fuel pressure for 1998

hi, whats the psi of the fuel at the rail with the vacum tube on fuel regulater and off the fuel regulater for the shoc 4.0??

Don't know first hand

I don't have first hand knowledge but my Haynes' manual states the fuel pressure at idle should be 30 to 45 psi with vacuum hose attached and 40 to 50 psi with vacuum hose detached.

I'm surprised that you can use the stock fuel pressure regulator with LPG but then I don't know much about using LPG as an automobile fuel.

im halfway of removing the LPG system now, just the large donut tank to remove from the spare wheel area and the wire loom through out the car. iv been running this on petrol for some time now. ill check the figures later today, thanks for the info

iv done the test,

fuel reg vac line on 35 psi
fuel reg vac line off 40 psi

Seems low

While the pressures you read are within specification, I would have expected a larger difference between the two readings. I assume the readings were taken at engine idle. Did you happen to check the pressure at higher engine speed (2000 rpm) with the vacuum connected to see if the pressure dropped? Have you replaced your fuel filter in the last 30,000 miles? Does the fuel system hold at least 30 psi for 5 minutes after turning off the ignition?

iv re-tested it again, also the fuel filter was replaced a month ago,

vacum tube on at idle 33 psi
vacum tube off at idle 40 psi

engine revved at 2k

vacum tube off 40 psi
vacum tube on 25 psi

After turning off the igintion with the vacum line connected (30 psi) the pressure dropped by 5 psi after 5 mins (to 25 psi)

Looks suspicious

The fuel pressure increases as the vacuum decreases. That's why the pressure reading is greater with the line disconnected than connected. When your rpm is 2000 the vacuum should be weaker than at idle. Hence, your fuel pressure should be greater. I plan to change my engine oil this afternoon. I'll try to check my fuel pressures afterward.

If anyone hasn't noticed, fuel pressure drops by exactly the manifold vacuum wich is typically about 7 pounds. I have a dead weight tester and can tell you that many gauges are off by quite a bit. I believe this is the reason they give such a variance for pressure readings in manuals.

Thanks for the help, i have a new walbro fuel pump to be installed when the weather gets a lil nicer, offers 255lph and can out put 80 psi, this should solve my power loss over 4500 rpms (on dyno shown to be 125hp). ill post the results after the install. i also have a FSE pressure regulater that can be installed to bypass the factory fuel regualter, giving me the option to set the desired psi at the rail.

2000StreetRod said:

While the pressures you read are within specification, I would have expected a larger difference between the two readings. I assume the readings were taken at engine idle. Did you happen to check the pressure at higher engine speed (2000 rpm) with the vacuum connected to see if the pressure dropped? Have you replaced your fuel filter in the last 30,000 miles? Does the fuel system hold at least 30 psi for 5 minutes after turning off the ignition?

Click to expand...

What are common causes for the pressure to drop faster than 5 minutes after shut off?

leaking injectors

According to my Haynes manual bleed off of the fuel pressure can be due to leaking fuel lines, leeking fuel pressue regulator or leaking fuel injector(s).

Well I dont think I have a leaking line, no puddles or smell.

The regulator potentially, while running at idle it does have 30+psi and if I disconnect the vacumm line jumps right to 40 psi.

Injectors they do have 200,000 + miles on them, they just stick open?

Haynes left out the probable leakdown source...

2000StreetRod said:

According to my Haynes manual bleed off of the fuel pressure can be due to leaking fuel lines, leeking fuel pressue regulator or leaking fuel injector(s).

Click to expand...

The checkvalve in the output side of the fuel pump is the most likely culprit here...It is not a leakproof seal but a method to prevent vapor lock issues when hot and hard starting issues due to lack of fuel in the system...

I would be more interested in the leakdown rate over 30 mins to 60 mins...Not maintaining pressurized fuel in the system up to the regulator could cause starting problems if the fuel system didn't repressurize when the pump spins up for the second or two when it primes before starting...

Oh, and if the regulator was leaking, you would probably see fuel in the vacuum line since you can get to the regulator which is mounted on the intake in your model year... It is mounted in the tank starting in the 99 model year and there is no return line back to the tank...And leaking injectors would cause rich starts [or restarts] with black smoke and carbon on the tailpipe along with diluted oil thanks to excess raw fuel...

During my fuel pressure check, Intermittently I would see the needle go from 30 psi to 0 in 60 seconds and bam no start. It would take a hand full of attempts. Then other times it would not loose any pressure...

There is a problem that can occur in the fuel pressure regulator due to a spring that id not centered. I had this problem with my 92 and it resulted in occasional stalling at stop lights. The engine would go into a death shudder and then recover. It also would start to die when taking high speed right exit ramp turns. Eventually I cut open the regulator and found seat wear was only on one side. This allowed the valve plate to slip to one side and stick open. High manifold vacuum opening the valve wider and lateral force would start the problem. This wear will also allow leak down with enough wear. Try tapping the regulator and then starting.

Wasn't aware

ranger7ltr said:

The checkvalve in the output side of the fuel pump is the most likely culprit here...It is not a leakproof seal but a method to prevent vapor lock issues when hot and hard starting issues due to lack of fuel in the system...

I would be more interested in the leakdown rate over 30 mins to 60 mins...Not maintaining pressurized fuel in the system up to the regulator could cause starting problems if the fuel system didn't repressurize when the pump spins up for the second or two when it primes before starting......

Click to expand...

Very interesting. I wasn't aware there is a check valve at the fuel pump output. Is it in the tank with the pump?

Absolutely...

The check valve is inline with the output port of the in-tank pump...When the engine is shutoff, the check valve keeps pressurized fuel at the injectors to facilitate hot starts to prevent vapor lock..

Without the pump check valve, the fuel system would have to rebuild pressure and that would cause drivability and starting issues...

Guys, According to the workshop manual for a 99 mountaineer and Explorer the fuel pressure should be 65PSI plus or minus 8 PSI Cheers Foxy

foxy said:

Guys, According to the workshop manual for a 99 mountaineer and Explorer the fuel pressure should be 65PSI plus or minus 8 PSI Cheers Foxy

Click to expand...

that is for the returnless models made after 1999, mine has the return system 1998.