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Underbody braces, turbos and more!

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PostPosted: Wed May 31, 2006 12:49 am 
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THE FOLLOWING WORK IS VERY DANGEROUS --AND SHOULD ONLY BE DONE
BY QUALIFIED SERVICE PERSONELL.




this write up will show how to install a pressure gauge on a G10 TBI system.

you will need the following tools/supplies:

a 0-60 psi pressure gauge rated for oil and gas

a 5/16" metal "T" fitting rated at 100 psi or more.

1 foot of 5/16" high pressure fuel line

the proper fuel line clamps to assemble the test gauge.



Image



Image


THESE PICTURES SHOW FUEL LINE REMOVAL- BE CAREFUL, FIRE HAZARD


Image



Image



Image


I know, we are all good mechanics, but PLEASE BE CAREFUL DOING THIS PROCEDURE

good luck :)


Last edited by 87t1 on Mon Jul 03, 2006 8:10 am, edited 5 times in total.

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PostPosted: Wed May 31, 2006 5:00 pm 
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Just out of curiosity - you Tee'd a pressure guage into the line supplying the fuel pressure regulator - would that show the pressure at the TBI or would show the unregulated pressure? Or are the two actually the same thing?


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PostPosted: Wed May 31, 2006 6:41 pm 
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hello, fordem

the pressure regulator is in the fuel return passage to the tank. the gauge reads the same pressure applied to the fuel injector (s). :)


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PostPosted: Wed May 31, 2006 8:08 pm 
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i think they would be the same thing because it will read the backpressure off of the regulator


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PostPosted: Wed May 31, 2006 8:56 pm 
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87t1 wrote:
the pressure regulator is in the fuel return passage to the tank. the gauge reads the same pressure applied to the fuel injector (s). :)


actually, thats backwards. to read pressure at the injectors you need to have it after the pump and before the tb.

fuel system goes fuel pump-filter- injector(s) - regulator- return to tank.

if you had the guage after the regulator, you should read zero pressure.

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PostPosted: Thu Jun 01, 2006 2:06 am 
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hi, tuffcar

a pic of the gauge install

Image

fordem wrote:
Just out of curiosity - you Tee'd a pressure guage into the line supplying the fuel pressure regulator - would that show the pressure at the TBI or would show the unregulated pressure? Or are the two actually the same thing?


i replied:


hello, fordem

the pressure regulator is in the fuel return passage to the tank. the gauge reads the same pressure applied to the fuel injector (s).


(here is a pic of the TBI, showing fuel passages.)

Image

the fuel comes in from the pump (red circle), into fuel galley through passage marked with red dots.
once here, fuel is either used by the fuel injector (blue dots) or returned to the tank using return passage (yellow dots).
the fuel pressure regulator closes off the return passage (yellow dots) to raise fuel pressure.

As connected in the first picture, the gauge will indicate fuel pressure to the injector..... :)


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PostPosted: Mon Dec 25, 2006 5:55 pm 
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Not trying to add fuel to this fire, but was the line bled of air before the pressure reading was obtained?

If not, then the pressure will be inaccurate as we all know that air bubbles will compress when pressure is applied. Hence it will cause an incorrect pressure reading.

Just like in a brake line when there is air present.

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PostPosted: Mon Dec 25, 2006 9:26 pm 
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metro freak wrote:
Not trying to add fuel to this fire, but was the line bled of air before the pressure reading was obtained?

If not, then the pressure will be inaccurate as we all know that air bubbles will compress when pressure is applied. Hence it will cause an incorrect pressure reading.

Just like in a brake line when there is air present.


During the leak test process the air is pumped back to the fuel tank via the fuel return line. :)


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PostPosted: Mon Dec 25, 2006 10:23 pm 
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metro freak wrote:
Not trying to add fuel to this fire, but was the line bled of air before the pressure reading was obtained?

If not, then the pressure will be inaccurate as we all know that air bubbles will compress when pressure is applied. Hence it will cause an incorrect pressure reading.

Just like in a brake line when there is air present.


Not necessarily.

The pump is an infinite pressure supplier. It doesn't matter if there is air in the system or not. The regulator will block anything from returning in the tank as long as the pressure hasn't been reached. No matter how much air you have in the line. Just like when you pump the brake again and again to finally have the required pressure to move the piston in the calipers and block the rotors. Air will compress to the exact same pressure as the fuel surrounding it. Compressing abilities are not relevant to pressure readings.


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PostPosted: Tue Dec 26, 2006 3:38 pm 
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87t1 wrote:
metro freak wrote:
Not trying to add fuel to this fire, but was the line bled of air before the pressure reading was obtained?

If not, then the pressure will be inaccurate as we all know that air bubbles will compress when pressure is applied. Hence it will cause an incorrect pressure reading.

Just like in a brake line when there is air present.


During the leak test process the air is pumped back to the fuel tank via the fuel return line. :)


In the picture it shows the length of hose from the tee fitting to the gauge. No way to pull the air out of that hose from the gauge while pressure is being applied by the fuel pump. :?

Not even if a vacuum was created in the line due to fuel passing the Tee in the line. :?

