Replaced the MAF sensor but still getting P0171 or P0174 codes, only in cold weather.

Just to note this truck runs on LPG & Gasoline so its probably not fuel related as it automatically switches from one fuel source to the other.

This is is common to both banks which sort-of suggest there is an air leak somewhere after the MAF sensor. It this is true could there be a gasket or pipe leak or somewhere?

If anyone has had a similar problem I'd be interested to know what you found.

Any comments welcome!

 

do you have a scanner to see the raw sensor data?

would be interested to see what the O2 sensors are reading and what the fuel trims are doing,

Neal

 

Indeed I have TECH2 and have looked at the MAF sensor, old and new with the engine warm (see JPG) seem pretty much the same. I assumed the O2 sensors would be OK since it happens to both banks 1 & 2 and its unlikely they both would be defective, also they're both only a couple of years old (new CATs)

Do the values seem reasonable?

I'ven test driven the H2 with the Teck2 monitoring the levels, of course no engine warnings happened whilts live.

 

Your MAF readings look a little low compared to mine. I dont think the 6.0 and 6.2 will be much different.

 

Fuel tank cap and oil fill cap secure? They will show up as a vacuum leak.

 

Thanks for all the suggestions guys.

Mo 2 MAFs produce similar results, the low readings suggest reduced air flow i.e. mixture too rich?

I'll look into the oil-cap scenario however as the hummer runs on LPG so the fuel/petrolium related issues are less likely I suspect.

LPG converted vehicles usually valve-care oil-additive systems fitted to reduce valve-wear, it involve drilling and fitting small micro-pipes to each of the 8 air-intakes which suck-in minute amounts of oil into the air-flow whilst the engine runs.

My suspicion is there is a leak in the pipe-array somewhere, will inspect it as soon as I can.

 

The oil cap and fuel cap allow the engine to pull in air through the PCV system, you can hear the engine change sound as you remove the caps and allow air to enter. It shouldn't matter what fuel your using as the extra air will cause a lean condition.

Can you check the fuel tank pressure to see what it is after warmed up? The purge solenoid can get hung open and allow fuel fumes and air to get pulled into the engine, usually this will cause a DTC. The vacuum in the tank is very small at idle but climbs as speed increases. Mine would hang open and cause the engine to idle very rough and the vacuum in the tank was rather high. I would imaging that even with the LPG system the PCV system would still be active.

Neal