Technical UT idle problems

[F R O S T Y]

Got a problem with my cars idling,
First if you'd start the car it would idle perfectly at 1000rpm but soon as the temp would get to a quarter way up it would go crazy, the revs would rise and fall from 1000 to nearly 2000rpm which is drinking all my petrol!!!!!!
But now its gotten worse the revs dont go higher than 900rpm for some reason and sometimes they fall below this and then the car cuts out. Other than that the car drives and boosts fine, its just cutting out sometimes when i come to a standstill in traffic etc. Any ideas????

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[jamie86]

Have you checked the usual suspects eg spark plugs, leads, fuel fiilter etc?

 

[F R O S T Y]

Its just had Brand new battery,spark plugs, leads rotor arm and oil filter fitted, changed the oil too. Havent changed fuel filter though...another thing today when i filled up with texaco high octane fuel the car let out some smoke which it hasnt done in aaaaaages, usually i use bp petrol, is there any difference between the two??? It only smoked on acceleration when i left the petrol station and stopped soon as i pulled over to have a look

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[bulldog5046]

do UT's have a vacume advance or somthing that controls the idle?

 

[alexGS]

No vacuum advance capsule but there is a vacuum pipe to the Microplex unit that has a similar effect - check for splits etc.

My 'usual suspect' would be the coolant temp. sensor (on the thermostat housing). Start by cleaning its connections? Helps if you have a known-good spare to substitute. The way the problem began (and then got worse) makes me think this is the culprit: the ECU didn't 'know' when the engine is warm, and now it doesn't 'know' when it is cold! (excuse the anthropomorphism)

Don't know about the smoke issue... maybe it is related but I can't think how...

-Alex

 

[Dunc Uno Turbo]

sound like a boost leak to me frosty. i had the same prob and one of my pipes was split. check the outlet on the turbo under and the back of the engine. this is where mine was after i spent ages looking

Dunc

 

[F R O S T Y]

Alex does the wire from the temperature sensor branch off into a Y shape and go to the idle control valve also?? I had a quick look earlier today and saw a wire which has one end going to the thermostat and the other to the idle control valve. I'll try cleaning the connections tommorow.

And also Can a faulty idle control valve itself cause my engine to rev so low??? Its idling at about 700rpm and struggling to get the revs higher. It seems to get worse as the car warms up. What i mean is when you start up the car it runs but the revs are below 1000rpm and the engine and exhaust shake a lot as the revs are trying to get higher. If you rev it hard as its doing this it will cut out but not always. However when the engine temp gets to a quarter way up it will almost certainly begin to struggle and then cut out when stood still. So while i drive it the car drives fine but at the traffic lights if i dont put my foot on the accelerator to keep the revs high it will cut out.
My friends golf is doing the same thing as it goes! lol

And dunc which pipe do you mean mate is it that one which comes out the block from the back??

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[Dunc Uno Turbo]

nope, just the outlet on the turbo is the one im on about

Dunc

 

[alexGS]

Frosty, yes, that sounds like the wire. Just pull all the connections and clean... then maybe consider swapping the sensor. You could test the resistance across the sensor terminals with a multimeter. I don't have the specs handy, but they're in the Haynes manual (which I've lent to a friend).

The idle control valve is actually a very simple device. It has an opening to allow air through when cold. As it warms up, a bimetal device closes the opening. The warming is done through heat conducted from the engine block, but also through an electrical heating wire on the bimetal strip (hence the wires). If you take off the valve and inspect it, you should see the opening (about half-open is 'normal'). If it's closed, then there's your problem.

I agree with Dunc now that it sounds like a boost leak (or vacuum leak, depending on your viewpoint! i.e. with engine idling there usually isn't any boost, though maybe Dunc's turbo is so massive... )

But seriously, the ECU usually runs the engine richer when cold (to give better performance). Assuming for a moment that the temperature sensor is working correctly, the ECU will lean out the mixture when the engine warms up. Now if you have an air leak (boost/vacuum leak) this will make the mixture leaner at all times. I think you can see that this could therefore have a more profound effect with the engine warm, since the mixture will end up too lean for the engine to idle at all.

So, as Dunc says, check all the boost pipes (listen for a whistling sound), check all the connections are tight, check the small vacuum pipes for splits (the ones that run to the Microplex ECU). Check that the idle control valve looks correct. And then if you really can't get anywhere, swap the coolant temp sensor.

Just my ideas of course

-Alex

 

[F R O S T Y]

Checked all the boost hoses. main hoses from intercooler, plenum etc are all fine, hoses off turbo look ok, hoses from the microplex ECU are brand new and also fine. Any others i may have missed where i may have a boost leak???
I do remember the car making a bit of a whining/wheezing type sound a while back and not boosting properly especially after the car reached normal temperature. Does this indicate a boost leak?? And can a cracked exhaust manifold cause these problems??? as i do remember having these problems before and then i checked my exhaust manifold and it had a few cracks, got them welded and everything was fine for a while but now the wheezing sounds are back?? possibly related or not???

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[F R O S T Y]

Alex you win the prize mate you were right about that thermostat coolant temperature sensor all along. Changed it today, started her up and she is idling perfectly once again Thank you sssssssssso much!! Damn i missed the UT sssssssssso much and its only been about a week or more gaaaaaaaawd knows how much Dunc must miss his one hehe

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[alexGS]

Thanks, I'm not sure I score the 'points' though, since I thought it sounded like a boost leak at one stage

I haven't driven mine for over a month! It's the damn Toyota stuck in the way...

-Alex