Wierd F150 bucking



My wife’s F150 XLT has been acting strangely for over a year now. The mechanic that rebuilt the engine has “given up” finding the problem. One other mechanic also declined the job.

300ci V6, fuel injected, 1990 1/2 (apparently this matters for some parts).

The truck runs in either “fine” or “badly” mode. “Badly” means it bucks when at a constant speed on level ground. Accelerating (or going up a hill) is smooth, and idle is rough.

Upon a cold start, it runs in “fine” mode. After a minute or two, it switches to “badly” mode and continues to run badly until it is thoroughly warm AND the ignition has been turned off. Typically, this means the second trip on a errand runs “fine”, but just momentarily turning the ignition off while moving, switches it to “fine” mode. It stays in “fine” until cold.

It feels similar to running one of the tanks dry, but since acceleration instantly smooths it out, I’m inclined to doubt a fuel problem. I also considered water in the gas, but it’s gone through dozens of tankfulls with no improvement. I read something elsewhereabout EGR valves, but the mechanic tested the valve. Also, the “reset” phenomenon points towards some component that “reboots” or something similar - maybe the ECU?

There’s no engine check light or other idiot light.

It’s had new fuel/air filters, plugs, distributor rotor (I think).

I’ve considered a bad ignition switch, but I would think the idiot lights would flicker if the switch was intermittent?

All suggestions considered!


is truck in good shape? rust? what city do you live in? 27yr old truck. you must like it? does it have good fuel pressure? pumps ever been changed?


A few body dings, but no rust. Engine was rebuilt just a couple of years ago for $3500. A little wear in the rear end, but not a problem for the use it gets. It only gets used to fetch manure, building materials, etc. Fuel pressure is unknown, but symptoms disappear if you just apply a little throttle. Pump never replaced. Hard acceleration works fine. Near Grants Pass, OR.

I’ve looked a replacements, but long beds are scarce in the used market, and spending 40K on a truck that gets used less than 1000mi/yr is sort of silly.


When I had a problem like this in my 1989 Ramcharger, it was finally fixed by a new distributor cap. There were carbon tracks in there shorting it out sometimes.


Did it ever run perfectly after the rebuild job? Or did these symptoms start immediately after you got it back from the rebuild and they haven’t let up?

Some ideas in the meantime …

Bucking most likely it is either a spark cutting out or fuel cutting out problem. I had some low speed neighborhood driving bucking problems on my Corolla one time, turned out to caused by dirty fuel injectors. The spray volume apparently was fine when they sprayed a lot of fuel, but they weren’t consistently good sprayers when only spraying a little fuel. You’ve probably seen garden and lawn sprayers that act like that when they clog up a little. Suggest to start by running a couple of bottles of fuel injector cleaner through a couple of tanks of fuel.

Another idea, some Ford engines of that era had a problem with the gadget that was involved w/ controlling the EGR valve, DPFE or something like that was its name. If your engine uses that part, might want to just replace it.

The heat/cold association could be cause by a faulty crank or cam position sensor. Those sensors are known to be heat sensitive.

You may have a problem with the compatibility of the engine computer to the rebuilt engine if this problem started immediately after the engine was replaced.


Owning a 1994 F150 with the straight 6 myself, it sounds like you might have an intake leak. A modest leak will not be present at start (it will be in “open loop,”) but will make itself known as the cat converter heats up.

If you coast down a steep hill, and “engine brake” in a low gear, does it misfire? I’m guessing it does.

Since your engine was freshly rebuilt, you may have a problem with the (combined) intake/exhaust manifold seal…either a warped intake, or bolts have backed off. That intake is HUGE, and (if one bolt is omitted) the whole thing cantilevers off of the manifold bolts, which will eventually result in leakage. Then there is the EGR (a big enough leak will give you a high idle, too), as well as vacuum hoses.

I’ve given you some places to start looking. Good luck!


…forgot to mention, if the exhaust hangers under the car (in the vicinity of the cat) are missing, then the whole front exhaust will cantilever off of the intake/exhaust manifold, which will result in an INTAKE leak, due to the non-crossflow nature of these engines. (If anything, this will happen even faster than with a poorly-secured intake.)

That would be consistent with an intake leak. At WOT, there is little vacuum, so very little air leaking in, so little misfiring. At idle, there’s a lot more vacuum, so a lot more leakage, so you get a lean misfire. The “worst-case scenario,” as I said, is engine braking.


Lots of info to respond to:
Ran well for a couple of years after the rebuild. The mechanic is retired after 60 years doing this stuff, so I’m confident he did a good job. He spent almost a month chasing this recent issue before surrendering.

No backfiring when engine breaking.

Anyone have a better description of the EGR related Part?

Ran lots of injector cleaner thru system.


Another idea, the throttle body needs to be cleaned; i.e. the throttle valve may be sticking a little.


If it’s running as poorly as suggested, you ought to have trouble codes stored. Refresh me: would a 1990.5 be OBDI? An “EEC-IV” system? If so, run the KOEO (key on, engine off) and the KOER (key on, engine running) tests. These ought to give you a starting point, especially the KOER.


It’s an OBD(1) system - I bought the adapter to read it with my OBD2 reader, but haven’t hooked it up since the light isn’t showing. The mechaninc said there were some nonsense codes when he looked. I’ll hook it up. I tried the flashing dash lights method early on, and none of the codes I got were documented for a 1990, although it’s easy to lose track of the flashing…


Folks here might be able to look up the meaning of diagnostic codes for an obd 1 1990 f150.


Need the rain to let up…


The problem might be with a bad MAP sensor.


The problem with OBI was, If a primary sensor to the computer malfunctioned it wouldn’t always turn on the Check Engine light.

Try this.

Unplug the vacuum hose and plug it, and unplug the electrical connector to the MAP sensor.

Then drive the vehicle.

If the vehicle runs better, the computer has reverted to a default mode for the missing MAP sensor signal.

Which means there’s a problem with the MAP sensor.



Where abouts is the map sensor?


It’s often on the firewall. That’s where it is on my Corolla. It’s easy to find b/c there’s a thick vacuum hose running from the intake manifold to the MAP sensor. And there’s a wiring harness connector to it too. We don’t hear of MAP sensor failures here very often. If you’re a diy’er, one thing you can do yourself possibly is test to make sure it holds vacuum to 20 mmHg. Might as well test the brake booster the same way while you are at it.


I tried my OBD to OBD2 adapter (for the first time) and I couldn’t get any of the various apps to connect to the vehicle. So I tried disconnecting the MAP sensor, and the engine smoothed out. I won’t know whether it is the problem or coincided with the engine getting warm enough to flip to the “good” mode. Need to try it after it cools off completely.


The engine seems to run much better with the MAP sensor disconnected. Does this mean the MAP sensor is bad, or does it possibly confirm an intake leak? Should I be able to hear an intake leak? If the truck goes a second day running okay, I’ll buy a MAP sensor just as a shotgun solution…


The second day of the disabled MAP sensor, it started running REALLY poorly. Won’t idle (just stalls without accelerator), bucking over much wider range. Now it only goes away at full throttle. Doesn’t seem to improve with warming up or cycling ignition.

I installed the new MAP sensor, and the problem hasn’t changed (except the Check Light for bad MAP has gone out).

I took the truck out for a short spin today, and barely made it back home. I put a timing light on a few spark plug wires, and saw no intermittent behavior - just blinks even as it stalls.

As I asked earlier, how do you find an intake leak?


I use starting fluid to find an intake leak.

Start the engine, and while it’s idling, give short bursts of the starting fluid around the intake manifold/vacuum hoses.

If the engine idle speed changes, you’ve found the intake leak.