91 Mighty Max Truck Missfire at Cylinders 1 and 4.

[mustangv12]

91 Mighty Max Truck running very rough. I have narrowed the problem down to an issue with cylinders 1 and 4. Cylinders 2 and 3 are firing properly. Getting spark, appears to be a fuel flow issue. Pulled injectors which appear fine and noticed 1 and 4 may be on the same circuit based on color of electrical connections. Looking at the wiring diagram it appears I could have an ECU issue going on. Any ideas appreciated? Where is the ECU on the 91 MMax? Can't get a straight answer from Haynes manual or Google searches.

Thx.

[geezer101]

Pennyman left a comment in chat - behind the glove box. Get your hands on a multimeter and check the continuity in the wiring from the ECU connection to the injector harness. Is it throwing any error codes?

[pennyman1]

that was Camoit, not me. The answer is the same, though - it sounds like the ECM is not firing the injectors or the injectors are plugged. Also change the fuel filter - was this truck sitting for a time before this issue started happening?

[mustangv12]

Thanks Geezer101. I finally found the ECU location in my Haynes manual. I stumbled across it in the Emissions Control Section. I have not tried to pull any codes yet. A little unfamiliar with this process on this vehicle since I cannot use my OBD II code reader. However, I do have an analog voltmeter. Perhaps it is not as difficult as it looks.

[mustangv12]

Pennyman1 thanks for the response. So, the ECM is the same as the ECU correct? The truck was damaged a year and a half ago when a 15 year old without a drivers license plowed into it in front of my house. It has been parked while I am working to straighten out the damage so I can replace body parts. What I have done so far - replaced fuel pump and filter (tank was full of white crystals coating the inside walls of tank, fuel sending unit, and fuel pump. Guessing from the crappy ethanol blends we use now days). Got the truck to start, but missing badly. So, I replaced all plugs, wires, dist cap and rotor and played with the timing a bit - but no change. I pulled compression check and all 4 are 150 - 160 range. Started pulling plug wires while running and vehicle died when I pulled plug wires 2 or 3, but no change with 1 or 4. You can hear the spark jumping, so the plugs are getting fire. That made me think the injectors were plugged up from the white crud I found in the tank. I have pulled the injectors and they appear to be fine. I have ordered a set of reman injectors for install since they have 150K on them. That led me to believe the injector solenoids for 1 and 4 are not getting a signal to disperse fuel. I pulled the plugs on 1 and 4 and they appear to be fairly clean when compared to plugs 2 and 3, which are sooty from firing with fuel. So, I'm pretty sure the injectors are not getting a signal to operate. So, now I'm thinking it has to be the ECU or a relay or something. I plan to pull the ECU now that I know where it is and check to see if I can locate any damage from capacitor leakage or shorting. So, should I still try to pull off any error codes?

[mustangv12]

Pennyman1, I just provided a very lengthy reply and I do not see that it posted. So, a very short version response to you. Yes, the truck has been sitting for about a year and a half. It was damaged in a wreck and I'm working on replacing body parts. I would start it periodically, but one day it would not start. I pulled the gas tank to replace the fuel pump and it was full of white crud, presumably from the ethanol fuels we now have to burn. I replaced the fuel pump, cleaned the fuel level sending unit, and replaced the fuel filter. I have replaced all the standard ignition parts and pulled compression check. Getting fuel and spark at cylinders 2 and 3, but only spark at 1 and 4. Remembering the white crud in the gas tank, I pulled all the injectors expecting to find them clogged, but they appear to be OK. So now I'm thinking ECU or an injector relay or something. Would computer show error codes for that? I do have an analog volt meter, but have never pulled codes in that manner.

[pennyman1]

You could switch out the injectors to see if the problem moves or stays - if it moves it is the injectors - if it stays, its the ECM or Bad clips / wiring.

[mustangv12]

I have remanufactured injectors on order and plan to replace all of the old ones. Put it all back together and see what happens. Although I have not had the time, I still plan to pull the ECU and look at it. Thanks for getting back with me.

[pennyman1]

no problem - I want to see your truck on the road again.

[mustangv12]

OK guys, Managed to get the stock fuel injectors changed out last night with 4 remanufactured OEM fuel injectors and the problem is gone. Apparently the stock injectors were clogged from the crud I found in the fuel tank, which must have occurred from sitting up for a year and a half. So, no ECU problem, thank goodness. Now, all I have to do is replace all the damaged body parts and get her back on the road. Thanks to those of you who responded to my pleas for suggestions. Have a great weekend.

[pennyman1]

Glad to see the truck is running again. We want pics - any condition is fine.