I have a 1995 F-150 4.9L 5speed.

The truck has been running very rough lately. It misses and bucks while cruising around 2000rpms. If you floor the pedal, it goes away temporarily, but then comes back. It seems it happens more while cruising or accelerating lightly.

I have replaced the following things:

Coil, Fuel filter, CTS (coolant temperature sensor) in the past week, and the plugs/wires/cap are less than a year old.

Here is the strange part. If I disconnect the CTS, it runs perfect, but the check engine light is on. This is why I replaced the CTS, as I thought that was the problem, but it did not work. Since it runs fine with the Check Engine light on, it must be a sensor, maybe the Oxygen sensor??

Any thoughts?

Thanks
Ray

 

I had the exact same problem in my '95 4.9L, except I didn't try unplugging the ECT (Engine Coolant Temp) sensor. It turned out to be dirty injectors. I was able to confirm this before getting new injectors by dumping two large bottles of a store bought fuel injector cleaner, in with a 1/4 tank of fuel. Note; this was enough cleaner for 2 or 3 tanks of gas, but I mixed it with only 1/4 tank of fuel, so it was comewhat concentrated.

I drove it around a little bit, and the next day the problem was almost gone. I ordered new injectors, and the miss/stumble/bucking while at a light cruise completely went away.

Take care,
~Chris

 

I will try that, but if my injectors were bad, it would seem it would still happen even though I have disconnected the CTS and thus bypassed all the sensors. I am leaning towards a bad O2 sensor since it does not happen at full throttle (when the vehicle is in open loop).

I also put it on a scanner, and got no codes.

--Ray

 

My truck would accelerate excellently, and never missed a beat at WOT. The only time the injectors were causing my problem would be cruising at a steady, lower speed, say, 40 mph as an example. The truck would cut out in a random pattern, and buck and jump. It reminded me of a bad TPS because if I accelerated a little bit, it went away. But as soon as the speed leveled out again, it was back.

I'm not saying whether or not this is your problem, I'm just sharing my experiences, and what I learned.

Best of luck, and please post the solution when you figure it out!

 

PKRWUD--Your truck was just telling you that you were driving TOO slow.

 

Could be the o2 sensor but those hardly ever cause drivability problems when you unplugged the ECT sensor the computer may have just richened up the fuel mixture making it run better even with clogged injectors.

Make sure your spark plugs are tight, they like to loosen up on the 4.9's.

-Jon

 

You obviously don't know me very well, do you?