Lumpy idle when warm

[GeraldGTE]

Hi all. My 16v starts lovely from cold and idles perfect for first five mins. It then starts to idle lumpy when it's warm. Have changed coolant temp sensor, and checked cleaned icv.
Any other ideas ? It's the later coil pack xe with lambda sensor. Could the lambda sensor cause these issues? It drives perfect and pulls amazing, just the lumpy idle!

[GeraldGTE]

Any suggestions? It's winding me up now!! Lol

[DarrenH]

on the cam cover theres two breather hoses, large one goes to the front of the throttle body, the other small goes to a tube/stud on the inlet manifold below the throttle. this small breather is a banjo bolt which has a banjo around it, the banjo has coolant fed to and from it. (dunno why the breather needs to be heated)

undo this small rubber breather hose from the cam cover end (with engine running) stick your thumb over the end and see if its sucking. if it isnt its probably gummed up with oil/petrol/slime, and could cause your issues. its a pain to remove and clear out as due to the coolant thing, but i've done it in situe with a ring spanner before. use wire to clean it out (not a drill like me and buggered it up)

on the lambda sensor theme heres a cut and paste from an older thread so you can test it

my lambda sensor has 3 wires on the lambda side of the loom. 2 white, one black. black is the wire youd need to check the voltage with ignition on with the positive probe, then with engine running (all wired up as it should be, youll have to use your imagination how to get into the wire/plug with the voltmeter probe). negative probe straight to the battery earth

Oxygen sensor health checks

A good lambda sensor should produce a fluctuating signal that changes quickly in response to changes in the oxygen level in the exhaust. The best way to check the sensor is to observe the sensor's output on a waveform scope or oscilloscope. A scope will display not only the sensor's minimum and maximum voltage readings, and average voltage reading, but also its back and forth voltage oscillations from rich to lean.

Sensor output can also be read directly with a 10K ohm impedance digital voltmeter, or some code readers.

CAUTION! Never use an ohmmeter on a zirconium O2 sensor in an attempt to check the sensor because doing so can damage it. And never jump or ground the sensor's leads.

The lambda sensor's voltage reading should have a minimum reading of 200 millivolts (0.20 V) and a maximum reading of 800 millivolts (0.80 V). If the sensor reading is averaging low (under 400 millivolts) or high (over 500 millivolts), the engine may be running rich or lean because of some other problem.

If the sensor's output voltage never gets higher than .60v and never drops to less than 0.30 V, it needs to be replaced. The same is true if the sensor's output is sluggish or doesn't change.

To check the sensor's response to changing oxygen levels in the exhaust, first create an artificially lean condition by pulling a large vacuum line. When extra air is introduced into the engine, the sensor's voltage output should drop to 0.2 V.

To check the sensor's rich response artificially richen the mixture by, if possible, clamping the return fuel line momentarily. This will force more fuel through the injectors and should cause the O2 sensor's voltage to increase to 0.8 V.

If the sensor's output fails to respond to the changes you've created in the oxygen level in the exhaust, it's time for a new sensor.

despite the above i used a plain volt meter, obviously cant see the wave form but can see the min max volts, and the speed its switching. to create the temporarily lean mix i just pulled the cam cover breather off the engine end (thereby letting a slug of unmetered air into the engine) block it with your thumb, release, rinse and repeat

to create temporarily rich mixture i did exactly as the text says, crimped the return hose coming out the back of the fuel pressure regulator (which creates a spike in fuel pressure, and thus over fuels)

to break into the signal wire for the voltmeter, i (crudely) stuck a sewing needel through the insulation and then hooked the positive probe to that. if its a long probe you can push through the rubber seal on the arse of the multiplug connector on the bulkhead

[GeraldGTE]

Thanks Darren. Checked pipes etc for blockage and all is good.
Have found this opened ended pipe. Look on my earlier GTE and it hasn't got one. So does anyone know where it goes?? Comes from a little valve and back right of engine, behind battery and to the front of engine.

[GeraldGTE]

[DarrenH]

not gonna lie the pictures are hard to see, but it can only really be the fuel tank vent valve. fitted to later detox engines where the fuel tank went to a carbon canister in passenger wheel arch, then to the valve, then to inlet manifold

it might cause your issue if the hose between the engine and valve were broken. because then essentially you have a vacuum leak. a break between the valve and atmosphere is "ok"

[GeraldGTE]

Well this is doing my nut in now !! New lambda and air intake temp sensor fitted and still the same !! Perfect when idling from cold, soon as any heat in engine idle is lumpy as hell ! I have also noticed that there is slight overrun when I turn off ignition, also my garage did say it was running a little lean when they did emissions. Am I right that when cold the engine runs rich help cold start and warming up?
What could be "kicking in" when warm to alter the mixture!!
Puzzled I am!!

[DarrenH]

what could be kicking in, well the lambda sensor is ignored until the coolant reaches 40 celcius. it measures that at the thermostat housing (2 pin coolant temp sensor). so basically its "open loop" running purely off ECU base maps until coolant warms up, then it goes closed loop, ecu base maps are trimmed and altered by the reading from the lambda sensor. this is called "block learn"

do you have any manifold leads, like cracks on teh headers, flexi gasket knackered at the sprung joint between the manifold and exhaust system ?

did you try the volt meter tests in previous reply ?

p.s try doing an ecu reset by unplugging leaving the battery off over night. this will whipe all the block learn settings from the lambda and start from fresh

[GeraldGTE]

Hi Darren. Didn't try the colt meter test. Thought bugger it and bought a brand new Bosch lambda sensor. No leaks, all new gaskets etc. will try the ecu reset tonight.
Any ideas if the afm is ignored initially on start up? Tried unplugging and plugging back in etc. engine does respond but not drastically and lumpiness is there in both cases.

[Astra Dan]

It's all very well swapping and renewing sensors but have you checked the wiring continuity from the sensor plugs to the ECU main connector?
On the other hand I'd be interesting to know if you sort yours, mine has always been kinda lumpy once warm.
You can ground pin 20 or 21 to get the ECU to ignore the lambda sensor.