:driving: Turbo Boost Gauge readings

[nvno1]

Good Day All,

I was reseaching and not able to get a true understanding with Boost gauge reading.

I have a aftermarket boost gauge and wanted to know what it should read vs what i am getting to help with some troubleshooting. At what Rpms should the Hg be in and when does it go to possitive numbers. Meaning when does the needle on the gauge leave the -Hgs and goes into the measurable boost numbers?

I know it seems like i'm:beatdeadhorse5:, however i cant find anything concrete on this matter.

Thanks in advance,
SJR.

[dudemaaan]

Hg " is vacuum you will see this at idle and off throttle. Anywhere from 14-20"hg, depending on your porting. Positive pressure will start as soon as you step on the gas and the turbo begins spooling. For stock twins you should see 10psi by like 2k rpms with a small dip to 8 psi at 4k rpms and then right back to 10 psi. Single turbos will just depend on their size. Some gauges read in bar, in which case you will have to convert my psi measurements.

[nvno1]

Hg " is vacuum you will see this at idle and off throttle. Anywhere from 14-20"hg, depending on your porting. Positive pressure will start as soon as you step on the gas and the turbo begins spooling. For stock twins you should see 10psi by like 2k rpms with a small dip to 8 psi at 4k rpms and then right back to 10 psi. Single turbos will just depend on their size. Some gauges read in bar, in which case you will have to convert my psi measurements.

WOW thanks for the prompt response. Well the car is a 87 TII,stock motor, RB downpipe and walbro fuel pump. The gauge reads in PSI, my gauge doesnt go that high. Well she is not on the road yet(no driveshaft right now).

[Phoenix7]

you won't build boost in neutral....with stock injectors you should not see more than 10 psi but, like I said, you won't build boost without load.

[nvno1]

you won't build boost in neutral....with stock injectors you should not see more than 10 psi but, like I said, you won't build boost without load.

great:driving:, however what should the Hg be at 750, 1000, 1300?

[RETed]

Vacuum reading don't really matter except for maybe at idle.
When driving, when you see vacuum on the gauge, it just means the turbo isn't working to produce boost.
Trying to see your vacuum numbers while driving is a royal waste of time.


-Ted

[NoDOHC]

I think a stock ECU rotary should run about -15 to -18 "Hg at Idle. As you gently rev the engine in neutral, this will dip a little about 1200 rpm (-18 to -21), but remain relatively consistant after that.

With an aftermarket ECU I run about -15 " Hg when cruising on the highway and about 21 " Hg at idle (Timing advanced over stock).

[dudemaaan]

Ignore what i said about 10 8 10 boost, thats for 3rd gen rx7's with sequential turbos. Didn't think about it being a 2nd gen. I wouldn't worry too much about vacuum provided you have good compression numbers. But stock ports usually pull over 16". My large street port pulls around 15" at 1000 rpm.

[nvno1]

Vacuum reading don't really matter except for maybe at idle.
When driving, when you see vacuum on the gauge, it just means the turbo isn't working to produce boost.
Trying to see your vacuum numbers while driving is a royal waste of time.


-Ted

Wow Ted, i feel so honored you replied on my post. I'm not driving the car right now, my main reason for asking about the vaccum is the idle is not set and i wanted to know if at 1000rpms, if the reading is right? I have checked for vac leaks and nothing has surfaced, however i dont have another way of verifing there is no leak. So i figured the vac reading on my boost gauge would fall as a back up.

[nvno1]

Ignore what i said about 10 8 10 boost, thats for 3rd gen rx7's with sequential turbos. Didn't think about it being a 2nd gen. I wouldn't worry too much about vacuum provided you have good compression numbers. But stock ports usually pull over 16". My large street port pulls around 15" at 1000 rpm.

:), Yeh i was still looking around and nothing seems very concrete on the matter on hand. I was just trying to see if there was a leak(still havent found any) and having low vac reading would state a vac leak right if the compression, fuel pressure and ignition is good right?

