Don't forget the lambda reading at mot isn't actually a reading from the sensor itself instead all it checks for is an unblocked exhaust (cat not blocked,silencer collapsed etc) this was how it was explained to me at my last mot. Best on lambdas is to use a multimeter and see how it switches ( not entirely sure how it's done local garage owner told me about this)

Just changed the lambda sensor because it's old and probably had it, car instantly felt like it revved even more freely but when it got hot it still had a slight occasional pop, however the dip/hunt in the idle was nowhere near as noticeable. I then switched the engine off and restarted and it seems to have now disappeared entirely.

Will now change the plugs and see how it is but regardless the issue is seemingly improved.

another small development that might not have been helping things too much, but the cinq developed a rather large fuel leak, priming the pump revealed it was coming from the top of injector one. took the injector out and found the seal had a tear on it, replaced the seal for one from a spare injector. Probably not related but i guess it's possible that if the seal was bad on that injector it might have been causing a slight difference in fuel pressure on one cylinder

Ok, problem is still there, not as obvious as before but certainly still there when it heats up. So can't mot until it's sorted because of emissions. Annoying because it actually drives beautifully around the garage compound.

Any other ideas? Changed plugs and the ones that came out were sooty and black, might pop a new one out tomorrow to see what it's doing in comparison to how the old ones looked.

swapped coils for known good ones, no difference. I'm missing a couple of nuts on the exhaust manifold to head (middle 2 at the bottom), could this be another thing that's perhaps messing the lambda's readings? doesn't sound like it's blowing and there's no sooty residue so i think it's unlikely but i'm genuinely getting stumped

woj said:

Did you try checking the diagnostics parameters with IAW ECU Scan 2 while this happens?

Click to expand...

i don't have the leads yet to plug it in, what info will it tell me exactly? i've never used it to now! will it give me numbers/values from all the specific sensors?

it can give live readings of every sensor imaginable yes and record them so you can export to excel etc. Never regretted for a moment buying all the stuff

Sweet just ordered the leads, will always be handy to have rather than working in the dark i guess

its my first port of call on just about every issue i have - unless its obvious of course

Just another small update, the diag leads haven't arrived yet but with a little poking around i think i found the problem.

The big brake servo vacuum pipe on mine was on it's last legs years ago and was collapsing so it was bodged with cable ties to make it less squishy. Noticed that since the rebuild, the brakes, whilst very good by most car standard in terms of stopping power needed a slightly harder shove than i remember.... lightbulb moment remembering that pipe, so took it off and had a look over it, it looked ok but did a couple of small tests. Plugged the manifold port for it and the idle seemed perfectly stable when hot, put the pipe back on the manifold and plugged the end where it would normally go on the servo and the idle didn't seem quite right again, popped it on the servo and could actually hear a faint whistle down the bulkhead, plugged the servo end off the servo and seemed a touch better than on it, then when i took it off the manifold end again and plugged that, it seemed perfect. So clearly i have a tiny vacuum leak on the pipe as well as it not sealing well to the servo.

As a temporary measure and a test, i cut off all the cable ties, trimmed the edge off the pipe from the servo end, sealed the pipe up with lashings of tape and stuck a few jubilee clips on the pipe to stop it collapsing, put the pipe back on and tightened up the clamps, result!, car idles so much nicer when hot and the brakes are absolutely razor sharp again!

so looks like the main issue was it dropping vacuum pressure through that pipe, confused the map sensor as a result and caused it to overfuel and become really apparent at hot idle.

Going to plug it on when the leads arrive just to check it all over and see what it says, but it seems to be fixed for now new servo pipe required!

Trying to run iaw scan 2, can't get it working. Have the leads connected, battery on them and can get the auto detect to recognise my ecu but it says ecu is not responding and gives me no info.

I assume it's a setting for the port, but what one

Bps-9600
Data bits-8
Parity-none
Stop bits-1
Flow control- none

Fifo buffers on
Recieve and transmit buffers both full, 14 and 16

Find mister TzOk on this forum and write him a pm, he is the author, he will know what the most likely cause for this is.

What cable are you using? Program works only with FT232 based cables, as other cables do not support non-standard baud rates used for communication with this ECU.

TzOk said:

What cable are you using? Program works only with FT232 based cables, as other cables do not support non-standard baud rates used for communication with this ECU.

Click to expand...

that'll be it then, i'll get another cable Thanks!

My non scientific tests for the issue.... whipping out the new spark plugs, is looking loads better than it did, however there may still be a tiny concern.

