Boost pressure problem, please help!

[Paul850TDI]

Hi there guys,

My car is a 1996 850TDI, engine code:D5252T.

Recently I connected Vol-FCR into my car and got the following values. Actual boost pressure is way bellow the desired value. Mass air flow is also strange to me.
I've checked all the vacuum and turbo hose, they are all ok. Fuel economy is also good, around 47mpg.

Any advice is highly appreciated!
Thank you in advance!

Paul850TDI

[brickman]

Yup, what you have there is how most of these engines currently run, OK but not ideal. It'll run just fine and most people won't notice the difference as the problem you have is a failing MAF

MAF values won't ever be exactly on the money, but up or down around 10-15% from expected is the limit IMO (Not sure what VIDA has to say on that though). Also note you won't get any particular meaningful numbers with the car sat parked + revving, you really need to take it out and do some 3,4,5th gear pulls from 1kRPM with someone else watching the numbers.

The fact at idle you have over twice the actual MAF value is exactly what happened to my failing MAF last month. One (or more) of the resistors on the tile inside the box will have blown, you can't replace them as they are within the surface of the component. there are no UK replacements any more, only a whole new MAF from VAG germany at around £250+vat (see my detailed thread in the 850/v70 forum section on this).

IF the value was wrong but not that far wrong. E.G, 4th gear, 10% throttle duty, 1500rpm, expected value 310gm/s and actual value 400gm/s
THEN you can adjust the tension on the spring within the MAF to bring it back more in line with the expected value. Again loads of detail on this, but essentially cut the cap off the box then push the wheel around with a screwdriver one click at a time then drive under same conditions. Mark the original position of the wheel incase you loose tension and need a starting point again.

My advice... start hunting through scappies for MAF boxes, you will find them on Audi 100/urA6 2.5 tdi and an Alfa of the same time (rare though!) and possibly some early VW LT vans (1993-1997). Don't pay more than £10-15 for one because MOST of them are also buggered.

Contrary to petrol engined cars, when the MAF fails the engine will still run mostly alright, just a bit flat and a bit smokey. Some scappies will argue the car was running driving fine before therefore it must be good, but nope.

I hunted all of mid scotlands yards last month (30+) and found ONE, that is also failing, but managed to add enough spring tension to make it reasonable for a while.

[brickman]

Because the MAF value is so far wrong the ECU might be going to default numbers (550gm/s over most of the range) so you will actually have decent power from 1000-1500 but then it will flatten off after that when it should take off!

Because of the fueling being out, the turbo will then not boost (as theres not enough fuel to accelerate) and your getting a low number, 10-20% either way isn't an issue.
If you find a scrap car, pull the MAP sensor as well, its sat on a bracket behind the fuel pump. £5 max each, get like 3 of them as they are mostly duff too

[Paul850TDI]

First of all thank you for the fast response!
I've started hunting for MAF and MAP sensors.
Is there any way to examine whether if a MAF or MAP sensor is in good condition or not? For example based on resistance values.

[brickman]

Yeah you can take a small battery to the connector and get a resistance value back from the sensor. There are lots of videos on youtube how to do it (not just for this car), that gets you the MAF sensor values

For MAP just get a bunch or buy a fresh one, about £27 from TPS I think?

[RoyMacDonald]

Yup, what you have there is how most of these engines currently run, OK but not ideal. It'll run just fine and most people won't notice the difference as the problem you have is a failing MAF

MAF values won't ever be exactly on the money, but up or down around 10-15% from expected is the limit IMO (Not sure what VIDA has to say on that though). Also note you won't get any particular meaningful numbers with the car sat parked + revving, you really need to take it out and do some 3,4,5th gear pulls from 1kRPM with someone else watching the numbers.

The fact at idle you have over twice the actual MAF value is exactly what happened to my failing MAF last month. One (or more) of the resistors on the tile inside the box will have blown, you can't replace them as they are within the surface of the component. there are no UK replacements any more, only a whole new MAF from VAG germany at around £250+vat (see my detailed thread in the 850/v70 forum section on this).

IF the value was wrong but not that far wrong. E.G, 4th gear, 10% throttle duty, 1500rpm, expected value 310gm/s and actual value 400gm/s
THEN you can adjust the tension on the spring within the MAF to bring it back more in line with the expected value. Again loads of detail on this, but essentially cut the cap off the box then push the wheel around with a screwdriver one click at a time then drive under same conditions. Mark the original position of the wheel incase you loose tension and need a starting point again.

My advice... start hunting through scappies for MAF boxes, you will find them on Audi 100/urA6 2.5 tdi and an Alfa of the same time (rare though!) and possibly some early VW LT vans (1993-1997). Don't pay more than £10-15 for one because MOST of them are also buggered.

Contrary to petrol engined cars, when the MAF fails the engine will still run mostly alright, just a bit flat and a bit smokey. Some scappies will argue the car was running driving fine before therefore it must be good, but nope.

I hunted all of mid scotlands yards last month (30+) and found ONE, that is also failing, but managed to add enough spring tension to make it reasonable for a while.

Don't know if this is any help but the VW LT was fitted with the same engine as the 940 TD. However this was a 6 cylinder engine. I believe the VW LT was also fitted with a 5 cylinder Tdi very similar to the first V70 2.5 diesel engine.

[brickman]

Yeah thats it, mid 90s LT's had the 5 cylinder and also in the T4 van, but AFAIK all the T4 AEL variants had a hot wire MAF and a totally different ECU system

[martin_r_smith]

Looking at your first post I see a couple of issues. 1 the car isn't up to temp so most figures will be off. 2 of course the boost will be lower as I assume you just revved it up stationery and not on load? You can't build boost pressure without load. A maf reading of 450-500 isn't perfect but acceptable. The reason the desired value is low is because the ecu is expecting inlet air from the egr so doesn't want more from the Maf. If you look at it whilst under load you will might find it matched up closer. My egr has been properly mapped out so my desired maf reading is 949mg as it always wants the full amount from the Maf and not through egr.

It's dangerously bad for paranoia this live data, and as I've said to brickman before, just keep the laptop shut and just drive the car until something actually goes wrong!

[Paul850TDI]

Thank you! I am going to check it under load and with warmed up engine.

Today I deleted the EGR with blank plate. There was no fault code. Is that enough or should it be mapped out as well?

[martin_r_smith]

Thank you! I am going to check it under load and with warmed up engine.

Today I deleted the EGR with blank plate. There was no fault code. Is that enough or should it be mapped out as well?

That's enough for what you need. Getting it mapped out is not an easy task!

Don't get hung up on trying to make the numbers match. My car is nigh on new in the engine department and my numbers are never what they are supposed to be! But does it run well.... Yes!

If your car is really out of speck it will tell you by way of a fault code or by not functioning properly.

[brickman]

It's dangerously bad for paranoia this live data, and as I've said to brickman before, just keep the laptop shut and just drive the car until something actually goes wrong!

Very true!