Trying to chase down what's causing my high idle and wanted to check on here before I pick up a new IACV. Out of the blue the other day when I started up the Disco, the idle shot up to 1700rpm in park, around 1000 in gear. Figured I could clean the IACV and resolve the problem but I'm not having any luck.

I cleaned it about a week ago and upon reinstalling it, it idled right where it was supposed to. I thought I had it fixed. Went out this morning and the high idle is back. Took everything apart, cleaned it again but this time it didn't remedy the issue.

I'm assuming the IACV is just shot and needs to be replaced but I wanted to ask and see if there's anything else I should check first? The idle adjustment thing under the cap on the intake manifold has been tampered with before. I tried adjusting it just for kicks but even all the way down, it doesn't slow the idle. Also checked the coolant temp sensor, its working. Any help would be great.

 

Adaptive reset is the only way to get the ECM to recognize the IACV. If there was enough carbon build up and you removed it guess what? Your pintle is no longer in the same position as it was and the ECM is now going to see it as a new one which needs to be reset to "zero" the stepper motor (it's not absolute zero but one that is interpreted). The screw under the tamper proof plug should always be left alone unless you have the right diagnostic box attached to your data connector (T3/4 or Autologic). You won't perceive the amount of air it adds or subtracts from the plenum chamber until you hear it. That is a fine granular adjustment device, not a wrench and ear adjustment one. If you mess with it then expect a long term fuel trim MIL of either too rich or too lean.

 

Got codes? Got a ultra gauge? There is a reason it went high idle , in the end you will need the ecm reset but need to know what caused it

I would hit a yard and get another plenum unmolested for one thing

 

New symptoms, looks like I may not be on the right track assuming the IACV was crap. I reset the ECU and upon start up it idled perfect, right at 700rpm after the initial higher rev upon first cranking over. Let it run for about a minute and all of a sudden it started hunting for idle. It would drop to around 250rpm then jump back up to 700rpm, then it would drop again and almost stall unless I kept my foot on the gas a bit. Turned it off, made sure I didn't miss a connection under the hood, started it back up and it went right back to idling high again.

I've got an Ultra Gauge but don't really know what I should be looking for as far as diagnosing the issue. Can someone point in me in the right direction? Thanks again!

 

Set page 1 to show TPS, rpm, temp, MPH, Intake air temp, maf

Seeing these should help for now

MPH should be 0 sitting still

Intake should be ambient temp after sits overnight

TPS at idle should be around 12% at idle

Are the hoses from valve covers good?
Is the pcv noodle in pass side valve cover clean?
Is the MAF boot on good? Tight? MAF o ring in place?
IACV gasket in place?

Mine ended up being the fuel pressure sensor shorted on the fuel sending unit, was canceling out my TPS

Do you have codes?

 

Not throwing any codes at all. Everything you outlined is spot on from what I can tell, I cant find an issue with any of it. I'll hook up the Ultra Gauge and see what it tells me.

 

Clean all grounds real good under hood, curious to see what the live data says

 

Fired it up just now after sitting all night and let it warm up so I could get readings cold and at operating temp.

Cold readings
tps 12.2
maf 0.17
rpm was 1000 for 5 seconds and then shot up slowly to 1900 over the course of about 30 seconds
engine temp 37 degrees
iat 31 degrees
mph 0

Readings after it warmed up
tps 12.2
maf 0.09
rpm steady at 1325
engine temp 163.4 degrees
iat 60.8 degrees
mph 0

 

Could be way off the mark, but I'll add this...
The description of your idle is very close to what I experience after my battery has been drained (usually because the kids left the dome light on in the back). Until I drive it for a good 30-60 minutes, allowing the battery to re-charge, and also turn off the truck and let it sit for another 30 mins, those idle characteristics persist.

So, I'd check the ground from the battery to the frame, volts coming from the battery, and the alternator.

 

Battery was recently replaced, probably less than 1000 miles ago. Alternator output is spot on as well. I'm still going to check all of the grounds though, this thing is acting super goofy.