50GT

Car is idling great, but on the way home today every time I let off the gas to slow down the car started to buck while in 1st, 2nd, or 3rd gear. I didn'tnotice it happing when going at highway speeds. The car did not hesitate when accelerating or as long as I was giving it gas. When I come to a stop the car idled very smoothly, When I got home ran the KOERtest and got a code 91 (HEGO sensor indicates system lean). I also ran the KOEO test and got a code 11 (system pass). With this info dose anyone know what might cause the car to run lean.It’s a 1990 Mustang GT 5 speed. Only thing done to it isCAI,Hypertech chip, under drive pulleys, 160 degree thermostat, Flowmaster Exhaust, Flow Tech Headers. All of the smog equipment is hooked up still. Also I only run 93 octane in it, Thanks for the help.

AdderMk2

take out the hypertech chip, and put a 180° T-stat in, and stop running 93 octane... its not giving you any more power

50GT

Do you think that could be causing it? Everything done to the vehicle has been done for over two years and this problem has just started today?

camaroeater

I don't think you need a 160 stat...I want to get a tester to run KOER/KOEO, does anyone know what tester I need and maybe a good place to get one...checker, Auto Zone?

AdderMk2

if you knew anything about EFI systems, you would think othrwise.
Having a 160° T-stat does not allow the car to completely warm up to full operating temperatures, and can cause all kinds of stupid problems

93 LX FiveO

adder is right, put a 180 in it.

im working on solving this code as well.

i suggest reading my thread and going from there.

50GT

ok thanks

aode08

The chips usually set that code when they go out.

50GT

Do you mean the chip might have gone bad, is there a way of testing that.

50GT

I'm going to put 180 degree T-stat in it this weekend hope that will solve this issue. The only reason I put the 160 degree T-stat in was because that’s what Hypertech recommended to keep the motor from possible detonation and to get the best preformance from the chip. Thanks for the help.