I have a 1992 F150 Lariat XLT 5.0 EFI that I've owned for over 20 years. The truck's had some issues in the past but most were wear parts or common maintenance issues. Lately I've been chasing a problem that has me stumped because it is only present for about 1% of the time that I drive the truck. The issue never presents itself when I'm running at cruise speed, only when I'm shifting through the gears from dead stopped, and even then only occasionally.

When it happens I've just started to accelerate , and when my RPMs hit about 2K the truck bogs as though it's being flooded. It will do this all the way through the gears each time I get close to 2K, and the only way to stop it is to aggressively accelerate through it, but if I don't change my accelerator position once speed catches up to RPM it will begin again at whatever RPM I'm currently at. Once I reach cruising speed it begins again (speed catches up to RPM) and I will have to wait it out until eventually the engine begins to run correctly again.

It doesn't sound like a vacuum leak to me (intermittent) but I checked for vacuum leaks anyway and found none. I replaced the IAC (no change) because it was cheap and I had read that it could cause my symptoms, although it was a shot in the dark as my issue is intermittent. I just had the shop drop my tank for a repair, and they drained and refilled the tank (no, I didn't have the problem before it was worked on). I know it's reaching, but I thought it may be gunk from the fuel tank even though they installed a new fuel filter. So far I've run more than a full tank through the engine, but I still have the problem at what appears to be the same rate of occurrence.

Any ideas? Could it be a bad throttle position sensor? Any ideas would be helpful as I'm running out of things to check/try.

Thanks!

 

Have you run a check for codes? First place to start.

 

You should put ALL the truck's details & history into your signature as this page explains:

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Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location in your profile & upload an avatar of the truck.Be specific; how long ago (time/miles) did it appear? Suddenly or progressively?Be specific - I've seen MANY distinct conditions described as "flooded/rich". What exactly do YOU mean?Be specific; time/miles.How exactly & where did you check?

With what (brand/PN/source)? What test did the previous one fail? Did the replacement pass the same test?Low-priced parts are no more likely to fix anything than high-priced parts, so that doesn't make sense. Either diagnose the problem, or pay someone who can - it's cheaper & quicker than throwing random part$ at it.Then you should take it back to them for diagnosis, since you've already paid them, and it's possible they overlooked something. What exactly was the problem that made you take it to them? What exactly did they do to fix it?Yes, or any of several other parts. It can also be caused by improper assembly of good parts, so until you KNOW what's causing it, there's no reason to think you need to buy anything. If you want to TEST the TPS or other sensors, read these pages & follow the relevant links:

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Thanks to you both for your replies. Steve83, terrifically detailed and well assembled post!

I've since resolved the issue, and it was completely my fault.

Truck is now running better than it has in years, thanks again for your replies!!

 

What exactly did you find, and what did you do to fix it?

 

Okay, I had hoped to carry my shame in silence, but since you asked...

Bear in mind as you read that the truck is a third vehicle and very rarely used. I mean like once a month, and half of those trips are just to run the old girl at least once a month.

The CE light had been on in the truck for years. A check with an OBD1 reader (by a mechanic years ago) indicated a bad O2 sensor. I replaced the sensor, but the truck still threw the same error code. My thought was that it was possible that the cat was bad, and I wasn't willing to spend the money and/or time to replace that. I had a suspicion that the EGR valve wasn't functioning correctly either, but I was unwilling to remove the plenum to replace the tube and the valve.

Fast forward to a few months ago, and a Youtube video on resolving O2 sensor CEL issues by installing what is essentially a metal extension tube that takes the tip of the sensor out of the gas stream. I figured what the hell. I installed the tube, but the stubborn light would not go out. Sadly, I didn't bother to remove said extension tube since it made no difference one way or the other.

Then a few weeks ago I decided to toss some cash at the truck and took it to a local mechanic to repair several things. One of the things that I had them do was remove the plenum and replace the EGR tube and valve, which I had suspected was bad (the CEL turned off after this repair btw). I tried a few things, begged for help here, then finally had an epiphany and removed the foolishly installed extension tube for the O2 sensor and installed the sensor properly.

The bogging issue went away immediately, and hasn't returned to this day. I believe that having a fully functioning EGR valve and an inoperative O2 sensor was causing the bogging problem. Oddly it wasn't throwing any codes, or at least not illuminating the CEL.

So that is how the problem wound up being completely my fault...

 

It helps the next person who reads this thread to see what you found.Codes do NOT identify "bad" parts - they identify "bad" electronic signals at the EEC. You always (even with modern vehicles) have to diagnose WHY the EEC thinks that signal is bad.That should have been a BIG clue. Never replace a part unless it's due on the maintenance schedule, or it fails a published test. And don't install the replacement unless it passes the same test.That's always possible. But it would have NO effect on the EEC, the engine, the oxygen sensor, or any code. You were blindly guessing that a relationship existed, but there is none. If you want to learn how to diagnose, repair, & maintain the truck, read the Haynes manual cover-to-cover at least once.

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