300rpm and above get 20in/Hg
- Coolant temperature is a steady 206-208F at temperature sensor when idling or driving, 160F intake runner temperature at idle in garage
- I've tried completely isolating plug wires into air as a test, but no change
- Spark plugs all look great PHOTO
- Correct firing order PHOTO
- Timing light on each wire doesn't appear to have any missing ignitions. I hoped to find one with occasional missing flashes...
- Replaced capacitors in A9P
- Binary Editor datalogs generally look good - the stumble isn't timing swinging around.
- Idle doesn't improve if I richen up the AFR. Leaner than stoich just starts to stumble, as expected.
- Fuel pressure 40psi without vacuum, 33psi at idle with vacuum. PHOTO PHOTO
- New alternator due to failing bearings. New alternator's AC ripple is 29mA.
- EGR block off

Things that I still want to check:
- Replace salt/pepper engine harness connectors with 12-pin deutsch connectors
- New coil again? Years ago I tried a new aftermarket coil, but it would breakup badly at 2500+ rpms. I went back to my old coil.
- Unplug Quarterhorse completely and run stock tune only with stock MAF/injectors
- Exhaust smoke machine test
- Try a completely different A9P?

Other plans:
- EDIS-8 to ditch TFI altogether. Just need a crank pulley to take to machine shop with trigger wheel for welding/balancing
- Install my Lightning MAF, for potential better metering than the aftermarket unit, unrelated to stock air/fuel parts and tune
- Upgrade from 130A alternator to newer 200A, 0awg ground upgrade
- Maybe removing the exhaust to see if my issue has nothing to do with the motor, but ignition in the cats sending waves back up to the motor, messing with cylinder activity?
- New seal on oil filler lid since I get a slight leak under pressure from the smoke machine
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #3
What's your tps set to at CT KOEO? When you replaced the IAC and TPS how did you reset base idle just out of curiosity. Also can you share the datalog .CSV file for binary editor? For when its acting up obviously.

Re: Troubleshooting persistent misfire

Reply #4
You still have the old stock coil? Check there first. Mine was behaving similarly when the MSD coil on the car crapped out.

https://youtu.be/91C8ese1LNo

Put a stock replacement Motorcraft coil on and it's been fine since.
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #5
What's your tps set to at CT KOEO? When you replaced the IAC and TPS how did you reset base idle just out of curiosity. Also can you share the datalog .CSV file for binary editor? For when its acting up obviously.

Binary Editor is reading 0.97V at idle.

Let me fire up the new BE version on my replacement laptop and get some newer datalogs (old laptop has BE2010), and I'll get a new cold to warm garage-idle datalog posted.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #6
You still have the old stock coil? Check there first. Mine was behaving similarly when the MSD coil on the car crapped out.

https://youtu.be/91C8ese1LNo

Put a stock replacement Motorcraft coil on and it's been fine since.

I thought your issue was a cracked spark plug?

It has a coil that has seemed to work (it may have been replaced 20,000 miles ago, but who knows at this point. A decade ago), but the coil is a part that I didn't have luck replacing a few years back. Rather than get a entirely new coil at what is likely $80-100, I'd prefer to get this EDIS-8 conversion underway.

I was tempted to tap into an inductive sensor and datalog the coil pulses via the dataq, so that I also have that data available to review. I'd really not care if the EDIS conversion simply fixed my issue though.

Edit: Coil from LMR is $60 shipped
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #7
You still have the old stock coil? Check there first. Mine was behaving similarly when the MSD coil on the car crapped out.

https://youtu.be/91C8ese1LNo

Put a stock replacement Motorcraft coil on and it's been fine since.

I thought your issue was a cracked spark plug?

It has a coil that has seemed to work (it may have been replaced 20,000 miles ago, but who knows at this point. A decade ago), but the coil is a part that I didn't have luck replacing a few years back. Rather than get a entirely new coil at what is likely $80-100, I'd prefer to get this EDIS-8 conversion underway.

I was tempted to tap into an inductive sensor and datalog the coil pulses via the dataq, so that I also have that data available to review. I'd really not care if the EDIS conversion simply fixed my issue though.

