Boost Gauge max reading not changing?

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Andy82GT
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Boost Gauge max reading not changing?

Post by Andy82GT » Tue Sep 25, 2018 7:36 pm

Did several fixes to the car:
-replaced master cylinder
-new vacuum lines
-new grounds
-fixed coolant hose leaks
-removed PCV and replaced with crankcase vent catch can

Now the AEM boost gauge and the factory gauge do not read higher than 8 psi no matter how I adjust Gillis manual boost controller. MBC does change engine performance though.

Both gauges used to vary with MBC adjustments, and I had it normally dialed to 18 psi.

BTW, it has PE SVO ECU, Big VAM, IC, header, 3" exhaust, A4LD tranny (yeah I know...)

I cleaned the MBC, double checked vacuum hoses, disconnected factory gauge - still the same. WTH? Any ideas?
82 Mustang GT, 408W install in prog
83 Ranger, 302, T5
85 XR4TI, need to do AT>T5 swap! Arrgh!

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Andy82GT
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Re: Boost Gauge max reading not changing?

Post by Andy82GT » Tue Sep 25, 2018 7:51 pm

Also I had disconnected and plugged vacuum lines to turbo overboost switch and EGR solenoid (EGR has been deleted).
82 Mustang GT, 408W install in prog
83 Ranger, 302, T5
85 XR4TI, need to do AT>T5 swap! Arrgh!

my8950
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Re: Boost Gauge max reading not changing?

Post by my8950 » Tue Sep 25, 2018 9:52 pm

The boost gauge is hooked directly to the manifold, or compressor outlet?
If not, do that, sounds like you have a leak somewhere in the line of the gauges.

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Andy82GT
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Re: Boost Gauge max reading not changing?

Post by Andy82GT » Wed Sep 26, 2018 5:34 pm

Gauge is hooked to vacuum tree that is connected directly to intake. Since both aftermarket and factory gauges were reading the same, I figured problem was somewhere else. But with my8950 suggesting possible gauge line leak, I guess if one was leaking it would affect the other, right?
And I replaced most all vacuum lines EXCEPT the ones feeding the gauges soooo.... I will test with air from compressor.

Crazy if that line JUST decided to leak after fixing other stuff, but then that's the magic of the XR4 gremlins. :evil:
82 Mustang GT, 408W install in prog
83 Ranger, 302, T5
85 XR4TI, need to do AT>T5 swap! Arrgh!

Ed Lijewski
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Re: Boost Gauge max reading not changing?

Post by Ed Lijewski » Wed Sep 26, 2018 6:35 pm

I suspect your catch can set up may be involved.

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Re: Boost Gauge max reading not changing?

Post by my8950 » Wed Sep 26, 2018 7:21 pm

Leave one connected where it's at and move the other one to compressor out.
See if the one you move changes.

Also, be sure to cap the spot where you remove the one gauge from... :lol:

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Re: Boost Gauge max reading not changing?

Post by Andy82GT » Fri Sep 28, 2018 8:19 pm

Applied 25 psi with air compressor separately to OEM and aftermarket gauges lines, both appear to hold ok.

As was suggested, connected OEM gauge to turbo compressor outlet and left aftermarket connected to original vacuum tree. Drove it a few times, adjusting Gillis valve up and down each time, OEM gauge always read 10 psi max, while AM gauge still read 8 psi max.

So as before, engine performance SEEMS to change with Gillis valve adjustment, but not the boost gauges. :?

Tomorrow gonna inspect all IC and vacuum line connections again, and probably start disconnecting/capping off vacuum lines to brake booster, tranny modulator, and other stuff to see if I can narrow down...
82 Mustang GT, 408W install in prog
83 Ranger, 302, T5
85 XR4TI, need to do AT>T5 swap! Arrgh!

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Re: Boost Gauge max reading not changing?

Post by Andy82GT » Fri Oct 05, 2018 8:39 pm

Update: borrowed a calibrated vac/pressure gauge from work and hooked up in place of my pillar mounted VDO gauge, still read no more than 10 psi.
Disconnected vacuum line to tranny modulator, still no difference. Disconnected brake booster, this time read no more than 8 psi (weird).

How the heck does boost increase/decrease with Boost Controller adjustment, but gauge does not???

I don't see how my having removed PCV/plugged intake port, and replacing with open air catch can would affect vacuum/boost, but I guess tomorrow I will switch back and see.

Starting to drive me a little insane already...
82 Mustang GT, 408W install in prog
83 Ranger, 302, T5
85 XR4TI, need to do AT>T5 swap! Arrgh!

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Re: Boost Gauge max reading not changing?

Post by my8950 » Fri Oct 05, 2018 9:06 pm

Does it peak up and drop off? Or does it just limp and go up to 10 or so and hold strong.
I'm wondering if you have a leak somewhere due to a hole in a hose.
Doesn't make sense, but we also cant see how you have it wired & what's really connected where on your engine to make sense of it.

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Re: Boost Gauge max reading not changing?

Post by merkurdriver » Sun Oct 07, 2018 1:54 am

How about the "new vacuum lines" you mentioned? What size/material, etc.?
I'm wondering if you could have a "new" vacuum line that is leaking around the fitting...but only at a certain boost pressure.
I use tiny black zip ties on all of my vacuum lines that change to pressure lines when under boost.

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