Thread: 2004 Discovery 2 TD5 overboosting at high RPM

Originally Posted by sierrafery

It's not about faulty MAF but about a real reading of 680 when it cuts out and if that's the case the symptom will go away with unplugged MAF cos it goes to default. That means that the turbo is spinning fast and the succrion is high while the boost which should be high as well at that sucction is not getting to the inlet for some reason or the MAP sensor is reading low. You really need a tester to properly diagnose this issue

Hi,

I did some multi-meter testing on the MAF today.

Fuse 2 (engine bay) is working correctly and supplying voltage to MAF and MAP

MAF sensor with Keys on:
Pin 1 - Ground - continuity good
Pin 3 - Ground - 12 V

MAF sensor with Engine on:
Pin 1 - Ground - 0V continuity good
Pin 2 - Ground - 2.15V while idling
Pin 2 - Ground - 4.72V while Revving @ 4000rpm
Pin 3 - Ground - 12 V (well closer to 14V due to charging)

These to me, are signs of a working MAF?

If the MAF is working (which makes sense as its new). When the ECU cuts the fuel it is because the MAF is sensing too much air? or could it just be "too much air for the fuel that is being delivered"?
Keeping in mind that with the MAF sensor unplugged, it doesn't overboost under heavy acceleration.

I will try take out the fuel pump tomorrow and check its connections and it is getting the required voltage. It has passed pressure tests on the 2 previous services.

I also re-checked the intercooler hoses for any leaks to no avail, they are in good condition.

In a nutshell. As long as the symptom goes away with unplugged MAF it means that when it occurred the reading exceeded 680kg/hr for some reason, it has nohing to do with the fuel delivery system... stationary voltage readings are not relevant all they show is that the MAF is not dead. I'm sorry but without a scantool to see what boost you get at that reading i can't say why it's happening but relaxing the wastegate rod might make it worst, better tighten i as much as you can aka to 11 threads left... there must be a correlation between MAF and MAP reading and normally at around 650 kg/hr the MAP should be maximum allowed at full throttle as overboost to not happen. main causes can be:
1. Boos leak which means high sucction and boost below the expeced value
2. the MAF sensor's reading is higher than the reality
3. the MAP's reading is lower than the reality

if it was overboost it would have occured even with MAF unplugged

It's common for maf sensors to read too high if either they were a cheap replacement or have been over cleaned .

Or they just die over time like mine did reading high.
I had a big miss at max revs in second.
I did a download of the real time values to my Nano com and the forum saw the MaF values going to zero.
New MAF no problem.
Regards PhilipA

Yeah okay, I wasn't aware they could be "over cleaned".

I did use proper MAF cleaner spray and have cleaned all the other related sensors aswell.

Next step is to test with Nanocom so might take a while to get access to one.

Hi all,

Thanks for the good advice.

Thanks to cjc_TD5 we did a few runs with the Nanocom today.
See attached excel file, we did 3 runs.

AA idling
BB Driving
CC Driving with MAF unplugged

Disco MAF 1.jpg
Disco 210320.xlsx
Aulro TPS ScotsD2.jpg

It looks to me like the MAF reading is reaching 600 g/s and then dropping to 0 g/s briefly.
After this occurred the TD5 was surging and limiting rpm (see the rpm column).

I have attached a TPS post from a while back that seems to confirm that the throttle valves are okay.

Looking at the BB plot, the waste gate, boost pressure and throttle position all seem to be okay?

Cheers

Hayden

yep same as mine.
New MAF fixed it.
BTW I replaced the dead MAF with a VDO but about a year ago I bought a $30 Chinese one and fitted it just to see what would happen.
Its still going strong! LOL
Regards PhilipA

The turbo pressure is too low, as i can see it didnt even reach 18psi along that drive