Limp mode after restart when engine is warm

tommydude13

New member
Joined
Nov 27, 2017
Location
Michigan
TDI
1998 Jetta TDI 265k
I have a 1998 Jetta with 265k manual trans.

The car will run perfect, build boost and drive completely normal if I start it in the morning and leave it running. But if I shut if off and restart it, it goes into limp mode with no codes thrown. If I let it sit for 4 hours or more and start it up it will run fine again. So it seems to not like restarting when warm.

Has anyone had this issue? or have any idea as to what would cause this?

And yes I have read through the sticky thread: How to troubleshoot low power on an A3/B4 engine.

Thanks everyone
 

Mongler98

Top Post Dawg
Joined
Mar 23, 2011
Location
COLORADO (SE of Denver)
TDI
98 Jetta TDI AHU 1.9L (944 TDI swap in progress) I moved so now i got nothing but an AHU in a garage on a pallet.
Limp mode is only triggered by a hand full of things. if you read up on that sticky, i think you probably missed something or do infact have an issue with Vacuum lines. maybe EGR.
 

tommydude13

New member
Joined
Nov 27, 2017
Location
Michigan
TDI
1998 Jetta TDI 265k
Finally found my issue. It ended up being corroded wires going to a sensor on the inter cooler tubes above the engine. I'm assuming its a boost pressure sensor or something, please correct me if I am wrong.

I had to take the connector apart to solder the good wire to the terminal and replace sections of the wire to get to good copper. 4 inches of the wire on the bottom and just a little bit of the other one. The lower wire that was the worst, the insulation was cracked and the wire was actually only hanging on by a few strands. After I fixed all of this, no more limp mode.

Not entirely sure the science behind how this was causing it to go into limp mode only after restarting the engine, and find it strange that this never threw any codes, but I'm glad it is working now.

Picture of the sensor: https://plus.google.com/photos/phot...6/6501717607771439970?authkey=CJXHwNHwt-q76wE
 
Last edited:

Mongler98

Top Post Dawg
Joined
Mar 23, 2011
Location
COLORADO (SE of Denver)
TDI
98 Jetta TDI AHU 1.9L (944 TDI swap in progress) I moved so now i got nothing but an AHU in a garage on a pallet.
Interesting. I would post this into the forum post about diagnosing limp mode.
Good find, glad to hear it fixed. It makes sense.
If its the one on the pipe going into the intake manifold, that is your Manifold Air Intake Temp
Limp mode is triggered by values that go past the set map limits in the ECUs programming. Voltage or rather the resistance of the amps running though the sensor is measured on all the sensors and the ECU gets its information. A cold copper wire has less resistance than a warm or hot wire. If the wire was literally hanging by a corroded thread, than you now have a wire that was supposed to be 20g but now has a spot that is like 50 gauges (size is example here). Resistants or Ohms is a factor is wire size over volts / watts. In other words, smaller wire when warmed up has a higher resistance and thus will send a voltage signal that the ECU interprets as out of value. All electrical components have a starting amps and running amps. This is usually never factored into anything other than heaters, AC units, compressors and what not where it matters, but the fact is that now you had a warm wire that can’t handle the starting amps when the car is started. In other words yes, a warm wire that small can’t give the right signals to the ECU. Very interesting to say the least.
 
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