16v Poor idle when cold

[mat_fenwick]

OK, the first confession is that the engine isn't in a BX, but I was hoping that someone could share some wisdom with me anyway...

The problem is that the engine will not idle until it has run for a few minutes, and even then it is very prone to stalling when you come off the throttle. The idle speed is also variable, it will speed up and slow down with no movement of the throttle.
You can adjust the throttle stop screw to allow it to idle when cold - but this is not much good unless you like a 3000 rpm idle when warm. I unplugged the idle speed control valve, which gave it a steady idle speed - but upon re-connection the engine immediately stalled.
I bought a new idle speed control valve, thinking that was the problem but I now have a slightly more consistant idle speed but the stalling problem has not improved.
My next thought is that the coolant temperature sensor is not giving the correct resistance, hence the engine thinks that it is warm all the time. Does this sound feasible? The connections are all good.
I don't like the idea of just replacing bits till the problem is cured, so I would be grateful if someone could help with a firmer diagnosis, or give me more things to check. All service items (plugs, leads etc) are good.

[Kitch]

Throttle postioning sensor. Should be more of a problem when warm, but basically it's a switch that tells the ECU to deliver fuel in Idle mode. Without it, the idle control valve doesn't really have much of a say in things!

Firstly check whether it's adjusted right, but rotating the throttle butterfly by hand. When it returns to it's shut position, you should hear a faint click, like a switch. If it doesn't do this, look down the back of the TB and you'll see a little black gadget with a multiplug going into it. It's held on with two 8mm (IIRC) headed bolts. Loosen these off (don't remove the unit!) and rotate the switch until it's aligned properly so that it clicks with the throttle closing. It's quite a common thing them going out of sync.

If it DOES click initially, there are some readings on the web somewhere for the resistances it should show, as it's possible it's failed.

Thats what I'd check first, although apologies if it's teachign you to suck eggs!

[prm]

Hi Mat

Maybe you should have fitted the V8 engine instead!!

Some 16v’s can be a right s-ds to set-up.
Have you recently carried out any engine/head repairs? Or has the problem just occurred? Their very susceptible to any air leaks on the inlet manifold.

A few points to check that may help as it sounds that you’ve a conflict between AFM and TPS setting.
Remove the AFM and hose assembly and reset the throttle stop. Can be slightly tricky with a well worn unit with play on the spindles. Back off the stop screw completely, wrap 5amp fuse wire round a cocktail stick, super glue together and leave a straight length of about 2 inches protruding at one end. Raise valve, insert wire and adjust stop until your able to just very very gently remove the wire. Then test and reset the TPS switch as Kitch has mentioned

The CTS and ATS have very similar resistance readings with a cold engine.
CTS should read approx, 3.0k ohms cold, to 250ohms at normal running temp.

Check for free movement on the AFM main valve. Might be worth removing the top square cover to clean the control tracking. These covers are sealed with a silicone type adhesive, a knife blade, ran around under the edge usually works. Then try resetting the mixture to 1.5 – 2.0 to see if there’s any improvement. Tie down any free floating throttle cables!! Had one where the RPM would increase when you closed the bonnet.

Regards

Pete

[prm]

Apologise - forgot to mention, to disconnect throttle cable when making adjustments to stop screw. Reconnect after adjustment, operate throttle pedal several times to ensure a minimal amount of free play on the inner cable stationary.

prm

[mat_fenwick]

Thanks to both of you for your replies, a good few things for me to check out. I'm not sure whether I will have chance to do much on it this week, I thought I would change the cambelt on the van before taking it to the National but it took me about half a day to get at the cambelt (and change the alternator) and the rest of the weekend to get the cam pully off!!! 4 allen bolts, 3 of them that required a 'special round allen key' which I don't have, so had to drill them out with a right angled drill. That was fun.

But when I get chance I will definately follow up the suggestions before spending any more money.