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Annoying idle problem - 940 LPT

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Old May 30th, 2020, 04:03   #11
TonyS9
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You are correct in the assumption that the idle valve can't react quickly. Actually its more the ECU software, the valve itself is normally very fast, fast enough to deal with engaging the automatic drive or several kW of AC provided said loads are signaled to the ECU at the same time.

If you let the clutch out without using the accelerator you can get a better sense of its reaction speed (very slow). Its really just for for the big loads and maybe a little compensation for the lights and other heaters.

Revs dropping are usually because the air flow is too high with a low signal from the MAF or just too low. This can happen after boost without any dump (as you have found in the archives) and if the throttle body is either blocking the idle air (deposites blocking) or allowing too much of it (lumps preventing closing).

No dump will definately stall it, but if you keep reving it up while it depressuries over a few seconds will eventually idle. Its just the throttle closed signal makes it go into a much leaner closed loop mode than open throttle open loop mode. Disconnecting the the throttle closed switch can stop it stalling, but it will run badly for other reasons then.

The dirty throttle body problem is normally related only to fresh start, I think the ECU remembers the idle point after that, not 100% sure. But do check it off the car, it should be spotless, and that all the little holes are clear. It can be hard to see them all.

You could have a slight air leak or low fuel pressure if its doing it all the time.

You should be able to close off the idle air by squeezing the pipe, it should idle / nearly stall at 500rpm then.

You might be able to infer certain things based on the fact the lambda light isn't coming on, but this can take a while before it gives up. A voltmeter or narrow band O2 meter will give a better indication.

Last edited by TonyS9; May 30th, 2020 at 04:07.
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Old May 30th, 2020, 10:26   #12
Cylinder 8
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Thanks again all, the coolant temp sensor is definitely on my list. I'll have a look at it this morning and check the resistance at various temperatures.

I'm fairly sure I've ruled out the ECU's idle control as being the cause. With the idle hose clamped and the idle set manually with the throttle body stop it behaved in the same way, I've since been through the procedure again to set it back up properly and thoroughly cleaned the throttle body (which was't as clean as I thought). I've also ruled out the turbo dump as I can come off the throttle and roll along on the overrun for ages (plenty of time for the recirculation to finish) and it'll still nearly stall when I press the clutch. It's got a pod filter on it so I can hear the dump working.

I'm still considering the MAF but it runs so well the rest of the time I'm fairly sure it's fine (and I don't want to spend £300 on a genuine new one just to find out until I've ruled out everything else).

An air leak is near the top of my list at the moment, I might see if my local garage has got a smoke machine to test it. Last year I'd done some work on the car and forgotten to reconnect the small pipe between the intake manifold and the oil trap. The only evidence of this was it stalled when I came to a stop occasionally (basically a more extreme version of what it does now). It came on boost, accelerated and cruised without a problem, so the lambda signal clearly allows the ECU to hide vacuum leaks very well most of the time.

The car's got a narrowband air/fuel gauge which flicks between rich and lean quite slowly at idle, more quickly at a cruise, stays in rich at full throttle, and stays in lean on overrun. This is exactly what I'd expect but I could be wrong.

I'll see if I can upload a video of the gauge and the idle later.

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Old May 30th, 2020, 12:26   #13
Cylinder 8
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The CTS has a resistance of 5.85 kilo-ohms when cold, and 0.53 kilo-ohms when hot, so I'm happy that's fine.

I've recorded some videos of how it behaves (hopefully the links work), make sure your volume's up high.

This is a cold start:
https://drive.google.com/file/d/1Wnq...ew?usp=sharing

And this is how it behaves when warm (watch to the end, the last time I rev it is the worst):
https://drive.google.com/file/d/1Awe...ew?usp=sharing

I noticed the air/fuel gauge stays in lean every time the revs drop too low and doesn't go back to rich until the idle has recovered. Does that point to a vauum leak or fuel pressure issue?
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Old May 30th, 2020, 12:51   #14
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Its running badly on the cold start like 3 cyls, and warm the needle 'vibrates'.

To be honest my 360 B230FT (97) runs a bit like how you describe. High idle or stalling sometimes, bad cold start. Other times perfectly normal. It is a transplant, so wiring or interference might be a problem. You have probably done more than me to diagnose. I felt I had a coolant weep into the cylinder, but not checked it yet. Fuel pressure good. I had found a bad inlet gasket (uneven tighening in one case and not removing the old gasket in another on 2 different cars). Will be checking compression and plugs soon.
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Old May 30th, 2020, 14:26   #15
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Is the ECU standard, I'm surprised it doesn't struggle to be honest as you have changed quite a lot. What about the exhaust ?
I don't know how the mixture reacts on a normal car but seems a bit extreme to go from normal to weak. My old favourite the injector relay located in the engine bay, wonder if an intermittent connection would cut fuel to the injectors.
Thats why I suggested checking the plugs, very clean or normal would indicate a lack of fuel, sotty might be too much fuel or no spark.
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Old May 30th, 2020, 14:30   #16
Laird Scooby
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Quote:
Originally Posted by Cylinder 8 View Post
The CTS has a resistance of 5.85 kilo-ohms when cold, and 0.53 kilo-ohms when hot, so I'm happy that's fine.

