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TDI Power Enhancements Discussions about increasing the power of your TDI engine. i.e. chips, injectors, powerboxes, clutches, etc. Handling, suspensions, wheels, type discussion should be put into the "Upgrades (non TDI Engine related)" forum. Non TDI vehicle related postings will be moved or removed. Please note the Performance Disclaimer.

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Old January 6th, 2010, 10:09   #1
t0m541
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Default Limp Mode??

Hi everyone, it's been a while since I have posted on here, so Happy New Year to all.

Before you all slap me down for my thread title and tell me I should be in 101, it is performance related.

As you will see from my cars spec sheet I have done a lot of mods and the old shed has been running well till late, after last Kerma chips It was pushing about 165whp, and I was quite happy, so I just left it at that.

About 5-8 weeks ago I began to notice that at WOT and full boost, my boost gauge is marked to 25psi and the needle would fly past it to the stop, the engine would lose power, turbo would drop to nothing, then jump back to life and boost would come back, not slowly but instantly, like flicking a switch, 3-4 times in quick succession, so I would back off the throttle and just drive "normally".

It was getting more and more frequent so I just kept on driving around gently, I did notice a whistling sound when boost came up like air either escaping or a blockage boost side of turbo, as I had the IC piping blow off on the M-Way a few months ago I decided to have a look in them to see if anything was "laying" around and causing a restriction, and apart fom a piece of cord, that I had used in making the seals, wrapped around the anti shudder flap, lucky it stopped there. The only other stuff I could see was a good deal of oil in the pipework.

After a while I began to notice the car was less reluctant to go to full boost and I would have to feather the throttle to get it there, but when it did it would take off and leave a huge cloud of black smoke which it has never done since the remap, it has always run clean.

I was putting the intermittent power fluctuations down to the turbo playing up, maybe sticking vanes, but as the car was always driven in a lively manner I cant really see that happening, then most recent, when the temperature dropped in December, the car would lose turbo completely going up hills at about 2k revs and struggle up till I could cycle the ignition.

I Vag-Com the ECU and it said bad MAF so I dioded it out, which helped a little, the power loss got a little less frequent but still happened, so I had a look on here and found Canadian Grizzly's limp mode diagnosis thread.

So I checked and changed vacuum pipes, swapped N75 for EGR controller which gave a little more boost but still got power loss, (limp), so I replaced N75/EGR controller back where they should be and plugged them into harness so the ECU would still see them and bypassed the N75, vacuum straight to turbo, (max vanes), but was still only getting 10-15psi boost pressure and still power was cutting back once 15psi was reached, I assume the ECU cuts fuelling to try to reduce boost.

This lead me to the MAP sensor possibly reading high or failing causing the ECU to read overboost, so have tried driving with it unplugged, the car struggled to make any boost, and plugged in but taken out of pipework and hole plugged so it reads atmosphere only, car drove better until it reached the 15psi limit then power backed off, ECU reduced fuelling?, which is restored by cycling ignition.

So how does the ECU know that the boost is reaching the 15psi limit to back the fuelling off, when everything is disconnected and also is there a electrical mod/bypass for the MAP, or even some kind of workaround for this.

Any input or new angles on where the fault could be would be helpful, the hybrid turbo was reconditioned about 18 months ago so the diaphragm should be good although I have not got under and checked if actuator is moving, damn snowy weather.

I realise I could just spend cash and buy some new sensors/N75 but I want to buy the right ones as I have been pricing the devils up and they aint cheap but if I have to do that, 3bar map sensor is probably a good idea, (part nos. and where from please if anyone knows).

So guys what do you think??

