Does the ALH have open loop/closed loop operation?

I am experiencing a hesitation issue with my ALH. It has a slight stumble under medium load. It seems like it goes away when you mat the throttle. It pulls completely normally when using full throttle. It kind of acts like a gas car having an issue with a sensor that is ignored during 'open loop' operation and shows up during 'closed loop' operation.

I'm wondering if my MAF is failing, but its readings are ignored when full throttle is requested?

The MAF readings from my scan gauge seem to be decent, but its not a great tool to use to diagnose.

No codes yet.

There is no combustion related feedback, if that is what you mean.

Are you certain it is in fact an engine power delivery issue, or a driveline vibration?

When does it occur? Road speed, engine speed, load... etc. Hot or cold?

There really is not much on the ALH that can cause bad driveability, which is why they are such a great package. Easy to make run perfect.

Its definitely an engine output issue. Boost drops slightly when it happens. This is the first time I have ever had an issue like this on this car (aside from a couple frozen fuel filters), it usually runs perfectly.

What I meant about open loop/closed loop was not worded correctly. I know there is combustion feedback. I am wondering if there is a load point where the MAF is ignored and the engine runs off the MAP instead. It seems like I get to some input level and the issue goes away.

It can be felt when climbing a steady grade around 2000-3000 rpms in higher gears. Not evident when accelerating or cruising on the flats.

Happens hot or cold, maybe slightly worse when air temps are cooler outside, but the issue could just be getting worse as fall sets in.

There is air in the fuel lines, but there always has been.

It starts instantly, and seems to run hard at 100% throttle with no hickups.

You could have an injector that is maybe not quite right, and has a less than ideal spray pattern in those mid range areas. This is always a difficult thing to really diagnose on the car, but sending the injectors to a qualified individual for evaluation on proper equipment could reveal some anomaly that could cause your issue.

Could be the pump, too, but it would be unwise to pursue anything there until the injectors were ruled out.

If the car has been modified software wise, you may ask your tuner what input the MAF is used for, as most tuners neuter the EGR, and the MAF is largely there to control that. One of my ALHs has a MAF delete tune, so it is not used anyway. Fuel and boost totally controlled by RPM and the position of my right foot.

I have a malone stage 3 on the car with EGR delete. Injectors have at least 100k on them, maybe much more. I have a spare set of OEM injectors mounted on bodies, I will swap those in as a test. Good idea.
I have noticed a small bit more smoke than normal at WOT and sometimes an idle that is not perfect, although this was intermittent.

I am not familiar first hand with that tune, but was the EGR delete done along with a MAF delete, or are they still using the MAF? It would be easy to watch the scan data (MAF deletes never change value, usually just "stuck" at 450 or 500 or whatever).

Is the EGR still physically on the engine? Sometimes they can rust from lack of use and cause some goofy problems.

EGR is physically gone. The MAF gives a readout in lb/min that makes sense and changes using a generic OBD scanner. My VCDS laptop currently died, so i havent been able to really look at it with a good program.

Injection pump issue. Maybe a fuel temp sensor?

i assume fuel temp could be monitored with VCDS?

In Group 000 and hit Go, fuel temp is easiest to find when using the timing graph.

I'm having the same issues with my "new" 98 beetle, mine happens around 1,800-2,200 rpms.
One trick I found that resolved it for me when it pops up is I run a little higher rpm for a few minutes and it never comes back for that trip, it only happens once in a while and it just hesitates a little, nothing too drastic but it's noticeable.
One thing a buddy mentioned when going over it with Vagcom right after purchase is my injectors flow was turned up so he put them back to stock, not knowing what the previous owner has put in this yet I'm wondering if what my buddy did is causing this.

I still have no solved this. Have not had much time. I did check the injector balance, timing and IQ with VCDS. it all looks good to me. The MAF actual vs requested looked a little suspect. I did run a can of diesel purge through it, but that did nothing. I just bit the bullet and ordered a new MAF. The on in there has at least 150k on it, probably much more.

Issue solved:

I pulled the fuel sender and the pickup had a bunch of crap in it. It had been gutted at some point, but it looked like there was a bit of plastic in there from the check valve being drilled out. The plastic looks like it collected some other crap over the years.

On my drive home yesterday, I really beat on the car and discovered that it would cut out extremely hard at high rpm and make whitish-bluish smoke, then come back online and smoke black for a second. This is reminischent of other times I have had fuel starvation issues with this car and other diesel engines. I was very happy to find a clot of crap in the pickup.

Double Update:
The issue was not solved after the above post. The hard cut was solved, but the minor hesitation continued like it has been.
I resigned to the fact that things would have to get worse before I could pinpoint the issue.
As the weather got cold, I have had fuel flow issues that have really given me a lot of headaches trying to get home over the mountain at night. Last week my car got to the point where it would only idle on a cold night. Temporarily bypassing the fuel filter on the side of the road did nothing. I opened up the fuel sender access hatch and pulled the line off, lots of fuel flow from there.
BUT.....someone has added an inline check valve in the supply off the tank (presumably when they gutted the check valve in the sender). I have suspected this valve for a while. I put my warm hand on it and the car started and got me home, while holding the check valve in hand.
I took the check valve out and opened it up. Full of dog hair and a few scraps of white plastic. It wasnt working as a check valve at all. I bypassed it and its been running great since. No more cold fuel flow issues, smoother acceleration than it ever has been since Ive owned it (100k) miles.

Yup, these things run better if they get their diesel unrestricted.
Glad you figured it out...thanks for posting the solution.

A case where a big issue turned out to be rectifiable with little/no money. Thanks for following up. Drive on!

It was below zero this morning, and no issues at all. Sweet.