Engine Car miss firing at 3200 rpm

I'm not sure where to start but I bought a 91 mustang GT off of eBay a few months ago. Never had any issues until a week and a half ago. The car is fully built strocker 347 with a procharger only running 7lbs of boost. Edelbrock Victor Jr heads and intake and a dart block. The whole fuel system and cooling system have been upgraded. It has tremec transmission along with upgraded drivetrain and rear end. Now to problems. It ran absolutely beautiful for first couple months then I noticed it started missing around 3500rpm. I changed the plugs with the same ones n there autolite ap3923. The car ran so much better like a new car. This lasted for bout 2 months then turned to :poo:. I started checking most components in the ignition system. I thought I found the problem when I found no.7 spark plug wire boot was bad either from header or the steel mesh oil dip stick. I tried something generic and easy. I wrapped electrical tape around the boot and it ran great for the night but knew it wasn't going to last cuz I could smell the electrical tape melting. I ended up replacing the spark plug wire and went to the next hotter stage of plugs 3924 since the plugs didn't look like they were burning off the deposits. The cap and rotor are good and tried a new coil but to no avail. I tried a bunch of other things like changing fuel pressure, adjust timing. Everytime I make an adjustment it will run good temporarily like half hour or so then go back to crap like a sensor took over. Someone mentioned to the egr valve but it has been removed. The car has an AEM wideband which I know has to have a oxygen sensor. This is farther towards the end of the x-pipe. It appears there are 2 more oxygen sensor, one on each side closer to the headers but I don't know if they are hooked up. I just remember eliminated them in my first mustang. The car will run great first thing in the morning for about 5-10 minutes depending on driving then it will start missing at 3200rpm. You can drive the car normal as long as you shift by 3200. I kinda have come to the conclusion that it's the oxygen sensor. I was told that they don't do much til they warm up to 600 degrees. Which would seem right. I was also reading on here that it could be the tfi module which my dad thought it was n the ignition system but he doesn't know a lot about Ford's. I could greatly use any information cuz I spent most of my savings on this car. I knew I would have some minor things to do but this is mechanical strong and pulls like heck. If you need any more information from me please let me know. Thanks so much and greatly appreciated
 
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Hi,
Sounds like an interesting ride. Like any motor, it requires the essentials.. Any questions, please ask. Can you pls.snap a Pic or two of the Motor to aid in diagnostics?
The Car’s running EFI, correct? It needs the O2 Sensors to initiate EEC’s closed loop, partake in A/F Ratio calculations.. You have an AEM Wideband Air/Fuel Gauge, correct? Aftermarket ignition components are significantly more prone than OE to fail, or cause issues such as this*.
Does the problem seem to be only RPM related, and occurs regardless of how much or little the throttle is open, & does the car “jerk” when this occurs, how does this ‘miss’ present?
Buy OEM replacement ignition parts/Sensors when required. I’d start with these, some are questions, other’s- Tests...
1) Is there a Check Engine light ? Locate any Codes, ether way, what did you find?. Pull with an OBD-1 reader, or by Jumping the DTC, reference: http://sbftech.com/index.php/topic,2471.0.html
2) Buy a Can of CRC MAF cleaner & spray the MAF Sensor clean & verify everything to the T.Body is clean/ clamps tight with no CAI leaks.
3) Have you altered Timing with the SPOUT jumper removed, then reinstalled? Run Timing conservatively at 10-12 Degrees until you locate the issue(s)..
4) What is your fuel pressure at the rail? Should be >/= 30-35lbs.
5) Pull the TFI, Autozone tests them for free. Bring a hairdryer and hit the TFI with heat during the testing to simulate engine heat.
6) What Color were the Plugs electrodes, what “deposits” are you referring to that made you jump up one heat range?
7) Sounds like the EEC goes to closed loop when the issue starts. 02’s need be the correct temp for closed loop to occur. Verify both 02’s are connected, may have a melted wire on a header.
8) Check your Engine Coolant Temp Sensor. Ones for the Gauge, Pass. side is for CPU. Pull the plug & measure Resistance with a VOM when Coolant is cold, also when hot.
Verify your Coolant level is topped off.
9) Pull & clean, apply anti-corrosive & reinstall your Grounds so they’re solid. Battery to Block, Block to Chassis, Battery to EEC, Battery to Chassis. Test Battery to Dizzy’s Chassis with a VOM.
10) Pull and inspect the #3924 Plugs. What color are the electrode(s)? If they appear Carbon fouled, I’d suggest you pull back the gap to 0.035.
11) Pop the Dizzy Cap off, pull the Rotor, remove the 2 screws below & clean any corrosion off any surfaces with light paper, blast the inside out with CRC MAF cleaner.
Hall Effect Sensor..
12) Is there any possibility there’s a 2 Step, or a launch control present that you may have enabled, that may be getting triggered?
FYI:
Top causes for this issue: Dirty MAF, Bad Plug Wire, TFI, fouled Plug(s) Dizzy or weak Spark.


