98 SOHC Fuel Pressure Test questions

[chefduane]

98 4.0L SOHC XLT 4x4 181k miles

I am trying to run down a hard starting problem on the above vehicle. I have previously had the upper & lower manifold gaskets done some time ago. I have also done a MAF and IAC cleaning and changed the Cam Position Sensor myself. Wires/Plugs were also done a few years ago. When she runs, she runs great. No hesitations, acceleration from a stop is fine and acceleration from 60-80 is fine. Seafoamed her a few months ago and all went well. And no timing change issues so far (knock on wood). Regular maintenance at recommended intervals. But she starts hard.

There is no "pump prime sound" at all when I switch the ignition to on. I have also completed the "whack it with a mallet" test procedure (!) to no avail.

Here is where I am today:

1. Checked voltages to the Inertia Switch. FPR & PCM relays are fine and all fuses are ok. I get a +12v (+/- .5v) spike at the Inertia Switch when ignition is switched to on. I HAVE NOT checked voltages at the Fuel Pump.

2. Using 2000StreetRod's Fuel Pressure Test Procedure (tnx 2kSR) I have zero (0) pressure at complete rest. No pressure to relieve when beginning the procedure.

3. When the ignition is turned to the ON position the pressure slowly comes up to barely 30psi but never above. Tested that a few times. No changes.

4. Pressure at idle is also 30psi, no changes. Again, tested a few times.

5. When slow revving to 3000RPM the pressure also NEVER moves from 30psi.

6. When fast revving to 3000RPM the pressure goes from 30psi to about 36psi but never gets to 40psi. When idling back down, the pressure settles back to 30psi. Again, retested a few times, no changes.

7. When the engine is turned off, the pressure quickly dissipates to zero within 30 seconds. There is no residual pressure that remains after 30 seconds or so.

This is where I hit a snag. Where the heck is the vacuum line at the rear of the intake? I know it is probably obvious, but not to me. Is it the vacuum line that comes off the brake booster? I can't really make it out in the pic in the test procedure. I know I need to test the pressure with the vacuum line disconnected and plugged.

That's where I am so far. Any input would be appreciated. I am thinking it is a fuel pump problem but am not sure. Since I am getting some pressure build when the ignition switch is in the ON position I assume that I am getting voltage to the fuel pump and the pump is putting out SOME pressure. Is that a correct or erroneous assumption?

Also, If I do need the check the voltages at the pump, I am considering doing the "floor pan fuel pump door access mod" and NOT dropping the tank. That way I can check the voltages and swap the pump if need be. (Besides, my wife wants me re-do the shower stall and I'll CERTAINLY need me a Harbor Freight Air Chisel remove the tile!!!)

Any thoughts or input would be appreciated!!

Thanks.

Chef Duane
Plano, TX.

 

[2000StreetRod]

voltage and ground at the pump

Since your pump is generating 30 psi I don't think it's necessary to check the voltage and ground at the pump. You should be able to reach down behind the coil pack and feel the vacuum hose connection to the fuel pressure regulator (FPR) and just pull it loose. The FPR is just barely visible in the photo below.

If fuel is present when you disconnect the vacuum hose to the FPR then the diaphragm in the FPR has failed and you need to replace the FPR.
Has it been more than 50K miles since the fuel filter was replaced? If so, I suggest replacing the fuel filter. Your fuel pump will last longer.

 

[chefduane]

The FPR is the diagonally mounted 'tophat' looking thing behind the coilpack in the pic? I'll check the pressure with the vacuum disconnected in the morning. I think on the 98 SOHC the FPR was integral to the fuel rail. If it has failed I assume I will have to pull the upper mani and replace the whole fuel rail? Yikes.

My fuel filter has about 23k miles on it so I think that should be ok. I can swap it out if need be.

Thanks for the info!

 

[2000StreetRod]

What's that on the fuel rail?

What's that on the fuel rail?

 

[chefduane]

Ok, so now I am thoroughly confused. I took a look at where the FPR should be located based on your photo and there is none. Nothing at the rear of the fuel rail on the drivers side behind the coil pack. There IS however this device (pics below) that is mounted on the front of the rail on the pass side. Its got a vac line going in one end and a braided fuel line coming our the other. My Chiltons manual indicates that it is the FPR.

BUT, what has me really confused is that in doing a pressure re-test today, I get 0 (zero) readings on the tester. Needle does not move at idle, rev, or anything. I re-read the test instructions and removed\re-installed the tester to verify connections. Same thing, no fuel pressure reading. I removed the vac hose from the device I mentioned above and still no pressure readings.

All during this time she starts up and runs just fine. Very odd.

I'm beginning think I should leave well enough alone and just figure she's a hard starter every so often.

Chef Duane

 

[2000StreetRod]

My mistake

I forgot that the FPR is in a different position on the return type fuel system.