Cylinder one looks like it still has a very small overfuel, there is a very small carbony residue on that plug, not covered in black, just a light grey dry dusting. plugs 2, 3 and 4 all look pretty much bang on perfect.When i whipped out the old plugs when the issue had been much worse, all 4 were covered in a deep black soot.

Might just stick it in for an mot as it is and see where i am on emissions this time.

Thread revivial!

Had a lot to rebuild so took my time, (also found in my rebuild i had leaky valves, so sorted them out), still a bit stumped by this, the problem just won't go away. Even ecu disgnostics are inconclusive.

The engine itself runs beautifully, revs up without hint of misfire and comes off throttle fine.

However when it gets hotter and the idle drops, it starts popping from the exhaust and the idle drops/hunts as it does so.

Finally, finally got IAW scan working after 3 sets of faulty leads and it seems the issue itself isn't quite as temperature related as i thought and more engine speed related.

The car when started cold is idling very smoothly, the problem totally comes in not as i thought, when it wants to switch to closed loop exactly (it will switch to closed loop according to the scan and when the idle is still fast as it reaches temperature, still run smoothly), the problem comes in when it gets hot enough that the ecu wants to set its target idle to under 1000rpm, around 1000rpm it is excellent still but once it tries to go under that, it just can't adapt for some reason and starts to do weird stuff. For example if the target rpm is 940rpm, it'll go down just above that, then as it approaches target, pop and drop to 750rpm (like it wants to stall for a quarter second), then come back up to where it was just above target at like 980rpm. And seems to get a little more frequent the lower the target idle is.

Another thing the scan showed, was when this happens, the ecu switches out of closed loop into open loop momentarily, before coming back to closed loop a second later. (Normal or not?)

All the sensors, as far as i can see are reading pretty normal, the only one i can see that might "not" be, is the TPS., with my foot off the throttle, it reads that it is sat stable at 14.3 degrees input. not sure if that's correct, but i'd assume it should be sat at zero? It has done the exact same thing symptoms wise on 3 throttle bodies, though i haven't yet plugged the car in with any others to see if they read the same angle at "rest".

The ecu is showing some stored errors on water temp sensor (short to +vcc), air temp sensor (short to +vcc) map sensor (Short to GND/Open), tps (short to +vcc), stepper motor (Short to GND/Open) (i assume from when i have been testing stuff and running with a few things unplugged), but none active aside the evap valve not being there.

All sensors are running stable at what i'd really expect them to display. There's no vacuum leaks, exhaust leaks etc i am aware of, compression is good and even, just had the head rebuilt... any ideas? aside from just thrashing it (hard with no mot) and seeing if it sorts itself out

And just to add some raw data, from the logs, Just from when it had dipped under target and is out of closed loop (i assume because it is under target)

PARAMETERS:
Engine Speed: 854 RPM
Injection Duration: 0.00 ms
Ignition Advance: 20.50 °
Manifold Absolute Pressure: 371 hPa
Air Temperature: 30 °C
Water Temperature: 86 °C
Throttle Opening Angle: 14.38 °
Battery Voltage: 14.39 V
Lambda Probe Correction: 1.00
Lambda Probe Voltage: 576.09 mV
Idle Stepper Motor Position: 55 steps
Idle Stepper Integral Gain: 0
Idle Stepper Proportional Gain: 15
Trimmer Position: 0 steps
Idle Engine Speed: 864 RPM
Idle Engine Speed Offset: 0 RPM
Autocalibration Injection Offset @ ZONE 1: 0 ms
Autocalibration Injection Offset @ ZONE 2: 0 ms
Autocalibration Injection Offset @ ZONE 3: 0 ms
Autocalibration Injection Offset @ ZONE 4: 0 ms
Autocalibration Injection Offset @ ZONE 5: 0 ms
Autocalibration Injection Offset @ ZONE 6: 0 ms
Autocalibration Injection Offset @ IDLE: 0 ms
Engine Running: YES
Sensor Signals OK: YES
Throttle at Min/Max: YES
Lambda Sensor in Closed Loop: NO
Air Conditioner ON: NO
Mixture AutoCalibration ENABLED: YES
Ignition Advance Decrased for Knocking: NO
Stepper AutoCalibration ENABLED: NO
Gear Engaged: NO

And when it is trying to behave, just above target before/after it does the above

