Battery out problems?

[cmdrfire]

I took my battery out on Sunday so I could install my boost gauge and ever since I put it all back together I've had some serious issues.
Symptoms are thus:
Car is not boosting much at all (I'm on stock map right now for safety, but it wasn't much better with my normal remap on) - gauge is indicating 0.3bar peak (which is a whopping 4.35 PSI only )
When under power, it "jumps" a bit from the 2500RPM-4000RPM range by 100-200RPM - as an example, if I'm on the motorway in 6th at 80mph, revs are at around 3000RPM... and then it'll jump to 3100RPM, car will shudder, and then it'll be back down to 3000RPM - originally I thought turbo boost pressure was dropping when this occurred but it doesn't, boost drops very slightly and then back to 0.3bar (4.35 PSI)
Doesn't occur when I'm accelerating hard (which isn't very hard now thanks to the terrible boost I'm getting)
Happens in all gears, every now and then, between 2500RPM and 4000RPM... beyond 4000RPM doesn't occur, under 2500RPM doesn't occur, and it occurs only intermittently...

Scariest part: Before I took my remap off, I was on the motorway at 80 noticing that this was happening. I put the cruise control on to make sure it wasn't driver-induced for whatever reason. It happens a couple of times and then suddenly engine loses all power, I get no acceleration or anything, my EPC light comes on and my ESP turns itself off. I'm able to coast to the shoulder and try putting on some power - nothing. I put the car in P, turn engine off, and restart. EPC light comes on, and car behaves as normal (albeit with this weird shuddering/jumping/noboost thing going on). I turned around and went home and put my OEM map back on. EPC light is off now, but problem still occurs. It's been doing this for four days now. It was absolutely fine before I disconnected the battery and connected the gauge's vacuum pipe to the Forge vacuum tap (I have the forge DV).

I checked under the cover for any torn or disconnected hoses in case it was just a boost leak... nothing. Everything tied down securely. So I'm kind of stumped, I'm not sure what to do now... VW had a look at it briefly yesterday and diagnosed an "electrical fault" but were unable to rectify and unable to install a new OEM map on the car... it's going back in on Thursday so they can try again.
Anyone got any ideas? Anyone encountered something similar on battery removal? :iono: help!

 

[cmdrfire]

Bump for update... I just disconnected the vac line to the gauge to see if that was causing the problem, and used the grub screw that Forge provided to close off the spare hole on the tap. Took it for a spirited drive... no joy still surging up 100-200RPM.
I've put the vac line back, but lost the grub screw doh... was careful to save it first time 'round.
Any thoughts from anyone? Please?
The Forge DV seems to be actuating fine, so I don't believe it's an issue with that...

 

[2zzge]

get the code(s) read. it will narrow it all down for you. (i know dealer has done that but just for your own sake and knowledge)

on with the guessing and e-diagnosis:thumbup:
i was gonna guess that there was a boost leak somewhere. this is assuming all was well before the install.
..but it seems you've gone over all that.
electrical would be another guess.
try disconnecting all the electical to the gauge, (make sure all is as before) and see if that helps.

without the codes tho' your just chasing your tail...as cute as it might be and all, you'll go nowhere :smoking:

 

[Groucho]

Man... that's just weird...

Is this an intermittant problem, or is it contrant? Could a stray voltage or accidental reverse polarity have fried your ECU?

 

[cmdrfire]

Unfortunately won't be able to get the codes 'til after the weekend, don't have a VAG-COM unit. However, the VW tech I spoke to Thursday said that there was an "electrical fault" which required further investigation... which is what I suppose must've happened.
A bit more info: I had the electrics of my boost gauge install done before I installed the vac line, and tested them. All worked fine, including the car. Car was actually running beautifully before this.
Then I disconnected the battery, connected the vac line and ran it into the cabin. Reconnected the battery, powered up, got a host of warning lights that went out after a few minutes - but oddly enough, gauge didn't work (LEDs were powering on and it was performing self test just fine before battery out). The gauge is a DEFI unit that connects to a DEFI control box; it draws 12V from the control unit which in turn is connected to the car power. Using a multimeter, I found out that the control box was not putting out 12V. It was at this point that I started encountering the jumping/shuddering problem, after the battery was reconnected.
I opened up the control unit, and found that an inductor on the 12V constant input was blown... I was able to replace that, and now the gauge/control box work fine. It's odd though because none of the fuses in the car blew.
But I'm wondering if the battery reconnect has caused some sort of surge which might've caused damage elsewhere.

@Groucho, it's intermittent, sometimes it's running okay at 3000RPM and then sometimes it'll just jump from there to 3100 or whatever. As the electrics for the gauge were installed and working fine prior to the battery disconnect, I've no reason to suspect that they are at fault; however, I'll pull them later on tonight and see if that makes a difference. I doubt it will.
I'm just worried that something's happened to the ECU... the car is back on the original map so it'll be a warranty issue if the ECU requires replacement, but nevertheless, I'm hoping it's something less severe.

 

[2zzge]

being intermittent, it sounds like it might be a loose wire someplace.

but you're completely safe in thinking that it might have caused another issue elsewhere such as witht he ECU or MAF.

here's to an easy and fast fix :drinking: :smoking:

 

[cmdrfire]

Okay, some more thoughts, car's in the shop Thursday and these are my current ideas:
Basically the problem feels exactly like a blown DV. Obviously not the case as I have the Forge DV fitted, and that won't just break because the battery's out.
Car is running okay apart from 0 boost and the engine is adapting to running without boost so is a lot smoother, except for the occasional judder... here is what I think is happening.
I beleive the car to be in a state such that the DV is always venting - I'm not sure if this possible, but this is my thought. The "shudder", an increase in revs of about 100RPM for a moment, is accompanied by a corresponding slight increase in boost, followed by a complete loss of boost (below 0, negative pressure). So: the DV appears to be always venting for some reason. Possibly it tries to close at certain times when power is required, resulting in the judder. However, it opens again, resulting in loss of all pressure built up.
I'm more convinced now that this is not a serious problem. I believe it to be a fault either with the Forge DV actuation (so with the electronics or the solenoid) as a result of the battery disconnect, or due to the ECU sending an incorrect signal.
Thoughts on my theories? Possible? way out?

 

[cmdrfire]

Just a little "closure" for those of you following this...
Problem was quickly found and rectified by VW today. A pair of components (charge pressure sensor and a/c/f valve and canister) had malfed and were replaced, and a software update performed. Car's performing nicely now, on stock map and peak boost touching 1bar (around 15PSI). I can't wait to put my map back on. It's great watching the little needle bounce around everywhere
So much for my theorising and conjecturing... car already knew what was wrong, just needed someone to read it and replaced the malfed components.
So it's running all fine now... except my airbag light came on ah well, she drives fine and that's what counts.