AHU m tdi low boost

I have an uncontrolled VNT15 on a Mk1 Caddy swapped AHU mtdi and I am seeing about 7psi on the boost gauge. Previously I saw 14psi but I decided to tinker with it and I adjusted the rod to get 18psi. At least once I turned it too far and got 20psi but dialed it in to 18psi. Then, it seemed to be mis adjusted the next day and only had 7psi, and has been that way since regardless of rod length.


Diagnostics:
Pulled vacuum on gauge and reads correct with no leaks. Reality check: engine is low on power. There would normally be turbo lag, then suddenly acceleration, but now it's just, are we going yet?
Reset vnt rod to 3-5inHg.

Pulled all boost and intake hoses and checked for holes and cracks.
Pressure tested the intercooler and hoses together to 15psi and corrected a very minor leak.
Replaced turbo with known working unit (ran it on an etdi swap).
Pulled the glow plugs with a hot engine and found #4 to be wet. Pulled them again today after cruising around and none were wet.
Compression tested with GPs out: 500, 400, 500, 450. I want to check #2 again since the gauge relief valve was leaking badly during 1, 2, and 3 but it's probably true.
EGR was blocked off, tonight removed egr and blocked hole to intake manifold, no change.
Checked cam and crank timing, correct. WIll probably adjust the pump both ways tomorrow to see if ti changes anything.

I also changed out the main seal, crank sprocket that was wallowed, and bolt that was super loose and looked to be reused, but I forget if I messed with the boost or changed the sprocket first.

Can leaky injectors or failing IP cause low boost?
I'm hoping someone stuck a banana up my tailpipe (can't see any foul play) or there's a rag in my intercooler (which I will check tomorrow). Possible cause?
What kind(s) of internal damage can cause this? Cylinder head cracks or bad valve seats?

Please can you just clarify something for me. Are you using Vacuum to control the VNT
turbo on your mTdi, or have you swapped the waste gate controller to a pressure one ?

I bought the car without anything hooked up to the VNT control, and I haven't added anything, it just sits there with the vanes stuck in one position.

So I peeked inside the intercooler and didn't see any obstructions. I was also thinking about putting on an alh intake manifold (which I have) and race pipe (that I'd have to order). I guess I have a spare EGR that I could slap on, can I leave the lower port alone or do I need to block it somehow? I'd think that no vacuum means it is closed.

Also installed translucent fuel feed line, only one small bubble that doesn't move, and new fuel filter filled with LM Diesel Purge, no affect.

I put on the ALH manifold with a new gasket, an ALH EGR/ASV with a new gasket, and rerouted the intercooler hoses. Same measurement of 7psi on the gauge. I think I will try a VDO boost gauge that I just got for my other car, have to find the right fittings though. I was trying to feel the lag/boost and now I'm starting to doubt the boost ever went away, and that the gauge may just be faulty. if I could jam the boost gauge tube up a tire valve stem then I could use the compressor to see if the gauge matches the compressor.

If the gauge checks out I will drive it to the shop next week and try the leak down tester I just got. Either that or injector swap with the other car. Then, it's time to convert it to etdi using ALH sensors and ECU.

The other gauge shows 9 psi tapering off to 7 at redline. I made the rod shorter by one turn and no change on the boost gauge. So not much progress. I think I'll try another run after disconnecting the downpipe. Also I could easily put a new gasket on the EGR delete and downpipe. Not as easily I could put new gaskets on the exhaust manifold.

Leakdown test showed 1%, 5%, and 4%. I couldn't get the fitting in the #4 glow plug because of the water outlet. I'll have to grind down the fitting or find a narrow one. Or make one out of a bad glow plug.

If the vanes are stuck only in one position then that is the reason for your low boost problems, the vanes are supposed to move to increase/decrease the boost pressure, if you're only getting between 7-9psi of boost I would assume the vanes are more fairly open. That is not an ideal way to use that turbo the vanes need some sort of control mechanism. I run an older 1.6td passat with a vnt turbo controlled by a pressure can and it will happily boost 1.5 bar all day every day, you need to free off the vanes and decide how you want to control them, going back to an ECU controlled engine is the safest option, although I have seen a stand alone controller for VNT turbos. Your choice.

This is how I understand the VNT15 to work: If you decrease the length of the rod on the vacuum actuator, it's as if the computer has pulled the rod, which happens as you step on it, opening the vanes. So the maximum pressure the turbo can get to, will go up when the rod is pulled, no? As well as reduce lag.

