Fuel Pressure Question

Trying to figure out a low MPG issue I am having, so tested the fuel pump per factory manual as well as tips from this forum.

After depressurizing the system, I removed the incoming fuel line connection in the engine bay and installed a pressure gauge. Few turns of the key and the gauge reads around 62 PSI. It drops down to about 59 PSI but will hold there for longer than 5 minutes.

The factory manual states pressure above 41.7 PSI the pump should be replaced.

This question might be dumb... but why would a "bad" fuel pump actually be putting out more pressure? Perhaps there is a pressure valve in the pump that fails?

Also... wouldn't higher pressure simply be rectified (dropped down) by the pressure regulator just before the fuel rails?

If I have to much pressure from the pump.... could this be overwhelming the pressure regulator at the rails and in effect send more gas than the system "thinks" through the injectors... thus wasted gas and lower MPG..?

 

Long term fuel trim would answer that question. get a little scanner/code reader for 60$

 

Did you connect the pressure gauge in a T Fitting as per the manual or did you just dead head it to the gauge? It would probably make a difference.



NOTE: The fuel pressure regulator is part of the fuel pump assembly in the tank.

 

Dead head gauge without a "T".

I saw the regulator is part of the pump... but I also find info that there seems to be a regulator before the fuel rails.

Can anyone confirm if there is just one regulator (on pump) or a second one at the rails...? Seems odd there would be two.

 

There is a fuel pressure regulator on the rear passenger section of the fuel rail as well. This has a vacuum hose that runs to your intake box.

 

Looks like this

 

Then you did it wrong.

See the image I posted above. That is from the service manual.

 

I think that is the bypass return valve to the fuel tank. Pressure comes to the rail from Fuel Line # 1, pressurizes the rail and the excess fuel is returned to the tank via This Valve and Fuel Line # 2. Which I suppose is sort of a regulator...

That is why when he dead headed the gauge without the Tee Connector he had a high reading, the fuel had nowhere to go.

See Image below:

 

Yes that sounds correct. Ive always heard this referred to as a fuel pressure regulator, but what you said makes sense. All this does is bypass the unused fuel back to the fuel bowl within the gas tank. I have my fuel pressure gauge tapped in using URDs fuel rail tap right before this bypass valve.

 

Thanks for the additional clarification... I was going off this thread:
https://www.tacomaworld.com/threads/how-to-check-fuel-pressure-w-pics.445975/

I am getting poor MPG and slow to start on occasion... like the fuel has drained out of the system. Was trying to figure out if I have a fuel pump, pressure regulator or... well, who knows.

 

Well that does show you what the pump is capable of doing, just not a real representation of what the injectors are seeing as far as pressure. Dead head pressure will always be greater than the true fuel circuit with the bypass valve.

 

The gadget you posted is an enrichment valve/pressure regulator the small vacuum port goes to the intake WOT drops the vacuum to raze the fuel pressure slightly. Deadheading the pump will give you very high pressure reading and it not real good for the pump. It is highly doubtful that is the cause of poor mileage.

 

Fuel trims are the answer to mpg questions

They allow you to split the system in half and narrow things down.

Mechanical stuff like pumps, injectors and pressure regulation on one side

Calculation stuff like sensors (maf, o2s) pcm, etc on the other side

 

This makes a lot of sense. I do have a scanner but to be honest not the most adept at using it. It's a

$56 Foxwell NT301 OBD2 Scanner Live Data Professional Mechanic OBDII Diagnostic Code Reader Tool for Check Engine Light

Foxwell NT301. I will give it a go... but I assume the best way to use this tool is why the motor is running / driving? See how the fuel trim and related sensors are working?

 

Look at the long term fuel trims. If they are stable/close to 0% (anything less than 8-10% plus or minus) then the fuel system has good control..

..and you can look elsewhere for mpg concerns like tire pressure or something mechanical

If it was over 8-10% that would indicate that the fuel system is trying to compensate for something. Like an intake leak or un-metered air that is "sneaking" past the maf sensor and not being detected until it gets to the 02 sensors in the exhaust

 

This might not be the best way to post this info, but this is the range of numbers my OBD reader has given me recording live data as I drove around town (no speeds above 35)

SHRTFT1(%): -8.6 to 5.5
LONGFT(%): -3.1
SHRTFT2(%): -8.6 to 3.9
LONGFT2(%): -4.7 to 0.0
SPARKADV: 30.5 to 10
IAT: 147 to 172
MAF(lb/s): 0.00
SHRTFTB1S2(%): N/A
SHRTFTB2S2(%): .115 to .935
02B1S2(v): .175 to .820
O2B2S2(V): .115 to .935
O2B1S1(v): 3.052 to 3.321
O2B2S1(v): 3.072 to 3.354

I am not that familiar with these readings... but a couple things that seem to stand out are the MAF(lb/s) staying at "0.0" throughout and SHRTFTB1S2 stating "N/A" throughout the test.

Opinions? Feedback?

I of course have more info but to be honest am not sure which PID data sets might be the most helpful.

 

Is the maf plugged in? Is there any codes stored? Is the check engine light on?

Those trims are less than 10% which is good. Do they read the same at idle, in gear, at a stop?

 

Did you check injectors on 5 and 6? Known to stick/leak or go bad. Could be just stuck open.

Also, what is your MPG at the moment?

 

MAF is plugged in.... but that "0.00" reading is odd right?

Codes: P0420, P0430

I would have to double check that (which I can tomorrow) but yet, I believe the trims are always no more than 10% regardless of what the truck is doing.

 

No, I have not checked the injectors. But have run various fuel system cleaning products through the tank. I will search on how to inspect / test / check the injectors... but ifg you have any tips that would be great.

12 MPG if I am lucky. It's like there is a hole in the tank. I can almost watch the fuel gauge needle drop as I drive.