Fuel pressure

[jahjah]

Alright, this is kinda weird.

 

Heres the setup.

 

Clean tank. sucker was vatted and recoated 1.5 years ago

New Fuel pump

New in tank filter

New Filter before pump

No pinholes or anything on fuel pump pick ups

Lines are clean

Aeromotive rising rate FPR

Magna manifold w/ Magna Fuel rail

Summit 8an filter

8an stainless braided lines

No fuel leaks anywhere. trunk doesnt smell like fuel either

75 lb injectors

 

Alright. Now a few days ago I had my pressure set at 38 Idle. It does not fluctuate at idle. However I noticed today my pressure is at like 35. the car drives just fine still i'm sure my ego control is adjusting the ve some to keep the car running fine. Now its doecreased the pressure and increased it on its own. the fitting on the regulator is tight and definitely not loose.

 

any ideas?

[BIL]

Where is the pressure guage, and is it always connected. If it is under the hood the pressure may vary due to temperature. If the ambient (air) temperature was higher on the day it was "set" or if the motor was hot when it was "set" and the guage is permanently attached under the hood it could cause the different reradings.

[jahjah]

Where is the pressure guage, and is it always connected. If it is under the hood the pressure may vary due to temperature. If the ambient (air) temperature was higher on the day it was "set" or if the motor was hot when it was "set" and the guage is permanently attached under the hood it could cause the different reradings.

the gauge is connected right on my FPR which is on the driver's side apron, like where the stock coil was. yes its always connected. The weather has only been a few degrees different the last few days. and if what you said is true everyday i should get a different reading when i drive it in the morning (cold like 70 degrees) and a different reading when i drive it in the afternoon (hot like 90 degrees) but for a week i got the same reading. then it suddenly dropped 3psi afternoon and evening.

 

I appreciate the help but i don't think it has anything to do with ambient temperature? please correct me if i'm wrong

[BIL]

Jahjah, you have seen mine..it's attached to the FPR under the hood. I have to set pressure when the motor is cold. When it is hot or when it is hot outside here the guage reads higher. This is not unusual in anyway. I believe there was another thread about this a few months ago. The problem that can happen is inaccurate readins when hot and even damage to the guage that will cause it to not zero out. You might also see this in oil pressure guages. You indeed might have a different problem. I was just offering a possible cause. Try checking fuel pressure when "cold" several mornings in a row and see what your pressure is.

[Indiana]

You can google liquid filled fuel pressure gauge and heat and read all the info on how they need to be away from engine bay heat. Those inexpensive 10.00 gauges may change pressure 5-10psi when hot Vs cold. Air pressure changes with temperature. What is inside your gauge is a C shaped ring tube that expand and contracts from slight pressure changes and on the end is some mechnical wheel or gear that moves your indicator needle. Liquid filled gauges have all that in the liquid and that changes the shape of the tube just from the temperature so you must set the pressure cold as mentioned or move that gauge inside where the temps are stable. Gauges that are inside with outside sending units cost some $$.

 

http://en.wikipedia.org/wiki/Pressure_gauge

[jahjah]

Jahjah, you have seen mine..it's attached to the FPR under the hood. I have to set pressure when the motor is cold. When it is hot or when it is hot outside here the guage reads higher. This is not unusual in anyway. I believe there was another thread about this a few months ago. The problem that can happen is inaccurate readins when hot and even damage to the guage that will cause it to not zero out. You might also see this in oil pressure guages. You indeed might have a different problem. I was just offering a possible cause. Try checking fuel pressure when "cold" several mornings in a row and see what your pressure is.

Good suggestion. I'll try to keep track of it.

 

Now heres the question, if the gauge is indeed changing some on a hot motor vs a cold motor. which reading is more accurate?

[Shelby]

the engine is gona be use'd when it's hot , so go with the hot reading,,

make all adjustments after the engine has ran for a peroid of time and every thing is at operateing temps includeing the fuel pump,, say a good 30 minutes of operation , and all readings are gona be diff say on a 40f morning and a 80f afternoon

durring cool months the air is gona be denser more compress'd then on a hot day so more air will actualy be entering the engine at the same boost lb'age

[BrazilBoy]

You can google liquid filled fuel pressure gauge and heat and read all the info on how they need to be away from engine bay heat. Those inexpensive 10.00 gauges may change pressure 5-10psi when hot Vs cold. Air pressure changes with temperature. What is inside your gauge is a C shaped ring tube that expand and contracts from slight pressure changes and on the end is some mechnical wheel or gear that moves your indicator needle. Liquid filled gauges have all that in the liquid and that changes the shape of the tube just from the temperature so you must set the pressure cold as mentioned or move that gauge inside where the temps are stable. Gauges that are inside with outside sending units cost some $$.