The above should yield you some results, any questions you answer will help.
Good luck!
-John
 
This thread so far appears to be one shot in the dark after another.


Go step by step and report your findings as you go.
 
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Hi,
Sounds like an interesting ride. Like any motor, it requires the essentials.. Any questions, please ask. Can you pls.snap a Pic or two of the Motor to aid in diagnostics?
The Car’s running EFI, correct? It needs the O2 Sensors to initiate EEC’s closed loop, partake in A/F Ratio calculations.. You have an AEM Wideband Air/Fuel Gauge, correct? Aftermarket ignition components are significantly more prone than OE to fail, or cause issues such as this*.
Does the problem seem to be only RPM related, and occurs regardless of how much or little the throttle is open, & does the car “jerk” when this occurs, how does this ‘miss’ present?
Buy OEM replacement ignition parts/Sensors when required. I’d start with these, some are questions, other’s- Tests...
1) Is there a Check Engine light ? Locate any Codes, ether way, what did you find?. Pull with an OBD-1 reader, or by Jumping the DTC, reference: http://sbftech.com/index.php/topic,2471.0.html
2) Buy a Can of CRC MAF cleaner & spray the MAF Sensor clean & verify everything to the T.Body is clean/ clamps tight with no CAI leaks.
3) Have you altered Timing with the SPOUT jumper removed, then reinstalled? Run Timing conservatively at 10-12 Degrees until you locate the issue(s)..
4) What is your fuel pressure at the rail? Should be >/= 30-35lbs.
5) Pull the TFI, Autozone tests them for free. Bring a hairdryer and hit the TFI with heat during the testing to simulate engine heat.
6) What Color were the Plugs electrodes, what “deposits” are you referring to that made you jump up one heat range?
7) Sounds like the EEC goes to closed loop when the issue starts. 02’s need be the correct temp for closed loop to occur. Verify both 02’s are connected, may have a melted wire on a header.
8) Check your Engine Coolant Temp Sensor. Ones for the Gauge, Pass. side is for CPU. Pull the plug & measure Resistance with a VOM when Coolant is cold, also when hot.
Verify your Coolant level is topped off.
9) Pull & clean, apply anti-corrosive & reinstall your Grounds so they’re solid. Battery to Block, Block to Chassis, Battery to EEC, Battery to Chassis. Test Battery to Dizzy’s Chassis with a VOM.
10) Pull and inspect the #3924 Plugs. What color are the electrode(s)? If they appear Carbon fouled, I’d suggest you pull back the gap to 0.035.
11) Pop the Dizzy Cap off, pull the Rotor, remove the 2 screws below & clean any corrosion off any surfaces with light paper, blast the inside out with CRC MAF cleaner.
Hall Effect Sensor..
12) Is there any possibility there’s a 2 Step, or a launch control present that you may have enabled, that may be getting triggered?
FYI:
Top causes for this issue: Dirty MAF, Bad Plug Wire, TFI, fouled Plug(s) Dizzy or weak Spark.


The above should yield you some results, any questions you answer will help.
Good luck!
-John
Thanks for the suggestions
1. #15 which I guess is not resetting codes correctly- not sure
#67 I don't understand. The thing I can think of is the a/c relay which I switched with fuel pump relay when it melted on header
#85- cang canister which was removed with the smog pump
2.Did do that didn't help but thanks
3. On my list to do
4. Right at 35
5. On my list. I really think that is the problem but part of my problem is the people that work in them around are not the smartest. Sorry not trying to be mean
6. Carbon build up and maybe little oil deposits. The car does run really rich. Need to get it tuned but can't find anyone around here
7. That's what I thought it was n the beginning but wouldn't throw a code for that
8. Need to check. Supposedly had new one just put in. Also I can't fill the radiator all the way cuz I have a small overflow tank that is only 14oz. If I do fill it the overflow tank gets filled then spits the rest of it on the ground
9. Need to check
10. They look better than the 3923 that were in there but still slight build up but it runs rich and I don't get on it at least until I fix the misfire. Gap is at .035
11. Did that no effect
12. Not that I know of
Plugs and wires are new and I checked all of it with inline spark tester. Everything was good. I appreciate all the suggestions and will let you know results of rest of them when I can
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