Your meter probably read 0 psi because the pressure relief pin in the Schrader valve core was not depressed. I have that problem and have to remove the valve core to get an accurate reading.

If your hard starts are due to low fuel pressure because of a bad pump eventually it will get worse and you can fix it then. I don't recommend leaving town unless you have a good road assistance program.

 

[chefduane]

I don't recommend leaving town unless you have a good road assistance program.

I've driven back and forth from Knoxville, TN to Dallas over the last few years and last week did a 500 roundtrip Dallas\Austin\Dallas. It does not seem to be getting progressively worse.

I'll check the pressure again with a different unit maybe. But I just don't feel my wrenching abilities are up to removing the upper intake manifold. I should say I probably could remove it, I just don't know about getting back on correctly!

 

[chefduane]

EDIT: A regular valve core removal tool is all that needed? I assume relieve the pressure first, remove valve core, then re-test?

 

[chefduane]

EDIT EDIT:

Must be the pressure gage. I removed the valve core from the Schrader valve on the fuel rail and got some minor seepage. Re-fit the tester and same thing, no pressure. So I removed the core stem from the tester adapter and basically has a straight thru to the test unit. Still nothing. Funny that it worked yesterday.

I guess I'll go swap the pressure test unit and check it again tomorrow.

Thanks for you input and help on this.

 

[chefduane]

OK, got some test results:

1. Pressure at idle is 30psi.
2. Pressure w/ slow rev to 3k rpm goes from 30psi to 40psi but not above 40psi.
3. Pressure w/ fast rev to 3k rpm - same result as #3 but quicker response.
4. Pressure @ idle no vacuum - 40psi.
5. Upon engine stop, pressure dissipates from 30psi to 0 (zero) within 2-2.5 minutes.

Test procedure was completed twice over 1/2hr period with no discernible differences between tests.

I'm thinking NOT fuel pump due to appropriate idle/rev/idle no vac results.
Most likely FPR due to rapid pressure dissipation after engine cut off.

What do y'all think?

Chef Duane
Plano, TX.

PS - I fully endorse the Harbor Freight cheapo tester #92699 . Picked it up today for $14.99 (19.99 -25% coupon). Worked great, just use Teflon tape for the couplings.

 

[2000StreetRod]

more testing

The FPR controls the pressure by allowing some fuel to return to the tank when the desired pressure is exceeded. I doubt there is a check valve in the FPR. I think the check valve is in the fuel pump assembly. Since your fuel pressure drops rapidly when the pump is shut off you could have a bad check valve or a leak between the check valve and the fuel rail. Hard starts occur because there is no residual pressure. The fuel pump runs for a few seconds when the ignition is switched to on or whenever the PCM detects the engine rotating via the crankshaft position sensor. The fuel pressure should reach normal in three cycles of the ignition key from off to on. If not, then the pump is weak, the flow is restricted, or there is a split in the hose from the fuel pump outlet to the tank outlet. Below is a photo of my stock return less fuel pump assembly.

Yours does not have the intank FPR. You may be able to get by without replacing anything by just cycling the ignition key twice before cranking the starter.

 

[chefduane]

I checked the pressure after I cycled the ignition switch a few times and I did not see any appreciable rise. It can still start hard after I've cycled the ignition. Back to fuel pump? Maybe its just weak as it is the original pump. I also have not noticed any fuel leaks anywhere so I don't think I have anything leaking.

I may just burn some money and swap the pump and FPR for the heck of it.
If that doesn't fix it then so be it.

 

[2000StreetRod]

test the FPR diaphragm

The FPR diaphragm could be leaking and fuel could be escaping into the intake manifold dropping the pressure and providing an overly rich mixture at engine start. I suggest that you test the FPR with a vacuum hand pump to see if it will hold vacuum.

 

[SoNic67]

I doubt there is a check valve in the FPR. I think the check valve is in the fuel pump assembly. Since your fuel pressure drops rapidly when the pump is shut off you could have a bad check valve or a leak between the check valve and the fuel rail.

Correct, the check valve is at the pump's output.
IMO his pump is bad, needs a new one.

 

[drdoom]

I agree w Nic, needs a pump. 30psi is not enough on a 98.

 

[Consoledoc2013]

Whatever happened with this was it a bad fuel pump?

 

[iCasper]

test the FPR diaphragm

The FPR diaphragm could be leaking and fuel could be escaping into the intake manifold dropping the pressure and providing an overly rich mixture at engine start. I suggest that you test the FPR with a vacuum hand pump to see if it will hold vacuum.

Question: how long should the fpr hold vacuum? My one drops from 10hg to 2.5hg in 7 minutes.

 

[2000StreetRod]

Question: how long should the fpr hold vacuum? My one drops from 10hg to 2.5hg in 7 minutes.

I doubt that's a problem. Try plugging the hose connected to your vacuum pump to see how fast the vacuum drops from 10hg for comparison.