[vex]

Wow Ted, i feel so honored you replied on my post. I'm not driving the car right now, my main reason for asking about the vaccum is the idle is not set and i wanted to know if at 1000rpms, if the reading is right? I have checked for vac leaks and nothing has surfaced, however i dont have another way of verifing there is no leak. So i figured the vac reading on my boost gauge would fall as a back up.

Ah, the mysterious missing vac leak, where you know you have one, but just can't find it.

Try to get a smoke machine. That should show you exactly where your vac leak is. I am in the process of doing this, but have a few things to do first before I do (mainly earn money).

as for boost and vac readings. Are you sure your gauge is grounded properly? Not having a proper ground will give you very messed up readings. If you have, expect to have anywhere from 14-20+inHg in vac. I'm currently pulling in around 14inHg on a fresh rebuild (though compression is absolutely fine). Good luck though.

[nvno1]

Ah, the mysterious missing vac leak, where you know you have one, but just can't find it.

Try to get a smoke machine. That should show you exactly where your vac leak is. I am in the process of doing this, but have a few things to do first before I do (mainly earn money).

as for boost and vac readings. Are you sure your gauge is grounded properly? Not having a proper ground will give you very messed up readings. If you have, expect to have anywhere from 14-20+inHg in vac. I'm currently pulling in around 14inHg on a fresh rebuild (though compression is absolutely fine). Good luck though.

Hey thanks for the info. The gnd is good but my Vac readings are not that high(if memory serves me right). With the smoke machine the smoke will be inserted to a opening in the intake somewhere and pressurized through the system with the vac leak showing up by means of escaping smoke?

[vex]

Hey thanks for the info. The gnd is good but my Vac readings are not that high(if memory serves me right). With the smoke machine the smoke will be inserted to a opening in the intake somewhere and pressurized through the system with the vac leak showing up by means of escaping smoke?

Not quite. With the engine running (and you're reading low on vac) start the smoke machine. Point the stream over the engine and look for locations where the smoke enters the engine where it shouldn't be. Gaskets, Hoses, connections, etc are good places to look.

[Flooder]

you won't build boost in neutral....with stock injectors you should not see more than 10 psi but, like I said, you won't build boost without load.

Sure you will, just not very much. You can still make your BOV vent when in neutral.

[nvno1]

Not quite. With the engine running (and you're reading low on vac) start the smoke machine. Point the stream over the engine and look for locations where the smoke enters the engine where it shouldn't be. Gaskets, Hoses, connections, etc are good places to look.

:biggthumpup:Iknew that:). not really..so like i was messing with the car somemore and my Hg reading are a lot lower than what i thought it was like -5@1100rpm and at 3-4000rpms the reading is about -14hg. diffy a vac leak somewhere. my oil pressure is reading good, temp is cool, when i press in my tps the rpms climb. i got it on video but my wk pc doesnt have a dvd player, i get my mgr to load i and send it to me email so i can post it.

[nvno1]

Sure you will, just not very much. You can still make your BOV vent when in neutral.


matter of fact it did...my boost gauage hopped up to nearly 5psi. it was pretty cool to see.

[nvno1]

OkAy SOoooo...like my Hg rading are low..i got video coverage of the issue, no dvd player on the wk pc and dont have internet access else where right now. so what i decided to do is block everything off with block off plates, get another BAC, change all the remaining hoses, upper to lower intake gasket and check the fuel injectors. from the information you guys have given me the Hgs are to low. i am sure that it HAS to be a vac leak somewhere. oil pressure good, not getting hot, no bubbling at the rad, no sweet smell out exhast, and hard to start unless i play with the trottle.:(

[Flooder]

"The HG's are too low"? WTF. lol.

[RETed]

from the information you guys have given me the Hgs are to low. i am sure that it HAS to be a vac leak somewhere. oil pressure good, not getting hot, no bubbling at the rad, no sweet smell out exhast, and hard to start unless i play with the trottle.:(

I doubt that's the case.
I put money it's something else.

