I have a 1990 Mustang lx that started off as a 2.3 AOD car that in the last 2 years I have swapped a brand new 5.0, T5 and all the GT parts to go along from a 1993 GT. Since I have finished the swap, it's been nothing but problems, firstly I accidentally wired the plugs in order for a carbureted engine and had a horrible misfire but noticed that only the #3 and 4 cylinders were not firing by feeling the exhaust headers. once I noticed the issue I reorganized the plug order to standard H.O firing order and once I did that it fired right up and idles on its own but I'm still noticing its headers are cold now on 2 cylinders on each side, I think cylinders 2,4,5 and maybe 7. it doesn't die off on its own since the timing is slightly advanced for a higher idle but I've put the cats on with the rest of the exhaust cut off just behind the transmission and even with those on I hear a ton of piston noise and the misfire is quite noticeable, I have videos that I will add onto this thread but I can't figure out why it would be misfiring in such a strange order, I have yet to dial in my fuel pressure so I have hopes that that's it but I can't see something that small causing that big of an issue, any experts please listen to the videos and tell me your thoughts. it sounds very rough in the first video because it was open headers at the time, but I just want to put my concerns behind me and be done with it. Any other ideas would be greatly appreciated, for reference, this is a 1993 convertible engine harness and ecu paired with a 1990 lx body harness. the wiring connected perfectly my only concern being the 2 fuel pump relays and the slightly different relay behind the ac unit in the dash, but the car still primes and runs, the engine and rotating assembly are brand new with rebuilt gt40p heads and a brand new t5 along with new mcleod clutch and flywheel. flywheel and balancer pully are both 50oz counter balanced. new fuel pump, injectors cleaned up fuel rail and lines. every part on this car has either been newly purchased or cleaned/rebuilt. Thanks again
Foxbody Misfire 5.0 Swap
- Thread starter DPORTER0729
- Start date
It won't let me add videos but please if you know about cars and engines, please leave your email so i can email you the videos and get a second opinion, once I get home, I'm going to check my fuel pressure and make sure that's dialed in then maybe try to restart it.
Post your videos on Youtube and copy/paste the links here.
I do not see a [single] EEC code listed. That's going to tell us a whole lot more than listening to knocks and pings through the cell phone microphone.
I do not see a [single] EEC code listed. That's going to tell us a whole lot more than listening to knocks and pings through the cell phone microphone.
Your first problem is going to be a mismatched dash harness to ECU harness.
The harnesses are not interchangeable from 1987-1993.
92.5 - 93 only
91 - 92.5
90 only
89
87-89
You can use an 89 ECU harness on a 87-88 speed density car to swap it to mass air.
O2 sensor harness are the same with the exception of auto and manual cars so you just have to repin them. If you don’t smoke will come out of the ECU. Mustang 5L5 has a really good thread on this here:
Where your issue will be is at the connectors from the ECU harness to the dash harness at the brake booster.
The other issue is you have a fuel pump relay under the driver seat and the 92.5-93 harness relocates it to in front of the passenger shock tower with the WOT AC relay. There pare other issues but you get the idea. You need a 92.5 - 93 ECU harness or you will need to swap dash harnesses which I would advise against as there are issues there.
Another thing is all of these ECU’s are 30+ years old and have timed out component wise. Caps, chips, and power supplies start failing and cause all kinds of weird issues.
Have you tried to pull codes? If not that is the first thing you should have done. If you cannot pull codes then that is usually a sure sign the ECU has issues.
The harnesses are not interchangeable from 1987-1993.
92.5 - 93 only
91 - 92.5
90 only
89
87-89
You can use an 89 ECU harness on a 87-88 speed density car to swap it to mass air.
O2 sensor harness are the same with the exception of auto and manual cars so you just have to repin them. If you don’t smoke will come out of the ECU. Mustang 5L5 has a really good thread on this here:
Electrical Thread 'O2 harness repin'
This thread is a HOW-TO for the process of repining the o2 sensor harness to match ECU/Trans, in the event the car is swapped. See post 15 as to what the jumper should be repinned to.
Step one. Remove the o2 sensor harness from vehicle. I find it easier to do on a bench plus it allowed me to degrease and reloom the harness. You can do this in the car if you want...it's that easy.
Here's the default loop setup in my 1988 AOD car
First you need to remove the red clip. Lightly pry up with a small pick
Next, look down inside with a light and you'll see small...
Step one. Remove the o2 sensor harness from vehicle. I find it easier to do on a bench plus it allowed me to degrease and reloom the harness. You can do this in the car if you want...it's that easy.
Here's the default loop setup in my 1988 AOD car
First you need to remove the red clip. Lightly pry up with a small pick
Next, look down inside with a light and you'll see small...
Where your issue will be is at the connectors from the ECU harness to the dash harness at the brake booster.
The other issue is you have a fuel pump relay under the driver seat and the 92.5-93 harness relocates it to in front of the passenger shock tower with the WOT AC relay. There pare other issues but you get the idea. You need a 92.5 - 93 ECU harness or you will need to swap dash harnesses which I would advise against as there are issues there.
Another thing is all of these ECU’s are 30+ years old and have timed out component wise. Caps, chips, and power supplies start failing and cause all kinds of weird issues.
Have you tried to pull codes? If not that is the first thing you should have done. If you cannot pull codes then that is usually a sure sign the ECU has issues.
I havent checked the ecc codes and im not sure how to as it doesnt have an obd port and only the pigtail in the engine bay to do the paper clip test or whatever. as far as the harness goes, i took the entire stock dash harness and the ecc harness for the passenger side from the 93' so the entire engine/trans has a stock 93 harness, the only harness from the 90' is the connectors under the kick panels through to the back of the car, everything had connected like normal and everything works as it should, pump primes, all lights work, even the trunk popper works....
If you don’t get any codes ECU is most likely bad.
this is the video of it starting up, correct firing order with the cats bolted up, couldnt upload it as a full video unfortunately... reminder that directly after taking this video, i felt all the header pipes and 4 of them felt like they were not firing.
View: https://www.youtube.com/shorts/NIsdEcBwj00
View: https://www.youtube.com/shorts/NIsdEcBwj00
Last edited:
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