CEL:P031 - Rough Idle

[Smigza]

Hi,

My 2009 Ralliart Lancer has thrown P031 CEL. Initial suspicions were that the front oxygen sensor required replacing and therefore it was replaced, however this didn't solve the problem as the CEL still exists and the idling hasn't improved at all.

Investigations showed the car is running slightly rich, and entering open/closed loop mode at the wrong time. It appears to almost stall during idle, and the ecu compensates causing the RPM to jump to 1500 rpm.

All of the intake piping has been inspected and no leaks could be found.

My research appears to tell me the ECU has died and needs to be replaced, furthermore it also tells me Mitsubishi released a service bulletin to rectify this problem stating that a dealer just needs to upgrade the ECU firmware.

Can anyone who is aware of this problem give me some advice as to what's actually going wrong? My mechanic is currently suspecting it could be another failed part casing the ecu to receive the wrong altimeter setting? I am suspecting this could be caused by an exhaust leak, or failing turbo exhaust manifold?

Thanks

[Viking]

A quick check of what's wrong is to get one of those OBD sensors and a free app on your phone (or find a friend who has one) and see what errors are being logged by the ECU.

A bluetooth OBD module costs about $15 on eBay - a shitload cheaper than randomly swapping parts hoping you'll find the broken one.

[Smigza]

Hi Viking. Thanks for your post. Thing is a friend has all the diagnostic tools, we used them to gather the information in the first post. All the codes are to do with the front oxygen sensor. This sensor was changed to hopefully clear the code and rectify the idling problem, however the problem has still remained.

Something else is clearly causing the issue.

[billyboy]

Might want to check this, solved my problem!

http://clubcj.net/viewtopic.php?t=17144&highlight=

[Smigza]

Hi guys, thanks for your help. Turns put it was the fuel pressure regulator hose.

Cheers

[Viking]

How did you work out it was the hose?

Not being a smart-arse, genuinely curious. I can see the link between the engine getting the wrong amount of fuel and the O2 sensor error / rough idle, but the detective work must have been a crap job.

[Smigza]

Hi Viking,

Did some research, which pointed me to inspect the FPR hose. Thing is this damn code has returned! However it's intermittent, so next step is to test the fuel pump relay and o2 sensor relay to see if either one has failed.

I'm aware that when the hose isn't secured to the fuel pressure regulator, the motor runs lean. My mechanic measured the afrs when this hose wasn't in place, and confirmed they were still at a safe level. Is this true? Could the lean condition have damaged something, causing the intermittent CEL?

[efod3]

Has the rough idle returned? The hose off would cause that. If it hasn't there may be two separate issues as the hose off may/will not throw a code but its effects may lead to a code.

Running lean is an issue, ideally you haven't driven the car hard when lean, your mechanic should do a compression test. Did it cause the intermittent cel, I'm inclined to say no, o2 sensors are famous for that if they are cleared. Did you mechanic clear the code or did it just magically go away?

[Smigza]

There is no longer a rough idle. The cel has been cleared by my mechanic, but it returns. It randomly clears itself but comes back hence why it's intermittent

[toc]

A few thoughts - give the intake a clean if it hasn't already.
Perhaps there's no leak, but more bypass air then expected is getting in?

Another suggestion - clear it, get it on the freeway and see if it comes back during that run..

A final one - you can monitor the O2 signal, it should oscillate between 0.4 and 0.7 on a regular basis, if it's not doing that, something is wrong with that circuit.