low boost, low mpg, low power

98 Jetta, AHU 197k miles

Normally if I jumped on the throttle boost levels would spike to 16-18psi then settle to 14psi (at highway speeds). For the last few weeks boost levels would rarely exceed 12psi, then one day it wouldn't exceed 6psi. I switched the car off then back on and it could hit the new norm of 12psi again. It had been a while since I've changed the fuel filter so I had that done and last night I put in a fresh air filter.

This morning I had max boost of 6psi until I restarted the engine then it was able to hit 12psi again.

I've got 36mpg on the last two tanks on b100 (which i've been running for months now), the check engine light is NOT on.

I will check the air box screens to make sure they aren't clogged, but what else should I check?

thanks for the insight

Have you changed the lines to/from the N75? Rather important items. If they look old, swap them out.

Also, the line from the innercooler to the ecu - and the tube inside the ecu.
If any wear (especially where it rubs right before going to the ecu) replace them, including the little one inside the ecu.

Also - get it scanned with VagCom. You may have a code and no light.

I've replaced the MAP line to the ECU within the last 2 years but I believe. I have never looked at the line INSIDE the ECU but that will be a last resort. I have not looked at the N75 lines lately (the green and red ones, right?). If I do need to replace them have any of you come up with clever tricks to keep the handy color-coding?

Thanks for the help

koabi said:

I have never looked at the line INSIDE the ECU but that will be a last resort.

Click to expand...

That should be the FIRST thing you look at, not the last. 10 cents of tubing could save you some headaches.

It takes, at most an hour (probably 1/2 hour) depending on if you have to search for the tools (why would I think of that - doesn't everyone have a neat garage?). I bet you have a boost deviation code, or a rom error (which I think does set the light).

Replace it. Less likely to cause harm that messing with the tubes on the N75 (only one to dis-engage).

Color coding - use colored high temp silicone of the proper color. Or use the dye job (wish I could remember the member that did it). I wish I could remember how to do it. I have the silicone hose myself, but dont' want to replace unless I see or feel a problem. I did replace the ecu hose as well as the MAP hose. That last one was worn through.

I did have a boost deviation code; good call. I just pulled apart the ECU and the rubber line looks dry but intact. I'll replace it, but I do not think it is the culprit. Where did you get your high temp silicone hose from?

and dayum the ECU connector was a pain to get off

McMaster? If you don't mind looking here
you'll confirm or correct my memory. Lots of other stuff too.

Oh here's the code readout:
1 Fault Found:
00575 - Intake Manifold Pressure
17-00 - Control Difference

And has that tube been replaced?

"Looks good" and "Is good" are worlds apart. Just think of the Willie Nelson Song "I Got In At 2 With A 10, And Woke Up At 10 With A 2"

Sounds like you've got a clogged intake manifold. Might I suggest cleaning it? The soot inside mine was horendous since I also have a 1998 Jetta with 156k miles on it. I could be wrong though, but Intake Manifold Pressure says a lot.

Here it is:

http://pics2.tdiclub.com/gwillie/VW/Sballintake/INTAKEMAN101b.htm

This took me a good 5-6 hours to complete, so I would verify that the code pertains to a clogged intake.

just replaced the internal ECU hose and took it for a spin. It hits 12 psi still so it's still not where it should be. At least I wont have to worry about that for a while now.

What is the thing on top of the intake manifold with a tube running from it? That's the worst looking hose of the lot.

I had cleaned intake manifold cleaned, but that was a long time ago (50,000 miles?). Supposedly running biodiesel helps keep them clean but I don't know if there's any truth to that. I hope I don't have to clean it again!

Have you tried just running Dino100 just to see if it`s the bio your using. I use B100 most of time but sometimes I feel more power and a little better fuel mileage when I keep about 2-3 gallons of dino in the tank. Some batches of bio I`ve seen a noteable hit in MPG not so much in power though.
One time when I changed my fuel filter. the connection to the MAF got loose. I thought for a long time my loss of power and economy was related to the bio, until I discovered all I needed to do was push the electric plug in tighter to thew MAF.

Also I believe your right about bio keeping intake cleaner. Less soot produced when using bio= less black smoke=cleaner egr/intake. Also I`ve noticed my oil doesn`t turn coal black anymore. The oil now more closely looks like the oil in our gasser Jetta....

I've never heard of dino100, but I'll look for it. I have noticed some loss of mileage sometimes with bio, but not to this extent (doesn't mean it's not possible, though). I replaced the hose that goes to that thing on top of the intake manifold - I have no idea what it's function is. Also replaced the hose ends that go on either side of the plastic line that runs right by the injectors. That hasn't improved anything, but those two hoses were sad looking.

I'm going to buy some silicone tube and replace everything, including the n75 lines that go somewhere under the turbo and disappear. Hope I can find out where they go!

Thanks for the help, all

koabi said:

I've never heard of dino100, but I'll look for it.

Click to expand...

Just refering to regular diesel with no added bio. Also unplug the connection to the maf, drive it around for a little the reconnect it and drive around a little bit. If you can`t tell the difference in each driving performance you have a bad maf.(mass airflow sensor)

sccasper said:

Just refering to regular diesel with no added bio. Also unplug the connection to the maf, drive it around for a little the reconnect it and drive around a little bit. If you can`t tell the difference in each driving performance you have a bad maf.(mass airflow sensor)

Click to expand...

haha, I'm an idiot The MAF connector felt slightly loose so I was hopeful that was it, but that woulda been too easy... I'll try your suggestion out, though

working in the daylight is a good idea: I found a split in the intake hose just off the intake manifold. I know I came across a thread that some people have found a radiator hose from a diesel v8 truck that can be cut and put in the place of the original hose. Once I find that, I am off to the auto parts store...

The car is boosting more fully now but fuel mileage still suffers (though marginally improved). I did run a tank of dino juice through it, but that didn't change MPG rating much (makes the engine more noisy, though).

I'll check for more leaks...

My N75 was bad for a long time, didn't realize it. My symptoms were reduced power at full throttle over 2K rpms. I was changing the vacuum lines and broke a nipple so I had to replace it anyway. about $55 shipped from TDI parts I think.
Solved my problem. Car runs great now at 240K miles ( just turned over today).

congrats!

Yeah I should just buy the thing because I've been putting off changing the lines because I am afraid I'll snap off one of the nipples as well.