High idle/varying idle: how do I properly adjust? (1 Viewer)

[Beej]

Hey folks,

How do I properly adjust the idle on my 97 fzj80?

Sometimes it idles at 1400-1500 rpms. Other times it's more like 700. I've read some stuff about ECU determining idle once engine is warm. The 1400-1500 can be present at start up (or maybe a little lower, like 1100, but then it still happens after driving for 30-40 minutes. But not all the time.

Is this an idle adjustment problem? Throttle cable problem? What is up here? I haven't experienced something like this before. I did remove the throttle body when I did the valve cover so adjustment may be off. But why isn't it consistently off?

Thanks is advance for any replies.

Happy holidays!

-- Beej

 

[blkprj80]

Look for air leaks, especially a vacuum leak in the intake somewhere.

Lift up on your gas pedal. If it idles down, it's a sticky cable.

 

[Tools R Us]

CEL on, if so, what codes?

 

[retrofive]

Look for air leaks, especially a vacuum leak in the intake somewhere.

Lift up on your gas pedal. If it idles down, it's a sticky cable.

x2

 

[Beej]

No CEL.

If it's the pedal, if it unsticks, what are rpms supposed to sit at?

 

[bloc]

650RPM. But, if it has been sticking consistently the ECU will learn incorrect adaptations and once unstuck the idle will probably be too low for a while.

I'd advocate turning the throttle valve itself back, vs the pedal. Technically if the cable is really sticking lifting the pedal alone might not let the cable reset to neutral.

If manipulating the valve itself doesn't change anything, it may be a sticky IAC valve, or a vacuum leak as others mentioned.

 

[Beej]

When I opened up the hood, I see that the throttle body assembly was not moved back to its original position. The little knobby was not up against the stop plate (reeeally wish a I had picture for that awful description).

Anyway, the strange part was that I couldn't push or pull it back into place. I could move the throttle body to be opened more but not all the way closed. Could this be a symptom of something other than throttle cable?

 

[ppc]

There are three cables that connect to the throttle plate that could be an issue.
1 - gas peddle
2 - cruise control
3 - cable connected to transmission

You may need to also clean the throttle body

 

[Beej]

Thanks for the help diagnosing!

1. For the gas pedal, when I pull on the gas pedal with my hand, it doesn't fix the high idle. However, this seems to be the one people talk about sticking.
2. Might be cruise control; I'll try disconnecting it. If its not that, then throttle becomes a more certain candidate.
3. Cable connected to the transmission pulls the opposite of the first two, yes? So unlikely?

Throttle body was removed and cleaned recently (within 1,000 miles).

 

[george_tlc]

Assuming the throttle is actually closed (not sticking cables etc), the idle is actually computer controlled via the IAC valve (idle air control valve...) which on some vehicles can stick as it gets gummed up with recycled PCV garbage. The TPS also can affect idle. When my TPS started flaking out, my idle was higher than nominal 650 (after engine was warmed up). The FSM explains how to test/replace/adjust the TPS.

So, if cables are ok, I'd be looking at the above.

cheers,
george.

 

[thatcabledude]

I went through a high idle issue at one time on my 94 and eventually figured out it was the engine temp sensor. Not the one that reports to the gauge, the one for the ECU. It was faulty and the ECU thought the engine was stone cold intermittently.... Raising the idle.

Eventually it wasn't intermittent and the idle was around 1800 at all times.

 

[skhochay]

had the same problem had to replace throttle cable

 

[Beej]

The TB is not closed. This is a certainty. I can see that the mechanism is not retarded back all the way to resting position. Weird thing is, I cannot push it back into position and lifting up on the gas pedal doesn't help either.

I would have thought that if it was just a "sticky cable", then yanking on it would get it unstuck. Still need to disconnect cruise and see if that affects it.

Thanks for replies.

 

[vinlc562]

I went through a high idle issue at one time on my 94 and eventually figured out it was the engine temp sensor. Not the one that reports to the gauge, the one for the ECU. It was faulty and the ECU thought the engine was stone cold intermittently.... Raising the idle.

Eventually it wasn't intermittent and the idle was around 1800 at all times.

Just following up on this, I had the same issue. Is this the sensor below the upper intake? If not, where is this located? I want to try to replace mine.

 

[thatcabledude]

Just following up on this, I had the same issue. Is this the sensor below the upper intake? If not, where is this located? I want to try to replace mine.

Yes. You can swap it out without removing the intake.

 

[vinlc562]

Yes. You can swap it out without removing the intake.

Will it spill oil if I remove it?

Thank you

 

[thatcabledude]

Will it spill oil if I remove it?

Thank you

No, it’s screwed into a coolant passage. Not much coolant will be lost.

Keep in mind there is two temp sensors in the head. One is for the ECM to read from, and the other is for the temperature gauge in the dash. IIRC, the ECM’s sensor is the third one from the end of the head closer to the front of the truck. Let me see if I can find a diagram.

 

[thatcabledude]

 

[vinlc562]

View attachment 3679558

I see, this is the part I’ve ordered. Is this the right one?

 

[thatcabledude]

View attachment 3679584
I see, this is the part I’ve ordered. Is this the right one?

Yeah