Low power after interstate drive?

Hey everyone,

Was curious if anyone has run into this problem or any hints to what it might be or if I'm crazy and this is all in my head.

It doesn't always happen but I've noticed the last few times on my way home from work on the latter portion of the drive after getting off the interstate; when I'm driving in town 35-45mph the car seems to have less pep. When getting upto speed I've even tried to floor it but don't really notice any change in acceleration although it still gets up to speed just seemingly slower than normal.

Is there some sort of extra soft limp mode? It starts up and drives fine around town and seems great on the interstate part of my trip it's just the last town bit home. It almost seems as though the car doesn't register as much peddle when I press 1/2 vs it on the floor.

Thanks

MIL come on? Any DTCs stored in the ECU? Is the brake pedal switch malfunctioning?

I do have some MIL's I've been slowly battling

-egr soli + valve
-good ol gp
-ECM fault is still there and I did replaced hose in ECM + all my vac lines

I don't think my brake pedel switch is malfunctioning because my cruise control works fine or is that not a good tell sign?

Well, that is one good sign (proper cruise control function).

I'd address the EGR bit, but that probably is not the source of your sporadic low power on that engine.

Of course, they could be related in the form of a failing MAF. The ECU uses the MAF to monitor EGR flow.

Check your vacuum hoses for "OLD AGE" especially the one going to the turbo control...Lie under the engine and look UP to see it!

Hm I replaced all the hoses on the N75, N18, & ECM (inside + hose going to it)

Are there more that can only be seen from the underside? Where would be the best spot to look around to find it? I'm guessing on the underside near the turbo?

If it was a hose I'm still kind of puzzled why I'd see this problem only after driving for a bit- hoses warm up get more bendy and leak more? ha

It would be nice to see MAF/MAP Actual/requested info during one of these trips.

There are a number of items that can have some sort of impact.

When was the fuel filter last changed? If it's clogging I could see it feeling worse after a trip. If it has done this several times, then I doubt the filter would clog and unclog after a bit of rest.

How about underboost? Clogging intake or air filter? I'm not sure underboost would give a code unless it was extreme, but it would cut your power.

Mine does this too. It's getting better as the temperature have gone down. Mine has a clogged intake that I attribute to it misty, as I have no faults or DTC's.

I also wonder if my timing is jumping sound. I confirmed my timing is advanced when it's up to temp, but when the cars coolant temperature is below 100*F there is drastically more advance knock (haven't checked it while cold though) but you can hear it., along with much more power than when it's up to temperature. I don't know if the stock tuning advances timing when the engine is cold, and if it does how much. The difference in sound tells me at least a few degrees difference.

Bah sounds like I need to get vagcom to monitor map/maf/boost >.<

I replaced the fuel/air filter and all that about 4k mi ago.

I haven't got to cleaning the intake yet since I've owned the car, but would that be a symptom? Runs fine;cold & warm-up temp just not after a prolonged drive?

Do you see black smoke while accelerating in town? You mentioned 35-45 MPH and full throttle. Maybe a very small leak/hole in one of the hoses, check the one coming from the inter-cooler.

I say that because after driving on the interstate the engine bay gets hotter and makes those small holes get bigger. They expand and you lose some of the power since that same hose is leaking air now.

I just realized you posted this statement also so what I just wrote is probably your problem.

Runs fine;cold & warm-up temp just not after a prolonged drive?

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Maybe one of the vac lines just popped out after you changed them all. Good luck with the fix

If you want to keep the car for a while, VCDS is worthwhile. If not, you can live without it.

Good luck with the diagnosing the problem.

Yeah I know a vcds will/is totally worth it. I was just waiting till I had a little more play $ sitting around.

Thanks for all the ideas so far. A little update- after my drive home yesterday the problem wasn't there, which is why this is leaving me baffled.

@TDI I haven't seen any black smoke.
I'm assuming there's no "fuel saving mode" or something weird of the sorts right? Both times I have noticed it happening is when I start to run low on fuel 1/4 tank and lower.

Other than that, the only thing I can think of is the waste gate sometimes getting stuck slightly open?

It's just bizzarr how the problem seems to "fix" itself and only happen on the last leg of my drive home between days.

I'd check the fuel filter first. When I've had them ALMOST clogged, what'll happen is, it drives around town just fine because the trickle of fuel getting through is enough to keep everything happy, but the higher fuel flow rate at highway speeds eventually outpaces the flow into the filter, and it starts starving and acts wimpy. I have a small clear pre-filter in the line just before the big filter, so I can watch that thing clog up and save the more expensive, harder-to-change big filter. In particular, I see this when I've run the tank almost dry and it starts vacuuming up all the old crud in the bottom of the tank.

foxtrot said:

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I'm assuming there's no "fuel saving mode" or something weird of the sorts right? Both times I have noticed it happening is when I start to run low on fuel 1/4 tank and lower.

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Really cold, low fuel, maybe fuel restriction at the pickup?

Thought I'd check back in and give a small update-

I had the same issue happen again, although this time the tank had plenty of fuel in it. What I did realize is both times it has happened when I drive 80mph on the interstate leg home (instead of my normal 70mph), then when I pull off and start doing town driving is when it gets wimpy. I'll try my drive home again today the same way to see if I can recreate the problem.

Would a fuel filter do that? Semi clog up then unclog for the next days drive ? Are there symptoms of a N75 going bad that would line up with this?


Edit: I forgot to add, the fuel filter has about 6k mi on it now so still kind of new'ish, but the PO did tell me he was running some 15 year old diesel in it (he scored a huge free tank of it). Since I've had it I've been running power service in it. Any way this has knocked some stuff loose making this new one clog faster?

Putting in a new fuel filter shouldn't hurt. High-speed uses more fuel, so more has to go through the filter. I would expect that, if it was the filter, you'd have some issues with hills (ok - Florida, no hills) or perhaps just keeping the car at 80. However, I can't think of much else that would give you this issue.

A filter won't unclog and clog up again either. Once it's clogged up, it's always clogged.

Well, I think that the problem is the N-75. It happens to me often with a less than new N-75 boost controller on a long trip. The symptoms: low power and high indicated fuel consumption on Scangauge. No MIL. Short term solution: with nobody driving right behind, depress the cluth, shut down the motor briefly, then ignition switch to "on" and release the clutch to start the motor. Normal power should be restored temporarily. Long term solution: to change the N-75 valve with a new one.

Perhaps the N-75 gets hot from the heavy use, and slows down? Might be it, but surprised that it doesn't throw a code.

I never got a code, neither with Scangauge, from that behavior. Personnally, I think it happens because of oil fumes that gets in the N-75 and get the internal mobile ball sticky. My first N-75 lasted the first 13 years of my car, the second one gives me problems recently even if it was cleaned.

The high low fuel economy has my ears perked. VCDS confirmed low MAF and MAP numbers vs requested. Plus, the same drive from Houston to Dallas and back resulted in mid 30's the first trip (low power), and 42mpg the second trip (much better power, but still lower than when the car is cold). Not to mention, much more noticable turbo whistle (still extremely quiet, but I can actually hear it the first couple of gear till the road noise gets to loud).

Might go run some logs when the engine is cold.

Thanks for the info/experience Ralphy. I didn't know a N75 could act like that.

Not being able to recreate the problem consistently is making this a bit of a pain to figure out

Maybe I'll give the N75 a shot. The only other off the wall thing I could think of is maybe some sort of fleck/debris in the tank that semi blocks at the fuel pick up sometimes lol.


Edit: if/when I do figure it out I'll make sure to post back!

Would be worth it to clean the check valve by the tank and the pickup in the tank as you suggested. Not a hard or long job.