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Old 10-24-2011, 02:30 AM   #1
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Default Boost/vacuum leak

Been chasing an issue with the car all day. Think I've finally got it pinned down, just wanted to see what you all thought and make sure I'm going down the right path...

Initially I was seeing a steady 12psi with boost gauge all day long, but MS's MAP sensor was showing something really weird- spools up to 100psi or so, then around 5k, it ramps up to 13 and stays there till redline. Does seem to start fluttering a lot though closer to redline. Boost gauge and MAP sensor using the same source. Using a hallman MBC. Does the same sorta thing spring pressure only..don't suspect the mbc being any sort of issue or contributing factor.

Upon further investigation (should have caught this earlier, but I didn't), I'm having to use usually high VE values in the 4000-5000 RPM range (where the low 11psi spot is in datalogs), in order to maintain target AFR. Around 5k the VE values actually decrease and seem to go back to 'normal' (I am running a bit rich up top, still need to finish the tune for sure).

So my logic is that, for whatever reason in that 4000-5000 region, actual air entering engine > measured by the MAP sensor. Spool up is fine, 12psi @ 3800 in a 2871 w/ big turbine housing. If it were a big boost leak in the IC piping I'm thinking it would be hurting spool a lot, and actual and measured air would probably be the same/very close. So, pretty sure I have some sort of leak in my vacuum tubing, probably close to or at the megasquirt, since the boost gauge (which is tapped off same source) seems to be reading steady and consistent. I'm still kinda perplexed as to why it would only be 'leaking' from 4000-5000 though, and appear to go away above that?

I did a quick check of all the fittings, hoses, IC pipes and everything seems OK.... going to make a real boost leak tester this week to actually test this stuff out. Am I on the track you think? Driving myself crazy today trying to figure all this stuff out. Here's a datalog that shows the Boost/VE stuff I was talking about....this is very repeatable. Boost gauge always showing a steady and consistent 12 psi.

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Old 10-24-2011, 12:10 PM   #2
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The logic that a MAP error could cause AFR problems is sound. However, I don't see any way that a physical leak in the vacuum tubing close to the MS could be RPM dependent. All a leak would see is pressure inside the tube vs. pressure outside the tube.

Just for testing, you might want to remove the MBC and operate on wastegate pressure to see if this wierd effect is still present.
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Old 10-24-2011, 12:43 PM   #3
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Apparently I didn't save the datalogs, but it was still there with spring pressure only. I'll probably go ahead and remove it anyways, just to keep everything as simple as possible until I sort this out. Thing that still really gets me is how the boost gauge reads constant for the whole rev range, while the MS map sensor is showing a 1-2 psi rise around 5k every time. The boost gauge seems to be working fine..it's a brand new Autometer full electric sweep model, with it's own MAP sensor and all that calibrates at every startup. Playing with the mbc ****, the boost gauge seems to follow along, showing even differences of 0.5 psi boost, and consistently always reading the same. If the boost gauge was just all over the place I would just disregard it's reading, but its not.

The fact that VE is so high at 4000-5000 RPM, to maintain something like 12.0-11.8 in that datalog... tells me the MAP sensor is reading low vs what actual boost is. Maybe this a good time to pull the ECU anyways and upgrade to the 4 bar MAP sensor.

Either way I just need to make my boost leak tester tonight.. I'll probably test the IC piping and intake manifold separate and see what results I get.
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Old 10-24-2011, 02:28 PM   #4
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You running some kind of custom firmware? Bugs do creep in.
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Old 10-24-2011, 05:19 PM   #5
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Yeah, I thought about that....

Gonna try 3.1.1 tonight
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Old 10-25-2011, 08:08 PM   #6
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just did a boost leak test... only leak pre-engine was EGR blockoff plate on the intake mani. Used some oem solid gasket...time for some RTV silicone, I'm starting to love that stuff more and more every day. Also had a possible leak hotside leaking into IC- if I pushed the pipes over the side a little I'd hear a leak, I'm sure it was doing that a little with engine torquing, etc.

But biggest discover of all was at the actual EMS/MAP sensor. Similar to the IC pipes that would leak if I wiggled them around a little...same thing for the vacuum tube connected to the EMS's MAP sensor. Thinking that's gotta by my issue, my mounting solution (It's up in the foot well on 99-00) isn't totally solid, and its highly likely it's moving a little when I get into boost rapidly around 4k or higher...and leaking a little pressure right at the MAP sensor.

I also just realized that the MAP sensor for my autometer boost gauge is the GM 3 bar... exact same thing DIY sells on their site, etc. Thinking about maybe just wiring that up to the MS, and use that as my signal for both boost gauge and EMS- thoughts? I won't have to run a long *** vacuum line into footwell anymore, and I'll need at least a 3 bar down the road when I build the motor anyways.
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