Popping at idle?

[AJC77]

I've noticed a popping from the exhaust, at idle once it's warmed up. It isn't constant, just every so often. Not sure if it's a mild misfire/backfire or perhaps even fuel igniting in the exhaust...

During my body off chassis refurb we replaced the manifolds with stainless steel ACTs, and the rotor arm was also replaced as it was burnt. Since noticing it I've also replaced the dizzy cap, HT leads and plugs (now iridium).

It starts up just as eagerly, first turn of the key, and if anything the car is a little quieter now, though the old manifolds were shot and one had holed.

Any thoughts on the popping? Perhaps it's not unusual? I've not had it before.
(It's the 2.9 in an 89 S2)

Thanks, Andrew.

[TVRees]

I suggest to check the idle air control (IAC) valve, clean the throttle plates and check that the vacuum hoses are all correctly connected and not blocked/impeded.

[AJC77]

So I've cleaned the throttle plates using a penetration oil, then wiped dry. Checked the ICV, can't see any problems with the vacuum hose. However, when I start up I get a fast idle at 2,000 rpm and it won't drop below. I've swapped ICVs, disengaged the ICV, disconnected the battery cable to reset the ECU but nothing will cure it.

Any ideas as I'm stumped :-/ ?

[ukflyboy]

Just out of interest, how does it run with the ISCV unplugged?

[AJC77]

ISCV unplugged still fast idle.
Throttle Pot unplugged still fast idle.
Just been for a run took it up to temp, up and down the rev range and gears, still fast idle.

Could it be drawing air in from around the throttle pot?

[AJC77]

Fixed it :-) yay!

Came to the conclusion it must have been drawing in air. Took the throttle pot off and had a closer inspection and it turns out my cleaning had shift so much mucky deposit, and that previously it had been adjusted so that the butterfly flaps fitted the muck... That it had left them just partially open even on the stop. So adjusted the throttle arm screw and roller till the flaps were as closed as they can go, and reassembled everything. Turned the key and immediately idled at 1,000 rpm, happy days. Probably a schoolboy error in hindsight but you live and learn.

And, so far at least, the popping at idle has subsided.

[TVRees]

AJC77 said:

.... So adjusted the throttle arm screw and roller till the flaps were as closed as they can go, ........

Good to hear that you fixed your problem.
When you say "as closed as they can go", were the throttle plates still slightly open ?
Only asking, since I just cleaned my throttle assy thoroughly and now I can see a small gap around the edges of the plates, when I hold it up to the light. This was not there before, due to the muck.

[AJC77]

There is a gap in places as the flaps aren't a perfect round in the inlet passages, but before they were slightly tilted and hence open whereas now they are as upright or closed as the adjustment will allow and as closed shut as they can get. In places no light was visible around the edges.
It surprised me to realise that a tiny amount of angle would add another 1,000 rpm at idle but hey ho.