No limp mode with vag-com?

TDI Mason

Veteran Member
Joined
Dec 13, 2011
Location
Seven Valleys PA
TDI
03 jetta tdi wagon
Hello all,
I'm relatively new here and have been living on this site for the past month or so trying to diagnose a limp mode problem. My car (03 tdi wagon 145000mi) has a limp mode issue with a pattern I haven't read about on here before. It seems to go into limp mode predictably very shortly after first start up of the day on a cold engine but if I pull over and shut the car off for a few minutes for it to reset I am fine for the rest of the drive and can't get it to go into limp mode no matter what I try until the car is shut off long enough for the engine to cool all the way down.
I have tried the following:
-replaced N75
-replaced MAF
-checked for vacuum leaks
-cleaned snow screen
-new air filter
-adjusted vnt control arm as per VW tsb i found on this site
-turbo was replaced within the past few thousand miles by the previous owner (so i am doubting sticky/carboned up vanes).
Enter Vagcom: So I decided I needed to view more data and purchased myself a vagcom cable to see what was happening and since I've had it (bout a week now) I've been driving around graphing requested vs. actual boost in VC scope and I've seen a few random spikes off the charts on the vc scope graph but the car won't go into limp mode. So yesterday I was tired of driving around with a laptop in the passenger seat so I unhooked Vag and what do you know... limp mode. This morning cold engine Vag hooked up, no limp mode.
So my question is (aside from if anyone can help me with my limp mode issue) will the ecu not send the car into limp mode when diagnostic equipment is plugged in?
 

dirtcar5

New member
Joined
Dec 15, 2011
Location
spring grove pa
TDI
2003 jetta
i also am new to the site and have been reading on limp mode(p0234) with a similar sounding symptom. i hope someone in the know will reply as a i too am interested in finding the cause. i would like to just adjust the act. rod longer even tho it will be out of spec per tsb,etc,,but if it makes the car/ecu happy .......what would it hurt? the car i am working on is stock with a rebuild engine. .040 pistons with a 2 hole gasket(1mm piston pop up). i am wondering if the increased displacement with the slightly higher comp. ratio is the cause?????
 

WillR

Veteran Member
Joined
Mar 2, 2001
Location
Honey Brook, PA
the following is what caused it on my 98, and my 99 is now showing the same symptom.

The turbo is designed to move X amount of air depending on vain position.
The ECU understands what it expects to be seeing for pressure at a given load/throttle position.

Now make a restriction in how much air can be pushed through the engine. The turbo spools up and starts pushing air, but the ECU sees too high of a pressure and tries to lower it by adjusting the Vains (or on my 98 case, dumping the waist gate).

On my 98 I was able to log this, including marking exactly where the event accrued that caused the limp mode. Pressure was always much higher then expected.

In both of my cars, this is/was worse on cold mornings, 10 minutes after start up (but I can also force the code by running WOT down a highway in the hot summer.

On the 98 it was a clogged intake. My gasket came for the 99 today.

My problem is I bought the 99 with 118K miles on it, from a dealer who specialized in VW TDI's. he said "I did everything this car could need, timing belt pulleys and tensioners, clean intake, etc, etc." and even gave me a receipt for all the work.
Now he is out of business and the car has 138K miles on it. If the intake is clogged it would indicate it might never have been cleaned. If that is the case, Did he ever do the timing belt? :eek:.
 

