Finally!
I know its a Ford, but I really should have posted something about it here. It was just such a slow motion thing that I did not want to drag it out.
Swapped in a rebuilt motor after about 250K. Still ran strong and had good oil pressure, but it had a persistent leak into two cylinders that would not go away with a head gasket .
Figured it was a cracked head or block, and didn't want to tear it down and find that out.
New engine ran strong, but had problems with stalling and wanting to rev up. Fixed most of that with a new fuel pressure reg.
Drove it to town, and that is when the weirdness started.
Pull up to a red light behind a car, and without warning the engine would go to half throttle.
This truck is built to run over or through just about anything in its way, so this was not a good thing.
I would stand on the brakes and not crush the car in front. Then, suddenly it might throttle down and often stall.
I was a nervous wreck when I finally got it home.
Checked all the obvious things. TPS, fuel pressure, swapped the computer, wiring connections.
It has a air bypass motor to control the idle, did not even check it, just swapped it.
Disconnected the cruise control just in case.
Took it to my mechanic, and we saw an erratic pattern from the distributor to the computer.
Aha!
Dist. bushings were so worn, the interrupter wheel was hitting the plastic of the pickup.
But, its a Ford, so it kept running as best it could.
New dist. Same problem. The truck sounds haunted. Crank it up, it idles fine as long as its in open loop.
As soon as it goes closed, the show is on.
Everybody in the shop would stop and stare. Nobody close by, it idles a little while, then suddenly revs to 4K. Maybe higher, or lower.
Then drops back down to normal. Rinse, lather, repeat.
No apparent rhyme or reason. Might go along for 5 minutes normally, then peg the tach, or just blip its self up and down.
A lot of the folks thought I was playing a joke on them. It sounded like someone was doing it.
It stayed in the shop most of the summer. I would visit it, and try some things.
My mechanic would get some spare time, and roll it in and do some tests.
We tested every sensor on the thing. Used a breakout box to examine all the inputs and outputs on the computer.
The computer was commanding it to rev up. It knew what the rpm was, it just wanted it to go faster.
Finally, reading a copy of the program code in the computer I found, I discovered exactly one instance where the computer was supposed to rev the engine past fast idle with no command from the throttle.
That was in case of engine overheat at idle. It would rev the engine up to speed up the fan and cool it off.
I could not read the temp input from the computer. But the sending unit tested good, and the voltage seemed right according to his meter.
I went to the house to get my Scope, so I could catch any glitches or spikes and record them.
When I got back, it was idling fine. I watched it for a while, and looked at my mechanic.
He had a big grin on his face.
Didn't say a word, just handed me the old temp sensor.
Its laying here beside me now. I'm going to do some small electrical tests and then cut it open to do a detailed autopsy this weekend.
Meanwhile.......ITS BACK BABY!!
Lots of stuff that has been on hold is going to happen now.
That tree that fell across the road by the pond?
Meet a PTO winch!
Oh Yeah, Specs.
1989 F250, Modded.
351ci multport FI.
C6 tranny, 205 Tcase, 4:10s 12.50/33s mud.
8" channel iron swept back bumper, diamond plate on top.
12K+ hydraulic PTO winch running off the Tcase.
Armed.
I know its a Ford, but I really should have posted something about it here. It was just such a slow motion thing that I did not want to drag it out.
Swapped in a rebuilt motor after about 250K. Still ran strong and had good oil pressure, but it had a persistent leak into two cylinders that would not go away with a head gasket .
Figured it was a cracked head or block, and didn't want to tear it down and find that out.
New engine ran strong, but had problems with stalling and wanting to rev up. Fixed most of that with a new fuel pressure reg.
Drove it to town, and that is when the weirdness started.
Pull up to a red light behind a car, and without warning the engine would go to half throttle.
This truck is built to run over or through just about anything in its way, so this was not a good thing.
I would stand on the brakes and not crush the car in front. Then, suddenly it might throttle down and often stall.
I was a nervous wreck when I finally got it home.
Checked all the obvious things. TPS, fuel pressure, swapped the computer, wiring connections.
It has a air bypass motor to control the idle, did not even check it, just swapped it.
Disconnected the cruise control just in case.
Took it to my mechanic, and we saw an erratic pattern from the distributor to the computer.
Aha!
Dist. bushings were so worn, the interrupter wheel was hitting the plastic of the pickup.
But, its a Ford, so it kept running as best it could.
New dist. Same problem. The truck sounds haunted. Crank it up, it idles fine as long as its in open loop.
As soon as it goes closed, the show is on.
Everybody in the shop would stop and stare. Nobody close by, it idles a little while, then suddenly revs to 4K. Maybe higher, or lower.
Then drops back down to normal. Rinse, lather, repeat.
No apparent rhyme or reason. Might go along for 5 minutes normally, then peg the tach, or just blip its self up and down.
A lot of the folks thought I was playing a joke on them. It sounded like someone was doing it.
It stayed in the shop most of the summer. I would visit it, and try some things.
My mechanic would get some spare time, and roll it in and do some tests.
We tested every sensor on the thing. Used a breakout box to examine all the inputs and outputs on the computer.
The computer was commanding it to rev up. It knew what the rpm was, it just wanted it to go faster.
Finally, reading a copy of the program code in the computer I found, I discovered exactly one instance where the computer was supposed to rev the engine past fast idle with no command from the throttle.
That was in case of engine overheat at idle. It would rev the engine up to speed up the fan and cool it off.
I could not read the temp input from the computer. But the sending unit tested good, and the voltage seemed right according to his meter.
I went to the house to get my Scope, so I could catch any glitches or spikes and record them.
When I got back, it was idling fine. I watched it for a while, and looked at my mechanic.
He had a big grin on his face.
Didn't say a word, just handed me the old temp sensor.
Its laying here beside me now. I'm going to do some small electrical tests and then cut it open to do a detailed autopsy this weekend.
Meanwhile.......ITS BACK BABY!!
Lots of stuff that has been on hold is going to happen now.
That tree that fell across the road by the pond?
Meet a PTO winch!
Oh Yeah, Specs.
1989 F250, Modded.
351ci multport FI.
C6 tranny, 205 Tcase, 4:10s 12.50/33s mud.
8" channel iron swept back bumper, diamond plate on top.
12K+ hydraulic PTO winch running off the Tcase.
Armed.
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...weird thing was the truck would run perfectly normal for about 30 seconds to a minute or so,then "floor" itself!...so you'd be unawate of any trouble till it was too late!..
