Cherokee 2.5 stumble at cruising & 1/8 throttle

[paultradex]

Hi guys, hope you can help me on this one, from across the pond!

I have had lots of Cherokees over the years so I like to think I know my Jeeps I have had lots of '70's 'yanks' as we call them over here.... AMC Javelin, 3rd gen Firebirds, 403 T/A's, Olds customs, Mercury's Montego, Chrysler Newports, C3 Vette etc and have always found the US forums real helpful when I had strange problems with those cars.

OK, the Cherokee is a 2.5 petrol (gasoline!) multi-point injection manual 1994 with 100K miles and good service history, she starts and drives fine from cold but when warmed up she develops a strange surging but only at 1/8 throttle...just off the idle stop at cruising speeds in 3rd, 4th or 5th gear....when you put your foot on the throttle further there can be a misfire back thru the throttle body like the timings gone out or the she has fuel lean but only for a moment then she is OK....this car drives 100% other than this. I have changed the TPS (throttle position switch) & no change...I have checked the leads, plugs and dizzy cap and done the water mist check, the fuel filter hasn't been changed for 20K miles but I guess that when car is cold and needs more fuel it can't be the cause as the problem is only on low load points when hot....I'm wondering if the Lambda sensor may be going bad as I don't think it's used when warming up (open loop?) but I have no check lights up to say the sensor is duff....I hear there maybe a diagnostic check using a flashing light on the dash' that can be done but don't know how to?

any help or ideas, no matter how wierd gratefully received!

cheers for now

[paultradex]

....the Lambda sensor was u/s..it ran better when it as disconnected (!) and the diagnosis light (red check engine light) confirmed this...£110 for a new one, fitted and now all OK! Strangely the O2 sensor light did not come on....?!?