Greetings - new here, with my first project in about 20 years. Last one was a Diesel VW van, and I learned a lot from a VW forum, so thought I'd sign up.

A friend's wife gave me her 1994 F-150 xlt, with a 302, 4x4, auto trans. They are competent witnesses, and report that intermittently, it won't accelerate when traveling at speed - from 40 to 60, say, as if entering a freeway. Sometimes it is fine, sometimes it won't speed up.

I had it towed to my place, started poking and prodding and cleaning. It starts and runs normally, but blows white when cold, misses a little at idle. Drives around the 'hood okay. It died a couple times when I was sitting in the driveway, listening to it, and revved it up and took my foot of the gas.

While it'll need thorough maintenance, it runs well enough it ought to accelerate, and it does except when it doesn't, so I figured I'd try to find a smoking gun on the acceleration problem first.

I started with an MAF multi-meter test. Sensor's a four-wire style and tested fine for power supply and battery ground, but the sensor-return ground to the PCM only read 5.5 volts when testing between disconnected MAF plug and the battery. I pulled the harness off the PCM, and got 0 ohms between the MAF plug and the the PCM plug, so figure both plugs and the wire between are good, so maybe the flaw is in the PCM - or, I accidently triggered or untriggered the intermittent aspect of the problem.

I reinstalled the PCM harness, and repeated the MAF tests, this time including the running voltage test. The sensor-return ground properly read battery voltage this time, and the sensor output rose and fell with RPMs, as they should.

Anyone else experience this? I gather the PCM cannot be bench tested, I don't have a spare to swap in for a test, and it's intermittent anyway...

My main goal? Find a smoking gun and solve the problem, so I can work through the other maintenance tasks and enjoy the vehicle.

Thanks for any ideas and advice!

 

Welcome,

Get you a Innova 3145 OBD I Ford code reader(best $50 bucks you can spend) get truck up to temp and then check for codes, before unloading the parts cannon at it.

 

Head gasket maybe.
Pull plugs, do a compression test.

 

I agree with getting a scanner - putting it off by a pay-period while I check a few obvious things and learn the layout. PO spent $1500 on having a mechanic scan it; all he did was make wiring repairs to two injectors, didn't solve the acceleration issue.

Haven't run it enough to tell if coolant is disappearing, but there's none visible in the oil, and the white emissions stop as it approaches operating temp. I'll run compression tests next, but am optimistic.... I suspect that wouldn't cause an intermittent acceleration problem - my limited experience with head gasket and cracked heads has involved catastrophic shut downs. No more driving until fixed. So, more backstory - they've used the truck for farm labor for about three years, once they quit trusting it over the road.

Found the three wires at the TPS were oddly bare for about the first inch or so, one closer to 1.5 inches. Bare as in stripped clean of any trace of insulation, and the end of the insulation was square-cut and clean where it stopped. Wires weren't frayed or broken, but were caked in oily crud. Can't image the device was working well, but what I read on TPS sensors is that if shorted, the engine runs rich. Not sure how that would impact acceleration, but at least bare wires touching might only intermittently touch.

Thanks for your help!

 

you can test TPS -->Throttle Position Sensor Testing, Replacement and Adjustment - Ford Bronco Forum (1)
may have been why wires were striped back, should have tapped them back up.

 

Ran a compression test today. As I've removed the throttle body, the engine was cold. Removed all the plugs, which were shot - not carbon fouled, but old, with huge gaps, some oily residue around the threads, rust on the metal body, rust stains on the insulator between the metal and the end of the boot.

Did the compression test dry first, then with a slug of oil squirted in.

On cylinders 1,2, and 4-8, the dry tests were all between 115 and 130, and the wet tests were between 155 and 170. #3 was 170 psi dry, and 185 wet.

Unhappy rings, and #3 has carbon, but the head gasket appears okay.

#7 wire, carefully tested repeatedly, had infinite resistance. #2 was 17,000, and #1 was 12,000. I didn't measure the length, but my Chilton say anything over 5000 ohms / foot is bad, and they're not two or certainly 3' long. Or infinite.

E-conversed with the PO, who probed his memory and said it would DO highway speeds, but sometimes, it would hesitate and kick, but not violently; then he'd back off on the throttle, and ease up to speed gently.

Coolant supply to the throttle body has been bypassed, which might make a difference in cold weather. Not sure about warm or hot weather, but it seems a minor thing.

Am beginning to think the intermittent acceleration issue is just an almost completely unloved motor protesting neglect. I'm about done poking and prodding, and haven't yet seen any sign of what I consider routine maintenance.

PCV valve rattled, but even that was a dull sound.