Hi guys.

My 1990 UCF10 has had most if not all engine maintenance carried out.

Idles and runs beautifully. HOWEVER - There is a flat spot at the very start of the throttle. With gradual acceleration it is not there, only when tapping the throttle. It is flat for a short moment then picks up revs as normal and revs great through the rest of the range.

Any ideas? I was thinking maybe TPS?

NOTE- After doing all the hose and line deletes, the trac light is on, but that was apparent way before this has occurred.

 

Line deletes?? Did you check whether stored trouble code(s)?

Could be throttle position sensor.

 

No codes stored. Just the PS line and a couple vacuum lines. It runs great. Idles lovely. Just that little spot right at the start as the throttle opens quickly.

 

Any ideas or tips on how to check the tps?

 

Is there any slack in the throttle cable? A 1990 model has had PLENTY of time to stretch...
It might just need adjusted.

 

+1 that is one area to check.

 

The TPS has two resistive tracks....so a 4 pin connector..one pair pins with ascending resistance value and the second pair of pins with descending resistance value.
Use an ohm meter to check resistance as you slowly move the TPS through it's sweep....resistance readings should increase or decrease linearly.

If you hear grinding noise or actuator lever displays excessive side play (due to worn plastic bearing) probably time to swap it out...

I believe early LS400's used more than one TPS' depending if TRAC and cruise control..

 

No slack in the cable. Throttle opens smooth, just not getting fuel at the fist second.

I will test the TPS today. Happy easter.