Hot Idle - Seems Too High

[Rotary Potato]

When I start the car from cold, it idles at about 800rpm. That doesn't seem unreasonable.

After a session of enthusiastic driving, it idles much higher. The lowest I've seen (bear in mind I've had the car 2 weeks and driven it 3 times) is 1200rpm at hot idle. The highest has been 2000rpm. The 3rd drive had a hot idle of about 1600rpm. So whatever it is, isn't consistent.

Checking when cold, there is a mm or 2 of slack in the accelerator cable at the engine end (i.e. if you don't touch the throttle pedal the throttles on the engine are not being opened by the pedal) which seems pretty normal to me. I haven't been clever enough (yet) to check whether this is still the case when the engine is hot. I had the plenum off on the driver's side bank for something else, and while it was off I checked the throttles were all fully closed (they were).

Has anyone had this before? Or have any clever ideas where to start the search if a check of the engine hot produces the same results?

The car is a 1998 4.2.

[andyvvc]

Had mine for about 5 days lol. Hot idle (reversing into garage, fans on, 95 degrees) the car then seems stable at a smidge under 1000rpm iirc sat stationary in the garage.

Kits and Classics did some wizardry on the car when he mapped it. I think he balanced the throttle bodies etc as part of the mapping. Might that be the issue?
Can't recall if my throttle pedal has any play (i dont think it does). I'll report back later this week when i check it

I also bought the posh throttle pots from Motaclan (Hall Effect ones) which theoretically dont need re-adjusting at all (or not as often as standard ones). Might it be a throttle pot issue causing the odd hot idle? (Others with way more knowledge than me will be along shortly to better help you I'm sure)

Edited by andyvvc on Wednesday 8th April 08:07

[camel_landy]

The slightly higher idle is normal, though I'd expect it to be consistent rather than randomly selecting somewhere up to 2000rpm.

M

[notaping]

Many things can affect idle speed, but here's a starting point. . .

TVR setup says Idle should be 950rpm. Personally - I go for about 1000/1050. That keeps the alternator charging. At 950 it won't be charging, at 800rpm - I'm surprised your engine still continues to run. Mine would bog down and stall.

Idle speeding up when hot (enthusiastic driving) would indicate a mechanical change - probably throttle pots. They can get very hot and the output changes. It doesn't take much of a change in TP value to increase the speed. That's where I'd start looking.

You can't check the butterfly's are closed without screwing out the idle screw. With the idle screw fully out and the butterfly's closed, the TP angle should be 15%. At idle they should be approx 19%.

The only sure way to find out what is going on is to download one of the available diagnostic programs and check the traces when the engine is cold/hot. If you see a change in TP values when it's hot then this will confirm dodgy throttle pots. Replace.

Good luck.

[FarmyardPants]

I think the last time I had that, my lambda sensors were buggered, which may have been caused by injector cleaner fuel additive. But diagnostic software should help narrow it down.