No Power at high speeds

Whenever I drive on the highway at about 70 - 80 MPH, I have this sudden loss of power, kinda like i'm not pushing on the gas at all. At first I thought that it was not making boost, but I installed a boost gauge a few weeks ago, and the turbo is working fine with about 9 - 12 PSI with full throttle. Anyone know what is causing this?

When was the last time you changed your fuel filter? If the symptom goes away right away when you slow down, or otherwise only afflicts you when you have a load on to start with, it could be a marginally clogged fuel filter. This will restrict fuel flow just a tad, and initially just under load, so you could, for example accelerate away from a stop light just fine, but cruising at 70 (which is the initial load) would see a drop in passing power.

VW Racer X said:

Whenever I drive on the highway at about 70 - 80 MPH, I have this sudden loss of power,

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Boy Racer, if you turn the car off/on, does the power return immediately and everything back to normal?

Yeah, if I take my foot off of the accelerator and slow down a bit, the power will come back in a little while. And yes if I turn off the car everything is fine when I turn it back on.

sounds like overboost condition then going to limp mode. What kind of car is it, MY, etc.? When was intake manifold last cleaned? What mods?

It's a 1998 VW TDI, engine code AHU, 1.9L. No mods yet. The intake manifold has not been cleaned yet and it has 113,xxx on the clock. I know that it needs to be cleaned, but I don't think that it would cause this problem as my dad's TDI has about 180,xxx on the clock, has never been cleaned, and although it doesn't have full power due to the clogging, it does not have the same problem.

I would not attempt a correlation between the two vehicles.

I'm not sure what you mean, they are the same engine code (AHU). I'm just saying that I doubt its due to the clogging, as that would be an overall performance issue, I don't see how it could just all of a sudden cause a sharp decrease in power. Not trying to be mean or anything, just my opinion I guess.

I was just suggesting two vehicles, even though they appear identical, could react differently under the same conditions. No big deal.

The cars can appear to run normally even with severe clogging. The degradation is so slow that most will never notice...until you hit limp mode and begin searching for causes.

My $.03: I'd be checking the condition of the IM and the vacuum hoses tomorrow.

Yeah, I should probably do a cleaning soon on the IM. The vacuum lines are all fine as far as I can see. I checked them all out when I installed the boost gauge.

VW Racer X said:

Yeah, I should probably do a cleaning soon on the IM. The vacuum lines are all fine as far as I can see. I checked them all out when I installed the boost gauge.

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Visual inspection or an actual vacuum leak down test


Guys, does the A3 diesel have that silly rubber tube within the ECU?

Yes it does have the tube, i used it to hook up my boost gauge. I was looking in my Bentley to find a place to hook up my gauge and it said that the tube going to the ECU was the one to tap in order to test the boost/vacuum.

i thought max sustained boost would be around 14, when applying full throttle. i definitely think the boost is kept down because of one of the sensors in the boost system telling the ECU to cut boost in order to save the turbo. the question is why?

i'd second Dorf on the IM cleaning being needed, as well as NEW vacuum(boost) hoses. Just because they "look good" doesn't mean they perform good. all it takes is a pin-sized hole and you will lose vacuum. do the safe thing and replace all boost lines to/from n75, clean your IM, then check to see the difference.

The Bentley manual says that the boost should average around 9 - 10 PSI under full throttle.

if Bentley says it, i believe it. it's been awhile since my AHU days, i still miss my old one. i would check for proper operation of the N75 valve as well, maybe it could do with some cleaning, besides the aforementioned IM cleaning and related items. good luck.

I've seen intake manifolds with 60,000 miles that can't have a large marble make it through there... I've also seen one with 132,000 that was almost spotless (almost!!)