My Hyundai Lantra 2.0 1999 has got a funny fault.
Original problem was an intermittant unstable idle - idle speed would oscillate between about 500 and 1500 rpm (instead of the normal 750) and this would happen when cold or hot. The problem would occur only with closed throttle - any hit of throttle opening and the prob would dissappear.
It's been to 2 different dealers, several times each and has had the ISC valve and TPS replaced. Despite this it's not right.
Although the unstable idle is much, much less objectionable - the throttle action is not good. There's a 'dead zone' for the first few degrees and then a catch-up surge.
Additionally, the overrun now has surging - when slowing down past 1500 rpm on the overrun, the engine will surge (as it's an auto, the revs will jump up to about 1800 and then drop to about 1300, and then oscillate a bit).
I tried a couple of very crude sensor tests:
MAP sensor disconnected: no difference to driveability. Same throttle response, same surging overrun and hunting idle.
TPS disconnected: Undriveable. Terrible bucking and loss of power at
2000 rpm. Listening to engine shows that fuel injectors switch off when engine speed reaches 2000 rpm.Any simple DIY tests that I could do before throwing good money after bad, having dealers replace parts that don't seem to help.
I'm guessing that what it really needs is a competant mechanic to take it for a test drive with a scan tool.
But any common probs that I could check at home. I've got a decent set of general purpose tools and a multimeter.
MQuick summary Hunting idle - attempted fix replaced ISC and TPS Now surges on overrun at 1500 rpm