Cheers

Tom
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Last edited by t0m541; January 6th, 2010 at 12:32.
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Old January 7th, 2010, 06:22   #2
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Ok, braved the snow today and had a poke under the bonnet and plugged in the laptop and and everything else under the lid as it should be.
What I found was that MAF was only reading 550mg/str, so dead MAF, the MAP was reading erratic to what was specified and not matching what the boost gauge was reading.
I re-diode the MAF (pins 4-5 bosch maf)and got 1275mg/str reading, no fault code flagged.
I took the car for a drive and I still got limp mode at 10-15psi on the boost gauge, cleared it on cycling on key.
So MAF and possibly MAP gone bad and the jury is still out on N75 although turbo is whistling away and obviously boost pressure is rising but not able to reach full pressure
MAF is bypassed now so need to work on either bypassing MAP or finding a 3bar one.
I read that there is a way of chemically cleaning the thrust ring and vanes on turbo, is this done in situ or out of car, should I find they are sticking.
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Old January 12th, 2010, 14:45   #3
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News..but not good...
I managed to obtain today a replacement N75 and MAP sensor.
Fitted MAP sensor first with N75 bypassed(vac direct to turbo). Test drove car, still got limp mode (reduced fuelling), up hill or flat ground 3rd gear, any more than 2000 rpm and about 1 bar boost.
So I tried replacing the N75 and connecting it as it should be, test drive thought things were going well till tried to put some power up hill, .. limp mode(reduced fuelling, no turbo(N75)).
I then tried again without the N75 connected and found that if I tried accelerating gently on the flat I would get limp mode, but if I buried the pedal 3rd gear slightly downhill, after a bit of stutter at 2000rpm and 1 bar the turbo would go to full boost and hold until revs maxed out then boost would fluctuate, so I would go to 4th and keep the boost up and it would keep going without cutting fuel back.
I tried this a few times when I could, kept running out of road mainly, but each time I did I would leave a black cloud that needed radar to see through.
Now black exhaust is usually overfuelling or lack of air caused by bad filtration or lazy turbo.
But could it be that due to the lack of action caused by constant limp mode that the turbo and actuator ring are sooting up and when I do get full power it's burning the soot off creating smoke and if can keep doing full power runs without getting arrested, would it clean the turbo of soot and carbon and free it up.
Why would I get limp mode under partial throttle but not WOT when done quick, I have eliminated 2 of the suspects and the MAF is dioded out to read 1275mg/str max.

Any thoughts guys and my thanks to Kerma for continued after sales care...good customer relations

Tom
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Old January 12th, 2010, 15:20   #4
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What you describe almost sounds like underboost. What is the specific code that it throws when it goes into limp mode?
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Old January 12th, 2010, 15:52   #5
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I havn't Vag-Com'd it in limp mode yet, it's something I will try tomorrow, my laptop has a dead battery so has to be on mains power, so I'll get the PC ready and take car out and put it into limp then keep it that way so I can get it on my drive to check it out.
I was getting a MAF fault (impluaseable signal), but cleared that with diode.
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Old January 12th, 2010, 16:28   #6
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as for the underboost, I was thinking that...if the ECU map is looking for X=boost and the turbo isn't spooling then it would throw it up, however the boost gets to 10-13psi quickly and is responsive to throttle input, so vanes must be catching exhaust flow efficiantly, also on quick WOT it will spool to full boost, now, corect me if I'm wrong, but exhaust flow is directly proportional to fuel/air input and combustion, so if the ecu is reducing the fuel input then the resultant exhaust gas will peak lower and fail to provide enough flow to spool the turbine up to full speed regardless of vane position, vis-a-vis underboost beyond the 1bar threshold, when the ECU map is looking for more.
Vicious circle, ECU then reduces fuel as it detects a fault and cuts vacuum to diaphragm closing vanes.
So could the original fault lie in the fuelling side, insufficiant fuel??
Just bouncing ideas here
Tom
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Old January 12th, 2010, 16:36   #7
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Quote:
Originally Posted by t0m541
as for the underboost, I was thinking that...if the ECU map is looking for X=boost and the turbo isn't spooling then it would throw it up, however the boost gets to 10-13psi quickly and is responsive to throttle input, so vanes must be catching exhaust flow efficiantly, also on quick WOT it will spool to full boost, now, corect me if I'm wrong, but exhaust flow is directly proportional to fuel/air input and combustion, so if the ecu is reducing the fuel input then the resultant exhaust gas will peak lower and fail to provide enough flow to spool the turbine up to full speed regardless of vane position, vis-a-vis underboost beyond the 1bar threshold, when the ECU map is looking for more.
Vicious circle, ECU then reduces fuel as it detects a fault and cuts vacuum to diaphragm closing vanes.
So could the original fault lie in the fuelling side, insufficiant fuel??
Just bouncing ideas here
Tom
It all has to do with the time limits and allowed deviation set forth in the software. The MAF issue may be related to a boost leak which could also cause the underboost error.
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Old January 12th, 2010, 16:48   #8
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The MAF, when I last VC'd it live data didnt show above ambient pressure regardless of airflow, 550mg/str constant and whilst plugged in showed fault code. How would a boost leak affect the MAF as they are in separate parts of intake system, pre and post turbine.
What about a software glitch in the ECU map, somehow cause by a voltage spike or a degradation of the EEPROM structure.
Or something more mundane, fuel filter collapse/clogging restricting supply to pump?
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Old January 12th, 2010, 16:57   #9
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Hmmm...fuelling fault....now I think about it, more fuel makes more compression, bigger burn, hotter gasses, more power, more exhaust, more flow to turbine...restrict that fuel amount and its going to peak...but why at 2K rpm, 1bar boost under load?? and not downhill fast action WOT, what controls fuel flow inside pump on fly by wire pumps apart from ECU/TPS inputs?
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Old January 12th, 2010, 16:58   #10
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Quote:
Originally Posted by t0m541
The MAF, when I last VC'd it live data didnt show above ambient pressure regardless of airflow, 550mg/str constant and whilst plugged in showed fault code. How would a boost leak affect the MAF as they are in separate parts of intake system, pre and post turbine.
What about a software glitch in the ECU map, somehow cause by a voltage spike or a degradation of the EEPROM structure.
Or something more mundane, fuel filter collapse/clogging restricting supply to pump?
Your MAF may well be dead, but 550mg is the default value. So, if it throws a code it will just stay at 550mg all the time. Also, if you happened to have a boost leak then the MAF may read very high in comparison to the boost pressure. In this case the ECU would see this correlation as being implausible. When your car is in limp mode it will not be requesting boost and bypassing the N75 may cause additional faults as it is not expecting to see it.

