Throttle surge at idol
surge, great post, what engine displacement.
open hood (my answers are USA only) look up, see big white
sticker that is the engine vacuum and type engine sticker)
surge hot or cold? engine.
if it surges hot, that means :
1: ISC system out of control , it job is to stop surge and 5 more things.
2: the engine is way lean and cause ISC to loose control
just 2 reasons, and can be 50 things. wrong.
is the CEL lamp out running> if not, pull the DTC codes and post them. (all)
as a mech ,we find out which it is.
(i make sure my engine can hold closed loop at idle , if not and lean it will surge. (may)
we make sure the ISC works ok (not sticking, clean it)
and then test to see , that if by my forcing it closed, by hand (ask)
i can get way below 800 RPM. most do 500 but 600 is ok.
if rpm stays above 800 closed, that means you have huge
vacuum leak or TV blackage (too thight throttle cable?)
finding vacuum leaks can be horrific, task .
there are many tricks (and tests) to do this, ask.
see that logic above, If I the mech, can not drop rpm below 800
by hand, then no ISC on earth can. not now or ever.
and proves the induction has huge air leaks
or some dolt set the spark timing 20 degrees advanced.
see it.
so as always , a tuneup is first, then as always , i always check timing,and for proper advance rates.
advances engines at idle race way too fast and the iSC will just go NUTS.
ISC = Idle speed controller , that valve that has wires.
the IAC is a thermal device and no wires and it too can stick open
when the IAC sticks open the ISC goes , yes, nuts,
nuts can be surge (ECU is hunting it)
or just racing way too fast hot, say 1500 rpm.
a slow RPm can be a stuck open EGR (50k service point)
lean will cause too low.
too rich , misfires and too low
the ISC is the last thing to work right, all else must be good.
what does your CEL lamp do?
PS: most cars, if the ECU/PCM enters Limphome, it cuts all idle controls. again, the CEL lamp. glowing while running.../????