Other causes that can "fool" your boost gauge to show improper vacuum while driving:

* ignition problems
* fuel delivery problems
* misfire
* hurt motor


-Ted

[NoDOHC]

Have you compression checked this engine yet?

[nvno1]

"The HG's are too low"? WTF. lol.

Flooder, i have the feeling that you are under the impression that i know a ton about Rx7s or perhaps you were just bored:). please feel free to provide useful information as to why what i stated was so funny.

From what i read and am lead to believe is that, if you have a VAC Leak ur car will not idle correctly. The internal pressure would be off thus causing the Hgs to not be at lets say -10hg min (on 85psi per rotor(front/rear) @1000rpms. I have good fuel pressure, set the timing to TDS and cas aligmnt as per FSM, good plugs, good A/F meter and no flooding. I have used two 1/2 cans of starter fluid to try and locate a vac leak and nothing have surfaced.

The troubleshooting help that I've recieved, mostly everyone is stating that it is mostlikely a vac leak. When i posted the ? as to what Hgs read under idle there was no response from you in that regards:) and the information that i got really has me believing this is the issue. please coorect me if im wrong.

[nvno1]

I doubt that's the case.
I put money it's something else.

Other causes that can "fool" your boost gauge to show improper vacuum while driving:

* ignition problems
* fuel delivery problems
* misfire
* hurt motor


-Ted

Ted i'm not driving;(, I got strong spark, getting good signal from the cas, plugs are good, coils seem good, good clear fuel flow, 85psi on front and rear and it doesnt backfire.

[nvno1]

Have you compression checked this engine yet?

yep, 85psi on front and rear before warm up.;)

[NoDOHC]

I would go and get a real vacuum gauge and put it on the car. Connect it to the port that comes out of the side on the manifold right next to the split between upper and lower manifolds.

I am inclined to mistrust your stock vacuum gauge.

If the vacuum gauge is reading identical to your boost gauge, you almost have to be running on 1 rotor or it should rev up really fast.

[nvno1]

I would go and get a real vacuum gauge and put it on the car. Connect it to the port that comes out of the side on the manifold right next to the split between upper and lower manifolds.

I am inclined to mistrust your stock vacuum gauge.

If the vacuum gauge is reading identical to your boost gauge, you almost have to be running on 1 rotor or it should rev up really fast.

Thanks NoDOHC, the gauage is aftermarket and i am starting to believe it might be operating on one rotor(fuel injector not pulsing). The reason i state this is because the car is hard to start(unless i play with the pedal) in addition to several turns of the key. When it does start it sometimes sort of stutters then rev good but the rpms drop then hop back up, then drop, then hops up again. This shit is getting on my nerves!!! When the car warms up at idle its does the same thing...hops from 1000 to bout 1600-1800 then back down to 1000.

What freaking me out is that the compression is 85psi front and rear, fuel pressure is good, no flooding, oil pressure good, plugs clean, oil clean, af meter reading good, once running revs up quick when i hold the pedal however the rpms will drop then come back to where it was previously.

This car is a common swap:
84 gslse, 87 TII(s4) complete pass harness(nothing was cut) still connected to the motor,drivers harness made and continu test done(ends tested), walbro pump, stock ecu, 87 TII tranny(still shopping for driveshaft).

[Rx-7fetish]

When it does start it sometimes sort of stutters then rev good but the rpms drop then hop back up, then drop, then hops up again. This shit is getting on my nerves!!! When the car warms up at idle its does the same thing...hops from 1000 to bout 1600-1800 then back down to 1000. Sounds like tps to me, have you checked it besides poking it with your finger, mainly the test lights or volt meter?

What freaking me out is that the compression is 85psi front and rear, fuel pressure is good, no flooding, oil pressure good, plugs clean, oil clean, af meter reading good, once running revs up quick when i hold the pedal however the rpms will drop then come back to where it was previously. Sounds like your afm is going bad or there is an intake leak after it. I had the same symptoms on both my 10th ann and an 87 gxl, the 10th ann had an intake leak between the afm and the turbo, and the afm was going bad on the gxl.

