Mk2 8v High Idling

Hi

I'd be really grateful if anyone on here could shed some light on an intermittent problem that I've been having on my Mk2 for the best part of 3 years...

Basically, for no apparent reason, it'll suddenly start idling at about 1500rpm. As you can imagine, it makes driving a real ball ache, especially when slowing down for lights etc and the car suddenly speeds up!

What makes this problem more annoying is that it can go months without any trouble and then it'll have a bout of high idling completely out of the blue. I don't know if there's any connection, but it only happens during the summer, usually on hot days like today.

The latest episode of high idling started about 5 weeks ago. It'd been perfectly fine for about a year and then started up to it's old tricks. As I'd only replaced the blue temp sender about 18 months ago I didn't suspect that, but as a check I swapped the plug over with the black sender and to my suprise everything settled down again. The blue sender was now wired up to the temp guage which duly stopped working. I only ever buy genuine senders so I was pretty peed off that this one had failed so soon. Anyway, whilst on my way to pick up a new blue sender, completly unannounced, the rev needle flipped up to 1500 and I was back down to 25mpg

To cut a long story short, the car has been into a garage for an MOT where the mechanic who's been working with dubs for years is completly baffled and has tried most things to get the idle back down to about 950, but can't sort it. So we have a new blue temp sender which hasn't fixed it. Tonight I swapped over the isv and that's made no difference. I've replaced some of the vacuum hose, even though it looked perfectly fine, but still idling too high. It's hovering around 1250 when stationary and bounces up to 1500when being driven. Interestingly, if I disconnect the electrical connector to the isv the revs drop to about 500 and it keeps running, although a but lumpy.

To be honest I'm at the stage now where I'm losing patience and falling out of love with my dear mk2 [:^(] I've even contemplated taking up Mr Brown's 2k offer against a new Scirocco but it would kill me to send it to the crusher. So if anyone can suggest something else I can try, I'd be really grateful.

Cheers
Matt

 

An update...

Went out for a drive yesterday lunch and all was ok for the first 20mins, idling nicely at 950. Then without warning, the high idling kicked in. I could literally pull off in 1st without even touching the accelerator. To make matters worse I was stuck in traffic, so everything got hot and was accompanied by the smell of burning rubber!

Went out again at about 10.30 last night and everything was perfectly normal. The half hour drive home was without incident.

I'm now almost 100% certain that this is being caused by hot weather, so surely it's something to do with whatever it is that measures or reacts to air temp?

Any ideas guys?

 

was the isv you changed over 100% good,have you tried cleaning it in petrol,could the throttle cable be sticking.Check out where the burning rubber smell is coming from it could be connected to the problem.

 

I would check check the ECU, the connector to it, and all grounds.

Also if you can catch it happening again, have a fiddle of the idle speed screw.

After cleaning my ISV, it went from completely buggered, to working quite well, except when its hot, the idle sits about ~1200 ish, and i have to blip the throttle hard to get it back down again to 900ish.

Dont you dare scrap that mk2! I have a '92 8v gti in the same colour

 

Thanks for the replies so far.

I can't be certain that the replacement isv is 100% good, it's one that was attached to an inlet manifold that I bought last year but have never done anything with. I soaked my original isv overnight in penetrating oil and have refitted it but the problem still kicked in this morning after 20mins of driving. Besides, the mechanic said he'd checked the isv and it wasn't 'sticking'.

The throttle cable isn't sticking either - I've had the bonnet up when the revs are high and the throttle is sitting up snug against the stop screw.

As regards the burning rubber smell, that only happened the once, when my oil temp reached 108 degrees! So I don't think it's connected.

I've not checked the ecu but to be honest, I wouldn't really know where to start with that one i.e. what to look for. Same for the grounds - where are they all etc?

The mechanic spent ages fiddling with the idle screw trying to get the revs down from 1500 but couldn't make any difference. His suggested next course of action was to take the trottle body off and 'have a look around'.

I've tried blipping the throttle (3 times over 3k) but again that makes no difference.

I hate to say it, but I'm really starting to dislike my car :-(

 

Meant the ECU as in replacing it altogether...they arent that expensive from the bay. This would be a bit of a last ditch tho.

Have you tried pulling them temp senders out aswell? Again unlikely but an easy test to get out the way.

If its happening and you turn it off then on again will it automatically do it again?

Tried spraying carb cleaner into the throttle body? Ive never changed one before but they look a right bast to do.

Something must be opening to get it to 1500 rpm.

 

There's a few ecu's on ebay at the moment, so I'll keep an eye on them and maybe pick one up if the bidding doesn't get all silly. I've also noticed a maf, isv and ignition module on there. Would you agree these are all potential culprits?

I did try disconnecting the blue sender whilst the revs were high and as soon as I did, the idle shot straight up to around 2500.

If I turn off, wait about 10 secs and then start up again the problem's still there.

I'll give the throttle body a good clean out, although I do try and do this fairly often. I agree that they look like a proper pita to remove.

Thanks
Matt

 

I thought the idle should go down if the blue sensor is disconnected?

 

My idle goes up when disconnected aswell? Only by about 200rpm or so, not almost 1000 like mattallens.

 

Thanks again for the pointers guys.

Just done my journey home (ok for first 10 mins then high idle) and popped the bonnet when I got back. Disconnected the blue sender and the idle increased by about 250rpm. My mistake for saying that it jumped to 2500rpm!

The idle switch definitely clicks when the throttle is closed. How do I go about checking the idle switch wires for breaks? Is this going to involve removing the tb, especially as one of the wires fits snugly to its underside? Can these be checked using a multimeter i.e. a continuity test or do they need to be physically inspected? If there was a break, then surely the idle would be affected all the time and not just during hot weather? Also, would it be possible for a broekn wire to cause the high idle to suddenly kick in 10-20mins into a journey?

Cheers

 

Cheers rubjonny!

Will get the multimeter out tonight and give the tb a thorough testing.

Fortunately it's a lot cooler outside today and everything's running as it should (I now officially hate summer!)

Matt

 

I had very simlar problem and sorted it by swapping the afm.

 

Good to hear that it's not just me who's experienced this one! Was your high idling also happening just on hot days? And did you swap out the air box as well as the AFM?

Cheers
Matt

 

to be honest i didnt really notice the outside temp. Mine would just randomdly idle at 1200 sometimes 1500. This usualy happen when i pulled up at traffic lights, i found that i had had the clutch down and one foot on the brake, if i slowly let the clutch up the revs would drop back to normal just as the clutch reached biting point. I kept my air filter.

 

Without air an engine will not rev, if the throttle is closed then air must be entering elsewhere, also, you say that the engine runs without the high idle solenoid connected and the throttle closed, where is it getting air from? have you tried bradex easy start?
I'm not saying an electrical problem is not to blame, but the problem has to cause excess air to enter between the butterfly and afm

 

An update, with success!

As per the suggestion made by Gareth20vt, I got hold of a replacement AFM (10 off the bay) and so far, touch wood, the problem seems to have been sorted. We've had a couple of hot days and the idle has stayed solid at 950. The car also feels like it's running a lot better generally. Would be interesting to know what role the afm plays in all of this, since I'm no mechanic!

A big thanks to all who offered help with this one.

Cheers
Matt