View Full Version : wierd boost readings

11-03-2010, 02:37 AM
ok, I think Ive finally run out of possible answers for this problem so Im looking for some new ideas.

88 ST165, 2nd gen twin scroll turbo, stainless DP, HKS knockoff SSQV, Home Depot ball and spring MBC, Sunpro -30/+20 boost gauge, radio shack "blackbox" FCD tuned to right around 15lbs.

The problem Im having is that I cant properly monitor my boost. If I remove the boost controller I build a dead on 10lbs of boost. My gauge will accurately read that. If I put the MBC in the loop, I can affect the boost level but my gauge goes wonky (I think thats a technical term). The needle will rise and I build boost fine but once the gauge hits about 12lbs it then "pops" back down to around 5lbs. When the gauge drops I do NOT feel a corresponding power drop, so I dont think Im losing that boost level. Also, I can dial up the boost to the point that I hit boost cut, so I know the boost sensor is at least seeing that peak.

Simple, it must be the gauge right? I tested the gauge using my compressor however and it reads just fine easily to 20lbs. It will hold that pressure for as long as I want it to. I dont believe the gauge is the problem at this point (even though it is a cheap gauge and I planned to replace it soon). Is there anything in the 1st gen 3SGTE system that "limits" boost mechanically outside of the "fuel cut" failsafe?

Im probably going to just order a new boost gauge anyway, but I was wondering if anyone else has ever experienced anything like this.

11-03-2010, 02:50 AM
Wel, the symptoms would make me think of an actual loss or fluctuation in boost pressure. Wastegate or boost controller, timing, weak spark... but, if you're thinking that it's just an indication and not an actual change in boost pressure or power, could only think yeah, issue with the gauge or the vacuum source for it.

11-03-2010, 03:07 PM
Are you running your boost gauge on a dedicated line? I had an automoter that wasn't reading right and it got a little better when I ran it directly from the intake with no other vacuum intersections. Turned out that my gauge was miscalibrated, but I've never had a problem with the prosport that I've been running for the last few years.

It just helped to eliminate the possibility of false readings due to boost leaks on the signal line.

11-03-2010, 05:52 PM
yes, someone else suggested I try that. There are limited options for running it on its own line but I came up with a plan last night to try that today.

I questioned the gauge right off the bat, but when I test it off other sources it registers pressure just fine. The other wierd thing thats so curious is why it only does it over 10lbs?
also the transition from 10 to 5 is so sudden you can actually hear the needle in the gauge slam back down. Its not like the reading "bleeds off"

I too planned to go with a nice set of prosports, but was waiting until I could afford to do the whole job at once with Boost, wideband, and EGT in the dash vent, and 45mm oil press, water temp and amp in the dash replacing the stockers. I'll try the dedicated line today and report back.

11-03-2010, 07:04 PM
Well sir, as you have been trying to help RedRkt troubleshoot his starter issue, the logic here is, if the gauge works on the bench test and not in the car, you know it isnt the gauge.

So, yeah, get as dedicated of a line as you can get. I always just Tee into the line that fees the stock turbo pressure sensor, OR unplug the turbo pressure sensors vacuum line, and plug your boost gauge up to the nipple that serves it.

In addition to that, there should be 2 aux nipples on the back of the intake manifold, and one is always open by default to be used by you for your own purposes. Use one of them.

11-04-2010, 01:22 AM
lol, yeah Im over that thread.

So I feel kinda foolish here. I pride myself on being able to sort out my own issues given enough time. When I do end up getting beat by something it almost always ends up being a simple fix that I just dismissed as "NAAaaa, couldnt be that"
I switched the gauge to the nipple that feeds the pressure sensor on its own line, nothing T'ing off that. I moved the pressure sensor to a T off the one next to it, that feeds the CC, regulator, etc. problem solved! Boost registers right up to 13 and at 14 Im hitting fuel cut. Im gonna tweak my FCD a little and shoot for cut at 16, but thats a different discussion.

SO the lesson here is, if you can put your boost gauge on its own line, do it. Thanks for the help. Sometimes I just need to write it down for it all to click, not to mention a second set of eyes always helps.

P.S. The two nipples on the back of my plenum are both being used. One feeds the idle up controller and the other feeds....a metal spider line...TVSV? If thats what that ends up being I can use that port in the future since Im not using the TVSV.