 

http://en.wikipedia..../Pressure_gauge

Maybe this gauge would do the trick and its fairly cheap too..

http://cgi.ebay.com/ebaymotors/MK1-SMOKE-LENS-ELECTRIC-FUEL-PRESSURE-GAUGE-SENDER_W0QQitemZ370268358174QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item5635b6c61e

[jahjah]

thats basicly what i'm running

[Indiana]

Might as well hire a deaf piano tuner too you missed the point.

[BIL]

I agree, you missed the point. The guage should be cool or it will be less than accurate. That is why I suggested the morning. Set it cold and check it on several separate days. Once you have established the right fuel pressure you do the tuning with the motor warm. If you do it with the guage hot it could be 5 lbs or so off. I'm sure you can do a better tune than I can, but I do understand the physics behind the guage problem. Yes an in cabin guage will be more accurate over the whole temperature range. I mounted mine in the bay just for ease of setting it up initially.

[jahjah]

I'm sorry you both feel i didn't "get the point." but i don't think you guys got the point. I'm just trying to ensure that my fuel pressure is not fluctuating. The actual reading of the gauge is not that great of a concern, the current thing I'm worried about is the fluctuation. I need the pressure to be set at 38 and not fluctuate.

and I have been taking note Bil. and will continue to monitor my fuel pressure on a daily basis. It doesnt seem to drop below 35 psi when hott.

 

On Tuesday I go outside and start the car its 70 degrees. I have 38 psi of fuel pressure.

On Wednesday i go outside and start the car its 70 degrees. I have 35 psi of fuel pressure.

 

Do you understand the scenario now guys?

 

So the gauge should consistently read the same psi under the same conditions right? well it doesnt seem to be doing its job.

 

I could have been imagining things, so I'm going to continue to monitor it. since i have been monitoring it, the pressure has not fluctuated. Thanks Bil, cold gauge gives best reading. thank you

 

by the way, i appreciate both of your inputs on this. Seriously, i do.

Edited October 11, 2009 by jahjah

[Shelby]

it may look cool but an under hood fuel pressure guage is redundent , the only place the pressures matter is on the road under boost

get your self a 5 inch real fuel injection pressure test gauge and monitor it for a few days hot and cold , then remove , no reason to have it there all the time,,the other gauges will warn if the fuel pressure gets out of whack

[spoolinturbo]

found this

 

Chances are good you have a liquid filled fuel pressure gauge. This type of gauge will change it’s reading as the temperature of the gauge case changes. Because it’s sealed to contain the liquid inside, a liquid filled gauge no longer compares fuel line pressure to actual atmospheric pressure, reporting the difference. Instead, it compares fuel line pressure to the pressure inside the gauge case. Unfortunately, as the liquid in the case heats or cools, it expands and contracts, causing case pressure to change as much as 7-12 psi plus or minus. As a result, gauge readings can fluctuate from 40 psi cold to as little as 28 psi hot! Fuel line pressure is NOT actually changing, just the pressure in the gauge case. To confirm if this is occurring, procure a heat gun or blow dryer and heat the gauge from cold to warm and then hot, while running the fuel pump. Observe if the pressure reading drops, if so, you found the problem.

[Indiana]

Liquid filled gauges have a big air bubble in them so that can't happen. You see a gauge and its filled with oil, hold it up and the upper portion of the face isn't even coverend. There is a rubber plug on most of those gauges and this is where the oil is poured in, is yours is too full dump some out.

 

You can not see those changes as you say even the car out in the sun ends up warming up the oil in the gauge. Your fuel pressure may be stable. What you might want to do is POUR that oil out of the gauge and only verify your pressure each day using the test port that way the motor isn't shaking and preventing you from seeing the pressure. What regulator are you using by the way? Maybe that is the problem or is that what you are trying to find out? We had an Aeromotive regulator go bad, it was not supposed to be effected at all by engine vacuum and something happened and it started to be effected, we sent it back and got a new one. The boost reference hose isn't supposed to PULL pressure down AT ALL and this includes the stock fuel pressure regulator.

[jahjah]

Liquid filled gauges have a big air bubble in them so that can't happen. You see a gauge and its filled with oil, hold it up and the upper portion of the face isn't even coverend. There is a rubber plug on most of those gauges and this is where the oil is poured in, is yours is too full dump some out.

