Boost outta nowhere

[Durandal]

Ok the title sucks but I'm having an issue with my boost pressure. Car is a stock jdm tt mkiv with only a catback exhaust (both cats still in), here are the symptoms.

 

1) While not boosting my boost gauge seems to be reading higher than usual but still in the vacuum area under 0 bar obviously.

 

2) First turbo seems to boost fine and builds up to around 0.5 bar but when the second turbo comes on at around 4000 RPM it doesn't seem as strong as if its not getting fully pre spooled.

 

I did a quick visual check on all my vacuum and boost lines but couldn't find anything.

 

I think there may be a small exhaust leak around my #1 o2 sensor but that wouldn't cause this would it?

[Scott]

Not sure on the normal boost figures but 0.5 on the first tubby sounds low, I thought 0.6/0.7 was the norm? I might be wrong though.

 

I don't think an exhaust leak would have that sort of effect to be honest.

 

I would have a good look over the VSVs. The very top one at the rear of the engine is often the culprite for what you are describing. Pay close attention to all the vac lines in this area. If you want to make sure you are seeing an appropriate level of boost you could run the car in TTC mode. You only need an 18" length of vac host in order to do it. This will bring both turbos in at the same time, in a linear fashion, and rule out the turbos (pointing to the sequential system inc vac lines, vsvs, pressure tank). It's always good to see some healthy boost in order to rule out the turbos, for peace of mind, so I would definitely strongly recommend giving it a go

[CanisLupus]

Boost should be over 0,65 on first Turbo i as far as i remember.

[Durandal]

I didn't have any tubing to try the TTC, but I went and checked over all the lines again pinching and looking for a small tear or crack. My first turbo is coming on a tad late and if i ever so gently ease the RPM up to 3K it doesn't even want to come online. If i mash it, it will come on around 2.8K and boost up to around 0.5-0.55 bar before the second turbo comes on. I'm sure it's a boost leak I just have to find it. It also got really hot today which usually exposes a boost leak a lot more.

Edited May 30, 2011 by Durandal (see edit history)

[Durandal]

Ok so I don't think it's a boost leak, I tested my IC piping today and it held pressure T_T. I bought some vacuum hose and replaced most of the lines around the VSV's, Actuators.

 

- At idle vacuum gauge reads -0.6 bar, pretty sure it was -0.8 bar before this problem

- First turbo seems fine and spools up to around 0.6 bar, although at one point while going up a hill in 5th I mashed the throttle, building boost with first turbo to around 0.6-0.7 before even hitting 3k RPM (Was around 2.8k)

- Second turbo sometimes doesn't come on until 4.5k RPM and when it does I don't get the woosh of power just keeps building pressure like it wasn't pre spooled, but I can eventually get up to around 0.8 bar

 

Is there a quick way to check the VSV with a multimeter? I guess I might have to hook it up in TTC although I would really rather not

[Durandal]

Just measured the resistance of my IACV VSV and it was around 68 ohms when it should be around 45, the heat of the engine must have finally cooked it. Assuming this is the problem, is the part I would need 25860‑46060? Vacuum switch.

Edited May 31, 2011 by Durandal (see edit history)

[Scott]

I would get a set of 2nd hand VSV's bud. They come up on the forum from time to time. Brand new they are VERY expensive for what they are, you can get a whole set 2nd hand for less than the cost of 1.

 

Your symptoms now do sound like it's the VSV at fault.

[Durandal]

The part of the VSV that craps out is the vacuum switch isn't it? I think they are only listed at 40-50$. The whole VSV assemblies are around 200 I think. Is there an easy way to get at the actuator down behind the second turbo? I really should double check those lines while I'm doing all this just in case.

[Durandal]

Actually now I'm thinking its a problem with the pressure tank cause I can here a hissing that sounds like its coming from the throttle body, but is probably from the tank below it. Please tell me theres an easy way to get at some of the lines going to the tank without pulling the starter or throttle body. I have small hands and can usually fit them into tight spots.

[Durandal]

I managed to reach my hand down to the pressure tank and pull off the 90 deg angled hoses and check them for crack and splits but they were fine so I put them back on. I also tried blowing in the pipe above the turbos that goes to the pressure tank from the "y" piping, and it seems to be solid too.

 

The resistance of my IACV vsv measured out to be twice what the TSRM says it should be, but it only mentions replacing it if the vsv is open and mine was reading 85 ohms (at 20 deg c) today which is not open.

 

It's a long shot but this started happening the same day I filled up with petrol. If they made a mistake and filled the tank with (87 AKI / 91 RON) would it cause all the issues I'm having or just misfire? I just can't find anything glaringly wrong that would have happened to my car while parked.

 

Going to disconnect the part of my stock BOV that goes back into the manifold and see if it is passing air while just idling.

Edited June 1, 2011 by Durandal (see edit history)

[Durandal]

Took off my stock BOV today and I could blow through the 2 big pipes on it so that was the problem. I put my HKSS back in and the problem is now solved although I'm not really fond of the venting to atmosphere sound when you gradually let off the gas.