C5 2.0 (not HPI) Petrol Misfire during warmup

[mlkey]

Me again, not having a lot of luck with the old girl at the moment. For many months I have experienced a slight misfire during warmup. It starts around a minute after starting the engine, you can hear it popping very slightly through the exhaust and the car will shake slightly. It feels as though it is just one cylinder. If I leave it to its own devices it will clear within a minute and all is well. It drives like a dream once warmed through. This morning I did something a little different, and increased the revs after starting to about 1500rpm. When the misfire started, it was much more noticeable, so much so it dragged the rpm down, put the MIL light on (anti pollution fault message) and ran really badly. Normally it does not put the MIL light on. I switched off and restarted immediately, and hey presto.. all was perfect again.

I put Lexia on it tonight to find it is reporting a uncharacterised misfire on cylinder 4, which is at least a start.

I have already replaced the plugs and coil pack, but there is no noticeable difference to tbe problem. I have read somewhere that the EW10J4 engine is prone to suffering with hydraulic cam follower issues, causing valves to stick open. It does feel very much like a sticking exhaust valve, and the fact it comes and goes so quickly, indicating that it is affected by a rapid heat up. On the other hand, why would my stopping the engine and restarting immediately resolve the misfire?

Any suggestions appreciated.

Thanks.

[mlkey]

I forgot to mention I have also tried a new coolant sensor.

[mlkey]

Ok, I have decided to document my findings here just in case anyone else has this issue. I did another Lexia session tonight, the car having stood since last night. It was 3c when I did the test, the coolant temperature sensor concurred this, but the intake air temperature is reading 6c higher at 9c. This was all checked before I started the engine. Once again I induced the misfire although it did not cause a MIL light this time. The misfire started about 45 seconds in, and was gone again within the next minute.

I am going to replace the IAT sensor as it is only about £12. I am not expecting this to cure the misfire, but hoping it will improve drive-ability a little. I am still confident on the cam follower theory, but want to check all other possibilities first.

[mlkey]

I have now replaced the Intake Air Temperature sensor, sadly it reports the same temperature as the old oily sensor. I now have to establish if the sensor should read higher than the ambient on a cold engine, or whether there is a wiring issue elsewhere causing the high reading. Watch this space.

[viorelovidiu]

Try this. uncouple lambda sensor cable and try a cold stard. if misfire goes away you`ll need a new sensor.

[mlkey]

Try this. uncouple lambda sensor cable and try a cold stard. if misfire goes away you`ll need a new sensor.

Hi, thanks for the suggestion. I have checked the lamda's with Lexia and they seem to be working correctly, but I can try disconnecting.

I have also given more thought to the IAT sensor. It can not be a wiring issue as to get a lower temperature to register it requires more resistance. I will check the voltage though.

[mlkey]

A quick update. Having pretty much decided it was a hydraulic bucket issue, I decided to have a second and final attempt with an engine flush. I drove the 40 motorway miles home and added a Wynn's engine flush. 25 minutes of 2000 rpm on the driveway, then drained the oil. For good measure I added a hydraulic lifter treatment to continue the cleaning action with the new oil. I have always doubted the "snake oil" trade, but have to say that this latest attempt has cleared the problem. Even the auto gearbox is benefiting from the smoother engine, downshifts being much less harsh. I have done 300 miles since changing the oil and the drive continues to improve.

[Xantidote]

Good result - fingers crossed