I cannot kill this stumble

xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
Hey guys! I need your help. I am dealing with a stumble (miss?) that I cannot seem to cure. My setup is a BHW, with a stage2+ standalone Malone tune, GTC1549VZ, intake flap delete, MAF delete, and dynamic EGR. I'm on the original injectors and cam, both of which are somewhere around 100k miles old. The cam is not showing signs of wear and the lifters look fine.

Here’s a video of the ‘stumble’: https://photos.app.goo.gl/vvhnREc5uLijYxCX6

The stumble is most prevalent when the engine is cold, and prevents accelerating past 2.5-3k RPMs when it hits. As the engine warms up, the stumble becomes less dramatic, and eventually only happens at 3k RPMs. If I somehow push past, say, 3.5k RPMs, everything is normal all the way up to redline. If I have been driving for, say, 30 minutes or more (trip into town), there’t no stumble. I have an occasional 16705 show up in VCDS. I’ve checked the torsion value, injector opening times, and idle stabilization values, all of which seem within range.

Now, what have I done? First and foremost, I installed an OEM crank and cam position sensor. The stumble exists when the cam position sensor is not installed, so its not the cam crank trigger. The stumble is a more recent development, so I am assuming the crank trigger wheel is okay. The stumble became less intense when I replaced the crank sensor. I have checked the CTS, MAP, IAT, and O2 sensors, and the 5v bus seems to be a stable 5v when the engine stumbles, so I’m assuming at this point that this is not a sensor issue.

I had read that EGR issues can cause a similar stumble, so I unplugged the EGR and went for a cold drive, the issue persisted.

Malone recommended that I check and clean the injector harness, so I ohmed every inch of injector harness, and cleaned all contacts. I ensured that the contacts at the injectors were tight. They have solid connections at all points.

I peeled back the looming and checked all grounds, powers, and splices throughout the harness, cleaned and replaced a couple, and the issue persisted.

Most recently I have run a full can of liquid moly diesel purge and installed a new 2uc fuel filter element, which also seems to have softened the stumble ever so slightly, but it persists.

What do you guys think?
 

xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
what kind of fuel supply do you have? as in lift pump and filter?
I am using a Stanadyne FM100. My fuel lines are the stock 3/8" stainless as supply and a 3/8" 30r14 rated stretch as return. In the tank I have a Dodge 2500 fuel pickup and pump, which were originally paired with the VP44. I had my wife watch fuel pressure while I brought then engine up to 3k, and the fuel pressure bottoms out at 5PSI on the pump side of the filter when the engine is doing its stumble thing, and sits solidly at 11PSI at idle.
 

xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
Funny you mention that, I have. There's an aux port on my fuel module that's just a 1/2" straw. I hooked it up and reprimed the system, and drove around for a while without the lift pump. No change. I used the existing fuel lines though. I had been wondering if the injector orings/seals could be related, and how I would rule them out without pulling them?

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xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
I used a BEW petal, and ran 6 new 20ga txl wires to the petal. I used new gold plated contacts and a new 6 pin plug. They are spliced into the VW harness with solder and waterproof heatshrink. They pass through the firewall, along with CAN and the clutch switch wiring, and go through a 10 pin Deutsch connector. I did at one point have a code related to "implausible" petal values, but I haven't seen that one in quite a while. I haven't seen that code since I had my MAF and intake flap deleted. What's your thought?

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jimbote

Certified Volkswagen Nut
Joined
Jul 10, 2006
Location
spiral arm, milky way (aka central NC)
TDI
Tacoma 4x4 converted to TDI
I used a BEW petal, and ran 6 new 20ga txl wires to the petal. I used new gold plated contacts and a new 6 pin plug. They are spliced into the VW harness with solder and waterproof heatshrink. They pass through the firewall, along with CAN and the clutch switch wiring, and go through a 10 pin Deutsch connector. I did at one point have a code related to "implausible" petal values, but I haven't seen that one in quite a while. I haven't seen that code since I had my MAF and intake flap deleted. What's your thought?

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did you match color for color on the wires? or go by the diagram? ... the colors on a few of the wires are reversed in the factory harness and don't mate to the corresponding color on the other side of the connector.
 

xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
I don't exactly remember. I'm pretty sure I went by colors. You wouldn't happen to know what doesn't match up, would you?

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xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
So' I've had a think about my last answer. In VCDS, throttle position, WOT switch, and idle switch all log correctly. I know I had a post in my build thread that was contrary to that statement, and I can't explain why it logged like that. It's not done that since I tore the entire harness apart and redid all splices I could find.

The throttle position sensor position seems to be irrelevant to the stumble. When it is stumbling, I can take my foot off the throttle or go WOT and the stumble persists. The only constant here is some strange combo of engine speed and engine temperature, since once the engine is warmed all the way up, the stumble virtually disappears. I've noticed 3 modes of stumble:

1) cold engine, just left the driveway and its 45 degrees out. Don't dare try to break 2.5k, it'll just buck like an angry bronco.
2) engine is up to normal temp but youve been putzing around town, you'll probably make it to 3k?
3) you just spent the last 30 minutes on the interstate, it will redline 95% of the time if you ask it!
 