However, if the rod is increased in length (or the rod moves out), the max pressure goes down and the vanes close, which the computer does when boost is built up or when cruising/idling.

myturbodieseldotcom explains it as I did in this video :https://www.youtube.com/watch?v=o1Hvt2b7PL0

So if this is correct, I should be able to decrease the length of the rod until I get to the max pressure I desire.

At the normal settings of 'starts to move at 3"-5" Hg' I've previously seen that at WOT from 3-4k RPM the uncontrolled VNT15 will push about 14psi.

I then decreased the length of the rod to try and get to 18psi, but even with a small movement of the rod it went over to about 22psi. So, I lengthened the rod to get it back to 18psi.

Then after a few days it is down to 9psi without having touched the rod.
Now when I decrease the length of the rod the max boost pressure does not change, so there is something else wrong that is limiting. Not a charge pipe leak, maybe an exhaust leak or plugged exhaust, maybe clogged oil feed line, etc etc.

How many miles are on this turbo?

Previous turbo that worked and then suddenly low boost: unknown miles.
Current VNT15 that worked fine on my ALH etdi swap and now on the m-tdi, low boost: 203k
So I really don't think it's the turbo, since two have the same symptoms.

I took it out of the other car to swap with a junkyard GT1749V, which I read on here that the only difference is the 46 vs 49 dimension, and also more importantly more robust.

Last night took off the oil feed line and sprayed clear with brake cleaner. Have to remember to prime it tomorrow (as well as put new copper seals on).
Took off the EGR delete plate and didn't see soot past the seal, so probably wasn't leaking, at least not enough to cause a problem. Put a new gasket on it anyway, haven't driven it yet.

VNT turbos with a vacuum can open the vanes when you take away the vacuum & close the vanes when there is vacuum, if you are making the rod longer or shorter you are changing when the boost occurs, either earlier or later. And looking at your 1st post i was under the impression you were running an mTdi ?

CasaEd said:

VNT turbos with a vacuum can open the vanes when you take away the vacuum & close the vanes when there is vacuum, if you are making the rod longer or shorter you are changing when the boost occurs, either earlier or later. And looking at your 1st post i was under the impression you were running an mTdi ?

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Changing when boost occurs, and maximum pressure, yes?
Yes an m-tdi. See first post first sentence. It's a VNT15 with no vacuum hose on the rod actuator. To clarify about me changing the rod length, I change it when the car isn't running, then take it for a spin to look for changes. The rod/vanes just sit there at that same position I left it when I go for a drive. There's nothing at all actuating the vanes when driving.

1749v's got a real slender shaft on them, so they're pretty easy to blow apart, so be careful

what is EGTs?
fuel supply to pump?

no fuel, no egt, no boost
too much vacuum on pump inlet, no timing advance, no power

[486] said:

1749v's got a real slender shaft on them, so they're pretty easy to blow apart, so be careful

what is EGTs?
fuel supply to pump?

no fuel, no egt, no boost
too much vacuum on pump inlet, no timing advance, no power

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Ok thanks, another idea, a problem with fueling.
I don't have an EGT gauge.
Fuel supply to pump is non assisted.

I have an unmarked injection pump. Could be going out, has fuel leaks but very slow, after a few weeks pump will get wet in some areas. Truck has a puff of white smoke on start so may have leaking injectors. Have had these conditions before and after current problem.

My next idea with fueling troubleshooting is to swap out injectors. I have a few sets from manual Mk4s. The idea is to run them in the sonic cleaner/simple green, drop them in the etdi and see what happens, if good put in mtdi. Previous owner said that the previous owner put bigger nozzles in. That's all he knows. I guess I could take them out and inspect the nozzles, never occurred to me that they might have part #s on them.

Until you sort out the control mechanism for that vnt turbo you're never going to get a decent level of power, all you are achieving is to move the power to a different part of the rev range, if you close the vanes too much the boost will be too high and if you open them too much you will have low boost.

CasaEd said:

Until you sort out the control mechanism for that vnt turbo you're never going to get a decent level of power, all you are achieving is to move the power to a different part of the rev range, if you close the vanes too much the boost will be too high and if you open them too much you will have low boost.

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This, a thousand times this.

