Recently had some pulleys and belts swapped out on my V8 SC. Since then, I’ve noticed it’s idling a bit high—anywhere from 800 to 1100 RPM when it’s warmed up. My mechanic took for a spin with the scanner plugged in and reckons a couple of injectors might be acting up. So, we're kicking things off with some fuel additives to see if that sorts it out.

But here’s the question that’s keeping my logical gears turning: is this just a coincidence, or could the pulley work somehow have triggered the higher idle speed? Just tossing it out there for a bit of insight.

Air leak somewhere?

Chris. L405 - 4.4SDV8 Vogue
P38 - 4.6V8 HSE
Land Rover series IIA V8

What sort of bits could leak air?

Is it idling smoothly, but varying its idle speed? Or it is jumping and stuttering between its idle speeds. ( My gut says that injector issues will more likely translate to rough running. )

High idle is going to come from either more fuel, or more air. But the if the ECU detects that too much fuel is getting in and running rich, it'll trim back the fuel amount. So that's less likely to be injectors. Plus, the work you've described is not likely to have touched any of the injectors.

Which leaves it being too much air ... or more air than what the ECU thinks should be getting in.

- Is the throttle body clean? is there any crap blocking the butterfly valve shutting cleanly? Even the slightest block to the butterfly closing down will greatly affect the air coming.

- Are ALL sensors plugged in correctly? Mass air flow? All the plenum pressure sensors around the supercharger etc etc ... if any of them are unplugged / mis-reading the ECU could compensate by allowing more air etc etc.

If there is a duff sensor reading, the ECU can do a great deal to keep the engine running by figuring out what it thinks the correct values should be based on all the other sensors ... so its not always obvious.

It may help to have the car plugged in to some form of diagnostics to watch what the sensors are saying when its idling high. V8 or else ...

Many thanks @Fisha this is great feedback. I'd say that it's idling smoothly, but varying its idle speed.

Another possibility is any joins in the air pipework going from the air filter box to the throttle body. If they aren’t sealed snugly , particularly after the air flow sensor, then extra air could be getting drawn in which the air flow sensor hasn’t registered (and by hasn’t calculated for) … which can increase the idle. V8 or else ...