Need help interpreting fuel pressure test results

[tracyballard]

I just tested the fuel pressure on my 2002 GT (140K mile original engine) and need some help figuring out what the results mean.

First - I was getting the Service Engine Soon light and the 0171 & 0174 engine codes meaning running lean on both banks, but before I tested the fuel pressure I disconnected the battery for 5 minutes (just cleaned the MAF which was probably oily from an overoiled K&N filter) and I am no longer showing the Service Engine Soon light...so I guess that's good. anyhoo, the test results:


when I hooked up my acutron pressure gauge to the valve on the fuel rail and turned the key to the on position it showed 12psi, turned the key back off and on again and it was at 34psi, turned it back off and on a 3rd time and it showed 40psi, where it remained no matter how many times I turned the key back then returned it to the on position. I relieved the pressure and repeated with the same results.

with the engine running it was showing a steady 34psi, moving maybe 1 or 2 psi as I revved the engine.

any thoughts??

 

[GDawg]

Subscribing... hoping WMBurns chimes in...

 

[nyuk98gt]

tracyballard,

I only have experience with the return style fuel system but here's what I can tell you.

34 psig may be OK. 39 psig is what is set in the return style calibration. Fuel injectors need a steady pressure so that they meter in the same amount of fuel each time they pulse open and closed. This is true for return and returnless style fuel systems.

Your returnless fuel system uses a fuel rail pressure sensor (FRPS) as an input to the computer which then ramps the voltage up and down to the fuel pump (i.e.--varies the speed) to maintain the pressure at the rail.

If the rail pressure is too low then the short-term and, eventually, long-term fuel trims will compensate if the variation in residual O2 (O2 sensors measure this) is not too great. If there is too much O2 then there is an air leak into the engine or there is not enough fuel entering even with the STFT and LTFT corrections applied. When the adaptive corrections are at the limit then the CEL pops on because the correction required is too big.

HTH,

Chris

 

[wmburns]

Testing the fuel pressure on a motor that is not running is worthless. Test with the motor running.

How to test fuel pressure with an external gauge. Disconnect and plug the intake vacuum reference from the sensor. Does the fuel pressure jump to 40 PSI give or take 1-2 PSI? If so, the fuel pressure sensor is basically working.

Reconnect the intake reference line. What happens to the fuel pressure? If it drops down to about 32 PSI, this is correct.

WHY?

Because the intake is under vacuum. The purpose of the vacuum reference line is to make the fuel pressure 40 PSI above the intake. This is called DELTA pressure.

Note, never drive the car with the intake vacuum line disconnected especially in a boosted application.

Bottom line, an external gauge has to be adjusted for the intake vacuum.

Bottom line2, the PCM's sole source of how to control the fuel pump is from the fuel pressure sensor. So testing the actual fuel pressure is only part of the battle. It's really necessary to know what the PCM "thinks" the fuel pressure is. This provides a way to cross check for a bad FP sensor.

 

[tracyballard]

question 1 - where is this fuel rail pressure sensor located that I'm supposed to disconnect and plug vacuum line from?

question 2 - does this mean that since I had 34 psi without removing the vacuum line, that my pressure is fine in that regard?

 

[wmburns]

question 1 - where is this fuel rail pressure sensor located that I'm supposed to disconnect and plug vacuum line from?

question 2 - does this mean that since I had 34 psi without removing the vacuum line, that my pressure is fine in that regard?

1-the FRPS is on the driver's side of the fuel rail. It's the only thing with an electrical connector AND a vacuum line.

2-Was the motor RUNNING when the test was performed?

 

[tracyballard]

2-Was the motor RUNNING when the test was performed?

yes, I was getting 34psi at idle.

 

[wmburns]

A delta pressure of 6 PSI equals 12" of vacuum. That is a low vacuum reading and may indicate another issue.

32 PSI equals 8 PSI of vacuum. 8 PSI equals 16" of vacuum which is a more reasonable value.

It would be handy to know the intake vacuum at idle as this could help trouble shoot the basic engine health.

As far as making a determination if the FP is correct, can't do that without knowing the true intake vacuum. To know for sure, repeat the test with the FRPS reference disconnected. FP should be 40 PSI.

 

[tracyballard]

I repeated the test with the FRPS disconnected from the vacuum line, with the vacuum line plugged, and I'm getting a reading of 44psi.

 

[wmburns]

That seems within a margin of error.

 

[trinity_gt]

FWIW, with regard to pressure drops across the injectors and fuel pressure:

You can approximate the absolute (psia) of the manifold under vacuum conditions with the following formula:

psia = 14.7 - (inHg * 0.491)

At 0 inHg indicated (such as when the engine is off), the manifold is at atmospheric (~14.7psia). If you could pull a perfect vacuum of 29.92inHg, the manifold pressure would be 0 psia. An idling engine will pull, say, 18inHg of manifold vacuum which equates to about 5.8psia in the manifold.

The fuel injectors are designed to operate with 40-psi across them. That is, the pressure drop from the fuel rail to the inlet port should always be ~40psi. A fuel pressure gauge typically measures psig (that is, the gauge reads '0' when exposed to the 14.7-psia atmospheric pressure and all other pressure readings are referenced to that value.) So with the engine off, if you see 40psig on the rail, the fuel pressure is actually 54.7psia.

The 18inHg idling engine sees 5.8psia in the manifold. The computer wants the rails to be 40psi above that to keep the injectors happy which is 45.8psia or about 31-psig. Since FP gauges typically read in gage pressure, you should see something close to this number at idle, depending on the idling vacuum inHg.

A reading of 34psig implies 8.7psia in the manifold or 12.2inHg manifold vacuum. Seems a little low...

I agree with wmburns: Measuring fuel pressure with the engine off has little utility Your engine-running numbers seem to be pretty close to correct. Regarding the codes: Have you cleaned the MAF recently? Running a K&N-style oiled filter? When were the front O2 sensors last replaced? Any exhaust leaks upstream of these sensors?

 

[tracyballard]

Regarding the codes: Have you cleaned the MAF recently? Running a K&N-style oiled filter? When were the front O2 sensors last replaced? Any exhaust leaks upstream of these sensors?

just cleaned the MAF after first replacing the K&N filter with a stock Motorcraft filter. I assume the O2 sensors have probably never been replaced, at least not recently, but since my fuel pressure numbers seem ok, and the Service Engine Soon light is no longer on, I'm going to just drive it for now and see what happens.

 

[trinity_gt]

just cleaned the MAF after first replacing the K&N filter with a stock Motorcraft filter. I assume the O2 sensors have probably never been replaced, at least not recently, but since my fuel pressure numbers seem ok, and the Service Engine Soon light is no longer on, I'm going to just drive it for now and see what happens.

If those really are the original O2 senors they are way past due for replacement: 100K miles would be about the max I would put on them before preventative replacement. O2 sensors can get lazy and slow and may affect driveability without failing outright.

See if the codes return but your may consider replacement as a matter of maintenance akin to replacing spark plugs...