Hello everyboby, I've been trying to finger this out myself for a while now, to no avail. I'm finally breaking down and starting my own thread. I've searched high and low, near and far, using about every conceivable keyword combo I can think of. I've found several issues that are similar, but none seem to be a match. Oh, the truck has been doing it since at least '99 or '00.

The problem is that my 1988 f150 300-6 5sp. 4x4 w/ 128xxx has a miss... sometimes. Then it bucks. then runs perfect. then is under powered with no other symptoms. I've tried my best to figure out a pattern, but there is no real consistency. The only thing that seems consistent is that it always seems to run well when the engine is cold. It also seems like driving reeeeeeally gently or extreme cold outside temps prolong the time it takes to start acting up.

USUALLY, and i say usually because it's not always, if I drive really nice, I can get anywhere from 10-50+ miles or so before it does anything, but USUALLY if I rev it around 3000 rpm it will start rather quickly.

Most of the time, once the symptoms start, they dont stop until the truck sits for a prolonged period of time. It's also seems like the longer it sits (days to weeks, not just cool down time) the longer it will last. However, one time it started to miss very slightly, so I slowed down and stayed off the throttle as much as I could, and it stopped for the rest of the 10 miles home.

Another example; It had sat for several weeks, then I used it to haul some furniture. It made the entire 30 minute drive in medium stop and go traffic, sat for 45 minutes while we loaded it, and made it back without missing a beat. I stayed under 2500 rpm the entire drive.

Now I've been driving it the past couple days to work, and it seems like as soon as it gets warmed up, it's short on power. Then today, it was missing and bucking for the last 10 minutes of my drive to work, despite my granny driving.

Now, I know the rear tank is filled with ancient gas, and the front fuel sending unit doesn't work. My mom was driving the truck while I was in the Marines, and recently switched it to the bad gas on accident, but she took it to the dealer and had it serviced. It was also doing this long before that happened.

The only code I get is a weak tps signal, which does flicker while I drive, and causes a feeling like I hit a slight headwind.

My theories have included:
---faulty TFI, but it sounds like that causes total shutdown until it cools
---faulty dizzy, dont know if that would cause my symptoms or not
---sediment in the fuel filter, but dont think that would explain the miss
---harmonic balancer, but dont think that would be so on again off again

My mom had brought it to the dealer several times, spending god knows how much to have it fixed, but the always claimed it wasn't doing anything wrong when they'd look at it.

I've checked any connection I can see, as well as vacuum lines. everything looks great under hood. nothing dry, rotten, or broken that i can see.

I dont have a lot of time or money to just start throwing parts at it. It's been well cared for since I got it in '99, and i have no plans to ever sell it. Will someone please just happen to know exactly what is wrong with it.

 

I'd look into the TPS if its throwing a code, it can cause a lean misfire if it reads wrong.

 

You might also consider checking the fuel pressure. Also if there is a problem with the tank venting system, you could have a partial vacuum in the tank, preventing adequate fuel flow. It takes awhile for the vacuum to build and while it sits, the vacuum bleeds off. Just a couple of ideas.

 

I've had similar problems with my truck; all of a sudden it will run out of fuel on the road and then will buck back to life (usually). It also wouldn't like to start after sitting off for a few minutes. I believe my problem was the canister purge valve on the intake manifold.

This valve takes fuel vapors build up from the engine, and allows them to sucked into the throttle body and burned off, instead of venting to the air making a safety concern and a gassy smell.

I haven't driven mine enough to confirm that was my problem (though I'm hoping so!), but I would recommend you bypassing it for now and see if it fixes your issue.

Here's a picture of what it looks like:

Duralast/Canister Purge Valve Solenoid (PV217) | AutoZone.com

Head up to your local AutoZone and buy a set of vacuum connectors like this

Dorman/AutoGrade 10 pcs. hard straight vacuum tubing connector assortment (47307) | Vacuum Connector and Tubing | AutoZone.com

The one you need will be 1/4" size. It might not be a bad idea to get a couple of feet of vacuum hose while you're there, your stuff now might be rotted. We sell it for $1.29/foot at our store.

