2.5TDI Flatspot

[davej]

I've just acquired a 51 plate Avant quattro 180 and after a couple of hundred thousand miles in diesel passats am disappointed with the low speed pick up. No boost until 1600rpm which can make junctions and roundabouts interesting if allowances aren't made with such a heavy old bus!
I've read a few places this can be a problem and admittedly I have not checked the MAF with the scope yet.
I'd like to hear from anyone who has found the same and maybe had chipped to get round this problem or know of other potential reasons for this lack of grunt.
Cheers.
DJ

[VanWheeler]

A remap on this will help big time, one of my customers had a similar issue with a 530D BMW and after the remap it picks up much better.

Generally after a remap the turbo will kick on 300-400rmp sooner and boost about 1/2 psi higher.

Feel free to call me, PM or email me.

Cheers Mark


I've just acquired a 51 plate Avant quattro 180 and after a couple of hundred thousand miles in diesel passats am disappointed with the low speed pick up. No boost until 1600rpm which can make junctions and roundabouts interesting if allowances aren't made with such a heavy old bus!
I've read a few places this can be a problem and admittedly I have not checked the MAF with the scope yet.
I'd like to hear from anyone who has found the same and maybe had chipped to get round this problem or know of other potential reasons for this lack of grunt.
Cheers.
DJ

[davej]

Thanks for the quick reply Mark. I don't have the seniority as yet to be able to either pm or email you mail me at djason@picotech.com, I'll get back to you.
Cheers,
Dave

[brickit]

I have same hesitation in my 2.5tdi 180 allroad, i put it down to the fact that it revs are lower due to engine size, and the drive is sent to 4 wheels instead of 2.
It would be nice to think it could be better, i have a hex-com - havent used it except for reading fault codes, i will do some research.

[James H]

DaveJ and Brickit - would you post to let me know the result if you do find an effective solution to this problem, please? I have a similar sort of issue (amongst others, possibly) with my 1999 2.5 tdi. It is the first V6 car I have had, and to be honest lack of grunt was the last thing I expected.......

cheers,

James.

[brickit]

I will do some vag-com dyno testing when i get time. I was thinking of MAF & turbo boost request/delivery. Is there any others that will help with this problem (if there is one)? This is also my first V6 TDI, and my 02 A4 TDi 130 went like a stink, even at low revs, i put the "blandness" i get now due to a 2 ton car and 4WD...

[davej]

With you on this brickit. I had a 51 130 Passat sport est followed by a 55 Passat saloon 140 both felt more torquey than this V6. What it lacks in low down torque is made up in other areas and I've a feeling I'm going to keep it a while. Just be nice to crack the niggle in the nether region.

[James H]

Yes - it is at least in part the contrast with the passat tdi that this car replaces that makes it generally seem to lack grunt. The flat spots in the torque curve are another matter, though - that simply cannot be right. Anyway, I will watch this thread just in case anyone offers a solution. Yesterday I ordered a cable to that I can use vagcom on my laptop and perhaps get some clues if there are any.

James.

[davej]

Good move James. I have the vagcom cable and used it with the freeware software from Ross-tech. The vagcom software is very good and many independant specialists sware by it. The measuring blocks are limited in the free version but it's a good taster. Last time I looked for $99 you can get the licence key for the full version.
I've not used it on the 2.5tdi yet as I've only had it a week but if you need to compare notes I can do that.
It's worth looking at the output waveform of the MAF if you have access to an oscilloscope as its' the best way to identify any sluggish responses. I work for a company that designs and manufactures them fortunately.
Cheers.
Dave

[James H]

Dave - thanks for that. I will have to see how I get on with it. I seem to recall being told, or perhaps reading , that a faulty MAFF doesn't give rise to any fault codes. Is that right?

If I disconnect the MAFF to see whether that makes difference, will it cause any further problem that can only be set right when I have got the vagcom up and running?

James.

[brickit]

If a MAF is out of spec but not totally duff there will be no fault code, the scale of an acceptable output is quite wide.
As regards to VAG-COM, i find a hex version is better as you can change computer hardware and get upgrades and it will activate immediately.
Maybe a mod could create another sub-section "VAG-COM diagnostic procedures" or something, which can cover investigative diagnostic routines (and come up with a fault flow chart) with possible fault / tests / solutions.This is someting main agent should have (but doesent).

