Diagnosing and Fixing Limp mode for A4 1.9TDI [low power troubleshooting]

[whitedog]

Originally Posted by slickfast

Turned out to be a piece of gunk in the check valve of the brake booster line that was preventing any vacuum pressure to the turbo actuator. Power restored!

Glad we could be of some help.

Really, nice job on troubleshooting. You knew something was wrong and dug in to find it.

That would probably have been a $200 bill at the dealer.

[slickfast]

Originally Posted by whitedog

Glad we could be of some help.
Really, nice job on troubleshooting. You knew something was wrong and dug in to find it.
That would probably have been a $200 bill at the dealer.

Thanks a lot for putting the knowledge out there... you were right on the money with boost issues.

My recommendation for anyone that's at the end of their rope: get systematic. I got a hand vacuum pump with a gauge on it (mity vac) and printed out the vacuum line diagram for my engine (alh, stock). I went through every line with a vacuum pump and checked that it would hold vacuum, and when it did I traced the corresponding line with a green sharpie to show that I had checked it and that it was good. After that I went through the check valves and T fittings to make sure they weren't working. The brake booster check valve is pretty much impossible to diagnose with it on the car, so I just took the 2 minutes to remove it. Main booster line was totally fine, but I couldn't get the turbo line to actually allow air through it... weird. Tried sucking on it (next time I'd recommend grabbing the right fitting and using the hand pump) and YECH: dirt and crap ended up in my mouth. Then grabbed a needle, bent it to a little "J" shape and dug the rest of it out. Blew on it again and now it freely allowed air through. Boom, that was it.

Even though the check engine light didn't go out right away I knew I had fixed it, because for the first time in forever I could hear my turbo. SO HAPPY.

[SmoothVW]

great post

[kblee27]

I've read the first page till the last. My TDI has a persistent P1550 code.
I've tried, checked, some replacement, almost everything. N75 solenoid, MAP, MAF, actuator, vacuum lines. Nothing.
Except I can't reached to clean the turbo or intake manifold.

However, I did note that when I plug the full vacuum into the actuator, I get boost. When I put it back to N75, nothing happens. I only get very small boost of about 5 psi.

Finally, after a couple of years of driving with low boost, and suspecting the boost control problem, my workshop change the ECU.
Instantly, the boost came back.

So it was a defective ECU which did not control the N75.

[rdkern]

What type of car?
Do you have the vac lines going to the proper locations?

[Joe Cool]

AFTER LOADING A MILD TUNE very mild my trans is now only using one gear. I guess a limp mode sorta.
I put the stock tune back in but of course its the same. I do not know what the fault code is. I was considering buying the VAG Com but have never used one before.

Will it clear a trans code..because I think this is what happened.

Thanks

[JASONP]

Originally Posted by Joe Cool

AFTER LOADING A MILD TUNE very mild my trans is now only using one gear. I guess a limp mode sorta.
I put the stock tune back in but of course its the same. I do not know what the fault code is. I was considering buying the VAG Com but have never used one before.

Will it clear a trans code..because I think this is what happened.

Thanks

That automatic tranny won't hold up to any sort of tune , made that mistake years ago with my 01M


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[Lensdude_com]

I've seen a lot of posts lately of guys breaking bits on their cars by trying to make them perform better...
I suppose the real problem would be one's personal definition of performance!

[CoolAirVw]

Originally Posted by Joe Cool

AFTER LOADING A MILD TUNE very mild my trans is now only using one gear. I guess a limp mode sorta.
I put the stock tune back in but of course its the same. I do not know what the fault code is. I was considering buying the VAG Com but have never used one before.

Will it clear a trans code..because I think this is what happened.

Thanks

You might just need to clear codes. Try a OBDII code tool if you have access first.