Hi guys,

Read through a lot of the advice on this and other forums but wanted to ask a few questions here and hopefully stumble across someone who's had the same issue (and solved it!). I'll try to structure and give as much info as I can below:

--Problem--

Car suffers from an inconsistent idle on start, it doesn't matter if it's a hot or cold start. You turn the key and the revs jump to somewhere between 1000 and 1500 and stays there. If I put some throttle on, it "settles" somewhere in the ~800 vicinity and occasionally a check engine light comes on for a short while and then goes off.

If I take the car for a short spin, only enough to warm it up lets say, as soon as I come to a stop the car idles at a buttery smooth 650rpm all the time every time, which makes me wonder if the jumping revs are something to do with a sensor and the intial cat-heating phase?

--Car--

Car is a 1998 Turbo 2000 model.

--Mods--

The car is de-cat for what it's worth. I do have a catted downpipe available, but I'm hesitant to put it on because the car is apparently tuned and I don't want to cause backpressure problems.

As far as other mods to the air intake system, it has a ram air filter in place of the box housing, but that looks like it.

--What I've done so far--

So far I've removed the intercooler, idle air control valve and throttle body and cleaned, replaced gaskets etc. The throttle body had a bit of build up but the IACV was awfully covered in soot. All cleaned and refurbed and put back in.

I didn't reset the ECU, only because I wanted to see if it could "re-learn the adaptations" however it doesn't seem to have taken.

-- Questions --

Should I reset the ECU? Would this potentially bugger with the tune?

Could it be spark plugs if the car doesn't misfire and the revs are smooth at all times?

What do we think it could be?

Thanks in advance everyone!

 

OEM Warm idle is set at 800 +/- 100 RPM
Cold idle is usually about 1000-1500 RPM

Have you checked your coolant temp sensor??

For MY97/98 models you will have 2 sensors - one for the gauge (21203AA030) and one for the ECU (22630AA041)

 

Cheers for that. Good to know that I'm within range at the very least!

I haven't checked the sensor, forgive me for asking the obvious but what would I be checking for? How do I know it's good or bad, what would I look out for?

Thanks in advance!

 

I see both sensors (I think) and they look a bit grubby but to pull them out I'd have to take off the alternator and belt which is a faff! Could that be the cause?

 

MY97/98 ECU pinout diagrams -CTS readings on first image.




DTC 21 Info

 

That's brilliant, thank you so much!

I've decided to go the replacement route. I managed to find a new ECU sensor for £20 and a gauge sensor for £5, both brand new.

To get to their location on my car I'll have to remove the alternator, so figured while I'm that far in I'm going to refurbish the leaking power steering pump and replace the aux belt while I have good access to that area. May as well handle the preventative maintenance eh?

All the best, thanks so much for the diagrams!

 

Hi everyone, just wanted to close the loop on this to hopefully help someone else down the road.

Turns out the rough idle didn't have anything to do with the air intake system, it was the coolant ECU sensor all along.

£26 and a morning's work later and it was all fixed. Replacing the sensor is easy, just getting to it is hard as it's underneath the alternator. I took the time to rebuild the power steering pump as well as it had a leak between the pump and reservoir.

My battery died during the wait to get the parts and fit them, so I didn't reset the ECU but all is well now.

Cheers for the help and suggestions along the way.