SO I got intermittent boost

My Car: 2003 Jetta ALH TDI 200K miles Auto trans

The turbo blew on my car a while back so I replaced it with one I had on an engine sitting in the shop. The "new" turbo was a reman which was in a parts car I had purchased. I had driven the parts car to my house and the turbo didn't work well and I adjusted the control arm and it work

So anyways, I installed that reman turbo and it worked well for a while. Now for some reason when ever it is hot it wont boost. During the winter I had zero problems but heat seams to cause issues.

"How could that be? You're crazy" and you're probably right.

Well I went through this forum and dutifully tested and or replaced all the usual suspects. I replaced the vacuum lines, the turbo had a new vacuum actuator and I've adjusted and readjusted that bad boy according to specifications listed on TDIclub.

I went ahead and put in a new N75 and even changed all the little plastic pieces that connect all the vacuum lines.

I have even built a leak tester as per TDIclub suggestions and tested all the piping for leaks. Nada, Nothing, zilch. No leaks and everything works.

I have a VCDS cable and tested the car. I can show you the data but the short version is that my specified boost and manifold pressures never will match my actual pressures when under full throttle.

Here is an excerpt in Excel format of specified and actual Airmass:

410 400
420 380
415 380
410 380
415 380
485 375
340 420
355 385
375 380
370 375
530 350
570 410
545 435
850 465
850 450
850 455
850 460
850 460
850 455
850 465
850 475
850 490
850 500
755 510
745 515
705 570
710 500
710 495

Here is my specified and actual Boost
1060.8 1030.2
1050.6 1050.6
1050.6 1030.2
1050.6 1030.2
1071 1030.2
1050.6 1030.2
1162.8 1030.2
999.6 1060.8
999.6 1020
999.6 1009.8
999.6 999.6
1193.4 999.6
1234.2 1060.8
1234.2 1101.6
1948.2 1142.4
1948.2 1162.8
1948.2 1173
1948.2 1183.2
1948.2 1193.4
1948.2 1213.8
1948.2 1244.4
1948.2 1285.2
1948.2 1326
1683 1356.6
1540.2 1397.4
1489.2 1417.8
1479 1346.4
1479 1285.2

Both of these are taken from an idle to heavy acceleration. I can show other variations of my driving prowess but its all about the same.


Long story short: Why does my turbo only seem to work when its cold? What am I missing? Help.....

I am doing everything in my power to avoid having to plunk down money on a turbo...but I think that may be what happens next......

Do your vanes always move freely? My vnt assy. Is worn, and vanes stick often, more so when its hot. Makes me wonder if its not also a map sensor causing the issue, and thr vanes are getting dirty because of it.

I don’t believe the vanes are sticking. It is an almost new turbo. But it is also a reman. Honestly I don’t know for certain but I have a suspicion that the turbo came from Hans auto. That infamous dude form Charlotte area that everyone loves so much

How would the MAP effect my boost? Since it’s seems to be reading when I plug in the vcds how do I determine if it is bad? Check continuity? Check resistance? On the actually probe?

I'm definitely no expert on reading logs..........

But your MAF numbers...... Something seems wrong there....

Have you considered your problem being the Mass Airflow Sensor?

I had "intermittent boost" with a bad VNT actuator but also I had the old style MAF. When i changed mine for the updated revision it made a night and day difference.

Unplug your MAF does it make any difference??

Here is a snarky comment, not meant to be real helpful. Nor am I being critical.

From reading here for years I thought everyone has a spare working MAF due to mis-diagnosis.

Looking at your requested and actual boost numbers it does not look like you are getting boost:
Request: Actual:
1948.2 1183.2
1948.2 1193.4
1948.2 1213.8
1948.2 1244.4
1948.2 1285.2
1948.2 1326
It's interesting the requested stayed the same in the time frame.

Have you tested the vacuum pump is producing at least 20 inches vacuum?
Is the vacuum getting to the top fitting on the N75 valve?

When you start the car does the actuator pull the arm on the turbo? It should.
Tee a vacuum gauge (or pump) into the line going to the actuator.
On mine I get about 10 inches vacuum (or so) and it drops when I take off.

BobnOH, I don't have a good spare MAF.
When I changed mine the car ran better so I assume I have a bad spare.

Well, in that case if you put the old one in and it doesn't run worse, then the new one may be bad too.

So the short answer is yes to all of these questions. Lol in answer to the previous post. I changed out the MAF and as of 6 months ago it was working great.

Anybody heard of a leaky turbo? Is that a thing to have an air leak at a turbo somehow? Kind of banging my head on the wall over this. Somebody tell me it’s something other than a bad turbo. As I mentioned before this turbo was a rebuilt one that came on an engine I pulled from car.

Final thought. Why does this problem go away in the winter? Car runs like a dream when it’s cold....

wonneber said:

Looking at your requested and actual boost numbers it does not look like you are getting boost:
Request: Actual:
1948.2 1183.2
1948.2 1193.4
1948.2 1213.8
1948.2 1244.4
1948.2 1285.2
1948.2 1326
It's interesting the requested stayed the same in the time frame.

Have you tested the vacuum pump is producing at least 20 inches vacuum?
Is the vacuum getting to the top fitting on the N75 valve?

When you start the car does the actuator pull the arm on the turbo? It should.
Tee a vacuum gauge (or pump) into the line going to the actuator.
On mine I get about 10 inches vacuum (or so) and it drops when I take off.

BobnOH, I don't have a good spare MAF.
When I changed mine the car ran better so I assume I have a bad spare.

Click to expand...

cfuzz987 said:

Anybody heard of a leaky turbo? Is that a thing to have an air leak at a turbo somehow?
Final thought. Why does this problem go away in the winter? Car runs like a dream when it’s cold....

Click to expand...

I don't recall anyone posting about a leaky turbo itself.
I found a crack in one of my boost tubes using soapy water.

I've read a few posts where the leak was in the core tubes of the intercooler but that was only a handful.

Makes no sense why the car is OK in the winter.

Better in winter? Okay, I am just throwing this one out there... As I am sure you know metal expands when heated and so does plastic in a lesser degree, so in theory something between the vacuum pump and the turbo is being affected by heat.
I just re-read this thread and the re a suggestion of possible heat causing this action. The N75 valve has a piston modulated by voltage and the piston could be hanging up in the bore, or misaligned with the seat when hot. Is is too warm out now for you to individually test the N75? If it is too hot out to just drive and check, start the engine, check the vacuum coming out of the N75 valve. Then pull the N75 out, put it in the freezer for an hour or so and check the vacuum coming out once again.

Just a thought...

It’s finally over

So I bit the bullet and bought a new Garrett vnt17 turbo. Magically all my problems are gone!!!

I popped open the old turbo for any evidence as to why it wouldn’t work and found the top of the exhaust was worn down. Additionally when I changed the oil there were some flakes (large flakes) of metal stuck to the magnet on my drain plug....

Did the install, cleaned all the plumbing up and she runs like a champ now!