Troubleshooting underfueling on '98 Jetta AHU

I've got an issue that I can't track down on my new-to-me MK3 AHU Jetta. It's had a ton of upgrades/maintenance done by the PO (also a TDIClub member), but on my drive home from his house, I started to lose power on the higher-end. It drives fine at partial throttle and at low RPM. Both the PO and I thought it was an exhaust leak, so I went home and dug into it. I tightened all the exhaust manifold nuts, verified the tubo nut/bolts, tightened EGR blockoff, tightened the intake and all boost pipes, etc... nothing leaking that I could see.

The actual issue was that injector #3 had come unseated from the head slightly, but I retorqued the yoke bolt to 20Nm and it's sealed again (I've got copper injector seal washers on order, and I'll probably change it the next time I'm doing work on the injection lines.) The PO did the nozzles a few months ago, and it must have backed out slightly. the yoke bolt was just more than finger tight.

However, I was running VCDS logs tonight, and here are the results: There's definitely a fuel problem.
The following graphs are of Measuring Block 015.
RPM in Red (0-5000 scale)
Actual injection quantity in green (0-40 scale)
Requested injection quantity in grey (0-80 scale)
VCDS Chart:

Excel chart of the beginning of that datalog chart, no split fuel scale:

So yeah... no wonder the car falls on it's face, the fuel can't keep up with demand. It's requesting 51mg/stroke, and maxes out at 23 right when you stomp the throttle, and slowly drops off. Leak, air in the fuel, clogged pickup, or a bad pump? The IP has a very slight leak, and I've got the seals, but I'm not keen on ripping it apart unless I have to.

Other stuff I've checked/verified:
Injector Balance is fine.

Injection Timing is still good.

MAF functional.

MAP/Boost sensor, and N75 frequency duty cycle.

First, Second, Third, paused, long third gear rip. I don't see anything hugely wrong here. It's showing 2260 mbar, minus 1000 for atmospheric, which is 18# in 3rd/4th gear.
Fuel filter was just changed before I bought it.
Tank is 1/2 full, same as when I bought it.
All vaccum lines were changed, boost piping is secure, and there aren't any boost leaks. It will hit 18# of boost at WOT in 4th, just doesn't have any balls.
Injection timing is good, and the IQ is low (extra fuel) at 1.8 mg/str. This thing should be leaving a haze with the fuel that high, but it's clean.
Upgrades/Specs:
Stock AHU, stock K03 turbo, 02A, 10mm pump, 205K miles
P150 intake, Race Pipe, no EGR
DLC1019 Nozzles, RC3 tune
Prior to the power loss, it'd spin the clutch in 5th. Now it basically drives like a stock MK3.

Thoughts?

I dunno but I don't see air causing the actual fuel to be lower than requested. There is no fuel "measurement". The IQ is derived from the QA position.

Air wouldn't cause the QA position to be low would it?

I don't know, honestly. I know other diesels can have IP external fuel leaks that turn into sucking air during high throttle.

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As for the coper washer, I have always had to reuse one when it leaks due to me fiddling with trying to prime. Just take a bit of 320 grip sand paper and sand the washer flat on both sides. Get a 10mm socket or one that works for you and put a bit of 320 on the end of it with some tape and put it down the hole and sand the surface a few times. Takes me about 5 times. Put it back together. This works very well as long as you don’t take any more than you have to off the washer.
Did the PO install nozzles himself? Where they pop tested? This could make a HUGE difference and might be one of your issues.
The leaky IP is not a power killer unless it’s gushing from the high pressure lines. Fix this later after you have this solved.
When the fuel filter replaced, was it done right? I suggest replacing it with a Nicktane or ditzle mod to get rid of air issues and better filtration if nothing else it working.
What is the IQ on the IP? Hammer mod it to 2.5 and see how that works.

Have you checked the boost line going into the ECU? You have to take it out and open it up.
This sounds like limp mode to me. Limp mode is a defuel mode map of the ecu when you don’t get the boost you need when the ecu expects it or a vac line or n75 valve is not working. Have you inspected your n75? Check your 109 relay and make sure its seated right and things are electrically ok with it with a volt meter. Look up keystone mod; I don’t think this is an issue for you but its one thing that can hamper fuel delivery. Is your shut off solenoid good? If it was failing this might cause a lack of fuel? What about the fuel lines? How do they fair?

