No boost on factory boost guage

xmxaxnx2

New Member
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I'm quite sure these questions have appeared here already so, I apologize if these are redundant. However, I'm about to pull my hair out. I'm trying to figure out why my '87 GN I recently purchased is having such a boost issue. Originally my RPM and boost gauges were completely max when I would turn the key "on". I sent my Tach to Caspers and they recalibrated it. However once I reconnected it I didn't get any reading. I found a plug behind the alternator that looked a mess and decided to unplug it and wham the RPM was now working. Would anyone have an idea what that connection could have been and if it will be fine if it remains unplugged? (Pic #1 is of the plug.)

Next, I was told to check the MAP sensor to see why the boost wasn't showing on the dash. I replaced the hose and checked for vacuum and there was. I tested the switch by connecting the b to the c on the plug with a paper clip and the boost shot up to max. So my next plan is to change the MAP and see if that fixes the problem. On my car there is a BOV and was told it might be affecting the vacuum which in turn could be affecting the boost gauge. I've disconnected the vacuum lines attaching the BOV to what looks like a device connected to the fuel rail on the right of the fuel pressure gauge in my 2nd pic and instead of the BOV making it a 3-way connection I excluded the BOV and capped where the BOV connected to the air intake just behind the MAF. To keep it looking clean I kept the hose that connects them on and now the car seems to idle much better but still no boost on the gauge. (Pic #2 &3)

I also noticed the wastegate was adjusted all the way to about 1" and I had read it needed to be 1/8". So I set it to 1/8". Could that play a part in all my troubles? How do you test the trap in the wastegate to make sure it's opening and closing?

Finally, I was told that the ignition module could b the problem, that it needs to be a stock AC delco which is black as the aftermarket brands are grey. I looked at mine and it appears to be black however the base has a grey look. (Pic #4)

Can anyone help me with these questions?
-What is that plug and why does the RPM now work after disconnecting it?
-Why is my boost gauge still not working on the Tach?
-Should I remove the BOV?
-Do I have the right ignition module?
-How do I check the wastegate to make sure it's working properly?

I'd appreciate any help you all can give. This has been a real challenge.
 
The factory boost gauge wasn't very accurate when these cars were new. You need an aftermarket boost gauge if you are adjusting the boost levels. The factory tach was about as accurate as the boost gauge so if you want correct rpm you will need another way to measure.
I would ditch the bov . I've never seen a very fast car with one( I dont care if yours is fast bison doesn't run one) so I don't think it is needed.
Do you have a scan master? Do you have a fuel pressure gauge and a long enough line to tape it to the windshield? You need to know the file pump is supplying enough fuel to make the pressure rise 1:1 with boost. This isn't optional.
Put a meter on the posts of that coil each pair should show resistance. 12k ohm I believe.
Scanmaster should be your next purchase. If you are having detonation you will be wrenching.
 
Those two connectors behind the alternator are not supposed to be plugged into each other. They are test points. The green connector with the white wires is the tach test point. The black connector is the fuel pump test point. The tach won't work if they are plugged into each other. That is a common mistake. The ignition module has been changed. Do you have any type of scan tool? Check the plumbing and wiring going to the MAP sensor. Do you have a 2 bar MAP or 3 bar MAP. Do you have alcohol injection? Alcohol injection uses a 3 bar MAP so the stock gauge is going to read low if that's what you have. How many pounds of boost are you running? Do you have an aftermarket boost gauge hooked up?
 
I just noticed in one picture it looks like the connector that feeds the boost gauge from the MAP sensor isn't plugged into anything. Pic#2
 
The stock boost gauge on my '92 MR2T was probably just as inaccurate as yours and in my opinion any turbo vehicle, especially one that has upgrades requires accurate gauges or they go BOOM! I also deleted the BOV intake tube and went atmosphere but I also modified my MAF sensor, no stumbling or driving issues.

Are you still running the internal WG or have you gone external? I would go external for the simple fact you can change springs that help keep boost in check. This options only draw back is that springs wear out but they are easily changed so extra's kept on hand is a wise choice.

Good luck.
 
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You guys are great with the helpful input. I don't have a scan master but, have seen many using it so, I'm thinking I should as well. I was told the car has a 2 bar MAP sensor so, I replaced it to see if that would help with the problem. The plug for the MAP sensor was unplugged at the time of the picture as it was taken before it was replaced. The biggest dilemma is having to reinspect all the work previously done on the vehicle before I took ownership of it. While not knowing much about this particular vehicle it seems everything I'm checking with all your help has been done wrong. I'm running the internal WG but wasn't sure how to tell if it's really working. I was considering going with an external one however. I'm also looking for a shop nearby that specializes in GN work so I can b sure everything's just right. Would anyone have an idea of any GN specific performance shops located in or around the New England area?
 
You guys are great with the helpful input. I don't have a scan master but, have seen many using it so, I'm thinking I should as well. I was told the car has a 2 bar MAP sensor so, I replaced it to see if that would help with the problem. The plug for the MAP sensor was unplugged at the time of the picture as it was taken before it was replaced. The biggest dilemma is having to reinspect all the work previously done on the vehicle before I took ownership of it. While not knowing much about this particular vehicle it seems everything I'm checking with all your help has been done wrong. I'm running the internal WG but wasn't sure how to tell if it's really working. I was considering going with an external one however. I'm also looking for a shop nearby that specializes in GN work so I can b sure everything's just right. Would anyone have an idea of any GN specific performance shops located in or around the New England area?
If you can't find a shop, find a knowledgeable member. Look down on the bottom of the forums, there's a regional area. Post in there for help as well. Most members are more than happy to help for a free, a few bucks, or beer.
 
You would have 30 psi of boost if the internal wastegate was not working. IF the car is over boosting. Take a 7/32 hose and run it from the turbo, to the wastegate. This will net you wastegate spring boost pressure. It SHOULD be around 12 psi it can be more if someone cranked down on the adjustment rod on the wastegate, OR drilled HOLES into the wastegate.
Guys are running super fast with a internal gate, no need to go external unless you have a super big turbo and cannot control boost creep
 
Thanks for all the insight. I've found Cotton online but everything I put in for Bison shows no results like they might be out of business or something. I'm also gonna recheck my wastegate as the way it was set was far off from what I found it should be set at. Then again I have no idea if it was set that way based on whatever crappy work was previously done to it.
 
Could it be your not seeing any boost indication on your stock (inadequate) gauge because your not generating any boost? you say you found the wastegate rod adjustment off? do you have it properly loaded at about 1/8 to 1/4" tension on it?. with the rod off the puck arm, have your puck flush to the wastegate hole, then adjust it so there is about an 1/8th in needed to pull the rod back onto the arm, that should load the tension for a starting point.
 
I did set it at 1/8 and still wasn't seeing boost on the factory gage. I'm aware it is typically inadequate but I do like to know the factory stuff is operating correctly. I went and purchased a new MAP to see if that helps.
 
another thought would be if you had a voltage source (calibrator) you could induce a signal (0-5Vdc) on the center terminal of the map sensor while unplugged and see if it lights up the factory turbo gauge. I work in at a calibration lab so i have that kind of stuff laying around, lol. might save some troubleshooting time.
 
i would think 2.5 to 5 volts would light it from 0 to full scale since its absolute pressure (about 1 atmosphere or 1 bar at ambient pressure)
 
I did try testing the gauge by bridging the b&c points on the MAP plug with a paper clip and the boost bar shot up to maximum.
 
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