Mapachurro
Well-known member
Hey everyone,
I'm happy to report that thanks to this forum and others, my Bentley, a friend who knows about electricity, and a lot of patience, I've solved an odd, persistent limp mode issue.
The basics/background:
2004 BEW Golf, ~280k on the body, ~160k on the engine. Put on a VNT-17 in January 2019, a VR6 clutch, 3bar MAP and Stage 4 Malone tune in July.
I had a couple of CELs after the clutch swap/tune, and from the moment I tuned it, I had awesome boost until the engine warmed up, at which point it would go limp and roll coal like a freight train. In particular, once warm, if I came to a stop, the engine note would change, get much deeper, and I'd be in limp mode. Every once in a while I'd get an unexpected boost spike. If I turned off the engine or cleared codes, I would get boost back -- until the next time I came to a stop. So, first step, I figured, was get rid of the CELs:
CELs/codes fixed:
-reinstalled O2 sensor after new cat install
-speedometer sensor (G22) and speedometer sensor fuse
I had the whole system professionally boost leak tested in Dec. 2018.
After fixing the CELs, same symptoms. Awesome boost and then limp rolling coal. Next step? MAF seemed to be going. So...
-Replaced MAF sensor.
Same limp mode symptoms, but now the boost spiked less and no coal. Next step:
-Check vacuum lines. Everything seems kosher. The EGR has been tuned out, so I redo the vacuum lines so that they only go to the N75 and the turbo. The hard brake line is giving 25" on my mityvac bang on the money, but no leaks. Next step:
-Adjust turbo actuator, previously began to move at 2-3" and ended at 15". Adjusted to 4"-18".
Same symptoms, but smoother, better boost. Next step:
-Unbolt downpipe, spray that bad boy full of oven cleaner, cycle the actuator, wait, etc. After a couple of hours, I heat up the engine and blow a huge blob of carbon out my tailpipe onto the driveway at 5000rpm.
...same limp mode symptoms, but more turbo whistle
So. At this point I'm thinking through the symptoms, and about the parts of the system I haven't touched yet -- including the N75 that might be factory original. It occurs to me that the N75 has three ports -- vacuum in, vacuum out, and... Vent. As in, vents when you slow down to a stop? I imagine? Like, when I stop it vents all the vacuum and gets stuck in that position once it gets warm? Next step:
-Pop off the vacuum line into the vent port on the N75, give it a spray of WD-40.
Solved. 22+psi boost peaks and plenty of sustained power throughout the proper range. I'm following the instructions about shifting at 2400/3000+ and loving my stupid fast 15 year old car.
I'm happy to report that thanks to this forum and others, my Bentley, a friend who knows about electricity, and a lot of patience, I've solved an odd, persistent limp mode issue.
The basics/background:
2004 BEW Golf, ~280k on the body, ~160k on the engine. Put on a VNT-17 in January 2019, a VR6 clutch, 3bar MAP and Stage 4 Malone tune in July.
I had a couple of CELs after the clutch swap/tune, and from the moment I tuned it, I had awesome boost until the engine warmed up, at which point it would go limp and roll coal like a freight train. In particular, once warm, if I came to a stop, the engine note would change, get much deeper, and I'd be in limp mode. Every once in a while I'd get an unexpected boost spike. If I turned off the engine or cleared codes, I would get boost back -- until the next time I came to a stop. So, first step, I figured, was get rid of the CELs:
CELs/codes fixed:
-reinstalled O2 sensor after new cat install
-speedometer sensor (G22) and speedometer sensor fuse
I had the whole system professionally boost leak tested in Dec. 2018.
After fixing the CELs, same symptoms. Awesome boost and then limp rolling coal. Next step? MAF seemed to be going. So...
-Replaced MAF sensor.
Same limp mode symptoms, but now the boost spiked less and no coal. Next step:
-Check vacuum lines. Everything seems kosher. The EGR has been tuned out, so I redo the vacuum lines so that they only go to the N75 and the turbo. The hard brake line is giving 25" on my mityvac bang on the money, but no leaks. Next step:
-Adjust turbo actuator, previously began to move at 2-3" and ended at 15". Adjusted to 4"-18".
Same symptoms, but smoother, better boost. Next step:
-Unbolt downpipe, spray that bad boy full of oven cleaner, cycle the actuator, wait, etc. After a couple of hours, I heat up the engine and blow a huge blob of carbon out my tailpipe onto the driveway at 5000rpm.
...same limp mode symptoms, but more turbo whistle
So. At this point I'm thinking through the symptoms, and about the parts of the system I haven't touched yet -- including the N75 that might be factory original. It occurs to me that the N75 has three ports -- vacuum in, vacuum out, and... Vent. As in, vents when you slow down to a stop? I imagine? Like, when I stop it vents all the vacuum and gets stuck in that position once it gets warm? Next step:-Pop off the vacuum line into the vent port on the N75, give it a spray of WD-40.
Solved. 22+psi boost peaks and plenty of sustained power throughout the proper range. I'm following the instructions about shifting at 2400/3000+ and loving my stupid fast 15 year old car.