The vanes on an VNT turbo are not just an advanced wastegate to keep the turbo from overboosting... they are integral to how the turbo functions and develops boost in the first place.

The good news is that there are (fairly) easy mechanical systems you can use to drive the actuator in an mTDI setup... head on over to vwdiesel.net and you'll find a few examples of cable-driven systems that cue off the throttle lever position. Look for threads by member "libbydiesel"... his design specifically is generally thought to be one of the better ones.

Another option would be to fit a traditional waste-gate turbo.

CasaEd said:

Until you sort out the control mechanism for that vnt turbo you're never going to get a decent level of power, all you are achieving is to move the power to a different part of the rev range, if you close the vanes too much the boost will be too high and if you open them too much you will have low boost.

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ehhh
near full open, turbine efficiency does go down a little, but anything less will be just the same as any fixed geometry turbine housing, just that you can adjust what the effective volute size is

you are limited in the fuel you can put to it though as it is a little baby thing, but by no means is it improper to use a VNT in a fixed position

I would certainly scab on a wastegate however, even if you've gotta put it on the little EGR port.

Near full open turbo efficiency drops more than just a little bit, think about how the ECU on a Tdi controls the boost pressure and at low rpm's with vanes near full open the power would be very low. A fixed geometry turbo works by bypassing the exhaust turbine so is not like a VNT in a fixed postion. How are you going to control the boost if you throw a lot of fuel in it with bigger nozzles and the turbo spools up to an over-boost situation ? If you were to route the egr port to by-pass the turbo you're back to square one, low boost, So as previously stated, control the vanes either mechanically, with vacuum or pressure can.

CasaEd said:

A fixed geometry turbo works by bypassing the exhaust turbine so is not like a VNT in a fixed postion. How are you going to control the boost if you throw a lot of fuel in it with bigger nozzles and the turbo spools up to an over-boost situation ? If you were to route the egr port to by-pass the turbo you're back to square one, low boost, So as previously stated, control the vanes either mechanically, with vacuum or pressure can.

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turbos without wastegates have been a very popular option in the MDT/HDT/tractor/stationary world for a VERY long time

diesels are neat in that you can choose a turbo that'll move enough air for the power you want, then it'll self-regulate without any sort of control at all
add more load/fuel, EGTs go up, it makes more boost and levels off the EGT rise (within reason)

the thing is that the gt1749V on an ALH is sized for like 60hp when treated like that
it gets by with the 90hp rating by kneecapping the turbine efficiency with over-opened vanes

ETA: and I was saying to add an external wastegate on the EGR port, not just an open pipe or whatever you seem to think that I was suggesting

I knew what you meant by waste gating the EGR pipe, I've had my say on the subject, if the OP wants to struggle trying to make a good set up that's up to him. MDT/HDT Tractors and stationary engines are another subject.

What I'm trying to do is restore the 14psi I was previously seeing. I set the rod back to where it was at that time (start moving at 3-5inhg) and I'm still getting 9psi.

The goal of originally moving the rod was to try and get a little more performance out of it. I have looked at some of Libby's posts but with so many pictures down its not possible to copy his design. I do see posts about an Arduino but haven't looked enough to see if there is adequate info for me to tackle that project. Then I have an AHU harness and ALH harness and I think actually all of the electronics required (however from the junkyard). Actually I need a mk4 80 pin ecu.

Sent from my SM-G900V using Tapatalk

Forget the 80 pin ECU, go for the 121 pin ECU so you can Flash Over OBD, A bit more work but worth the effort

I opened the LDA today to take a look, hadn't opened one before but I expected the boost pin to have a different taper around the circumference when this one appears to be symmetric:


I'm not sure what the star wheel does? It doesn't appear to be connected to anything but a metal tab that it ratchets on, on the left.

There's also a little hole in the aluminum LDA housing, below the diaphragm, that leads to the outside. The hole is in the middle of a fitting that's screwed into the side of the housing, not sure what's inside the fitting.

I did screw down the adjuster on the top outside of the lid, looks like it pushes the diaphragm down and adjusts where it the boost pin starts before pressure is applied by boost but it drives no different.

The star wheel can be moved up and down, up increases the pressure on the spring and down decreases it. If you're looking for more fuel then you need to refit the pin with the steepest slope facing towards the front of the pump. The hole you describe is just a vent . When you adjust the top screw it pushes the pin further in wich increases fuel too, also if you remove the plastic shim under diaphram that also allows the pin to go down further. Be warned though, too much fuel will push the EGT's up.