[davej]

I've attached a couple of scope traces of good and bad mass air flow meters. With good reason, petrol engines MAF output look different to diesels even though the sensors are similar electronically. With a snap throttle test as here the initial peak is the sensors output responding to the rush of air over it from driver input opening the throttle butterfly. This initial response is the area where most sensors fail by not signalling to the pcm the true level of air entering the inlet plenum. With effectively unmeasured air entering the engine
will run lean. As brickit said there is a tolerance of failure where the pcm makes adaptions to injector duration to try to compensate for the lean out so a bad MAF won't put a dtc out immediately but you will notice drivability issues. After all the dtc is only reported because emissions could be in breach not because throttle is not crisp.
This MAF was bad enough to put on the MIL and report P1128 (system too lean)
I 'll look out some diesel MAF traces to show the difference.

[brickit]

Another reason is the MAF wire has oxidised, causing insulation between the wire and air passing accross it, the ECU measures air volume by the temperature change (cooling of the wire will cause the resitance of the wire to fall), the ECU will also need the ambient air temperature to use as a reference point.
That's how i see it - please correct if i am mistaken.

[James H]

Thanks, both of you. I don't have access to an oscilloscope. Those thumbnails from the faulty MAF are interesting - makes the issue easier to understand when you can see it like that.

VAGCOM cable has arrived today, so I am going to download the software and have a play.

James.

[James H]

Sorted out the VAGCOM, and hooked it up. It is showing 00575 intake manifold absolute pressure - 08.10 - control limit surpassed - intermittent.

As I understand it, this can be essentially 2 things - the variable vane mech on the turbo sticking in full boost position (or possibly moving off boost too slowly so that the overboost is still hit ?) or a faulty N75 valve.

I know that the VVT mechanism was sticky when I got the car, but thought I had freed it off. If I don't find a fix with the N75, I guess the turbo will have to come off and be cleaned out. I am hoping that at least if it is overboosting, it doesn't indicate a need for a new turbo. (?)

Anyone know what an N75 would cost? If it's cheap it may be worth just doing that first to see what happens.

I am right in thinking that overboost cannot be caused by leaking vacuum pipes, aren't I? (since if the vacuum fails the VVT should default to minimum boost)

James.

[davej]

Hi James, There are some suggestions to possible cures here if you didn't see them already. Hopefully someone with some tech info will respond with some clues. I've been looking for a technical description of this engine for a while now to understand it more. Was the MIL on ? What happened when you cleared the stored fault ?

http://autos.groups.yahoo.com/group/VAG-COM/message/22529
http://tech.bentleypublishers.com/thread.jspa?threadID=10955&tstart=1065
Dave

[adamss24]

The 2.5 tdi has a bigger turbo than the 130 pd tdi and it will have lag under 1600 rpms, but, when its starts spooling, it will boost all the way to the rev. limiter ! Also, the 2.5 tdi engine is heavyer and the quattro system/gearbox weigs another few hundreds of kg...but its worth it because of the way the car handles on bad weather and twisty roads and the way it puts all that extra Bhp on the road !

[James H]

Thanks for those links Dave - I have pretty much seen those (or others like) before. However, the pics of the stripdown are more numerous and detailed than what I have seen before. What is the MIL?

When I cleared the stored fault, I am not sure that it made any difference at all to the car's behaviour. However, as soon as the car is next driven, the same fault code is back again.

Anyway - as of half an hour ago, I have some more concrete information. I took off the vacuum-operated boost control device from the top of the turbo so that I could get hold of the turbo vane control arm without an y interference from the diaphragm and (I presume) spring in the control device. The vane control is very stiff and doesn't move at all smoothly - so I am fairly sure that is the reason for the overboost, and also the cause of general driveability issues - flat spots in the torque curve, followed by plenty of power, etc

I am wondering whether I can remove, strip and rebuild the exhaust side of the turbo without removing the whole unit from the car. Perhaps that would be asking for trouble even if it is possible.