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PostPosted: Tue Dec 26, 2006 3:40 pm 
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whats all this big deal about? the way he has listed to check the fuel pressure, is 100% the correct method.

air is of no concern, at all, none.

also, the fuel pump does not provide infinite pressure, it provides infinite FLOW. there is a marked difference in that.

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PostPosted: Tue Dec 26, 2006 3:54 pm 
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Infinite pressure! Infinite flow!! Where do I find a pump like that !?!


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PostPosted: Tue Dec 26, 2006 3:55 pm 
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Quote:
["sportage4x4"]whats all this big deal about?


Obviously with the multiple replies to fuel pressure and fuel pumps, there is some discussion to be had as there ARE some things that others know about them and the way they work.

Quote:
air is of no concern, at all, none.


??????

Quote:
also, the fuel pump does not provide infinite pressure, it provides infinite FLOW. there is a marked difference in that.


Jard made that statement right there.......

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PostPosted: Tue Dec 26, 2006 4:13 pm 
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metro freak wrote:
In the picture it shows the length of hose from the tee fitting to the gauge. No way to pull the air out of that hose from the gauge while pressure is being applied by the fuel pump.


My bad..I was looking for a fast way to respond to your question without a lot of talk.
The pressure gauge install is out of the FSM and does indicate fuel rail pressure.
The air trapped in the hose will compress until it reaches pressure equlibrium with the fluid around it.
The air volume will remain constant until the fluid pressure changes.
Air trapped in the lines has no effect on the pressure gauge reading.


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PostPosted: Tue Dec 26, 2006 9:40 pm 
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87t1 wrote:
The pressure gauge install is out of the FSM and does indicate fuel rail pressure.
The air trapped in the hose will compress until it reaches pressure equlibrium with the fluid around it.
The air volume will remain constant until the fluid pressure changes.
Air trapped in the lines has no effect on the pressure gauge reading.


87t1,

if we still had karma points , i am sure you would have a TON!!! :D

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PostPosted: Tue Dec 26, 2006 9:45 pm 
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If I would have explained myself better the first time, none of this would have happend :shock: :lol:


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PostPosted: Wed Dec 27, 2006 1:02 am 
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metro freak wrote:

Quote:
air is of no concern, at all, none.


??????


You can see air as an energy accumulator. Just like your shock absorber, it temporarily stores part of the pressure supplied by the fuel pump. It doesn't affect the reading as long as you give a few miliseconds to the system to compress the air in the line.

Ever hooked a mechanical oil pressure gauge? the small plastic line remain air-filled all the time because air has no place to escape. It doesn't impede on the readings.


Last edited by Jardamuth on Wed Dec 27, 2006 1:11 am, edited 2 times in total.

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PostPosted: Wed Dec 27, 2006 1:09 am 
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sportage4x4 wrote:

also, the fuel pump does not provide infinite pressure, it provides infinite FLOW. there is a marked difference in that.


Thank you professor. What I meant was that the pressure will be there forever as long as the pump is running. From this point of view, flow or pressure are the same thing. You cannot have one without another, just like volts and watts.


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PostPosted: Wed May 21, 2008 5:59 pm 
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Now we know how to test fuel pressure... what PSI is it supposed to be at?

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PostPosted: Sat Jun 07, 2008 2:28 am 
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G10 n/a 13-20 psi at idle, depending on coolant temp. :)


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PostPosted: Thu Aug 11, 2011 12:40 pm 
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Why does the coolant temp effect the pressure ?


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PostPosted: Mon Aug 15, 2011 1:29 am 
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Pressure fluctuation due to coolant temperature could be the result of several factors.
Some could say it is because the regulator is a part of the throttle body which heats up making the regulator spring expand, increasing pressure.
I'm sure on the same note, others would argue fuel density is lower in a hot regulator, allowing more fuel to squeak past the regulator causing a pressure drop.

The truth is neither paths of thought are true. (I just want to clear that up right now)
The reason fuel pressure fluctuates at the line relative to coolant temperature is quite simple. Fuel pressure regulators regulate pressure relative to engine vacuum, not atmospheric pressure. A cold engine in warm-up mode idles faster than it's base warm idle setting so it can warm up faster. As a result, the vacuum within the intake manifold is lower than warm idle as the cold idle circuit is allowing more air to be consumed. The result is a increse in fuel pressure at the line, as the relative vacuum is lower. Once the engine is warmed up, vacuum increases as not as much air and fuel are needed to keep the engine running. Cold idle circuit closes, vacuume increases and fuel pressure decreases.

Remember, the regulator regulates fuel Pressure so that pressure and flow rates are consistant no matter what vacuum the manifold is seeing.

So, if you need 30 psi pressure at the head of an injector, and the head is seeing -10 pis, you only need to feed the injector with 20 psi to meet the 30 psi requirement. However, if the head is seeing -25 psi, all you need is 5 Psi to feed the injector, and maintain the required flow rate. This is why the fuel pressure fluctuates. If we assume the same figures as previously mentioned, but with a maximum fuel pump pressure of 35psi, the regulator would be dumping enough fuel to lower the main line pressure to 5 psi. A difference of 30 psi from max output. But the pressure hat the head of the injector stays at 30 Psi.

This is all done to maintain injector flow rates. Otherwise the ecu would have no idea how long a pulse is required to adequately fuel an engine.

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