Find and pull your purge valve off your engine (trace the vacuum line that comes off the two nipples on the top of your throttle body and is kind of draped over the intake manifold, it's pretty much in plain sight), then add in the connector where the purge valve used to be. Drive the truck around and see if it worked. You may want to replace the valve itself if that fixes your truck, but I haven't had a CEL from it since.

Good luck, and let us know how it works out! If you can't find the valve, I can get some pictures for you.

 

Thanks guys! I dont think its the tps because the issue at hand was around long before the tps started going bad, but I plan on replacing that soon anyway.

The vacuum ideas made a little bit of sense, and now that I got my taxes back, i think I'm gonna head out and grab a vacuum gauge and fuel pressure gauge to check those out. I'm also going to check my timing again because I think i did it wrong last time.

If anyone has any other ideas, keep em coming. There's still a lot about this truck i dont know about and may not think of.

Thanks again.

 

You'd be surprised I have had to replace and adjust quite a few TPS when working at walts, your running a OBD1 and in my experience its not the most reliable for throwing codes quickly or diagnosis. I have seen tps sensors that weren't adjusted correctly or faulty not move into the correct circuit when the throttle body is rotated and that will cause all kinds of problems with your A/F ratio and timing. You say it gets worse quicker if you rev it near 3k? When you push it towards 3k are you going WOT because if you are your TPS may be telling everything else your foots only half way to the floor so if you sucking in all that air and your computer isn't advancing the timing or doing something it should in conjunction to the throttle body opening the tps it will throw the whole system out of whack. I really haven't worked on Computer controlled L6s much so it could possibly be something else but if your TPS was flagged its alot more crucial then you'd think and just because it wasn't throwing a code doesn't mean it wasn't working incorrectly it may not have been bad from the gate just out of adjustment.

 

Hm. Well I'll have to prioritize that tps then to rule that out for sure. I'll do that and update whether it works or not. Thanks Jo.

 

Great Scott!! That canister purge valve is pricey!! I hope that aint it!!

 

Just got home from work which is a 15mi drive. I unscrewed the gas cap a little bit to rule out tank vacuum, and just like its done for the last few day, it started missing after about 5 miles. It sort of feels like the timing might be a bit erratic or something. It doesn't cut out or anything, it just misses or bucks so I don't really suspect a fuel problem.

Does anyone know for sure if the TFI module can cause erratic timing? I read that it can only ONCE, but that was a website selling distributors so I though it might just be a sales ploy. Everyone else says a faulty TFI will just cut out until it cools off.

 

If your rig was a carbed vehicle I would say the EGR is suspect...dirty or burned out.

 

My 89 used to act like that. I described it as a hiccup. It drove me crazy for years, Until the engine got a dead miss one day. I figured it was a piston, and parked the truck. A few years later, I got ready to pull the motor, and go through it, decided to pull the valve cover first. Turns out an intake rocker arm was worn out, and had finally let the push rod come out. New pushrod and a new rocker arm, it's running again, and the hiccup is gone.

Your valve cover is probably leaking anyway, so it won't hurt to check that out

 

i don't have much experience with EFI motors, so i'm not saying you should buy a new TFI module. BUT- if you do, shop hard. you can find these cheap. i've bought a couple from this outfit:

NEW IGNITION CONTROL MODULE ICM FORD LINCOLN MERCURY LX226 | eBay

 

I already have a TFI module that I plan to use when I convert to remote mount TFI. I've also looked into the egr issues because of the thread that talks about restricting it, but I ruled that out a while ago. I don't really see how the valve train would be so inconsistent. You may need to explain that a little bit.

Thanks for the help everyone btw. Im taking it all into consideration.

 

The rocker arm was worn out, it would work fine until the rpm's came up, then it would let the lifter pump up until the valve wouldn't close. Once the oil pressure went back down i would behave. I'm not saying that's what's wrong with your truck, Just that's what was wrong with mine

 

Ah. Kinda makes sense. Might have to look into that. Thanks