I have an '89 Bronco XLT, 3 speed auto and the 351 engine. The truck has about 120k miles on it. Heres the deal. Sometimes when I get to around 35-40 miles per hour, when accelerating, the engine seems to buck. Not sure if its the engine missing, or the tranny "hunting" between gears. There is a definite lurching feeling. I watch the tach, and the RPM's swing up and down as well. Any ideas on what exactly might be happening, or ways to narrow it down? ITs very consistent in that it happens at those speeds, but it does not occur all the time.
The truck idles a tad rough too. Not too bad, but not perfectly smooth. Havent done it yet, but a new cap, rotor, plugs and wires are going on next week, just to cover the basics.
This truck is new to me, so I dont know the history that well. I'm pretty sure it was well maintained, and not offroaded much at all. Everything is tight on the truck and in good shape. Any ideas?

 

It could be the computer. I am having pretty much the same problem with my '90. I too have have the 5.8L engine, but I have the E4OD tranny. I have replaced just about every thing to try and fix this problem that I have, except for the TPS sensor and the computer. When I had replaced the MAP sensor, it helped somewhat. My rig does it more when it warms up and after Tranny shifts into second and more so when in third. Mine is still driveable until I try to get the RPM's past 3grand at any speed. You might want to try and pull the codes.

I just purchased a TPS sensor last week that I haven't installed yet. I just ordered a mass airflow conversion kit for my rig two days ago. The kit comes with a new computer and all the hardware & goodies to convert over from speed density to mass airflow. It's not cheap. I'm afraid to say how much until I receive the kit in about two weeks.

Hope this Helps

 

Do the tune-up before trying to diagnose this. It sounds like the plugs have burnt to a wide gap.

 

Thanks guys. Yeah the tune-up comes first obviously. I have a sneaking suspicion that there is more going on though. I would think if it was just a tune-up needed, the trouble would be more consistent and over a wider RPM range, who knows. It realy feels like the tranny cant decide between 2nd and 3rd gears. It may just be the engine though. Tite, keep us posted on what happens with the new tps sensor, I'm curious if this helps or not. IF my tune-up doesnt solve it, I'm going to look for vacuum leaks, test out some sensors<better get me a manual>, check the tranny kick-down< I assume it has one> and go from there. Thanks for the tips guys, keep em coming if you all think of anything.

 

I had a similar problem...rough idle, stalling at stops, but I also noticed mine lurching like you described only it was while I slowed down instead of accelerating. I had AutoZone pull the codes and it called for throttle position sensor and EGR valve sensor. Swapped them out and it idles fine and no more lurching. You do need to get a new throttle body gasket because you need to take the throttle body off to get to the TPS screws. Keep us updated.

 

I had same type of problem with my 93, I did major tune up and then started replacing electronics. One thing I never checked as I thought is was computer controlled was the timing. Guess what, that's all that was wrong was the timing was way off. I'm not sure if chain stretched or what, but she runs like a top for the last year.

 

Funny you mention the timing thing BB, I ran across something about that earlier today. I thought it was comp controlled too. A few more things to check.........Zuk if the tune doesnt do it, then I'll probably check for codes. I sort of have bigger fish to fry now though. The rear main seal took a dumper today. Was driving around town making a few stops here and there and noticed a puddle under the rig. Crawled down there, sure enough its right at the back of the motor. Lost about a quart and a half of oil in just a few miles. shes parked for now, using the ole trusty beater Honda for now. I made one call to a shop and he wants over 500 bucks to do a rear main seal OWWWW!!! I'm going to call around for better prices. I live in an apartment at the moment so I cant go droping trannies and transfer cases out in the lot. All my real heavy duty tools are at dads house WAY far away too, I just brought the basics with me. Anyhow, toying with the idea of just rebuilding the motor while I'm at it. Its got over 120k on it. Who knows, I may look into pricing a rebuild as well. Man I miss my garage......thanks for the help guys

 

I understand; I'm faced with the prospect of having to move out of the house and into an apartment for about a year, and I hate the idea of losing the garage. I can't just run the compressor in an apartment parking lot at 3 AM like I can here...

 

Let me tell ya its not fun Bigric. I'm trying to fine someone to let me use their garage . I'm hoping to start looking for a house this fall. But if I want to drive the Bronc in the meantime, it may mean paying someone . Oh well, off to call for prices. I'm torn. I feel funny spending 500+ on a motor just for a seal, then have the motor blow a few months down the road. On the other hand, it may last for years yet, its hard to say. Going to try and bum a compression tester off someone and test the motor that way. Might help me decide. Man, labor charges make me want to cry. If its 500 labor for a 50 dollar seal, imagine what it costs to have a motor dropped in:-staun