[NoDOHC]

Hopping is common around that speed, the stock ECU advances the timing quite a bit at 1200 rpm. Try adjusting the idle down to 750 rpm. It could be the TPS, but it is not uncommon for a properly tuned car to hunt if the idle speed is set too high.

[nvno1]

Sounds like tps to me, have you checked it besides poking it with your finger, mainly the test lights or volt meter?

Sounds like your afm is going bad or there is an intake leak after it. I had the same symptoms on both my 10th ann and an 87 gxl, the 10th ann had an intake leak between the afm and the turbo, and the afm was going bad on the gxl.

Thanks for the response without the doggish dialog...i have check the TPS w/lights and a meter. when i press the TPS in all the way the motor heads to redline quick i mean fast dude.(;) love the TII motor bwoy)

we checked the afm per FSM and it fine according to the FSM..I plan on blocking off everything and triple checking the hoses. going to be replacing all vac lines that remain and the gasket for the Upper to Lower. can it test okay and still be bad?

[nvno1]

Hopping is common around that speed, the stock ECU advances the timing quite a bit at 1200 rpm. Try adjusting the idle down to 750 rpm. It could be the TPS, but it is not uncommon for a properly tuned car to hunt if the idle speed is set too high.


...NoDOHC thanks for your feedback always a pleasure to hear from you....the BAC is bad i believe, the plunger is not moving...just tested it..the BAC was not plugged in either at the time and i tried it plugged up and not plugged up same rpm hunting...waiting to get the block off plates and gaskets now.

[NoDOHC]

If the idle is above 750 and the engine is warm, the BAC should be shut. If you unplugged it and the pulsation didn't go away, I doubt that it is the issue. The stock ECU has a 'hole' in the timing map to make the engine idle down while getting excessive air and fuel (5 degrees ATDC). If I program a similar hole into my Haltech, it will do the same oscillations if I set the idle too high. Seriously, try adjusting the idle down to 750 with the check jumper connected and then see what it idles like.

This still doesn't explain why the engine is running at such low vacuum. You said that it revs very quickly from idle if you floor it?

Are you sure that the timing is correct?

By press the TPS in all the way, you mean floor the gas, right? Pressing in the TPS without moving the pedal shouldn't make the engine rev to the redline.

[nvno1]

If the idle is above 750 and the engine is warm, the BAC should be shut. If you unplugged it and the pulsation didn't go away, I doubt that it is the issue. The stock ECU has a 'hole' in the timing map to make the engine idle down while getting excessive air and fuel (5 degrees ATDC). If I program a similar hole into my Haltech, it will do the same oscillations if I set the idle too high. Seriously, try adjusting the idle down to 750 with the check jumper connected and then see what it idles like.

This still doesn't explain why the engine is running at such low vacuum. You said that it revs very quickly from idle if you floor it?

Are you sure that the timing is correct?

By press the TPS in all the way, you mean floor the gas, right? Pressing in the TPS without moving the pedal shouldn't make the engine rev to the redline.

This is going freak you guys out..

the car starts after a few cranks(no bac connected) ..however if i play with the pedal it starts up..stutters a little then she picks up...i have to play with the gas at little to keep her running. after about two mins of that she will run with no pedaling of the gas pedal. she is still looping.

when under the hood and i press in the TPS plunger, not by way of the pedal, the rpms start climbing.......crazy i know.. the timing was set by way of the FSM...aligning the second notch(if facing the car,second notch to the right) on the main pulley to the needle on the front cover(gslse) and the cas has been removed and placed back with the little circle mark on the gear wheel with the pointer on the shaft on the cas.

[NoDOHC]

I hate to say it again, but this sounds like a vacuum leak (it may be a mal-adjusted bypass screw in the AFM - if S5 Turbos have it).

