Fuel Pressure

[scotch1]

I’m having trouble with my L98, so I checked around and found that my fuel pressure is well above 60 to 70lbs and the fuel pressure needle vibrates a lot. The book says that the pressure should be around 40-44 lbs.
Any ideas or suggestions as to why this is happening?
Or what to check for next?
Any input would be greatly appreciated, before I go crazy and start tearing apart the intake and fuel runners or fuel pump and replace unnecessary parts.
Tom.

 

[G Winter]

Make sure the vacuum line didn't come off the regulator.

Glenn

 

[Hib Halverson]

Make sure the vacuum line didn't come off the regulator.

Glenn

Generally, when the vacuum lines comes off the FP goes up 7-10 psi. The 60-70 is way more than that. In fact, it's hard for me to believe that the stock L98 pump could make that much pressure.

But, if it is, the engine would be rich...actually, not just "rich" but fat-pig rich. So rich, it ought to be setting fault codes.

Is that what the problem is?

 

[scotch1]

After about a minute it starts to run really rough, the engine vibrates as though its missing. The engine starts right up at the touch of the key. It lacks low end power. The timing is right on and the fuel filter has been replaced and I also replaced the spark plugs.

 

[speedmaster4]

you are seeing max pressure that the pump can develop...should NEVER happen as the pressure regulator ''should'' allow anything over abt 43 psi to bleed off to the return piping and back into the tank, but ain't happening...needs early repair as the pump requires constant fuel flow thru itself to carry heat away from the pump motor.

my guess is your pressure regulator diaphram is leaking and fuel pressure is adding to the regulator spring to 'close' the return flow...but checking return piping with compressed air (REMOVE THE TANK FILL CAP) would be first before digging down to the pressure reg.

 

[scotch1]

you are seeing max pressure that the pump can develop...should NEVER happen as the pressure regulator ''should'' allow anything over abt 43 psi to bleed off to the return piping and back into the tank, but ain't happening...needs early repair as the pump requires constant fuel flow thru itself to carry heat away from the pump motor.

my guess is your pressure regulator diaphram is leaking and fuel pressure is adding to the regulator spring to 'close' the return flow...but checking return piping with compressed air (REMOVE THE TANK FILL CAP) would be first before digging down to the pressure reg.

Thanks for your reply.
Good thought. A blockage somewhere is causing pressure to build up. The problem, then, lies with the return fuel line, or the regulator, or after the regulator or in the fuel rails. I recently replace the all the injectors, and the rails were clean. Could defective injectors cause something like this?
Sounds like the easiest thing to do first is to disconnect the two fuel lines and blow them out. Then remove the plenum and get into the regulator and rails.
Tom.

 

[speedmaster4]

Thanks for your reply.
Good thought. A blockage somewhere is causing pressure to build up. The problem, then, lies with the return fuel line, or the regulator, or after the regulator or in the fuel rails. I recently replace the all the injectors, and the rails were clean. Could defective injectors cause something like this?
Sounds like the easiest thing to do first is to disconnect the two fuel lines and blow them out. Then remove the plenum and get into the regulator and rails.
Tom.

injectors are highly unlikely cause this problem.

''blasting'' the fuel rail with compressed air at high pressure could rupture the pressure regulator diaphram...short time ago NAPA sold just the diaphram for much less than a complete regulator

DANGER...easiest way to burn down the shop is blowing gasoline out from piping with compressed air...DANGER...suggest lung power (waiting to exhale, do NOT inhale, chemical damage of lung tissue by gasoline liquid or vapor can be fatal) thru a short length of new hose clamped on the return line to check for blockage... do agree that checking return for blockage before removing plenum would be most efficient (contrary to my wife's statements, i am not lazy, just efficient)

 

[scotch1]

How do I get at the pressure regulator diaphram? How do I replace it and where do I buy one?..

 

[vetteboy86]

I am pretty sure they offered one at autozone a while back. It really is simple to replace once you get the plenum off. Make note of how the old one came out, and replace that same way.

 

[speedmaster4]

regulator diaphram/spring assy currently shown on NAPA web as p/n DEMFP10210 for $67...have seen diaphram only for less but can't recall where(old age- the mind is second to go).

must get under the plenum to access the fuel press reg...there are six torx head screws that hold the diaphram cover to the reg body...those screws are ''tamper-proof'' torx, with a tiny ''pin'' in the center of the torx hole that was intended to keep you from shimming the reg spring or otherwise modding the reg to tune injector flow in ''open'' loop if you mod the engine (?), but the ''special'' torx head tool bits are now available at sears and most auto parts stores...remove old diaphram and any debris from reg body (usually none there but do look) and use care when pushing the diaphram cover down on the new to go in ''square'' --not difficult until after fourth round of johnniewalkerblack or such.

 

[scotch1]

I like the idea of the jonnie walker, doesn't matter which color.
But I think that they are talking about the other fuel pressure regulator, which cold very well be the problem.
Thats the return fuel line regulator, located in the front of the engine, passanger side, by the radiator, under my AC. How does one repalce or check that one.??

 

[speedmaster4]

fuel pressure regulator on all l98's is between the fuel rails at the rear of the engine, right in front of the distrib...and under the intake plenum...not sure what you are referring to that is up front under the a/c, under the a/c at the rear of the engine/pass side is the fuel filter.

 

[scotch1]

Speedmaster4, yes, there is another fuel pressure regulator in the return line. Here is a picture of it.
The theory, so far, is that this diaphram in the return line regulator is malfunctioning, and although the engine runs, I am getting extremely high pressure because this part is causing a back pressure.

How can I check this out and prove it, before tearing my car apart.??

 

[speedmaster4]

haven't cut one open to check (yet, just added to gotta-do list) but been told that is an ''accumulator''...non-serviceable, would need to pay for that whole piece of return pipe (only abt 3 ft long tho) with the gizmo included, if replacement were req'd...not used on 85-87...function is rumored to be damping of pressure spikes as injectors operate.

check pressure regulator / return piping function by disconnecting coupling in return line at front of intake manifold and installing temporary piping to a 5-gal gasoline container, activate fuel pump and observe pressure in fuel rail and IF any fuel flow discharging into temporary container.

 

[scotch1]

Good thought Speedmaster4, makes sense, I'll start on it tomorrow. Been shopping around for the fuel line part, today. Local Chevy parts shop wants 80 dollars for it and can deliver in 2-3 days. Its a discontinued part, not surprised. I have no choice at least it can be had.
I'll let you know result of the test you suggested.
Thanks for your input.
Tom.