[Engine] Hunting

[Jehu40]

Hi folks. Ive a 54 2.0HDi which started hunting in 5th gear on accelerating at about 50mph, which disappeared as revs rose. Lately I get it on 3rd and 4th too after changing up and accelerating. Now Ive noticed no accelerating at all in 5th. Any clues?

[oilyrag]

Cheap options - take the air filter out and see if things improve, change fuel filter, add injector cleaner.

[Jehu40]

Hi Oilyrag, thanks. However the car has been well maintained and has had a new air filter and fairly recent fuel filter. Acceleration is good at higher revs in 3rd and 4th, but suddenly not in 5th. NB The car was remapped and de EGRd years ago.

[Sol]

MAF sensor I'd say.

[Jehu40]

Thanks Zion, its a possibility; I cleaned the MAF recently and it coincides with the problem getting worse... I'll check it out today.

[Sol]

Cleaning them seldom works, the hot film resistor ages and the output profile of airflow Vs temperature probably drifts.

The ECU heats a resistor of known value which changes its resistance as it heats (meant to do this) and the cooling effect of the airflow brings it back down in temperature and up in resistance and this is how the ECU measures the volume of air entering the engine.

If the resistor has aged through years of heating and cooling, the value it reads could will be wrong.

If you have DiagBox / Lexia you can read live airflow. With the MAF in place at idle it's around 400 but with it disconnected it defaults to a fixed 400 units, so when connected it should rise pretty evenly to a maximum value at full RPM, you could log the airflow and do 2 full RPM runs in 2nd or 3rd gear with the MAF connected and without it connected.

The readings should be kind of similar between the measured value and the table estimated values but if they are wildly different or the MAF tops out early when connected, it's gone wrong.

[Jehu40]

Great thanks.

I unplugged the MAF with the engine running - no difference. Conclusive?

[Sol]

I'd start it with the MAF already disconnected and see if it's any better. If that gives the same result, I'd maybe look at the fuel pressure to the injector rail or one injector itself could be failing or partly clogged.

Ideally you need DiagBox to read live data for this kind of stuff.

Wavering RPM has to be one of only a few things, fluctuating air flow readings, fluctuating fuel pressure or something like a loose or failing cam timing sensor? As the rpm rises the cam sensor could be missing pulses. With DiagBox you can check cam to crank sync but without that it would be very hard to tell.

[Jehu40]

Result! Changed MAF and car running great, acceleration much more responsive even in 5th. Hunting no more. Yes it did run noticeably better with the MAF unplugged so I didnt mess about after that. Many thanks for your help.

[Sol]

Cracking, no worries mate.