Resolving the Stammer

In the 2 major trips I took this Summer (2021), my road reports focused mostly Hapy's handling, the wind and the traffic or road conditions. I mentioned, sort of in passing, about the engine stumbling or stammering, having an error code thrown, and the occasional blanket of smoke. Today's post covers how I resolved this condition.

I should call out that the last trip of the season (for Hapy's birthday) did not include a code being thrown. He struggled into the headwind on the way there, but cruised with a tailwind on the way back. In neither direction did he throw a code nor dump smoke, but there were bumbles. So, I did this maintenance so the next trip would not be wanting for power.

P1550

Let's start with the code that was getting thrown: P1550. This indicates: Solenoid Valve for Boost Pressure Control (N75) - Control Deviation. in short, the thing that controls the turbo is not behaving consistently. So, the turbo is dumping boost to protect the engine. The result is a loss in power, sometimes a considerable loss. When I first noted the stumbling engine, I assumed it was fuel related. I replaced the clear fuel filter, because it had some stuff in it, but while that may have helped with consistent fuel delivery, it did not meaningfully address the engine stammering.

As I reflect back on the drives, we should have had WAY more power. I just got bigger nozzles and had the computer chipped. We should have been flying down OR-22, limited solely by the completely soft setting on the adjustable shocks. Instead, I was back to climbing hills in 3rd. It didn't register while we were driving, but it is now. Since we were sporadically getting a P1550 code, the turbo was sometimes dumping boost on the ground.... and creating a nice smoke blanket in the process.

Vacuum Lines First

stock hoses

Throwing parts at a problem is one way to solve it. Most people don't like resolving that way, though. When you have vacuum issues, which a P1550 error code usually means, there are some parts you kind of need to throw at it. If you only want to change the one thing that's wrong, you need to check all of the vacuum lines and appliances (turbo wastegate, N75, N18, EGR, ASV, vacuum ball, brake line, etc) and replace the one that is not holding pressure. The least expensive solution is to disconnect and re-connect all of the hoses. Sometimes they work themselves loose, and when you put things back together again, they just work. While cheapest and least guaranteed, this is also the least satisfying path.

If your vacuum lines are over 10 years old, like the ones on Hapy are, then you are due for replacing the lines anyway. Many of Hapy's hoses removed with little effort, almost falling off. Others were brittle and baked on and some others looked like they could have been replaced a couple of years ago. My hoses were following the stock pattern, but the other 2 valves (for Exhaust Gas Recirculation - EGR and Anti-Shudder Valve - ASV) were just blocked off. So, I switched to the simplified version (image below, courtesy of DaveLinger of TDIClub), leaving the unused valves in-place and wired, but without a vacuum source. This should reduce the opportunities for vacuum leaks to appear in the future.

There are kits on the 'net for a couple of meters of 3mm hose and a couple of meters of 5mm hose. The 5mm is for the line from the N75 to the bottom of the turbo and from the N75 to the air cleaner. The rest on Hapy are 3mm, but from looking at the diagram, I would expect that the line from the "T" to the N18 should also be 5mm. All of the lines I removed from Hapy were 3mm: I didn't have any 5mm when I did these 10 years ago. I am sure the VW engineers used the bigger diameter hose for a reason, though, so I installed 5mm hose to/from the N75. Anyway, replace one hose at a time, cutting the old hose off by slicing along the plastic nipple (to avoid breaking it). This will cost, like $30US, in hoses and in about an hour your hoses will be good as new. Possibly better. P1550 possibly fixed.

Brake Booster Line

So, you still have a code or lack of power? Next, I would replace that big vacuum line from the vacuum pump to the power brake. These age and crack like the others, but the replacement is not in the hose kits. For another $30US, you can replace this. I was about at this point when I kinda remembered that my brakes were not as responsive, and I had to push harder to get the bus to stop. Sometimes. I had thought it was weight-related and even hit a trucker scale on the way beck from 4Peaks to see if I was especially heavy (4400 pounds, btw). I wasn't. Perhaps I was stopping with only partial vacuum boost or even power boost free. Hoping the power booster on the brake was not the cause, I continued down the path of parts I had on hand. My hose was original, and while there was a little cracking at the vacuum-pump end, it looked pretty good. Still.. you can't tell if a hose is failing by looking at it. Glad I replaced it.

Replace the N75

no EGR, no ASV

Last "inexpensive" thing to look at is the N75. I put quotes around it because it is still $70US. I suppose the vacuum ball or the little grey/white check valve could be the issue and these are fairly inexpensive. If you are still getting the p1550 after all the replacements above and the N75, you may have an issue with the wastegate actuator, or your brake booster. Remember the booster failure on Oliver, the MGB? For reference, here are the links for the finding and the fixing posts. 

Replacing the N75 is usually as easy as replacing the hoses. 3 hoses and a 2-pin plug-on-a-cable connect to it. It is held to the firewall with a couple 10mm bolts. I ordered both the hose kit and the brake booster line as well as a N75 when I attacked this problem. Since my engine is a first-year ALH, the 2-pin plug is different from most others, so the N75 on-market does not direct swap in. The plug needs to be re-wired or the pins pushed out of the old / into the new. I left the N75 in my supply heap and just did the lines. I figured that if I need to cut some wires or play with the pins, I want to be sure that it will actually solve a problem, rather than just create work. I had to remove the air intake to get to the vacuum lines so I cleaned the air filter while it was in-hand.

Test Drive

With the vacuum lines replaced and simplified, I was ready to see if the stammering issue had resolved. Prior to doing this work, I was still driving Hapy around as my "daily" driver (quoted because with the pandemic I don't actually drive everyday. Maybe I should say my "weekly" driver), and I was still getting sporadic stammering issues. So, I figured if I could just drive to my favorite burrito place (Taqueria El Gordo) without a stammer, I'd have solved the problem, and gotten dinner. I say that because it is just far enough away for the stammering to appear. Even if it didn't solve, I would still have a killer burrito. It was on a drive to Gordo's that I could easily re-create the stammer problem within a short trip, so I figured it was a decent test-track.

What's that saying about "the best laid plans"? Well, instead of going to the burrito place, Boo and I drove right past it and out to Cornelius (west of Hillsboro) to see some live music. Hapy suffered no stammering, and he had plenty of power. I didn't really put my foot into it, but he popped off the stop lines well. At no point on the drive there and back did he stammer, nor lay down smoke. I am calling this solved: failing vacuum lines. 

That's it for today. Thanks, as always, for following along-