erratic idle again gti 8v

[rutter123]

took the gti for mot today, only 1 failure on emmisions being 50% more than the limit, also erratic idle after a good blast, will only stay steady at 1300rpm sometimes a little higher and seemingly running rich, the head was rebuilt only a few miles back by citroen classics so will rule that out as a poss cause.

all breathers have been checked and all ok
new plugs leads cap and arm
throttle body has been thoroughly cleaned
good even compression on all cyl.

firstly what is the correct way to install the throttle position sensor and how should it be set?
how many turns should the air mixture screw be wound out, mine currently just about 1 full turn out from the stop
also looks as tho the butterfly is not resting on the idle adjustment screw but wont close any further?
seems to make no difference with the TPS disconnected
Any ideas?

[Way2go]

also looks as tho the butterfly is not resting on the idle adjustment screw but wont close any further?

Any ideas?

If it's the second butterfly then that closes by spring pressure alone and not by linkage to the first. If it;s not closing then it points to the shaft needing lubrication, perhaps by penetrating oil first and then normal 3in1. The idle should only be adjusting the first butterfly as the second comes in only on later demand .

[rutter123]

its 1st butterfly that dosent seem to be closing, looks as tho the tps is stopping it before it gets to the fully closed position, it will close a little more with some finger pressure on the linkage.

[mat_fenwick]

Which emissions are 50% too high? That will be a HUGE clue to what actually may be wrong...high CO will have a different cause to high unburnt HC for example.

[rutter123]

CO is high @5.70 limit 3.50.

have searched the net looking for poss cause and most of them point towards the ignition system or a faulty oxygen sensor, but this is a pre cat car, i have a spare known good fuel pressure reg, will change that first, and will also check the operation of the throttle position sensor.

[mat_fenwick]

OK, that confirms what you thought - too much fuel, but the fact that (presumably?) the HC reading is OK would suggest to me there isn't an ignition problem. Fuel pressure regulator is a good shout; is there somewhere to measure the pressure, like a tyre valve? As you think, there must be an air leak of some kind too for the idle speed to be so high...always fun when you have more than one problem!

Assuming it's a Jetronic system (and I think the Motronic is the same), I'm sure all the TPS does on these is signal when the throttle is either fully open or closed (for fuel shut off on the overrun). I.e. it's just a simple switch which closes one set of contacts at closed throttle, and the other at full throttle. Easy to check!

[rutter123]

mat, have checked the tps seems working ok, as u say its just an on/off switch, u can hear the internal micro switch operating just as the butterfly opens, have currently got the inlet manifold off to once again check breathers etc for air leaks, but surely if there is an air leak on the inlet manifold side of things wouldnt this cause it to run lean ? i,m at a bit of a loss as to what the prob is as the last mot co reading was 3.00 and passed, the car has done a mere 100 miles since. i have just removed the spark plugs and they are quite sooty. it could prob be leaned out a bit by turning the air mixture screw another turn out but this still doesnt solve the high idle (1500rpm) speed. the FPR is rated at 2.5bar i have a spare but its 3.0bar so this wont help. any more suggestions would be most appreciated as time is of the essence. any other sensors/switches that would be worth checking?

yes it is bosch jetronic.

there is a temp sensor of a sort mounted on a housing directly below the dizzy (2 pin plug) seems to have water and air pipes attached, poss faulty, how to check??

[mat_fenwick]

Yes, an air leak along would cause it to run lean but the reason I suggested it is because extra fuel alone wouldn't make it idle quicker, so air must be getting in somewhere. What you describe is the coolant temperature sensor, so if that is telling the ECU the engine is cold all the time, it will be run rich and probably raise the idle speed...

From memory it should be around 2000 ohms at 20 °C, dropping to a few hundred ohms when fully hot. I'm not certain but if there is a failure in the wiring to this sensor it may go into a default safe/rich mode. Another way of proving if the extra air is a leak or intended, would be to unplug the idle air valve which bypasses the throttle body. If the ECU is holding this open then unplugging the electrical connector to it should drop the idle speed.

EDIT - not worked on a Jetronic system on a BX but had L-Jet on an old Alfa, so things are coming back to me bit by bit! The basic fundamentals should be the same though. Is there an extra injector in the inlet manifold somewhere? If so that's the cold start injector which is used during warm-up. If that's chucking in more fuel it could be unplugged to prove it one way or another.

[rutter123]

have refitted the inlet manifold and checked all breathers, now it wont idle at all!!!!! will only run at 3000rpm or above, below that it is backfiring in the inlet manifold and exhaust and generally running like shit. cant see where ive gone wrong as everything has gone back into its original place, turning into a bit of a nitemare this one.

[mat_fenwick]

Must be more air than before...confident the inlet manifold gasket isn't leaking seeing as you've had it off?

[rutter123]

inlet manifold has been refitted with gaskets i made as none are factory fitted. i suspect a breather has somehow come astray or worse, its certainly something significant, have ordered new water temp sensor as a precaution. run out of daylight so will resume in a few days hopefully with better results.

Have since found out the CO mixture adjustment is on the top of the air filter unit so all of this could have been in vain. DOH!!!!

[sdelasal]

Brian - does this car have a large alloy 'flap' type air meter? If so, the co adjust is on that unit - it acts as an adjustable by-pass path for through the meter.

[Way2go]

It's probable that the problem is with the injectors, especially if injector cleaner has been used previously. They have different modes and duty cycles controlled by the ecu. Any gunging up affects idle, balance and performance.
Best to take them out & get them serviced by a bosch agent with an ASNU ultrasonic machine. It's quite amazing the smoothness & power available afterwards.

[Jaba]

Once you have got it to return to a situation where it will idle at a normal speed you can the tweak
the CO adjustment on the flow meter. It needs to be turned OUT to reduce/weaken the mixture. It changes by about 0.7 per turn. So about 4 turns should bring you in within limits at the normal idling speed that is.
You can remove just the throttle housing to see what is going wrong with the butterfly closing.

[rutter123]

Jaba thanx for that now all back to normal (problem caused by breather pipe from the big rubber air intake hose had come adrift from the water temp/air temp housing just under the dizzy) idling ok but still not perfect seems pretty stable @ 1000rpm, prob as good as its gonna get, maybe some CO adjustment will steady it a little more.