I have searched and seen a few threads on this but wanted to get some input, I have a 99 Disco 1 and it has a high idle like close to 2000 rpm on a cold start up, now i can drive in highway spped for about 30 min or so and turn it off then bacck on and it will idle below 1000 like about 800 rpm,. a little background here i just replaced the intake gasket and all the o-rings on the injectors. what could be causing me to have a high idle? a vacuum leak? im wondering if i have a vacuum leak on the intake manifold or on the plenum or plenum cover. and would this cause these symptoms? as it seems to dissapear after warm up. it has new pugs new wires as well and a new fuel pump. any ideas would be great,, thanks guys,

 

My money is on a vacuum leak, use brake cleaner and spray all around the intake, plenum, etc. and if the engine speed changes you find your leak.

 

If not a vacum leak then you need to clean iacv, and or reflash ecm

 

If you have a scanner, take a look at the coolant temp. Dash gauge is driven by a separate sensor. If coolant temp sensor unplugged, it tells the ECU it is -40F... you want cold start colant temp to be about ambient, rising in logical fashion. Vac leak is also highly likely.

 

Is the high idle intermittent or constant?

I just had this problem and it appears for me the truism that you "Check the simple things first" was proved:

I'd read about checking the throttle cable and to make sure the throttle plate was actually going back to fully closed and I noticed that my throttle body has one broken throttle return spring and the other one's main leg was out of the slot cut for it. I reseated the leg in it's cut and the idle problem has not so far returned.

I'll be looking for a discovery in a junk yard to find the other spring as it appears that to get one you'd have to buy the whole throttle body.

 

Check out MrBeamerdave you tube plenty on 3.9 EFI

 

99 D1 of North American Spec was 4.0 GEMS engine, different parts. Here is a short booklet from LR that details the different parts for Lucas, GEMS, and Bosch fuel systems.

 

ok here is what i have so far,
tried the brake cleaner but it was ideling so high was hard to tell. should i use a starting fluid or something more flammable ( when cold)
i do have a pocket computer but it does not read the coolant live, dont think it can read it at all.
im leaning twards a vac leak only because the issue seems to go away on a up to temp engine, it usually will idle at around 800 when the engine is up to temp. always in the am it will jump to 2000 or more.

where is the coolant temp sensor that you are talking about that i can check to make sure it is connected well?

any other ideas out there? it real is not intermittent it is pretty constant on when it has the high idle. the throttle cable is good as well, i also cleaned out the intake to make sure it was seating well. thanks guys, for all the replies

 

A bad coolant temp sensor will not cause a high idle.
Do not use starting fluid, you dont need something more flammable, you could also use water.
All you are doing by spraying the engine is adding more fuel than it already has so the engine speed will lower when it sucks in the carb cleaner, or water, or brake cleaner...
You are basically flooding the engine because it will suck in whatever you are spraying through the vacuum leak.