So here it goes, a few months ago I bought a 1989 f150 with the legendary 4.9L and the Mazda 5 speed, factory no A/C truck from the original owner. The plan was to DD it for a year than rip the motor out next year to put in my 1966 Unimog since the stock motor is underpowered, the 4.9L shares the same geometrical tolerances, I want EFI for easy starting and off camber situations and lastly because I am a Ford fan. Anyways when I got this truck for 1K it ran great but was ugly, had crap tires and shifted like crap so I replaced the shift bushings, gave it a new paint job, hoses, belts, radiator, shocks and new tires, good truck for 70miles or so. But the gas tank gauges did not work and it left me stranded one day on the side of the road with full tanks. Pulled the tanks and was amazed that one in tank pump caught fire and the other was seized up. Both tanks were rusty inside so I replaced both brand new tanks with new sending units, pumps, main boost pump, and filter. Truck ran excellent and passed emissions with flying colors, that is until I tired to make any trip in excess of 20+ miles.

Literally 30 seconds after passing emissions a new problem reared its ugly head. The truck literally shuts off and will not restart for an hr or two. I literally just got done replacing every single component of the fuel system so I thought spark right? heat=shutoff=TFI So I thought the TFI went bad, replaced the TFI, coil, entire distributor and it seemed better but it kept shutting the truck off. The truck left me stranded on the side of the freeway, gangster hoods, middle of intersections etc, but I thought I was making headway, I live in phoenix and this is an extreme environment with 120degree+ temps everyday so whatever. Frustrated, I bought my 4th TFI unit (motorcraft this time) and electronically shielded it and relocated it to the fenderwell on it's own heatpipe copper heatsink with dedicated fan, didn’t help. I gave up, threw in the towel and bought a brand new daily driver since I had school starting back up and it's my last semester... I could not rely on a lemon.


I took the thing off my insurance, I would like to sell the truck or use the motor for another project. But now I am not reliant on it so I have time, I cleaned all the relays, checked all the fuses and checked all connections and cleaned all of them. I can recreate the no start and die condition by letting the truck idle for an hr or drive it around 1500-2600 rpm for 30 mins. When it does die, the truck has good spark but the fuel pumps and injectors get no voltage, I did a stand alone test supplying voltage to the pumps and it wouldn't run. Injectors do not fire and only 30psi to the fuel rail. (good pressure 55psi to the fuel rail with standalone test or when truck "works") The truck throws no codes or CEL. Truck will magically run after letting it sit and cool for 2-3hrs. When it runs it runs excellently and has gobs of power like it was brand new.


I am extremely frustrated and disappointed at this point, this truck has driven me more miles on the back of a tow truck then it has on the road. I think my last and final step is to replace the ECM and then it’s off to the Junker for it, or maybe a slim chance of going to DSII and Carb. What do you guys think I should do or what my next step should be? I have alot of time, work and money into it and cant really go back now.

 

I believe the eec power relay supplies power to the injectors as well as FP relay, I have seen them go bad intermittently. I would carry a voltmeter and wiring diagram with me, when it shuts off trace the circuit and see where you are losing power.

 

2X on the ECU relay, it runs the ECU,injectors and if the ECU has no power, the fuel pump relay will not engage the pumps.

I had that same problem with my 89 F250, intermitantly would die. May start back up right away or not for a few min to hours. I replaced the ECU relay and problem was gone. For me it took about a year of intermittant issues to hard fault to find that stinking relay, thats after checking,swapping and replacing many other things too......

 

I had an 89 that an ECU relay problem as well. It would happen all the time and I replaced my relay 4 times. I finally ended up selling the truck, the person who bought it had to replace the relay AGAIN and to my knowledge, never had a problem after that. That year was the worst 4.9 I've ever had.

 

Sounds like you have a bad ECU computer.

 

He just might. Did those ECUs have problems with the capacitors leaking too?

 

They did but I have seen a few post where the truck did the same thing and it turned out to be the computer.

The other possabilty is the PIP sensor in the distributor is bad. He said he replace the entire distributor but if it was not a new distributor most of the rebuilt distributors have bad PIP sensors in them.

 

Brand new, not a reman.

 

I can't help you on the technical issues, but I wanted to say that your frame looks awesome (from the photo showing the new gas tanks).

Did you re-paint/POR-15 it? Or was it just like that (because its an Arizona truck)?

 

Thanks, it's originally from CA and spent most of its life in AZ, I didn't do anything besides taking the tanks out and towed it to an RV powerwash station. Just regular paint from ACE hardware, but I did use 3M undercoating under the cab and for the front portion of the frame, it's worth every penny

 

Alright I ordered the ECC relay and a couple others I never replaced. Going to try this thing again, then the ECU or she is goin carbed.

 

My computer took a crap in my 93 w/300-6, but I lost the reference voltage for most of the sensors. I pulled the computer and some of the capacitors on the board were fried. Mine were pretty obvious.

Don't feel too bad about your truck, my uncle bought a brand new BMW suv that would randomly shut off and leave him stranded-but would start right up as soon as the tow truck dropped it off at the dealership...never figured out the problem.

 

Same here. I've found that the 300's are plagued with capacitor failure.