Annoying idle problem - 940 LPT

[Cylinder 8]

Hi All, I'm just about at my wit's end with this, I've tried so many different things but my 940 is just refusing to idle properly. Specifically, it nearly stalls almost every time it goes from overrun to idle. Whenever I press the clutch when coming to a stop, or just rev the engine in neutral, as the revs drop back to idle speed they keep dropping to about 500rpm, the whole car shudders for a second or two, and then it goes back up to a (fairly) smooth idle. It's like the ECU can't react quickly enough to stop the revs dropping too low.

So far I've:

Cleaned the entire PCV system and replaced the oil trap
Replaced (almost) all the vacuum lines
Replaced the plugs, leads, and distributor cap
Cleaned the idle control valve
Replaced the idle control valve
Checked the throttle body is clean
Checked the setup of the throttle body (500rpm idle with control valve hose clamped, plus throttle position switch set correctly) - this has smoothed out the idle generally, but hasn't fixed this particular problem
Checked the TPS with a multimeter
Cleaned the TPS connector plug
Cleaned the MAF
Compression tested the engine just to make sure it's not something more serious.

I don't think it's the turbo recirc valve (as I've seen suggested in other threads) because with both the original turbo and the 16T I've just installed the engine behaves exactly the same. It also does it when I've been driving gently off boost.

The car does have a lot of modifications, including an IPD cam and a lightened flywheel, but it behaved just the same when it was standard, so I don't think it's anything I've caused.

Apologies for rambling on, if there's anything else I should be looking at I'd be very grateful.

[Laird Scooby]

What's the cold starting and running like?

Since doing all that remedial work, have you pulled fuse #1 at all? If not, pull it and leave it out for a few minutes then refit. Take it for a drive, if you can for about 30-40 miles.

What fuel are you running on? Brand and octane rating please. Have you filled up anywhere different in the time just before this happened? Do you normally run with half a tank, full tank or keep it fairly low?

There are other possibilities, i'm trying to eliminate some of the more common ones first.

[Cylinder 8]

It's interesting you should mention cold starting. Unless the weather is particularly warm, cold starts are generally poor. For the first 30 seconds or so it has a strange sort of surging idle, the best way I can describe it is like an engine with really aggressive cams, or the way a rotary engine idles. It's also reluctant to rev. It gets gradually smoother and after 30 seconds or so it's fine and runs really well. I've just remembered, a few days ago it wouldn't start at all until I gave the throttle a quick prod, at which point it fired right up and was fine.

Yes, fuse 1 has been regularly pulled to reset everything after work has been carried out.

Fuel is normal supermarket 95 rating, usually Sainsbury's but sometimes others. I usually fill the tank and refuel just before the light in the gauge comes on. This problem has been there for over a year and I haven't noticed any connection to different fuels.

Cheers

[Laird Scooby]

Originally Posted by Cylinder 8

It's interesting you should mention cold starting. Unless the weather is particularly warm, cold starts are generally poor. For the first 30 seconds or so it has a strange sort of surging idle, the best way I can describe it is like an engine with really aggressive cams, or the way a rotary engine idles. It's also reluctant to rev. It gets gradually smoother and after 30 seconds or so it's fine and runs really well. I've just remembered, a few days ago it wouldn't start at all until I gave the throttle a quick prod, at which point it fired right up and was fine.

Yes, fuse 1 has been regularly pulled to reset everything after work has been carried out.

Fuel is normal supermarket 95 rating, usually Sainsbury's but sometimes others. I usually fill the tank and refuel just before the light in the gauge comes on. This problem has been there for over a year and I haven't noticed any connection to different fuels.

Cheers

Buy yourself 2L meths from Toolstation and when the gauge next gets near the point where it brings the "feed me" light on, add it to the tank.

