Land Rover Repair: 98 Disco startup, fuel pressure regulator, plenum chamber

Question
QUESTION: John,

On my second Disco now, a '98 with only 120k miles, Ive had for 5 months, or since 114k miles. The issue is that when cold - relatively speaking, as in any time it has been hours since last driven, I have learned that I have to turn the key to "on" for a few seconds, then start, or if I attempt starting as soon as I turn the key, it takes several, several turns before a start, sometimes even a second attempt. I have a mid-grade scan tool showing all is well, I have replaced the speed sensor early on to correct the "bucking" issue, and a crank position sensor that had a bent shaft. I pulled my plugs tonight and all were gapped correctly and dry. I connected a fuel gauge to the fuel rail and noted that with the key turned on, the pressure quickly rose to 40 psi, then dropped to 32 psi when the engine was started. When the engine is cut off, it abruptly drops to zero. Is that normal? I think the fuel pressure drop may very well be the issue here, but do you have any idea how to stop it? Shouldn't residual pressure remain in the fuel rail?

Thanks,
Michael

ANSWER: Hi Michael,

Q: did it maintain a 40psi for very long BEFORE you actually started the engine?

Once the engine is started, the fuel pressure should be maintained between 2.4 - 2.6 bar (34 - 37 lbf/in2) on the fuel rail.   Although the pressure went to 40, it did drop back to 32, which is within the 10psi drop wit.  That being said, I've heard other owners reporting that a minimum of 35 and a maximum of 60psi AND it should idle at about 40.

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Fuel pressure regulator:
The fuel pressure regulator is mounted in the fuel rail at the rear of the plenum chamber. The regulator is a mechanical device controlled by plenum chamber vacuum, it ensures that fuel rail pressure is maintained at a constant pressure difference of 2.5 bar above that of the manifold. When pressure exceeds the regulator setting excess fuel is returned to the fuel tank.
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BEFORE you start swapping parts, check the inertia switch and replace the fuel filter (an inexpensive part) just in case.  IF not,  I suspect it could be either your fuel regulator or pump.

Best of luck,
JohnMc

---------- FOLLOW-UP ----------

QUESTION: John,

If I do not start the engine, but simply leave the key in the "on" position, the pressure drops to zero after about 10 seconds. Not gradually, but quite abruptly. Yes, I thought to do that test as well, just did not mention before. Fuel pump and filter are new since February, and I was suspecting the fuel pressure regulator, but I do not throw parts at a problem. Especially when the least expensive regulator I have seen is a closeout for $40 at Atlantic British. Based on that though, would you suspect the regulator as well?

Thanks,
Michael

ANSWER: Hi Michael,

interesting.  Were there any OBDII codes trapped?

The pressure drop should not exceed 10psi after 1 minute, pressure should rise quickly and hold steady.  IF this is not the case, check for voltage reading at the pump.  

If you have good voltage at the pump but the pump is still not working, it's usually the pump.  

When the fuel pump is turned off (or stops running) the pressure should hold reasonably well, as mentioned above, a 10psi drop over 1 minute is common.  This is called the Residual Pressure test.  IF it drops quickly, as you mention, it could indicate a problem with the fuel regulator, leaking fuel injector(s), leaking fuel line, or a leaking fuel pump check valve.

NOTE: low residual pressure test results can cause hard starts and vapour lock during hot weather.   

Test the fuel pressure regulator to determine if line pressure changes exist in response to the engine vacuum.  

Note: caution should be taken with the following test. Be ready to cut engine power quickly as fuel may be present.

While the engine is running, remove the vacuum hose from the pressure regulator and watch to see if fuel pressure increases (10psi ?).  IF no change in pressure is present, it could be either the regulator or a vacuum problem.  You should also examine the vacuum hose for fuel - it should be dry.  If it's not, then the regulator diaphragm is gone - replace the regulator.

Since you mentioned that the Running Pressure test had 32psi (below normal), it could very well be the regulator.

Best of luck,
JohnMc

---------- FOLLOW-UP ----------

QUESTION: John,

Ok, so I tested tonight again and I have about 30 psi with engine running, climbs to 40 psi with vacuum removed. This indicates a functioning regulator and good vacuum. I did note that when the key is first turned on, the pressure is slow to rise to 30 psi, and again, falls to zero sharply once the key is turned to "off" whether engine has been running or not. I'm going to go ahead and plan on swapping in a new filter Saturday as that is the least expensive approach, even though I just did this 5 months ago. It does seem there is a restriction on the input side, and in my experience, a pump either works, or it doesn't as in, either alive or dead so a weakness is most likely an obstruction, with the prime culprit being the filter. Any thoughts?

Thanks,
Michael

Answer
Hi Michael,

slow to rise and quick to drop off fuel pressure is not good, usually means a fuel pump problem.  It could be an obstruction within the filter or on the line but I wonder...  

I forgot to mention another fuel regulator test - pressurize the fuel system then clamp off the return line. If the pressure holds, you have a leaking fuel pressure regulator.

Best of luck with the filter replacement,
JohnMc