Is it time for an AHU engine exchange in my car?

I've got an AHU engine with about 180,000 on it I believe that became hard to start, smokes white (the type that burns the eyes and stinks of diesel) which dissipates when warm, and has a tick, that sounded loudest near the number 3 cylinder at the valve cover from what I can hear. The noise started 900 miles from home while trying to wrap up a 5000 mile road trip. I chose to drive it home, it made it but it isn't a happy engine. The power seemed ok, but the ticking never went away and when I got it back home it wouldn't start in about mid thirty degree weather easily without the block heater. The glow plugs are functional, but it would still require cranking and result in a very smokey start.

I have another AHU that has about 235,000 sitting in storage. I bought that car as a non-runner. I got it running by fixing the jammed fuel shutoff solenoid. The timing is erratic, I believe because the crankshaft gear has worn from the PO loosening the crank bolt trying to change the timing belt and troubleshoot the no start issue. I didn't have an extra bolt and gear on hand when I pulled the running engine and scrapped the rotted body. So, I'm looking for opinions, should I swap this engine in and put my smokey hard starting engine on the stand, or troubleshoot it further?



Here's what I've done so far.

1 can Diesel Purge fed directly. No change.

Loosened injector nut at each injector. They all caused the engine to stumble even more, but the ticking noise always persisted. The ticking I can hear at idle, I can't tell if it goes away at higher RPM, or if the engine noise just mask it.

Checked the timing in VCDS. It is slightly advanced, which is where I set it when I did the timing belt less then 8000 miles ago. (This vehicle is relatively new to me.)

I checked group 13 for injector balance. 2/3 numbers were always at 2.99 and the other was about 1.5 most of the time. Not good. I swapped injector 1 and 4 to see if the 2.99 would follow the injector, and I believe it did. Sometimes the numbers would change drastically, but then the engine would idle rough and tick even louder, so it was hard to tell.

Another reason I swapped 1 and 4 injectors is because there was a misfire DTC for number 4. After I swapped injectors, I was unable to reproduce that problem, but haven't had it running for very long and no road driving. I was beginning to learn that it wasn't just a nailing injector like I thought it might have been when I first heard it.

I swapped the number 3 injector with a backup that was pulled from a running engine about 10 years ago. The group 13 numbers and engine ticking persisted.

I scrolled through the other groups in VCDS, but nothing caught my eye. I'm no expert though.

Next I got my compression tester out. By this point I didn't want to run it very much, so I tested on a cold engine in about 40 degree weather.Oiled means I added a couple drops of oil to the cylinder to see how it affects the numbers. I did 2 cylinders to learn how it affected the low cylinder and a "good" cylinder. (I know all my numbers are low.)

I do have a home made adapter which doesn't take up the volume of the glow plug in the cylinder. To get a feel for how much that affected things, I tested on a AHU that runs ok, but is probably pretty worn out too with about 400,000 on it. I only tested 1 cylinder there because the glow plug harness was crunchy and this one is my current daily driver. I also tested it cold and it had a compression of 210 in about 45 degree weather.



I thought I should treat myself and bought a Harbor Freight kit for $25 after discount to get the proper adapter. The adapter has a different size connector then my gauge. I used the Harbor Freight gauge on cylinder 1 and it got to 210 on my smokey hard starting AHU, released the pressure and the gauge won't hold pressure anymore. Usually it is the schraeder valve that leaks, but I couldn't figure out how to tighten this one. I'll just return it and trust the numbers I have unless someone has a better idea.

I don't have a leak-down tester, so I did the poor man's version of it. I aligned the cam lobes on the number 3 cylinder upwards and used my compressor to blow air into the number 3 cylinder. I believe I heard air via the dipstick tube and the crankcase vent tube that goes up to the hockey puck on the valve cover. I don't think I heard it out the intake. First time I've done this process so I may not have it right.

I didn't pull the cam, but I did inspect the cam followers and cam. They did look fine to my untrained eye. I also made sure that the followers for cylinder 3 rotated via my fingers in the bore. I pressed down on them with my fingers and they didn't wiggle, but I didn't get a piece of wood to be able to apply more force.

I also didn't notice any damage on any glow plugs or injectors I pulled.