If the engine revs when the TPS is pressed, it must be getting air from somewhere. It is probably running really lean and the Idle adjust screw is turned out so far to keep it running that it really is idling with that poor of vacuum.

Are you pushing the part-range or full range TPS? (I think that the part-range TPS is pressed when the throttle is shut, so I am guessing that you are pressing the full range TPS).

My theory is that you are pushing the full-range TPS in, which is causing throttle enrichment, which is allowing your woefully under-fueled engine to rev up. This also explains your idle vacuum issue (a large vacuum leak has no air going through the AFM when there is significant vacuum because it all comes in the leak).

Places to check:
Input to BAC Valve
Injector Pintle cap seals
emissions removal plates (if you have them)
cruise control (if you have it)
Vacuum Booster for brakes.

Let me see if I can describe what the engine is doing.

If you try to crank it without pushing the gas pedal, it will not start.
If you rapidly pump the gas pedal while cranking, it will try occaisionally (almost always as the gas is being pressed down) and will eventually start.
Once running, it revs to about 2,000 rpm, at which point it is misfiring so bad that it dies.
On the way down, the engine catches about 800 rpm, it fires several times in a row which causes it to rev to 2,000 rpm again.

If this is what it is doing (which I think you have already described) then it is running too lean to fire as it revs up. This is almost a dead giveaway for a vacuum leak. The only other possibility is that the FPR is referenced to a ported vacuum source (which shouldn't make this drastic of a change).

Edit: Things to try.
Press the Full Range TPS in and hold it in with the BAC unplugged. If my theory is right, it will rev for a little while, reach 3,000 to 4,000 rpm and start to stutter and eventually return to the loping idle.
Take off every vacuum line except the one to the MAP sensor and the FPR and plug the holes.
Make sure that you have the correct AFM for the car.
Get more carb cleaner or a smoke machine
Watch your fuel pressure (I doubt this is it).

[TitaniumTT]

you have an engine that produces 85 psi of compression. That is below what Mazda considers the minimum for a running engine to produce. It's no wonder that your engine hunts, idles poorly, and pulls lower than average vacuum. It's becuase the engine is old and tired or improperly rebuilt. My ported RE makes 120-125psi of comp on all 6 faces and pulls ~15" vac at a 750rpm idle and it hunts a little. I bumped the idle to 1000 rpms and she'll pull 19" of vac all day and not hunt around much at all.

I think you're chasing a problem that doesn't exist. How did you run your comp test?

By the way, I noticed that you said you're adjusting things per the gsl-se FSM. That is WRONG! You need to adjust things per the FSM that the engine belongs to. If the motor came from a 86-86 TII car, that's the FSM that you need to be looking at for adjustment.

[Max777]

I had a very simmilar problem with my 91 N/A.... that motor tested out 82psi compression. I never did figure out what the problem was before parting out the car, but I did alot of the same stuff that you did, and it just never liked to stay running.

One thing to check: Check the adjustment for the thermowax pellet on the throttle body, mine was out of adjustment and ran a lot better after I readjusted it to FSM spec.

[RETed]

front cover(gslse)

Ar you using the GSL-SE CAS too?
If not, can you verify that the GSL-SE and FC front cover pin locations are identical?
I don't like the idea that you're mixing and matching parts like this... :(


-Ted

[nvno1]

Sounds like tps to me, have you checked it besides poking it with your finger, mainly the test lights or volt meter?

sorry for the delay in responding...the tps has been checked with lights and multimeters..it appears to be good.

Sounds like your afm is going bad or there is an intake leak after it. I had the same symptoms on both my 10th ann and an 87 gxl, the 10th ann had an intake leak between the afm and the turbo, and the afm was going bad on the gxl.

I just got my block off plates, new waterpump housing and upper intake gaskets. when i take her apart i will change the clamps with some clamps i can tighten down...thanks for the input..if all else fails i will change the afm and the tps even though both tested fine.:(

[nvno1]

I hate to say it again, but this sounds like a vacuum leak (it may be a mal-adjusted bypass screw in the AFM - if S5 Turbos have it).