TDI Mason

Veteran Member
Joined
Dec 13, 2011
Location
Seven Valleys PA
TDI
03 jetta tdi wagon
the following is what caused it on my 98, and my 99 is now showing the same symptom.
The turbo is designed to move X amount of air depending on vain position.
The ECU understands what it expects to be seeing for pressure at a given load/throttle position.
Now make a restriction in how much air can be pushed through the engine. The turbo spools up and starts pushing air, but the ECU sees too high of a pressure and tries to lower it by adjusting the Vains (or on my 98 case, dumping the waist gate).
On my 98 I was able to log this, including marking exactly where the event accrued that caused the limp mode. Pressure was always much higher then expected.
In both of my cars, this is/was worse on cold mornings, 10 minutes after start up (but I can also force the code by running WOT down a highway in the hot summer.
On the 98 it was a clogged intake. My gasket came for the 99 today.
My problem is I bought the 99 with 118K miles on it, from a dealer who specialized in VW TDI's. he said "I did everything this car could need, timing belt pulleys and tensioners, clean intake, etc, etc." and even gave me a receipt for all the work.
Now he is out of business and the car has 138K miles on it. If the intake is clogged it would indicate it might never have been cleaned. If that is the case, Did he ever do the timing belt? :eek:.
This would make sense but I forgot to mention that I also have a new intake installed.
Maybe I should elaborate a little on this car because it has an unbelievably troubled past. First off I bought the car from a friends dad who is the original owner and basically the history of the car goes like this: somewhere around 70k mi a coolant line broke and from what i gather he must have drove it quite a while and eventually to the dealership for repair. Long story short, they tell him the engine is toast and he has it rebuilt to the tune of $8500:eek:. Somewhere between 100k and 140k turbo is making a noise and is replaced??? At 145k timing belt brakes and trashes the head, and having had enough of the stealerships expensive bills it was taken to a local clown for repairs, has a new head put on and never ran after that. After 2 years pursuing legal action and the mechanic pulling a "Houdini" and disappearing the owner is fed up with the car and I buy it for $1000. I had hoped to take it apart and deal with some head issues and have it back on the road in no time but oh no... The car obviously had a new head installed but looked to have sucked some shrapnel into cylinder #2. Possibly leftovers from the TB failure made their way into the intake and where sucked into the engine upon start up after the new head. We also come to realize that this mechanic honed the already over bored cylinders and put stock bore pistons back in (.026 in clearance!!!), and not only that it had a cylinder 3/4 piston in cylinder #2 and cylinder 1/2 pistons in all the other cylinders (all very mind boggling). So in order to try to salvage the block I sourced 1mm over bore pistons (not available through oem :mad:).
Short story long - the car is back together after a total rebuild and running fine except for this limp mode issue. One last theory we have is maybe the extra displacement from the bigger pistons is pushing more volume through the exhaust causing the over boost condition and I need to make the actuator rod even longer then oem spec to reduce sensitivity?
Sorry for the long post, but maybe a few of you will find it a comical read, and others see it to be a warning to be wary of any old mechanic claiming to be able to "work on a TDI" (kind of scary what is out there...).
 

WillR

Veteran Member
Joined
Mar 2, 2001
Location
Honey Brook, PA
Also, when dealing with computers, you have to remember "garbage in, garbage out".
Maybe the sensor that tells the ECM how much boost pressure it sees is really sending a wrong reading? Very common for the Temp sensor to fail this way.
 

TDI Mason

Veteran Member
Joined
Dec 13, 2011
Location
Seven Valleys PA
TDI
03 jetta tdi wagon
Also, when dealing with computers, you have to remember "garbage in, garbage out".
Maybe the sensor that tells the ECM how much boost pressure it sees is really sending a wrong reading? Very common for the Temp sensor to fail this way.
Which temp sensor are you refering to? fuel temp, or maybe coolant temp?
Right now I am making adjustments to my actuator length and am 2 turns longer than where it was previously and limp mode seems to have gone away.
Back to my main question... I am now convinced that these cars will not be set into "limp mode" when vcds is hooked up to the car unless someone "in the know" can tell me otherwise. I was trying to read the graphs to see what boost pressures need to be seen for the ecu to send me into limp mode and I am seeing a few random spikes off the chart on the graph (over 2500 mbar) and not getting limp mode at all. As soon as I drive the car without vcds plugged in I fairly consistently get limp mode.
 

WillR

Veteran Member
Joined
Mar 2, 2001
Location
Honey Brook, PA
I was not associating the temp sensor to your problem, just that they are known to have problems and it's a perfect example of garbage in, garbage out.
It's the 4 wire one that controls both the dash gauge and the ECM uses to time the glow plugs.

Example. It's 10 deg F outside, and the glow plugs stay on for 1 second. This is that sensor sending garbage information to the ECM. Sensor tells ECM car is warm, so ECM only cycles the glow plugs for 1 sec.
Then you unplug the sensor, ECM sees sensor gone so it defaults to cold and cycles the glow plug light for like 30 seconds.
 
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