Thanks for reading this novel of a post. If you got this far, please share your opinion on what to do next or what additional information I should try to gather. (I'll hold off sharing on what I think I would do for a bit so I don't taint people's opinions.)

I started it tonight to move it into position for long term work. I decided I might as well get it warm and check the compression. While I was in there I checked the timing and group 13 numbers again. Timing was still good. Group 13 looked a little better too. Not sure why. It was about 60 here today.





Now the real shocker. The compression numbers while the engine was warm using my compression tester and adapter.

1 -255
2 -260
3 -260
4 -250

Why is number 3 magically better when the engine is warm?

I did check them twice. I'm going to let the engine cool and check again tomorrow evening.

Your compression seems awfully low, like 50% low.

A few things,
AHU's have a #3 piston fenomina, least restricted path for ait on the intake and thus the one that gets the most abuse, bearings, rings, all get more wear on them over time. 1 and 4 get the least.
Now the CRAZY part is that your compresison numbers are WAY WAY LOW.

https://www.myturbodiesel.com/wiki/how-to-do-compression-test-on-vw-audi-tdi-engine-results/
VW compression for all 1.9L diesel non pumpe duse (ALH AHU,1Z) and pumpe duse engines (BEW)
New: 363-450 psi
Minimum limit: 276 psi
Max difference between cylinders: 73 psi

Why on earth are they that low for an engine with that few miles? Are you not keeping up on oil changes or something?
To me those compresison numbers means the engine is garbage. Maybe im wrong but i have never seen any that low and across the board too. Maybe your head is jacked up

You should do a leak down test. See if you get any pressure air flow out of the exhaust or intake, you should get some though the CCV and with those numbers for sure but not as much as a leaking valves.

I was hoping someone would tell me that since my compression tester doesn't fill the volume that the glow plug does, that would be the reason the numbers were lower on all cylinders. That is why I included the photo on my first post and did the test on one cylinder in likely worn out, but ok starting engine. Perhaps that was wishful thinking.



I checked cylinder 3 again this morning in about 45 degree temps on a cold engine.



3 - 120 psi


I don't have a leak-down tester. I suppose I could take the air cleaner off, to get better audio, and try my poor mans version on all the cylinders. I only did number 3 before. (Or maybe get the leak-down tester from Harbor Freight.)

Most of the service history of this engine is unknown. I think the PO only had it about 30,000 miles I believe and I'm at less then 8000 I believe. The PO did keep records of parts purchases but I don't have oil change records. I know the PO had 2 oil starvation events because the intermediate shaft pulley came loose. I had one oil starvation event because the vacuum pump retaining bolt came loose.

One interesting note. When I was warming the engine, I was running it a while with the crank case ventilation was disconnected from the hockey puck on the valve cover. (What a mess that made.) After I reconnected it the ticking noise became more pronounced out the air intake.


I was thinking about pulling the oil pan to see if the cylinder bores are scored. To me that is easier and less costly then pulling the head. Is that worthwhile, or is the view not good enough because of the obstruction from the crank?

A: leak down test is supper easy and you only need a few bucks worth of parts from lowes with your compression tester. and if you dont have an air compressor, go to a gas station where there is free air and do it there. its not rocket science. Just hook up and air line to the compression testing rig and put 150PSI to it and rotate the crank so you can open and close the valves as needed.

Compression that low is total devastation.
Dont bother checking out the bores. a simple egg overlimit or taper can cause this issue and worn rings dont always scare the walls very much.
Leak down to test the valves, if no air comes out of the closed valves, then you know 100% that your rings and or block is toast and needs a rebuild.
Dont spend hours tryng to figure it out when a simple $10 testing rig will tell you all you need to know here.

I cobbled together some fittings and got a direct air feed to the glow plug holes. If I put more then about 40psi to it, it would rotate the engine. I didn't think to counter-hold it until just now and I'm done in the garage for tonight. If it matters I can try that tomorrow.

I didn't hear anything thru the air cleaner snorkel or tailpipe that I could say was definitely a leak. I did add air to a piston when a valve was open to get the gist of what I should be hearing. I couldn't hear any air movement like that with my mechanics stethoscope shoved in the air intake snorkel or tailpipe when the valves were closed. I could hear air moving thru the crankcase vent tube that goes up to the hockey puck on the valve cover.