If the engine revs when the TPS is pressed, it must be getting air from somewhere. It is probably running really lean and the Idle adjust screw is turned out so far to keep it running that it really is idling with that poor of vacuum.

Are you pushing the part-range or full range TPS? (I think that the part-range TPS is pressed when the throttle is shut, so I am guessing that you are pressing the full range TPS).

My theory is that you are pushing the full-range TPS in, which is causing throttle enrichment, which is allowing your woefully under-fueled engine to rev up. This also explains your idle vacuum issue (a large vacuum leak has no air going through the AFM when there is significant vacuum because it all comes in the leak).

Places to check:
Input to BAC Valve
Injector Pintle cap seals
emissions removal plates (if you have them)
cruise control (if you have it)
Vacuum Booster for brakes.

Let me see if I can describe what the engine is doing.

If you try to crank it without pushing the gas pedal, it will not start.
If you rapidly pump the gas pedal while cranking, it will try occaisionally (almost always as the gas is being pressed down) and will eventually start.
Once running, it revs to about 2,000 rpm, at which point it is misfiring so bad that it dies.
On the way down, the engine catches about 800 rpm, it fires several times in a row which causes it to rev to 2,000 rpm again.

If this is what it is doing (which I think you have already described) then it is running too lean to fire as it revs up. This is almost a dead giveaway for a vacuum leak. The only other possibility is that the FPR is referenced to a ported vacuum source (which shouldn't make this drastic of a change).

Edit: Things to try.
Press the Full Range TPS in and hold it in with the BAC unplugged. If my theory is right, it will rev for a little while, reach 3,000 to 4,000 rpm and start to stutter and eventually return to the loping idle.
Take off every vacuum line except the one to the MAP sensor and the FPR and plug the holes.
Make sure that you have the correct AFM for the car.
Get more carb cleaner or a smoke machine
Watch your fuel pressure (I doubt this is it).

Hey NoDOHC, sorry for the delay...dont have access to a pc right now..in my last post i stated that i have a block off kit and will change my gaskets while i have the car apart i willdouble check the hoses and the AFM...I am hoping that it is a vac leak.


the starting thing is mainly just holding the pedal down a little bit. and i only see one TPS i have a 87TII S4

[nvno1]

you have an engine that produces 85 psi of compression. That is below what Mazda considers the minimum for a running engine to produce. It's no wonder that your engine hunts, idles poorly, and pulls lower than average vacuum. It's becuase the engine is old and tired or improperly rebuilt. My ported RE makes 120-125psi of comp on all 6 faces and pulls ~15" vac at a 750rpm idle and it hunts a little. I bumped the idle to 1000 rpms and she'll pull 19" of vac all day and not hunt around much at all.

I think you're chasing a problem that doesn't exist. How did you run your comp test?

By the way, I noticed that you said you're adjusting things per the gsl-se FSM. That is WRONG! You need to adjust things per the FSM that the engine belongs to. If the motor came from a 86-86 TII car, that's the FSM that you need to be looking at for adjustment.

must be nice to have a freshly built motor...i miss those days (except the break in part;))..the way i tested the compression was with a reg compression tester with the valve taken out and cold. i got 3even pulses. A friend of mines who builds motors told me the compression is okay , not great but okay. you noticed i was adjusting things per GSLSE FSM? where did to get that? i have the 86-88 FSM on hand.

[nvno1]

Ar you using the GSL-SE CAS too?
If not, can you verify that the GSL-SE and FC front cover pin locations are identical?
I don't like the idea that you're mixing and matching parts like this... :(


-Ted

Hey Ted,

i had no choice but to use the GSLSE front cover it a TII swap in a GSLSE. and the GSLSE uses a dizzy. im using the cas that came with the 87 motor. the pin looks to be in the same location.