Higher fuel consumption after "upper engine clean"

[TPatS]

Hi guys,

So bought a WM Series 2 LFX V6 Caprice a few weeks back. It's been great but a few days ago a check engine light popped up so I took it into my mechanic. He said it was a misfire (funny cause it didn't really feel rough or anything on the drive to the mechanic) and he would firstly do an "upper engine clean". Apparently this took care of the problem and the error codes went away but he said I should monitor the situation if anything new pops up. But the past few days I've been noticing that it seems that the fuel consumption has gone up a noticeable amount. It was around 11L/100km before, now it's around 14L/100km and when coasting to a stop, the INST fuel consumption drops to 0 like it normally would with the fuel cut off but during the last 15km/h or so it would spike up to like 15L/100km suddenly then suddenly drop back to around 1.8L/h which would be the normal idle fuel usage. The car drives fine and there's no warning lights or anything so i'm wondering if there's something I should do here.

Thanks guys.

 

[MYVESSV8]

o2 sensors

 

[Immortality]

The upper cylinder clean might have contaminated the O2 sensors as above. Give it a couple of good hard runs and see if it clears up.

 

[lmoengnr]

The upper cylinder clean might have contaminated the O2 sensors as above. Give it a couple of good hard runs and see if it clears up.

Might have blocked the cats as well.

 

[lowandslow]

o2 sensors

Might have blocked the cats as well.

Both excellent suggestions to follow up on.

 

[TPatS]

Thanks for the suggestions guys. I'll have a talk to my mechanic about it. On a second note, do you notice your V6 upon cold start idling around 1100rpm and about 8L/h for about 20 seconds before settling down to around 600rpm and 1.8L/h or is it just me?

 

[vr304]

Dunno about those exact figures but cold start will always run fairly rich fuel air mixtures compared with normal operating kinda like how a choke would operate in a old school carbed car

 

[wetwork65]

Thanks for the suggestions guys. I'll have a talk to my mechanic about it. On a second note, do you notice your V6 upon cold start idling around 1100rpm and about 8L/h for about 20 seconds before settling down to around 600rpm and 1.8L/h or is it just me?

Yes, exactly the same in my SV6. It settles down after a while. I believe it is due to some emissions management requirement.

 

[Immortality]

Thanks for the suggestions guys. I'll have a talk to my mechanic about it. On a second note, do you notice your V6 upon cold start idling around 1100rpm and about 8L/h for about 20 seconds before settling down to around 600rpm and 1.8L/h or is it just me?

Sounds fairly typical, it's like the old carb choke function.

 

[Skylarking]

When you start a modern fuel injected car, it will start and run in open loop mode for a short time until the ECU decides to go into closed loop mode where it then adjust fueling based in O2 sensor readings amongst other things.

While it’s in open loop mode, it will run a set injection strategy based on rpm, air pressure, temperature and throttle position and doesn’t even look at O2 sensors. Such open loop strategies tend to be on the safer side so they run a rich mixture to cope with various cold start issues. Its just like your old manual choke cable going to the carbi which made the car run rich so it could cope with cold start/run conditions (like fuel condensation within the intake piping which resulted in too lean a mixture getting into the cylinder and the engine not being able to start or run)

Blips in instantaneous fuel consumption can be caused by anything since its calculated from wheel speed sensors (abs sensors) and fuel injected volume (proportional to fuel pressure and injection duty cycle) ... if a sensor is starting to go bad, you could get occasional/sporadic measurements which make the calculated number seem rather big... after all dividing anything by zero is a very very big number

Best not to worry about short blips in instantaneous fuel consumption and focus more on how the engine is performing and the kms driven on a per tank or per multiple tanks basis. Also worth getting a cheap OBD2 diagnostic dongle which you can use via a phone app to check fault closed and see real time O2 sensor and other sensor readings. Much more probable to crash gawking at such info than just looking at the instantaneous fuel consumption

Oh, and as for upper cylinder clean... what did the mechanic actually do, inject a chemical into the intake system as the engine ran or pour a chemical into the fuel tank and drive it around?