TDI Sudden Power Loss! Later higher MPG?

:1zhelp:I just experienced a sudden power loss while pulling a long mountain pass in a 2004.5 B5 TDI with 60,000miles. Vehicle was pulling 6% hill just fine at about half throttle position and 65+mph for about three miles, then a rapid loss of power, no Check engine light, no rough running, no high temps, just a rapid loss of power down to about 20 to 30mph on the 6% grade.

Power loss was similar to what I experienced with my old 97’ B4 TDi a couple of times when I left the Mass airflow sensor connector off after changing the air filter. This condition continued for about 10 miles until I pulled off. Checked the MAF and restarted the engine. Power seemed to come back and I had a nearby Volvo shop check, but they found no codes. Did not seem like a plugged fuel filter or turbo or waste gate problem.

I later disconnected the MAF sensor to see if a similar condition resulted. Surprisingly no power loss but I did get a check engine light. I reconnected the MAF sensor and restarted the vehicle, Check engine stayed on but I got a “EMMISIONS WORKSHOP” message but noticed the engine seemed a little more peppy with more clatter, and indicated (computer) MPG came up about 5 to 7mpg to well over 40 mpg at 60 to 70mph from the typical 32 to 37 MPG as always before.

My Questions:

What might cause the power loss without generating a fault code? MAF Sensor perhaps or something else?

Could a pre-existing problem have caused a low mpg condition , or could a fault condition cause the MPG to read wrong or perhaps disable something like the EGR and resulting in more MPG?

Does VW cover a MAF sensor under the 80,000 mile warranty or other components related to an emissions warning.

Thanks in advance for any ideas? :1zhelp:

If you were at very high altitude, this may help to explain the 'limp mode'.

Your turbo has to spin faster & work much much harder to achieve a fraction of the relative 'sea level' pressure with the less dense mountain air.

You possibly 'underboosted', if the turbo simply could not supply the required boost pressure.

Also fueling may have been cut by the mapping at that altitude.

Thanks for the thoughts,

The summit was 2,000 FT + or - at about standard Baro.

Would an "underboost condition" cause a fault code? I still had a considerable amount of power left so I would not suspect an underboost unless something like an Anti-shudder valve closed or wastegate suddendly started leaking.

2000ft does not seem high enough for surging or reduced fueling etc.

If it only threw a temp code, or no code, and doesnt happen again, i would not be concerned.