Edit: Coil from LMR is $60 shipped

Coil crapped out. Plugs ended up being fine. I changed the plugs, it ran fine then started acting up again. Swapped out the MSD coil with the old stock one and the engine ran fine again.

If you're doing the EDIS conversion the coil might be a waste. On the other hand, based on what you have described, this seems like an ignition or electrical problem. There are only so many mechanical things that can cause the issue, and you seem to have checked them all. Do you know anyone local with a Fox you could borrow the ignition coil from to test?
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #8
No, I don't do anything with other local 'car people', as most are usually less about vehicle engineering, and more about "mods" to vehicles that often hinder performance, and they don't understand how. Not my type of crowd. A typical truck forum shows the demographic I'm talking about, where these types seem to run rampant.

I'd be curious about a coil, but I've gone down the route of "throw parts and time at it" before with various things. No sense in buying new parts if Ill be changing it up.

Anyone have a stock crankshaft pulley for not-$50? Need to get one for trigger wheel install.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #9
I'd be curious to see what the ignition spark/wave looks like on an oscilloscope (Sun Performance Scope?).  I never used one but remember the mechanics using it to chase ignition problems to see if it was only one cylinder and what was happening.
1988 Thunderbird TC, 5spd
Stinger single exhaust, Cone Filter, Adjustable Cam Pulley, Ranger roller cam conversion,
'93 Mustang Cobra replica wheels on 235/50R17

'16 F150 XLT 5.0
'17 Husqvarna TX300

Re: Troubleshooting persistent misfire

Reply #10
Is that like an inductive setup? I'd imagine something could be put together, or a timing light hacked up, that would work to read from an oscope. I may put together something this weekend.

I forgot that I shipped my Quarterhorse off to Moates to get the updated revision. I won't be able to get datalogs for a couple weeks until I receive the replacement unit back.
1988 Thunderbird Sport

Re: Troubleshooting persistent misfire

Reply #11
Do you have a shop near by that has an oscilloscope? Since you're ditching the TFI system eventually it would be best to scope the current system to see if you can figure out the cause. Is the sheathing of the TFI connector wiring still intact? 

If you can't get to a shop with an oscilloscope I may have a spare ignition coil in the garage. Let me look tonight. If I do you can have it for the cost of shipping, just so you can check and see if the coil is the issue.
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: Troubleshooting persistent misfire

Reply #12
I have a couple portable 2-channel oscopes (20 and 60MHz), and a benchtop 100MHz 4-channel. Just need to see what type of signals I can probe at, from where, and have an idea of what I'm looking for on the ignition side. I don't normally scope beyond microcontroller work, and low Voltage vehicle signals.
1988 Thunderbird Sport

 

Re: Troubleshooting persistent misfire

Reply #13
The ones I remember looked like this but I bet you can do it with a regular scope too.  I'm not an electrical type person so never learn how.  Sorry.
https://www.google.com/search?rlz=1C1GCEA_enUS864US864&sxsrf=ACYBGNQ4knTBFeLShHAsEKBR_jkNLegPJA:1570559048797&q=Sun+Performance+Scope&tbm=isch&source=univ&sa=X&ved=2ahUKEwj57M6xpI3lAhUqTd8KHelaCmUQsAR6BAgOEAE&biw=1920&bih=937
1988 Thunderbird TC, 5spd
Stinger single exhaust, Cone Filter, Adjustable Cam Pulley, Ranger roller cam conversion,
'93 Mustang Cobra replica wheels on 235/50R17

'16 F150 XLT 5.0
'17 Husqvarna TX300

Re: Troubleshooting persistent misfire

Reply #14
The ones I remember looked like this but I bet you can do it with a regular scope too.  I'm not an electrical type person so never learn how.  Sorry.
https://www.google.com/search?rlz=1C1GCEA_enUS864US864&sxsrf=ACYBGNQ4knTBFeLShHAsEKBR_jkNLegPJA:1570559048797&q=Sun+Performance+Scope&tbm=isch&source=univ&sa=X&ved=2ahUKEwj57M6xpI3lAhUqTd8KHelaCmUQsAR6BAgOEAE&biw=1920&bih=937

Oscilloscopes are great - Voltage over time, with an adjustable time-scale. Just an extension of a point-in-time Voltmeter.
1988 Thunderbird Sport