Mongler98 said:

As for the coper washer, I have always had to reuse one when it leaks due to me fiddling with trying to prime. Just take a bit of 320 grip sand paper and sand the washer flat on both sides. Get a 10mm socket or one that works for you and put a bit of 320 on the end of it with some tape and put it down the hole and sand the surface a few times. Takes me about 5 times. Put it back together. This works very well as long as you don’t take any more than you have to off the washer.
Did the PO install nozzles himself? Where they pop tested? This could make a HUGE difference and might be one of your issues.
The leaky IP is not a power killer unless it’s gushing from the high pressure lines. Fix this later after you have this solved.
When the fuel filter replaced, was it done right? I suggest replacing it with a Nicktane or ditzle mod to get rid of air issues and better filtration if nothing else it working.
What is the IQ on the IP? Hammer mod it to 2.5 and see how that works.

Have you checked the boost line going into the ECU? You have to take it out and open it up.
This sounds like limp mode to me. Limp mode is a defuel mode map of the ecu when you don’t get the boost you need when the ecu expects it or a vac line or n75 valve is not working. Have you inspected your n75? Check your 109 relay and make sure its seated right and things are electrically ok with it with a volt meter. Look up keystone mod; I don’t think this is an issue for you but its one thing that can hamper fuel delivery. Is your shut off solenoid good? If it was failing this might cause a lack of fuel? What about the fuel lines? How do they fair?

Click to expand...

If I'm pulling the injector, I'll put in a new washer. 90 cents isn't gonna break me.

The PO installed them himself, and I don't think they were pop tested. However, it ran way better when I left his house, and they don't just spontaneously get weak, right?

He's been working on TDI's forever, so I'd wager that the filter was installed correctly. Again, it ran way better a week ago, and I can't see a way that the filter would cause this unless it got blocked up with crud from the tank. It's possible that the cold weather has something to do with it, but it's only dropped about 10° from when I bought it.

The IQ is low at 1.8. The QA has been hammer modded previously. Again, nothing changed in that regard from when I bought it, and everything is tight on the IP.

I'll check the vac line going to the ECU, but I'm pretty sure he replaced it. If not, I've got a pile of McMaster vac line in the right colors to replace ALL the lines in the car. I've vacuum-checked all the lines in the car aside from the ECU line, and they all hold vac just fine.

Relay 109 is fine, there's no fluctuations in any voltage readings anywhere in the car. I'll swap it with a new unit that I've got in the glovebox, but I don't see that doing much.

Keystone mod? I'm not spinning 6000RPM, the car falls on it's face at 2500RPM. No thanks, even RocketChip has said this is unhelpful and they've tuned it out in the RC6 software.

I haven't checked the fuel shutoff solenoid, but that's worth looking at.

Fuel lines are old, but look decent. Some cracking on the rubber in places, but no worse than 80% of other MK3's I see. No fuel leaks from any of them in the engine bay.

I was under the impression that limp mode limited the boost to zero or near zero, not cutting fuel back. Is this incorrect? I'm making full boost pressure, but the fuel isn't being delivered.

:EDIT: There are no codes when scanned with VCDS, forgot to mention that. If the ECU vac line is bad, doesn't it throw a code?

I agree, if you have it on hand lol. It’s not going to hurt to reuse one in a pinch. It’s about results. I had a leak develop at the autocross event I was at and fixed it right there. I bring tools!
So from what I have read, if they are installed improperly or with ANY dirt what so ever, I mean even from a fingerprint dust spec, you can trash the nozzle. I would not count them as being good. You must pop test them if you want reliability and performance.
Maybe you have a gel issue. Put a bottle of diesel heat into the tank and see what that does. I was talking last year with a manager at a gas station (we do the fireworks display for 4th of July together) He said that the diesel can’t sit for too long. There are some bacteria that loves low sulfur diesel and can clog up filters on big trucks that sit to long. He said that the fuel has about 6 month shelf life before the bacteria make an issue.

Limp mode is a map of the ecu that limits fuel. Limp mode protect the engine from damage if too much boost or not enough. How could it control the boost if it was the boot that has issues, IE failed boost line, n75, waste gate etc...? You defuel the engine so there is not enough to spool up the turbo.

I have a post on another page about running an external or lift pump. I ran a wire to the brake switch. Next time you’re driving, keep your foot on the gas, and then put your left on the brake and press, within 1/4 a second the ECU defuels the IP. It’s to help with runaway or is a safety feature. I tapped into that circuit with my Auber relay so when the EGT gets too hot it defuels the IP so I don’t have to watch the EGT gauge.

Maybe it’s an issue with the wiring to the IP? Maybe an issue in the controller? I have no clue how to diagnose that sort of thing. Past this I have no further information I could think of. This sounds like an issue with the IP or the ECU. I seriously don’t think that the nozzles are the issue here but I don’t think they were done right and would fix that ASAP if it was me.