vtpsd

Veteran Member
Joined
Aug 15, 2013
Location
Vermont
TDI
03 jsw TDI, audi 90 AHU swap
As a last ditch effort, you could try a can of diesel purge bottle fed directly through the pump. I had a similar issue on my audi with an AHU swap, except it was constant, but it had a horrible stumble at a certain RPM. I believe something in the timing mechanism was sticking and the diesel purge fixed it immediately and permanently. Your situation does not sound the same, but its a cheap thing to try. Diesel purge has solved a couple strange TDI issues for me and its worth a shot.
 

jimbote

Certified Volkswagen Nut
Joined
Jul 10, 2006
Location
spiral arm, milky way (aka central NC)
TDI
Tacoma 4x4 converted to TDI
As a last ditch effort, you could try a can of diesel purge bottle fed directly through the pump. I had a similar issue on my audi with an AHU swap, except it was constant, but it had a horrible stumble at a certain RPM. I believe something in the timing mechanism was sticking and the diesel purge fixed it immediately and permanently. Your situation does not sound the same, but its a cheap thing to try. Diesel purge has solved a couple strange TDI issues for me and its worth a shot.
he has a PD BHW.
 

xerootg

Veteran Member
Joined
Oct 20, 2015
Location
USA
TDI
Mk7 Golf Sportwagen, BHW Jeep XJ
New data!

tl;dr; This stumble is both temperature AND RPM dependent. When the engine is cold, it stumbles earlier. If you slowly accelerate once its warm, its less likely but still happens 9 out of 10 times. Since IQ is temperature, throttle and engine speed dependent, I'm confident this is a fuel issue. I need to choose whether I spend my money on injectors or other unknown fuel system components, and would like some advice.

I tweaked the 'torsion value' to the the lowest 2k RPM fuel consumption. I found that it made the stumble less dramatic than before. The compromise here was that when it stumbles, it cannot make it past the point at which it stumbled, whereas before if you were persistent with throttle (VERY persistent), it would eventually accelerate past the troublesome speed band.

I had an extra BHW PD element from an unknown car that I swapped in for the injector with the worst idle stabilization and most unbalanced injector timings (group 13 and 23). No change.

Last night, I finished putting a cam, timing belt, injector harness, and all new seals on the PD elements, and adjusted the injector rocker lash. No change.

The old cam showed signs of typical wear ("tree rings" from wearing through the hardening) but definitely had life left in it. Actually, when I threw my micrometer on it, the lobes had evenly worn, and were fractions of a millimeter off from the new one I just put in. Some lifters had wear and some pitting, but much of the black coating was intact, and spun freely in their bores. The bearings also had life left, with one showing signs of oil starvation. At the end of the day the cam needed to be done but was not the root of my issue.

The injector harness has been a root of problems on many PD TDIs. My harness had some plastic fatigue on the #2 injector plug in the form of one of the pins locking tabs breaking and allowing the pin to push out of the socket, so again, I'm just gonna write this off as necessary maintenance.

Throughout this adventure, my idle stabilization numbers haven't been perfect, but still reasonable. After the cam swap, my deviations seem more dramatic since in my mind they should have evened out. They are currently -0.07, -0.07, 0.21, and -0.05 mg/str. I've logged some group 23 data, since that's been used by others to diagnose fuel starvation in the past, and noticed those numbers are all over the place, but always within 20us of each other. Some are consistently faster, and others are consistently slower.

There's only a handful of variables left to prove unrelated. To quote Owain@Malone out of context, "irregularities under consistent throttle is always fuel supply". I have not proven the injectors are healthy, I have not proven I have enough fuel supplied.

I have anecdotal evidence that there's enough fuel supplied, but not conclusive. 1) the stumble is engine speed dependent, NOT consumption dependent. When its pulling hard, driving up a grade at highway speeds for example, I do not have the stumble below 3k RPM. Significantly more fuel is consumed (and therefore volume) at highway speeds than when I'm driving up and down my street. 2) The tandem pump is brand new - It's a Bosch not a LuK though. 3) I bypassed the lift pump, and connected the fuel filter directly to my lift pump's unpressurized aux port, and saw zero change in behavior.

I know for a fact that these injectors are not new. In fact, they probably have somewhere around 210k miles on them. After talking to an injector guy familiar with these units, apparently the pumps wear and instead of failing, just don't push enough fuel under specific circumstances. Since the volume of fuel they push is largely dependent on engine speed, they seem to me to be most likely my issue. I am talking to an injector rebuilder about having my injectors refurbished, but that's big money.

On one hand, refurbished injectors will not be a wasted effort, but if it does not solve the stumble, it pushes off spending money on other parts of my fuel system significantly given the cost of such. What sort of smoking guns should I be looking for to choose where I spend my money and time next?
 

jimbote

Certified Volkswagen Nut
Joined
Jul 10, 2006
Location
spiral arm, milky way (aka central NC)
TDI
Tacoma 4x4 converted to TDI
one other thing to check, and probably not the issue, is the fuel distribution pipe inserted into the head...you access it by removing the tandem pump. I get them out by tapping the center hole to m6 or m7, (can't remember exactly) inserting a bolt and working it out... on another note, did you double check your throttle wiring?
 

jimbote

Certified Volkswagen Nut
Joined
Jul 10, 2006
Location
spiral arm, milky way (aka central NC)
TDI
Tacoma 4x4 converted to TDI
i just went through a similar issue on an 05 bew. Stumble under load, no codes. Seemed like fuel delivery so replaced filter. No good. Unplugged cam sensor and it would die every time after revving it. Took.a chance and replaced crank sensor. Problem solved.
 
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