Hello,  Ive got a 91 JLX with 104,000kms on it,  truck is absolutely mint but it isn't running right.  While idling the idle lopes a little, and engine runs quite rough, and when driving, it "jerks" on light acceleration mostly noticable in 1st and 2nd gears.  I have changed the plugs, wires, cap, rotor, coil, and even the entire distributor, I don't see any vacuum leaks and everything else looks ok.  She will run down the freeway with no problems, but in town has these problems..  can anyone help me?  I don't want to get left stranded somewhere just incase it gets worse.   Thanks for any info.

Lance

the mass air(little box on left side of firewall)  Mine ran bad once because of it. lata stu

But wouldn't the mass air sensor set a trouble code?  I have no trouble codes stored.  

throttle position sensor.  it has an open or short in it, will not give you a trouble code because the computer just thinks you temporarily took your foot of the gas

sorry, I also forgot to mention that I changed the entire TBI unit with another one and still had the same problem.  

I agree on the TPS being the culprit. I don't think the 16v was an option in '91. All 8v engines were speed-density, not mass airflow delivery systems, so that eliminates the mass airflow sensor theory.

The little box is a MAP not MAF
and it won't run without it.

Check the tps any way. To do so use your voltmeter on low scale voltage and probe one of the wires til you get a low voltage reading.(with key on engine off) The move the throttle slowly and the voltage should increase steadily with no flat spots or drop outs of voltage. Check wiring diagram for exact wire but it should be easy to find.

Haven't had a chance to look under the hood yet, but from what I'm reading in the Haynes manual I have, they say only 92's thru 98's have TPS...  Seems kinda strange that the 91 wouldn't have it though..  

my 89' has it so a 91' should