You have made a fantastic post here, for a noob; you know what you’re doing. Glad to see so much information presented.

My hunch is that the nozzles were done improperly and are damaged. I hope not, for your sake anyways. You need to send them out to Kerma and have them pop tested and or redone. Your peace of mind over $0.90 washer vs a $100 bill from Kerma is worth it IMHO. You can burn a hole right though the piston if they go bad from debris in the plunger.

I'm only a "noob" here on TDIClub, note the reg date. I've spent a LOT of time reading, just never had anything to say. I've been working on VW's since '02, including a couple of TDI's, just never any that were mine or needed anything more than "standard" diagnostics. My mom had an '01 TDI Beetle that she loved, and I did a lot of the maintenance/repair work using info from TDIClub. She bought it used with 127K, and sold it still running at 205K.

I've spent a lot of time working on diesels, from 1-cylinder Hatz 1B27's in generators to Volvo TAMD41B's, CAT 3406C/E, C18, and C32 motors in boats. Heck, I've worked on Detroits from a 1-71 (look it up) all the way to 12v149's and 16v92's. I know all about bacteria/algae in fuel and filters, gelling, etc. I'm not looking to cheap out at all, but if there's a mysterious issue, I don't like throwing money at it hoping that the fix will just appear. This issue really stumped me and my standard troubleshooting kept uncovering little things that made me think I was headed the right direction.

Thanks for the encouragement, this is why I posted here.

In response to your list, I pulled the cowl cover and checked the ECU boost line with a vac pump and gauge. It's fine, and was very recently replaced. I checked the wastegate operation with a 7PSI air line, and it's fine. I triple-checked the boost line from the turbo outlet, it's new and fine.
I thought for a bit, then had an inkling...

I figured out the problem, based on a hunch. I realized that when I test drove it before buying it, it was about 3/4 full of fuel. After I bought it, I drove very reasonably for about half an hour before I filled the tank for the rest of the trip. When I noticed the loss of power, it was nearly 1.5 hours later, and the tank was around 1/2 or slightly above. Plus, the "exhaust leak" was worse. Putting 2 and 2 together and not getting 6... I went out and filled the tank with fresh diesel, and it was like a completely new car. I'm 99% sure that I've got a cracked pickup or a cracked in-tank fuel line. The car hauls @$$ and leaks a haze behind it, plus overboosts to 20# before quickly dropping back to 18 like it's supposed to... just like when I bought it. I lit the tires up leaving the fuel station from 1st through 3rd...

It's tough when there are 2 issues and one (the exhaust leak that turned out to be the injector not sealed) clouds your mind's ability to see the second.
I'm going to get the car over to Chris at Schnell to have the injectors pop tested and checked out. I seriously hope there was no damage done, but I can't expect to hand him a car with a problem that he has no background on, and expect that he'll be a magician. My time is effectively free, his is not.
Thanks again for the effort in helping.

AWESOME DUDE.
I knew you were not a noob but whenever I see that on the profile I always assume some 16yo just got an mk3 and wants to roll coal! It was evident from your level of knowledge and your VCDS info that you were not unfamiliar with TDI's. I’m glad it works now. Reminds me from sling blade "It ain't got no gas in it!" How the simplest things skip past us when you are confronted with other more complex issues.
Injectors are probably fine but a pop test will help out a good amount.

Lift pumps have issues too. If the lift pump is the problem, may I suggest you upgrade to a Walbro 12V, 8-11 psi, 45 gal/hr fuel pump. It will not restrict your flow if it fails and is one beefy pump to boot. It does need to be in the upright position so it takes up more room than most but it fits under the hood easily. Add in 12 row oil cooler and you’re in business to gain a tad bit more power! I updated my lines to Stainless braided 1/2" from the tank to the filter (nictane) and -6AN on everything else with high quality lines from Earls. If you up to an 11m pump you can have the inlet drilled and weld a -6an fitting to it. I’m getting mad power and you don’t need all this but the Walbro is 10/10 and solves the issues with the lift pump failures.

Thanks.

I kept looking for air in the lines, but because it's only sucking air at high throttle, the bubbles had passed by the time I could pull over and check.

I ran a Walbro 255lph in-tank in my EJ1 turbo build. It handled 225WHP no problem. I've also run the Bosch 044 pumps similar to the ones from the old CIS Rabbits on higher power builds.

Until I start reaching for more power, I'll likely stick to the stock pump, but if things go the way I'd like... a Walbro may in the cards.