Drive it for a day or two before filling up but my guess is that you have condensation in the tank.
The ideal concentration is 10% meths to petrol, when the gauge is getting near the "feed me" point, there will be 3-4 gallons left in the tank (feed me light usually comes on at about 2 gallons) and 4 gallons is 18L so adding the meths makes 20L - 2L in 20 is 1:10 which is 10% hence the method above.

If you can, use BP Ultimate instead of supermarket petrols. On the face of it, they are cheaper but in every car i've ever had, fuel economy has suffered on supermarket petrol (except when Sainsburys were supplied by BP - they no longer are! ) and the improved economy far outweighs the extra cost.

[TonyS9]

Not trying to be cheeky but everything you have done has been changing things. I'd suggest stop changing things and diagnose the fault with testing.

Measure the fuel pressure, check the spark, take out the air filter and listen for the dump, check you are getting vacuum at idle at the dump valve (the nipple could be blocked).

[Laird Scooby]

Originally Posted by TonyS9

Not trying to be cheeky but everything you have done has been changing things. I'd suggest stop changing things and diagnose the fault with testing.

Measure the fuel pressure, check the spark, take out the air filter and listen for the dump, check you are getting vacuum at idle at the dump valve (the nipple could be blocked).

There is something he hasn't changed (or at least not listed) Tony, that's why i asked about the cold running.

My money is on condensation in the tank at the moment, if nothing else it will eliminate water in the fuel as a potential cause.

Then if the fault is still present, any tests will be that much more valid, especially if fuse #1 is pulled after the meths has done its thing and another drive taken. Any fault codes then will be the result of whatever else may be causing the problem.

[Cylinder 8]

Thanks for the help guys.

The car already had a full tank of fuel and isn't used much at the moment, so I've added a fuel treatment that's designed to remove moisture from a full tank. It's supposed to be very effective but hasn't made any difference to my idle yet. I'll keep an eye on it over the next week or so.

Assuming it's not fuel moisture, I don't really know where else to look. I've done my best to rule out vacuum leaks. I did an experiment earlier in the week, I clamped off the idle valve pipe and set the throttle body stop to a nice idle speed (and readjusted the TPS correctly) and it didn't make much difference, so it's definitely not a problem with the ECU's idle control.

I can rule out most of the things Tony suggested but fuel pressure is something I haven't looked into, so maybe that should be my next step.

[Laird Scooby]

Originally Posted by Cylinder 8

Thanks for the help guys.

The car already had a full tank of fuel and isn't used much at the moment, so I've added a fuel treatment that's designed to remove moisture from a full tank. It's supposed to be very effective but hasn't made any difference to my idle yet. I'll keep an eye on it over the next week or so.

What was the fuel treatment you added? If it's the one i'm thinking of (can't remember the name but would know it if i saw the bottle) it's 99.9% meths in a tiny bottle for about £4-5 which IMHO is a rip-off.

Also it won't do much because it's such a tiny concentration in a full tank.

Until you've got the fuel level down to about 1/4 tank and added 2L of meths, i wouldn't start messing around with anything else as you could end up making other problems so won't know if you've actually fixed the first one.

[Steve940estate]

Hi, sounds like you have gone through a lot of the normal things. Possibly a red herring but the poor starting when cold might be something to do with the coolant temp sensor. I can't remember which one is for the gauge and which is for the ECU though.
Can you tell from the state of the plugs how it's running ?

[Laird Scooby]

Originally Posted by Steve940estate

Hi, sounds like you have gone through a lot of the normal things. Possibly a red herring but the poor starting when cold might be something to do with the coolant temp sensor. I can't remember which one is for the gauge and which is for the ECU though.
Can you tell from the state of the plugs how it's running ?

The one on the 'stat housing is for the gauge, the one under the inlet manifold between #2 & #3 inlet tracts (usually) is the one for the ECUs (ignition and injection) but i think you might be right about the red herring bit.

Thing is, it is starting, it's just running rough as i understand things which ponts to either fuel quality or perhaps duff plugs, leads, rotor, dizzy cap etc