4.9L EFI Electrical Problems

Asked by Apr 12, 2010 at 08:16 PM about the 1988 Ford F-150

Question type: Maintenance & Repair

Can't get it to idle smoothly or run without bucking.  Replaced plugs, wires, distributor, coil, - basically everything but the wiring harness.  Fuel isn't the problem, fuel pressure in the rail is within spec. Vacuum is within spec wherever we've tested it. Timing is in spec.  This engine has had an intermittent problem like this for over ten years.  We have rebuilt the entire truck and now we can't get it to run right.

11 Answers


Clean out the idle control and spray some wd 40 in it, and it will idle.

3 of 3 people found this helpful.

Thanks, we'll try that. We had that thing off and tried adjusting it, but I don't think we sprayed any WD40 in it. I mean, we've cleaned or replaced the EGR, the ECM, the PCV, you name it, we fiddled with it. This is really annoying.


Ya I would think so. And if that still doesn't do it, check the wires and connections to the computer.


try the mass air flow sensor, there's nothing else left.

2 of 2 people found this helpful.

mine had the same problem and i changed the mass air sensor and it worked

2 of 2 people found this helpful.

i disconected the EGR and plugged the vacum line going to the EGR on the one i used to own,it was ok after that.you could also check your TPS.

I have the same type of issue. I have checked several things. What i have found was when i put the fuel pressure gauge on it at idle i was fine but when i drove it under normal conditions the fuel pressure would suddenly drop off and and the truck would then begin to buck. Sometimes it would gain back the pressure and run fine. I disconnected my regulator vacuum line and the pressure went up to 60psi and the truck idled better, but it would still would drop off pressure as i drove it down the street.I haven't figured out if the pump is losing pressure because it loses power, fuel flow(air), or pump is just failing. Check your pressure with the truck driving down the road. You can lock the hood partially and as long as there are no major bumps it should stay down. You will need a longer line on the gauge that can allow for the gauge to sit on the windshield under the wiper. so you can watch it as you drive. Let me know how it works out!


Mine did the same thing I pulled the vacum line off the regulator itself and found fuel flowing out the regulator, replaced fuel pressure regulator and it ran great.

2 of 2 people found this helpful.

my 4.9 l 6cyn did same ,change oxygen sensor mounted on exhaust pipe before catalytic .ran way lot better,still have unstable idle ,buy ill be checking air flow sensor and idle sensor next.

1 of 1 people found this helpful.


I have a 1989 4.9l engine in my 1983 F150, and I've found fuel issues are the top choices. Both of my in tank fuel pumps are bad, but my electric pump keeps up just fine. Since I have dual tanks, I have the tank selector valve, which when I took it apart and looked, I noticed that it was actually gutted inside. Basically, my truck would draw fuel from whichever had less restriction, and then it would return to either tank. I found that, generally at low RPM, it would draw from one tank, and when I got moving it would tend to draw from the other, giving it a very unsteady idle and the worst bucking when trying to drive. I took the valve out and created my own manual switch with some brass fittings and a couple quarter turn valves, which works great for switching tanks. I didn't have the correct fuel lines for the change so I made a different hose work, but it does not seal properly, so occasionally I have the issue again. One last note; the other day when I was driving, it felt like I was running out of gas, but I knew I had about half a tank. After my truck died and left me on the side of the road, I got out and switched tanks over. I happened to look to the front of the truck and I saw fuel running out. Well, it turns out that the fuel line, still a stock hard line with the clip, had fallen off. I'm unsure if it just wasn't on all the way or if pressure just got that high, but the line popped off with the clip still in place. Along with cleaning your IAT valve, make sure you put a new gasket on, and consider using some hi-tac gasket sealant to prevent a vacuum leak from that area.

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