Just like the subject line says.

'88 F250, 4.9L. Has an stumble, or "hiccup" at slight throttle openings. The RPM doesn't matter. Drop the hammer and row the gearbox and it runs like a top, idles fine too.

I also notice that its less likely to do this after the motor is completely warmed up.

Any ideas?? I was thinking faulty TPI.

Its not any of these, as they've been all replaced very recently.

-plugs
-wires
-cap/rotor
-fuel filter
-air filter

 

Do regularly use injector cleaner? (ie at least every oil change) If not may need to overhaul or replace the injectors.

Have you cleaned the throttlebody/intake manafold?

Replaced the HEGO?

Is the exaust clogged?

Checked for a vaccum leak?

Pulled codes?

 

My '91 does the same. We named the truck "Bucky" because it bucks when going down the road. I am still working thru my very long list of possible causes. My next thing to check is the TPS and then if that's not it I am gonna work with the EGR.

 

There is one main cause for that sort of behavior that has popped up many times in this forum. That is the throttle body gasket. If you take off the intake tubes, hold the throttle wide open and look inside with a flashlight, you will probably see it sucked up inside the intake creating a very difficult to find vacuum leak.

 

I have the same problem on my '92 F150, it only seems to go away after it has warmed up anywhere from 3-10 minutes...weird, but didn't think about the gasket either.

 

I wish it would be that easy. I yanked the TB and cleaned it. The gasket looks good. Checked the TPS and the IAT sensors and they also check good. I am going to try the EGR restrictor next.

 

Speaking of IAT sensors.......mine did buck and jerk just off idle a while back. It actually started about 2 years ago and got progressively worse until I decided to put some effort into finding the source. What I found was an IAT sensor that glitched when it experienced an increase in airflow. It would momentarily gain about 70 degrees of temp and lean out the mixture. I found it purely by chance when it glitched while I was looking at the scanner. It tested good when checked with an ohmmeter.

 

It's fixed! The problem is I don't know what I did. I don't think it was the TB, since it wasn't very dirty at all. So that leaves a bad connection at either the TPS or the IAT sensor. I think it must have been the IAT sensor because a) I think I already disconnected the TPS one time before and it made no diff and b) the IAT sensor contacts have dielectric grease on them. Aren't you NOT supposed to use that on these low-voltage (i.e <5VDC) sensors? Anyway, after I cleaned the TPS and unplugged/plugged the 2 sensors, the truck runs really great. It is so much smoother now and the trans even shifts right (it was banging the 1-2 shift). It zips right up to 60mph whereas before it used to get to about 50 and then start surging and I would have to push it to go faster.

I am glad it's fixed but I hate it when I don't know what I did. I'd feel better if I had replaced something.

 

i have a similar miss or hic-cup at idle/in gear stopped. what and where is this IAT sensor, and is it easily changed/cleaned??

 

If you have an earlier 300 I-6 then it should be located just to the left of the engine on the intake manifold and it's quite tough getting to it with convential tools. Properly named the ACT, Air Charge Temp Sensor. Mine was all gunked up, so I cleaned, but then changed my mind and got a new one. Looks like the ECT, Engine coolant temperature sensor. Here's what it looks like. Also, this site is useful in diagnosing problems with sensors and senders. Hope this helps!

http://fordfuelinjection.com/?p=10
http://fordfuelinjection.com/index.php?p=29

The bottom link take you to the ACT page. The top link gives you the different types of sensors and senders.