Well, for too long now ive been trying to figure this truck out. Id like to apologize for the lenght, but you know. BTW 1991 f150 efi 302 4x4 v8

NO mechanics know how to fix this, and ive spent bucket loads of money replacing everything and having throttle bodies cleaned, seafoam, fuel injection flush, system pressures, codes, cap, rotor, plugs, fuel filter, and all to no avail. One even replaced the tranny sensor saying it could be that b/c it controls shift points and fuel adjustments. Another 400 down the drain. It did make my tranny shift smoother, but didnt fix the poor drivability!!

Well, heres the first problem. This first one dose not happen all the time, just every now and then, and it matters NOT how long the truck sits. Sometimes when I start the truck up, most of the time the motor will turn over and run good. But half the time, it will start, and then drop off like its dead, but will somehow cycle itself over again seconds later and start running. Weird. So the truck will idle good and rev up, but its just missing something. You can hear it in how the truck runs, the exhuast note is VERY different, kinda like a 6cyl, almost sounds like the truck is missing, but its running great.

And heres the other weird thing, once I wait until the truck has been driven awhile, (usually by about 20 minutes of driving, sometimes it takes almost an hour) the truck will do a 100 percent turn around and it will run great and the exhuast note will totally change back too the deep v8 grumble and the truck will start to haul *** again. When its not running right, it will still go, but its SOOO slow, and instead of the nice v8 motor revving up noise, I get a loud "whooshing" rev up noise from the motor, and I cant hear the actual motor rev up when its running like that, just like alot of air, like the fan turning hard. Every mechanic tells me that it goes great but makes alot of noise, and thats just how it is. But then I can leave and drive it home, and most of the time on the way home, it starts running right again. I have a flowmaster 40 series on it, so believe me, when its not running right, it sounds like a 1985 chevrolet, like loud crap. But when the truck runs good, sounds like a nice 302 with flows. I cant take driving this truck and half the time its a slow turd with no power, and the other times it runs like a bat out of hell. What the hell is this truck doing???

Everyone thinks im nuts except for my friends and family b/c theyve heard and seen it. Mechanics just dont want to spend the time with it to see how well it SHOULD run. Any thoughts about it?????.....BTW, its a brand new motor, only 12,000 miles on it. 302 5.0 HO.

 

I would guess you have a bad <!--StartFragment -->Engine Coolant Temperature (ECT) (the one with two wires on it) or Air Charge Temp sensor (ACT) sensor.

 

Jeeze I didnt even think about the ect. The both are pretty inexpensive so I guess it wouldnt hurt to look into them both. Thanks for the reply.

 

Is it running on speed density still? The MAP sensor could be bad but not throwing a code. Might want to get out a vacuum gauge and voltmeter and check it.

Thats why I never go to mechanics anymore, or atleast avoid them at all costs. Most of them will not correctly diagnose a problem, and simply have the owner throw money at it as in "it could be this, it could be that" and never fix it. They do that so they can make money off the parts too. There's almost always a way to dianose a part and find out if it's working and could really be your problem.

 

Yea machanics are crap man. Ive come here for my stuff and just read and such. It may take a little more time but I learn the stuff myself and if I have to Ill take it to a machanic and tell them what to do since I dont have a lift. I hate them so much.

 

It sounds like the same thing that happened with my Bronco-1992 speed density 5.0. Mine would do the same thing, but sometimes wouldn't downshift, or upshift, and I could put my foot to the floor and it would just drown out. Sometimes it would want to die. Anyways, it ended up being the Throttle Position Sensor. I replaced that-took about 30 minutes due to the location- and she ran like new. I was amazed at how big an impact that sensor had on the entire truck. It was like $15 too.

 

This may not help in any way, but I had a similiar problem when a chunk had broken off inside a cat. As the piece moved around, it caused very similiar performance/sound characterestics to the ones you described.

 

yup, I'm with pyrobob on this one. sounds like something is loose in the cat.
my 86 302 did the same thing. gutless wonder one second, and tire burning fool the next. finally plugged solid one day on the interstate. i got off at the next exit, rolled into the ford dealer at the bottom of the ramp, and traded it for the 88 diesel. made out good on it too. bought new for $9,500 cash, drove it for 18 months, and traded it for $11,000 off the price of the $18,000 diesel.

 

The TPS sensor is a main EFI sensor - the computer relies on this sensor quite a bit so therefore a defect can have significant performance changes. Normally, without this sensor working correctly, the engine wouldn't even work much at all. This EFI system has a fault mode that if a sensor is failing, it will override that sensor's input and get you home with a check engine light.

A bad TPS will throw a code - since he said he checked codes and didn't have any - we're assuming there are no codes (hopefully code check was done correctly) and that we're looking for problems that do not throw a code...

Such as the cat, MAP can cause problems w/o a code, etc...

The ECT and ACT sensors will throw codes if there is a malfuction - but is a good place to look given the situation.

 

Make sure your cat isn't plugged up. Although if it was I think your truck would always run crappy.

 

I agree, I'd say the cat is probably fine and wouldn't be at the top of my check list.

 

The cat is easy to try. Loosen it up from the head pipe, jamb some steel in there about 1/2 thick (spacer) and go for a quick drive. If it is better, get a new cat, if it is the same, remove the spacer and bolt it tight again.

Kenny

 

The truck has a full exhaust system on it, the cats were removed! So I dunno what it could be. 3 mehanics said it was not throwing any codes.

 

Then you need to consider all items that don't throw a code. There are a few of them from TPS, Exhaust, IAB.

Thinking about your problem...my wife's 2000 Stang had similar issues. It went in 4 times before I figured out the problem and then they fixed it. It was a sticky Idle Air Bypass valve. Funny thing, it made the woosh sound and the car would fall flat on its face, sometimes it would run, sometimes it wouldn't even start. It was terrible when it was cold, but as soon as it warmed a bit (2 minutes of so), it would run like a champ. You could feel it as soon as it released, the engine just felt like it caught a breath.

Kenny

 

Ill probably look at this first. Seems with it not throwing any codes it would be the place to start, since everything else should throw codes. Also the ECT and ACT are inexpensive fixes, probably 25 bucks for the both of them. So ill try those and let you guys know what happens. I appreciate all the replies and input.