Voltage spikes causing corruption of the eeprom seems rather unlikely. And, if such a thing were to occur I believe your first indication would be a checksum error. In this case the car either would not run at all or would run with a constant error code that won't go away. Let's not get into the unlikely just yet and stick to the more likely scenarios.
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Old January 12th, 2010, 17:18   #11
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I do have the MAF dioded out to read 1275mg/str, I have read the thread on here regarding MAF/Diode mod but I don't think there was any reported cases of limp mode, I could be wrong, will re-read thread.
However, surely once the rpm and demand reach into the higher band the MAF report would then start to match fuel map required flow, hence reported, in MAF thread, "once rpm reached 2K the car took off", it would as intake flow(what diode/MAF says)/MAP sensor reading and fuel flow match the ECU curve.
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Old January 12th, 2010, 18:54   #12
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Quote:
Originally Posted by t0m541
I do have the MAF dioded out to read 1275mg/str, I have read the thread on here regarding MAF/Diode mod but I don't think there was any reported cases of limp mode, I could be wrong, will re-read thread.
However, surely once the rpm and demand reach into the higher band the MAF report would then start to match fuel map required flow, hence reported, in MAF thread, "once rpm reached 2K the car took off", it would as intake flow(what diode/MAF says)/MAP sensor reading and fuel flow match the ECU curve.
It may well be that it won't throw a code because of the MAF reading too high. The only way that you're going to know for sure what's happening is to check the codes and do some logs.
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Old January 13th, 2010, 13:48   #13
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Hi, Ok did a limp mode fault code reading tonight and this is what it found :- 16485 - Mass Air Flow Sensor (G70): Implausible Signal

P0101 - 35-10 - - - Intermittent.
Without cycling the ignition, the fault code refused to clear and MAF was reading default 550mg/str, when I cycled the ignition, the MAf went back up to 1275mg/str and I code cleared the fault and it didnt return.

So any thoughts and ideas
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Old January 14th, 2010, 02:00   #14
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Hi Tom- Maybe it's time for an updated tune that totally disregards the flaky mass air sensor and uses the MAP sensor to control the smoke based on your boost signal. True Speed density control in your case would be cheaper than buying a new maf. Please E-mail me and we can discuss the particulars.
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Old January 14th, 2010, 02:27   #15
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Quote:
Originally Posted by KERMA
Hi Tom- Maybe it's time for an updated tune that totally disregards the flaky mass air sensor and uses the MAP sensor to control the smoke based on your boost signal. True Speed density control in your case would be cheaper than buying a new maf. Please E-mail me and we can discuss the particulars.
Did you figure out how to block the MAF codes? If not then the CEL will still be there.
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