I also took the gauge head off the crappy HF gauge and put it on my setup to try to see if my gauge was really off. I only checked #1 cold and it was still in the 250psi ballpark. The HF gauge head leaks so I can only say it was in the ballpark from what I could see while cranking.


Conclusive enough that the cylinder bores are out of spec or the rings are shot?

I gave the poor man's leak down test another try today. I counter held the engine so I could get 100psi instead of the 40psi like in my earlier attempt. I still couldn't hear much of anything in the intake or exhaust. I did hear much more pronounced rushing of air from the CCV hose from the block on cylinder #3 then the others.

I did foolishly use put the counter hold on the pump sprocket at one point which caused slack in the timing belt and caused it to jump at least one tooth. I stopped at that point. I'm not sure if 100 psi in a cylinder would cause damage to the valves. I do think I've learned all I can without tearing into the engine at this point.

So decided to inspect the potential replacement engine. It was pulled as a running engine from a body rotted A3. I started with a compression test. The engine is cold, outside temp about 60 and the starter sounds noticeably slow. (I was using jumper cables from a running car to power the starter.)

1 - 215
2 - 230
3 - 230
4 - 230

I'm pretty sure part of the reason the reason the readings are so low is because my cobbled together fittings don't take the volume of the glow plug in the head and there is no check valve down near the fitting.

I tore apart the HF gauge and made it so there is a check valve that doesn't leak near the fitting at the cylinder head. I'm using my gauge head that doesn't leak. Here are the compression numbers for the potential replacement engine after the reconfiguration .


1 - 280
2 - 300
3 - 310
4 - 310


I did put the reconfigured compression tester on my warm AHU engine in my B4.


1 - 320 (the old configuration with a cold engine was 210)


I've got enough confidence that my compression tester setup just reads low and my backup engine will be good enough to run while I tear in to the bad engine to see what is up with cylinder #3.

I think since I'll be pulling the engine and disassembling anyways, I may pull the cam to get a better look a the lifters and then the head just to see if I get lucky. All evidence collected so far though suggests it is rings or cylinder bore though. But since it seems to magically cure when it warms up, who knows?

WELL, seems like your testing rig might be subject

it really does not make a hill of beans difference if you have 500 feet of hose from the gauge to the head or 5 inches with or without 1 or 10 check valves. All you need is the gauge, it has a valve built in it. You crank the engine untill you reach PEAK pressure, volumetric displacement has NO effect on this test at all. You also need to crank at normal cranking speeds so a drained battery is not good. One way to help DRASTICALLY with this is to pull ALL the glow plugs so the starter has very little resistance and you only test one at a time. MUCH EASYER. You are building pressure, not checking for compression ratio or volumetric displacement.
You should go test it on a few other engines.
Just because it runs, does not mean it has worn pistons.
did you try the grease or oil trick? you did mention that you used testing with and without oil.
the trick is to do a dry compression test cold and then add like 4 good squirts of diff oil or any very thick oil like 90w into the engine while it is at BOTTOM of its stroke to allow for MAX oiling of the rings. that should DRASTICALLY increase the compression for a few strokes enough to test and see if the rings are really bad. Normal 5-40 is not going to do enough to "seal" the leak.
Ofcourse you want to do an oil change soon if you do.
did you fix your TB?

Thanks for the tips. When I oiled the cylinders it was with 5w40 synthetic. All glow plugs removed in the bad engine and replacement engine during testing. I didn't pull them all on the b4 because the glow plug harness was crumbling and I still need to drive it.

I did ask around to see if I could find another diesel compression tester but didn't have any luck.

Tomorrow hopefully I'll have time to pull the head and see if that reveals anything. There's something odd with number 3.

I could maybe pull the starter from the bad engine and try it on the replacement engine. I was jumping it from a car with the engine running so it should've had plenty of voltage.

I'm not sure if the oil ring was broken before removal or not. I haven't found the pieces. The compression rings are stuck. Maybe they free when warm which would explain why the compression changed so much between warm and cold.

I did see a post from Franko6 about attempting to see if a bore has highs and lows by removing the rings and measuring the gap on the top compression ring at different bore depths. The piston is used to make sure the ring is in the bore square. I tried this and measured 0.014" at all locations I tried.

The second compression ring came off in 3 pieces. I didn't use as much heat or force to remove it as I did the top ring. I'm not sure what is up with that.