 

You can not see those changes as you say even the car out in the sun ends up warming up the oil in the gauge. Your fuel pressure may be stable. What you might want to do is POUR that oil out of the gauge and only verify your pressure each day using the test port that way the motor isn't shaking and preventing you from seeing the pressure. What regulator are you using by the way? Maybe that is the problem or is that what you are trying to find out? We had an Aeromotive regulator go bad, it was not supposed to be effected at all by engine vacuum and something happened and it started to be effected, we sent it back and got a new one. The boost reference hose isn't supposed to PULL pressure down AT ALL and this includes the stock fuel pressure regulator.

 

Maybe that is the problem or is that what you are trying to find out?

 

Bingo. The pressure is changing from day to day even when the car is the same temperature and I'm not sure if its regulator/pump/gauge or what.

 

I'm using the Aeromotive. So wait... the pressure should not drop below what i have the idle/base set at? Even under decel when my vacuum drops below what idle would be it should not drop below my base setting? cause it is definitely doing that

[BIL]

If that is the problem and the FPR is out of warranty you can get a rebuild kit. My local speed shop carries them so I'm sure you could find one in Hot'Lanta easily.

[Indiana]

What model # is your regulator?

[jahjah]

What model # is your regulator?

pretty sure its this one

http://www.summitracing.com/parts/AEI-13109/

 

If that is the problem and the FPR is out of warranty you can get a rebuild kit. My local speed shop carries them so I'm sure you could find one in Hot'Lanta easily.

. we call it "the A T L" only ppl who aren't front atlanta call it hot'lanta

 

but yeah i'll look into that.

[BIL]

come on now...lighten up...you know us old folk from Tampa don't know where the caps key is on the keyboard. Edited October 12, 2009 by BIL

[jahjah]

come on now...lighten up...you know us old folk from Tampa don't know where the caps key is on the keyboard.

lol its alright. I'll have something cool to show ya next year at the mega meet

[pitboss]

I have the FIP pro2 kit and an in cabin electric fp gauge and when I set it to BASE idle pressure (i.e. vacuum line unhooked and plugged and car at cold or hot idle) of ~40 psi that I set it to years ago, it doesn't change at all. FPR is a MSD 2222 I think it is. As soon as I hook up the vacuum line, my pressure changes a little and fluctuates. It doesn't change much when I first start it at cold, but it does if I've benn driving it for a little while, or when I restart the engine after driving it around town, like it'll drop 4 psi and come back up and jump around at idle and while driving. My gauge reads XX.X psi. I was told along time ago that some fluctuation was normal as there is just a little vacuum at that point. My TB is ported to 52 mm also.

 

When I go on boost it jumps to 1:1 and holds all day long also.

Edited October 17, 2009 by pitboss

[Chad]

the fuel pressure should fluctuate, but it depends MPI or TBI what it should do.

 

The injectors are after the throttle body on MPI, so the FPR refrence port should also be after the throttle body, and see the same full vacuum as the injetor tips.

 

On MPI, the FP should follow the boost gage exatly, if not, there is a problem and your fuel maps will all be jacked-up trying to makeup for a non-linear fuel supply.

 

At full decen, the FP gage should dip 6-8 PSI, and on full boost, the FP shodul read higher than neutral baseline (with the FP refrence port disconneted) by the same amounnt as your boost.

[pitboss]

the fuel pressure should fluctuate, but it depends MPI or TBI what it should do.

 

The injectors are after the throttle body on MPI, so the FPR refrence port should also be after the throttle body, and see the same full vacuum as the injetor tips.

 

On MPI, the FP should follow the boost gage exatly, if not, there is a problem and your fuel maps will all be jacked-up trying to makeup for a non-linear fuel supply.

 

At full decen, the FP gage should dip 6-8 PSI, and on full boost, the FP shodul read higher than neutral baseline (with the FP refrence port disconneted) by the same amounnt as your boost.

 

Chad, you meant CONNECTED right, because if it was DISconnected the fpr wouldn't see a boost reference, and wouldn't then increase fp. That's what I meant too when I said 1:1 Jahjah. For every pound of boost, you should be seeing 1 psi increase in fp, so 40 psi base fp (set with vac line disconnected from fpr to get number you want, then reattached after your set)+ 20 psi boost = 60 psi at gauge and should stay fairly consistant at 60 psi. If your fp reading starts to decrease, like more than a couple psi and continues dropping as your on 20 psi boost, you may have a clogged return line and you will soon get fuel cut... and whiplash...

Edited October 18, 2009 by pitboss

[Chad]

Neutral baseline is only acchieved when the refrence line is disconnected. That "disconnected" part in my post is in refrence to how to propperly set baseline.