I've recorded some videos of how it behaves (hopefully the links work), make sure your volume's up high.

This is a cold start:
https://drive.google.com/file/d/1Wnq...ew?usp=sharing

And this is how it behaves when warm (watch to the end, the last time I rev it is the worst):
https://drive.google.com/file/d/1Awe...ew?usp=sharing

I noticed the air/fuel gauge stays in lean every time the revs drop too low and doesn't go back to rich until the idle has recovered. Does that point to a vauum leak or fuel pressure issue?
You've only half-tested the CTS - you've checked the resistance between the pins but have you checked each pin to earth? Each should come out about half of the total at the time.

However, what was the actual coolant temperature at the time you tested it? Usually the (rough) figures for each half of the CTS are 2.5K +/- 10% @ 20C so if the coolant wasn't quite 20C that would account for the higher reading on the cold setting. The hot reading is about right @80C, at -10C it's 9410 Ohms (each leg to earth) +/-1150 Ohms which is about 12% but if the earth connection has failed (or if someone has wrapped PTFE tape round the sensor threads to aid sealing) then the ignition and fuel ECUs will see it as high resistance all the time.

All that said, i've just watched your videos, it sounds very much like you have a slight air leak somewhere. The AFR gauge is doing what it's meant to.
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Old May 30th, 2020, 15:17   #17
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Quote:
Originally Posted by Steve940estate View Post
I don't know how the mixture reacts on a normal car but seems a bit extreme to go from normal to weak.
That's exactly what it should do under closed loop Lambda control. Lean-rich-lean-rich-lean constantly on idle and also cruising. Should go lean above 1200rpm with a closed throttle and rich on WOT then return to the lean-rich-lean-rich cycle.
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Old May 30th, 2020, 16:43   #18
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Quote:
Originally Posted by Laird Scooby View Post
That's exactly what it should do under closed loop Lambda control. Lean-rich-lean-rich-lean constantly on idle and also cruising. Should go lean above 1200rpm with a closed throttle and rich on WOT then return to the lean-rich-lean-rich cycle.
Its a narrow band O2 sensor, with wide (0-5V)/narrow(0-1V) band gauge. They don't normally show sensible indication on narrow band mode as the face has been calibrated for wideband even though they have a setting for it.

FYI the lean (fuel cut) on closed-throttle will only happen if you start over about 1800 or 2000rpm. Any significant load should be rich.

Do you know the conditions it goes into closed loop on cruise? By my mpg meter I'd guess it is below 50mph on the flat, with the throttle teased out to not accelerating or slightly decelerating. She will average 40mpg at 30mph in 5th.
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Old May 31st, 2020, 01:25   #19
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FYI the lean (fuel cut) on closed-throttle will only happen if you start over about 1800 or 2000rpm. Any significant load should be rich.

Do you know the conditions it goes into closed loop on cruise? By my mpg meter I'd guess it is below 50mph on the flat, with the throttle teased out to not accelerating or slightly decelerating. She will average 40mpg at 30mph in 5th.
Something in the back of my mind says 1800rpm Tony for the fuel-cut trigger. It then stays cut (no injector action at all) until 1200rpm which when coasting in gear will normally have enough momentum to overcome the hysteresis of the fuel cut to fuel again scenario.

The system is closed loop untl WOT (on n/asp), the idle switch in the TPS only tells the ECU to operate the AICV and switch to a "tighter" fuel map for idle only (hence it won't rev above about 3000rpm if the idle switch stays made or thre's a short in the loom) so when cruise conditions allow, it will run as lean as possible to achieve good economy.

As there is no WOT switch in the TPS on turbo cars, it would be fair to assume the ECU works out the load from the engine speed Vs airflow with feedback from the Lambda sensor to tell the ECU if it's running lean, rich or what.

Purely as an experiemtn, see what your mpg gauge at 30mph in 4th - i'd suggest it will be higher than 40mpg because the engine is revving higher more in its torque band.
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Old May 31st, 2020, 13:36   #20
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Higher RPMs are always lower mpg, even when the engine feels more relaxed. Engine friction is the dominant force. You do need a very light throttle to avoid engine vibration at those rpms though. Lower speed is also a consistent benefit.
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