1990 UK Coupe, 2.0e 4 cylinder 3A engine, 140,000 miles.

The engine idles fine when started, barring the occasional stall
initially. Engine pulls great, feels smooth when applying & releasing
throttle. Economy 32mpg (UK) about town, 38mpg on motorway, which I
reckon is pretty good. Stopping at lights, the idle is around 800 revs,
a bit lumpy but basically ok. Temp is about 1/3 up the guage, or 1/4 on
motorway.

After driving for a 'while' (anything from 5 mins to 30 mins) the
engine mode will change - applying or releasing the throttle makes the
engine jerk, and when stopping at lights, the idle will bounce between
1000 & 1100 revs, once a second. These two symptoms are always
concurrent. The economy is the same as before. This switch in modes
seems unrelated to time, or temperature. It often follows a sprint of
heavy throttle / acceleration. The engine will stay in this mode until
it's shut off. when restarted, the first (smooth) mode occurs, and the
cycle repeats.

I've changed the ISV, without major difference. I also found and
repaired a bad vacuum hose, which helped the 'good' idle a bit, but
didn't change the second 'bad' engine mode.

Any ideas on where I should look next?

Many thanks,
Jamie

Any ideas on where I should look next?

fish... (AT) gmail (DOT) com wrote:

Exhaust gas recirculation (EGR) valve if your car has one?
Any fault codes reported by ECU?