What causes 3500+ rpm miss?

[Aliate X]

Hey guys, I have a 93 5.0 LX with a new 255lph fuel pump, AFPR, and full exhaust. My car used to run like garbage until I replaced the fuel pump, now it runs a hell of a lot better but it still likes to hesitate at higher RPM's, like it will be accelerating just fine, then sometimes at the same exact RPM (3500) it will just drop until I let off the gas a little then nail it again and its fine. I dont understand where this problem is coming from.

The fuel filter has been replaced, along with cap, wires, rotor, and plugs. Any suggestions would help a ton, I was thinking my ignition was messed up, time for full MSD?

 

[stang22]

It could be a number of things. What is your fuel pressure and TPS set to? Have you had the TFI module tested yet or pulled any codes? The list is quite long, but these are some starting points, being you already replaced the common items.

Tim

 

[jb89coupe]

I would look into your TFI module. Also, people have had a lot of bad luck with the modules that come with MSD distributers, so I'd get and OEM replacement if it was bad.

 

[Aliate X]

Whats a TFI moduel?

My FP is set to about 40. Ive never checked my TPS, nor do I know how to.

Neither do I have anything to check out some codes. I dont have much confidence in those, lol.

Would bad O2 sensors cause any of the problems Im having?

 

[jb89coupe]

Whats a TFI moduel?

My FP is set to about 40. Ive never checked my TPS, nor do I know how to.

Neither do I have anything to check out some codes. I dont have much confidence in those, lol.

Would bad O2 sensors cause any of the problems Im having?

The TFI is the ignition module bolted to the outside of your distributer.. it's notorius for doing stuff like this, and I had one do a similar thing to me, it would buck and bog down around 3k.. replaced the module and the problem went away.

To set the TPS, just measure the vDC between the green and black wires coming off the sensor with the key on, engine off. You're looking for around .98v while the throttle is closed. To adjust it, loosen the two screws that hold it down and pivot the whole sensor until it's close to 1v without going over. You should also check the sensor for a smooth range throughout its sweep by opening and closing the throttle while keeping an eye on the voltage. If you have any dead spots or platues, it's probably time for a new sensor.

All you need to check codes is a paperclip or a small piece of wire. I can post a pic of the two connectors you have to bridge if you'd like.

It's not too likely that o2 sensors would cause such an rpm specific problem.

You should also check all your grounds.. a bad ground can cause weird problems like this too.

 

[crunchie12268]

spark plugs gapped too much

 

[Aliate X]

The TFI is the ignition module bolted to the outside of your distributer.. it's notorius for doing stuff like this, and I had one do a similar thing to me, it would buck and bog down around 3k.. replaced the module and the problem went away.

To set the TPS, just measure the vDC between the green and black wires coming off the sensor with the key on, engine off. You're looking for around .98v while the throttle is closed. To adjust it, loosen the two screws that hold it down and pivot the whole sensor until it's close to 1v without going over. You should also check the sensor for a smooth range throughout its sweep by opening and closing the throttle while keeping an eye on the voltage. If you have any dead spots or platues, it's probably time for a new sensor.

All you need to check codes is a paperclip or a small piece of wire. I can post a pic of the two connectors you have to bridge if you'd like.

It's not too likely that o2 sensors would cause such an rpm specific problem.

You should also check all your grounds.. a bad ground can cause weird problems like this too.

TFI module sounds like a good shot, its directly bolted ON the dist? And how much is one?

 

[HISSIN50]

if not mentioned, dont overlook a weak coil. it is strange that it lets off and then comes back on (with good power above 3500 RPM right?). it is almost like an engine mangeement issue instead of a faulty component per se. the TPS is good to go, right?

tough to diagnose things that happen in such a specific range of perameters, huh? (i think so).

good luck.