Engine Speed: 902 RPM
Injection Duration: 1.02 ms
Ignition Advance: 24.50 °
Manifold Absolute Pressure: 371 hPa
Air Temperature: 30 °C
Water Temperature: 86 °C
Throttle Opening Angle: 14.38 °
Battery Voltage: 14.45 V
Lambda Probe Correction: 1.00
Lambda Probe Voltage: 603.26 mV
Idle Stepper Motor Position: 56 steps
Idle Stepper Integral Gain: 0
Idle Stepper Proportional Gain: 14
Trimmer Position: 0 steps
Idle Engine Speed: 864 RPM
Idle Engine Speed Offset: 0 RPM
Autocalibration Injection Offset @ ZONE 1: 0 ms
Autocalibration Injection Offset @ ZONE 2: 0 ms
Autocalibration Injection Offset @ ZONE 3: 0 ms
Autocalibration Injection Offset @ ZONE 4: 0 ms
Autocalibration Injection Offset @ ZONE 5: 0 ms
Autocalibration Injection Offset @ ZONE 6: 0 ms
Autocalibration Injection Offset @ IDLE: 0 ms
Engine Running: YES
Sensor Signals OK: YES
Throttle at Min/Max: YES
Lambda Sensor in Closed Loop: YES
Air Conditioner ON: NO
Mixture AutoCalibration ENABLED: YES
Ignition Advance Decrased for Knocking: NO
Stepper AutoCalibration ENABLED: NO
Gear Engaged: NO

The going in and out of closed loop when the idle is hunting is normal I'd say.

What I wonder about is:

Stepper AutoCalibration ENABLED: NO

This in essence should be on, but I do not remember if IES has a way to switch it on (there is a way in general).

First I'd clean up the trouble codes and see if any of them reappear, just to be on the safe side. Then, per previous advice above, I'd disconnect the lambda sensor altogether and see what happens, also using diagnostics. Then I'd do the same with the stepper motor - disconnect it while the hunting happens and see if this changes anything.

Oh, and the TPS thing - a bit of a puzzle. 14 degress or so is not unusual at closed throttle (I do not want to know what the IAW ECU designers had in mind writing down the formulas for TPS calibration). However, the question is whether the ECU knows that the throttle is at closed position, that is, whether it calibrated itself correctly according to the values it sees. So, while watching the diagnostics, pay attention to this one:

Throttle at Min/Max: YES

(Switch off other parameters and see if this one is stable or goes out of the YES state from time to time).

woj said:

The going in and out of closed loop when the idle is hunting is normal I'd say.

What I wonder about is:

Stepper AutoCalibration ENABLED: NO

This in essence should be on, but I do not remember if IES has a way to switch it on (there is a way in general).

First I'd clean up the trouble codes and see if any of them reappear, just to be on the safe side. Then, per previous advice above, I'd disconnect the lambda sensor altogether and see what happens, also using diagnostics. Then I'd do the same with the stepper motor - disconnect it while the hunting happens and see if this changes anything.

Oh, and the TPS thing - a bit of a puzzle. 14 degress or so is not unusual at closed throttle (I do not want to know what the IAW ECU designers had in mind writing down the formulas for TPS calibration). However, the question is whether the ECU knows that the throttle is at closed position, that is, whether it calibrated itself correctly according to the values it sees. So, while watching the diagnostics, pay attention to this one:

Throttle at Min/Max: YES

(Switch off other parameters and see if this one is stable or goes out of the YES state from time to time).

Click to expand...

Thanks.

Will try that tomorrow.

I also have 2 1.2 16v TB's to test with, so will try and get readings off of them along with trying the above.

The stepper autocalibration was my mistake there, i was testing things out on the adjustments on iaw scan and trying the idle hot with that off, it does exactly the same with it turned on (as it is in my earlier logs from the day, the ones posted were the latest ones)

For the tps, i know it's detecting throttle input, it goes up and down according to what i do with the pedal and always returns to 14.3 at rest. I guess it's really only there to detect movement of the throttle so the base value doesn't really matter "too" much?

Forgot i also had this, taken about 3 weeks ago before head was removed and rebuilt, still doing it with the top end rebuild.

P.s kinda ignore the light rattle noise after the popping, it's just the temp exhaust mount of a jubilee clip on the central hook behind the spare wheel well, the fact i have a rear arb means space is tight around that mount (it runs just above it) and a rubber won't fit properly without modification/cutting), the rattle is evident when the idle drops and it causes the engine to make a very light shake as it drops/picks up again.

Hopefully gives a better idea what i mean by it hunting/dipping/popping