I am going to go back to the pics of the other rebuild from your link and see if I can get a better understanding from that.

Will also look at the cost of an exchange unit - which may be worth some saved time and certainty.

Any thoughts or comments would be welcome.

James.

[James H]

Didn't think I would replying next - but having had another look at things, - forget my thing about stripping the exhaust side of the turbo without removing from the car. Daft idea!

The only difficult thing in removing the turbo from the car, from the looks of things, would seem to be detaching the lower oil pipe from the turbo. Possibly getting it parted from the exhaust header, - but depends on how flexible the flexible joint is.

Somewhat alarmingly, I can now see that the turbo is also missing at least two of the bolts that hold the exhaust side of the turbo together. :(

More on all this later.

James.

[davej]

Oh dear, sorry to hear that James. Missing bolts = turbo been taken off in the past do you think ? How long have you owned and what's the mileage ?
Malfunction Indicator Lamp although after my last post realised the glow plug symbol likely gets illuminated when there is a fault logged ? Is that right ?

Are you working from any kind of workshop manual ? I see the Bentley A6 manual annoyingly does not cover the diesels. Anyone know of a hard copy manual for the 2.5tdi ? It's about time Haynes got their act together for this motor.
I take Adams point about the weight and bigger turbo hinderiing off boost performance. Still be interested in driveability improvements from someone whose had a remap/tuning box experience.

James keep us posted about the turbo.

Dave

[James H]

Dave - had the car since early May (now seems like a bad buy). Mileage 95k ish. Don't know whether the missing bolts indicate previous dismantling/intereference, or just came loose. I suspect the latter actually.

I am waiting for a Bentley manual to come through the post - I thought is was a paper manual and I thought it covered my car. I will be well p*****d if it doesn't!

I think I have a solution. I have found a place near Leeds (ie near me) where they will rebuild/service the turbine for 195 plus VAT - and do it in one working day. I think that may be a worthwhile amount of cost for me.

James.

[davej]

Hi James, That does sound ok for the rebuild assuming there is good warranty with it.
The Bentley manual does not cover any diesels I'm afraid. The US is a bit slow on the uptake. I thought we might get a response from this forum about diy technical references but it looks like most folk these days either pay to get the work done or have a copy of Autodata and prefer to keep it to themselves.
Dave

[James H]

I have been trying this evening to remove the turbo from the car - with some success, though not complete. I have 2 remaining problems stopping the removal;

- getting the turbo to lift away from without removing or at least loosening the exhaust system because of hte angle of the header flange bolts. I suspect that there is no other answer, and I am going to have to get the car in the air and the exhaust at least loosened. If anyone knows otherwise, I would love to hear.

- the other problem is detaching the lower oil pipe from the turbo - as far as I can tell, it is held on by two small allen head bolts, and I am struggling to get my hand and any sort of tool in there. And even if I were to decide to sacrifice the pipe, and cut it to remove the turbo, I can't see how I would manage the reattachment. I have half an idea that I may have to buy an new one anyway, but I am not sure about that yet.
Again - anyone know about this, please?

thanks,

James.

[James H]

Update on my recent whingings - got the turbo off, with help from Adamss24 - and today got the turbine serviced just the other side of Leeds - dropped it off about 09.30 and picked it up again mid-afternoon. £150 plus VAT. 12 months warranty. Having refitted it to the car, it all seems to be doing what it should. What a different car.......

- smooth boost. Perhaps not as much as I would like, but then when the A6 is off the road, I am in a Porsche 968 - but it pulls smoothly all the way through the range. No fault codes after a test run.

Hmmm - I hope I am not tempting fate by announcing that it is all sorted out.........


Anyway - thanks all for all help



James.

[snapdragon]

I recently had similar issues and replaced the turbo.
I was still not happy, and eventually found that the one way valve which is located where the vacuum pipes go through the bulkhead was faulty. It is a small plastic thing, half black and half white. It has a thin pipe pushed on each side. This was not holding air back in one direction. After replacing this, the car was finally as fast as I had always expected it would be but was always disappointed. Worth checking for the 60 secs it takes and for people who find this thread in the future.