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Hello. I'm having an issue where my car has hunting low idle when in gear, but in neutral it instantly fixes itself. It appears to happen randomly. Anyone know where I should start looking? Spark plugs and leads have been replaced.

Hydraulics? Which model?

Oops, sorry.

1990 NA

I should add that there is no issue with acceleration and no hesitation.

So when you say "I'm having an issue where my car has hunting low idle when in gear, but in neutral it instantly fixes itself.", is this when holding the clutch pedal down and with a gear selected but stationary? If yes, then it could be that your clutch hydraulics are failing and is partially engaging that gear.

JBT wrote:So when you say "I'm having an issue where my car has hunting low idle when in gear, but in neutral it instantly fixes itself.", is this when holding the clutch pedal down and with a gear selected but stationary? If yes, then it could be that your clutch hydraulics are failing and is partially engaging that gear.

Yep that's correct. Do you think I'd be able to feel it if it was doing that? Didn't feel like it was engaging or anything. What should I be looking for in terms of wear/issues - and what requires fixing if this is the issue?

I've owned the car for about 2-3 months - today is the first time I've seen it and it hasn't happened several trips after it happened.

Check clutch hydraulic fluid level and for leaks at the slave cylinder or, less commonly, at the master cylinder.

JBT wrote:Check clutch hydraulic fluid level and for leaks at the slave cylinder or, less commonly, at the master cylinder.

Fluid level is fine and no visible leaks anywhere.

I tried putting it in all gears when it was happening, there was no change in RPM - If it was the clutch hydraulics failing, wouldn't it change rpm with the higher gears being engaged slightly?

i'd be more inclined to think it was a vacuum issue related to the idle air controller valve under the throttle body.

project.r.racing wrote:i'd be more inclined to think it was a vacuum issue related to the idle air controller valve under the throttle body.

Suppose it's just a clean? I replaced some coolant lines under there during my service and that's when I noticed the issue (fluctuates 100 rpm "normally" since service, but nothing like the issue described in the OP which was fluctuating about 500 rpm ), so maybe I have bumped something or upset it. Any write ups?

There's a switch on top of the clutch pedal housing which goes on/off when you press the clutch pedal. There a switch on top of the gearbox that goes on/off when you put the car in/out of neutral. Both these switches feed to the engine management system, which uses them to determine engine idle speed. Idle goes very rough when both fail. I'd ask an auto electrician to check them.

apegrip wrote:Fluid level is fine and no visible leaks anywhere.

I tried putting it in all gears when it was happening, there was no change in RPM - If it was the clutch hydraulics failing, wouldn't it change rpm with the higher gears being engaged slightly?

No. Other possible cause is electrical/ECU as per manga_blue's post.

I should have mentioned too that there's an on/off switch in the throttle positioning sensor that tells the ECU when you've got your foot off the accelerator pedal. Those three switches, plus the engine coolant temp sensor, are what the ECU uses to set idle speed.

Alright so I should check these switches too. Any way to test them other than an auto electrician?

You need to have a multimeter and be a contortionist.

The clutch switch is near the top of the clutch pedal and you need to test across the terminals up there.

The gearbox neutral switch is near the top of the gearbox on the left hand side but the two wires from it come back over the top into connectors beside the centre of the gearbox on the right hand side. You can disconnect these and test across the connectors there.

Given that it sounds like there's a difference between idle when gearbox is in neutral and when it's not in neutral it looks like the neutral switch is OK, so I'd focus on the clutch and TPS. Does anyone know the pinouts on an NA6 TPS?