Where did my boost go?

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  1. #1
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    Where did my boost go?

    On a road trip into BC, Canada right now, camping around different places and sometimes my phone doesn't have good signal. I've got a good assortment of hand tools with me, but left the laptop with the Bentley manual PDF's, VCDS-Lite and the KKL cable at home in Portland to have one less thing to worry about possibly getting jacked if we left the camper unattended someplace.

    '98 Passat AEB engine, swapped into a '75 VW Bus high-top camper. Stock ECU, only real mods are an ICM delete with ATW bolt-down Hitachi coils and the rearranged intake & exhaust plumbing needed to fit the motor into the engine compartment. Plugs about a year old, exhaust is 3" through the cat then 2.5" through the muffler, cat only a couple years old. It's been running great for a couple years, did fine on the drive up from Portland on Monday, did fine driving around Vancouver for a few days. Couple of hours after we got to Vancouver Island the other day I noticed the power and acceleration were down, and that I was getting maybe 5PSI boost at best. The next morning it was acting fine for about the first hour and easily hitting 10PSI boost, then back to what it was doing before and hasn't changed since. My first thought was that I got some bad gas from a no-brand station I stopped at before the ferry, but I've run through that tank and burned through a tank and a half of Shell premium since then with no change. I've looked for leaks in the turbo/IC/TB piping and connections and didn't find anything, plus the silicone IC-to-TB hose swells when I open the throttle like it always has .. so even without the gauge I know I'm getting *some* boost, just not what it should.

    I'm suspicious of the diverter valve (OEM plastic, 2+ years old), maybe the knock sensors and the Chinese fleabay MAF I've got in there .. though I've been running that fleabay MAF all along with no problems. I've got a multimeter with me so I can test electrical things (wire continuity to ECM, ohms of N75 valve, etc) if I know what I'm looking for. Drives OK so it's not like we're stuck. If I can find what's wrong and it's an easy fix, great! If not, I'll dick with it when we get home in a few days.

    Thanks!

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  3. #2
    in dire need of an organic chemistry lesson
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    Quote Originally Posted by tristessa View Post
    I'm suspicious of the diverter valve
    That's an interesting hypothesis. My diverter valve is factory original after 18 years and my car also suffers from serious lack of boost and power. I have bought a new valve but due to cold weather in Michigan haven't been to change it yet. But will do so very soon and see if the boost returns.

  4. #3
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    My thinking is that if it's not holding like it should, it could be bleeding pressure back into the inlet pipe, but TBH I'm kinda grasping at straws. It's original to the motor so probably 80K miles on it, was clean and working when I drove the donor Passat around for a month before stripping it. Probably only came to mind because the OEM DV on an AWM I've rebuilt to install in a Vanagon was literally plugged with sludge and wouldn't open, and I was just dealing with that two days before hitting the highway so it's fresh in my mind.

    Off to Timmy's for coffee...

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  6. #4
    in dire need of an organic chemistry lesson
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    I still have my doubt if the diverter valve can significantly affect the boost. Unless someone familiar with it can chime it and enlighten us.
    Another source of low compression is usually clogged piston rings. Using a bottle Power Restore in the engine oil may remedy this.

  7. #5
    PZ
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    A bad DV can kill the boost. You can pull it and check it to see if it stays closed (blow through it).

  8. #6
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    Got home to Portland OK, down on power but still runs fine and holds freeway speeds except when crawling through JBLM south of Tacoma.

    DV is the original 710A from the '98 Passat and the diaphragm is shot, sucking on the small vacuum line is like sucking on a straw with no resistance at all. Also saw that I had it installed backwards. Oops.

    Plugs are a little sooty but otherwise look fine, still going to replace them since I've got another set on the shelf. I seem to have a wiring problem between the ECM and the OBD2 port, can't connect to the ECU with either my VCDS-Lite system, the ELM327 dongle or my generic OBD2 code scanner. Going to have to sort that before I can check for stored codes, but that'll have to wait at least until I get back from the phone repair place. Dropped it on a rock at a campground and busted the screen.


    Edit/update: new plugs installed, no-name rebuildable billet DV installed. Drives much more like it should but still only seeing 4-5PSI max on the VDO boost/vac gauge, which is teed in between the FPR and the manifold. Went with the billet unit for the rebuildable factor, left the "stock" blue spring in it and made sure it wasn't installed backwards this time.
    Last edited by tristessa; Today at 01:38 PM.

  9. #7
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    Bad ground on the OBD2 port, fixed that and pulled codes with VCDS-Lite. Misfire codes on all cylinders, pre- and post-cat O2 sensor codes (low voltage/bad sensor) and cat converter efficiency codes, N75 wiring fault code -- probably my bad, unplugged the N75 with engine running trying to troubleshoot on the trip -- along with an implausible VSS signal code. Cleared codes and ran for a short test drive, got my usual 9-10PSI boost back and the only code that returned is the implausible VSS signal.

    Taking it on a longer drive to see if any of the codes return and/or if the boost disappears. Going to log some IAT data while I'm at it -- I know my intercooler is less than ideal (even worse airflow than stock Passat) and while there's an always-on fan pushing air through it, I've got a suspicion that my IAT's are still on the high side.

    Thanks for letting me ramble.

  10. #8
    PZ
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    The backwards DV was actually the subject of debate and testing years ago. It seemed there was better boost holding with the DV backwards due to the design. I never bothered to switch mine, I just went with the higher rated 710N when I chipped my 1st Passat.

  11. #9
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    A few of the codes came back. From VCDS-Lite:

    ----------------------------------------------------------
    3 Faults Found:

    16885 - Vehicle Speed Sensor: Implausible Signal **** this one has always been there. No cluster on the swap, just engine/trans/ECU/wiring
    P0501 - 35-00 - -

    17544 - Fuel Trim: Bank 1 (Add): System too Lean
    P1136 - 35-00 - -

    16518 - Oxygen (Lambda) Sensor B1 S1: No Activity
    P0134 - 35-10 - - - Intermittent

    ----------------------------------------------------------

    Looking at the possible causes of the other two, both are mention fuel pressure, and I did notice that the pump -- external pump -- started making noise partway through the trip. Guess I'll be putting a fuel pressure gauge on it tomorrow...

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