CasaEd said:

The star wheel can be moved up and down, up increases the pressure on the spring and down decreases it. If you're looking for more fuel then you need to refit the pin with the steepest slope facing towards the front of the pump. The hole you describe is just a vent . When you adjust the top screw it pushes the pin further in wich increases fuel too, also if you remove the plastic shim under diaphram that also allows the pin to go down further. Be warned though, too much fuel will push the EGT's up.

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Thanks for the advice. Regarding EGT, I had a probe on my 1.5L idi, with just the load screwed turned in all of the way which was an additional 1/2 turn from factory, I could make a James Bond smokescreen in a split second, and get EGTs to 1300F in just a few seconds. Now with an ALH and light mods (OMI, 2.5" exhaust, FMIC, slightly advanced timing, IQ=2.0) in a <2000lb car, I have to floor it for about 15 or 20 seconds up a steep grade to get to 1300F. I feel like that temperature is conservative though after reading about how high some people here push it.

Right now the mtdi does not smoke at all, so I'm being pretty brave without an EGT gauge, and I just won't push it too hard up the steep hills here.

Also, I figured out the boost; I finally got the bright idea to hook up a vacuum hand pump to the turbo and pull vacuum while driving. The rod was adjusted so long that it was beyond my comfort zone to shorten it. It took 10inHg to get it to 15psi while floored, and now have adjusted the rod to be there all of the time. Now I have a much less concerning problem; the intercooler pipes are blowing off.

Here's a good resource for adjusting/tuning our pumps.
https://www.landroverweb.com/Pdf-files/Tuning_Land_Rover_Tdi_engines_Rev_2.pdf

Combine that pdf with this link http://dodgeram.org/tech/dsl/more_power/Power_ve.htm and you can get a pretty good idea of tuning a ve pump

So I was wrong. When I hot pipe the truck it boosts to whatever I set the control rod to. A quarter turn either way gives a massive change in pressure and in what RPM range.

When the intercooler is in the loop the gauge reads 9psi max regardless of rod position. I took the intercooler out, capped it, pumped it up to 25 psi and submerged it in water to find no leaks.

The gauge is reading at the intake manifold. Maybe it's just pressure drop? But this setup was working before. I might put a nipple in the hot side piping with the IC installled to measure the pressure drop. But it also feels slower with the IC installed. And i can hear the turbo spool much clearer.
Maybe there's a leak in the hoses that connect to the intercooler? I included those in the IC pressure test. And I replaced them with brand new as well.
Maybe the intake manifold is bad? I took it off of my E-ALH swap so it is known good, with a new gasket.
A new thought: maybe while WOT, the engine is rotating enough due to torque, that it is yanking some of the hoses apart at the connections and causing a leak? When it is hot piped the hoses are all floating around in the engine bay and not mounted to anything, so they wouldn't be pulled on by the motor. I put all new engine mounts on while filling all of the gaps with window weld. The same setup holds on my eALH swap.
What is it about installing this IC that is causing the problem?

Make a coupler to put on the turbo inlet so the entire intake system can be tested while it is installed? I made my own adapter with a rubber coupler, pvc end cap, and a tire air valve. This covers the general process https://turboboostleaktesters.com/how-to-boost-leak-test/

glenn1179 said:

Make a coupler to put on the turbo inlet so the entire intake system can be tested while it is installed? I made my own adapter with a rubber coupler, pvc end cap, and a tire air valve. This covers the general process https://turboboostleaktesters.com/how-to-boost-leak-test/

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I did the same from the turbo outlet hose to the intake manifold inlet hose, had minor leaks but tightening the clamps solved that, but adding the intake is an idea I hadn't though about.

But, I did put in another nipple right after the turbo and ran two gauges, to see if it's just pressure drop. Turns out the cheap gauge is failing and reading low, sometimes zero. The VDO gauge I just got reads the same on either side of the intercooler. So I started at 9psi and tweaked the rod to get 14psi, yay! But then it went down again? Adjusted the rod, it's back, next run, gone, was the vacuum actuator getting hot and changing position or something? No, the lock nut was coming loose because I was hand tightening it and I guess the engine vibes and the spring pressure of the vacuum can were making the rod shorter as it drove. I think. That was last night, so tonight we will see if it's still sitting pretty at 14psi (after tightening the lock nut).