Fuel Pressure Fluctuation

jmille · #1 (**permalink**) 05-28-2003, 05:47 PM

Have an interesting observation.

Just installed a fuel pressure gage on my Holley 4160 fuel log. Car is equipped with a 392 stroker with a Carter mechanical fuel pump and a fuel filter back by the tank.

When I first start the car gage shows 6.5 psi fuel pressure. Every thing looks normal. Close the hood and go for a little spin. Return home pop the hood and read 2 psi. Let the car sit for an hour or so start the engine now I have 3 psi. Also note that when I shut the engine off fuel pressure bleeds to 0 in 3 or 4 seconds.

Engine seems to run OK on street and track. Didn't suspect any problem till I installed the gage. Still don't know if I have any problem. But what I expected was constant fuel pressure some where around 5 to 8 psi. I expected that the valves in the fuel pump would prevent pressure bleed down. Or at least it would take several minutes to bleed off.

Things I think it might be.

Bad fuel pump valves.

Plugged fuel filter causing excessive suction on tank line which in turn causes some sort of vapor lock????

Its just normal and don't have any thing to worry about??

Thanks in advance for the help.
Jerry

mr0077 · #2 (**permalink**) 05-28-2003, 07:04 PM

Jerry, that's definitely not normal fuel pressure readings. I think you've listed the likely cause(s); a few other possibilities are a partial obstruction in the tank pickup, a kink or other obstruction in the supply line and a plugged tank vent. It sounds like you're pulling a vacuum somewhere in the system on the tank side of the pump. I'd start with the easy stuff first, replace the fuel filter, and maybe blow the line out from the fuel pump while you have the filter off. Also, see if you can get a good fuel flow out of the tank while the filter is off (be careful with the fuel, of course).

cwmcobra · #3 (**permalink**) 05-28-2003, 08:11 PM

Just curious.....is your pressure gauge liquid filled? I have observed the same situation in my car and consulted with Aeromotive on it since I'm using their fuel system. They tell me that many liquid filled gauges will read lower pressure as they heat up when, in fact, the pressure is really at the original (cold) level. I'm going to replace my liquid filled gage with a good AutoMeter mechanical gage. Aeromotive is confident that will solve my problem.

Now, if your gauge is not liquid-filled, disregard this suggestion and proceed with a design of experiments to determine the root cause of the problem. Engineering 101.....

Good luck!

Chuck

wilf leek · #4 (**permalink**) 05-29-2003, 02:01 AM

I have the same fuel pump. I think that at least one of the symptoms you describe is actually not a fault.

That is the speed with which the fuel pressure decays when you switch off. Mine too runs down to zero in 1 or 2 seconds. I do not believe there is any non-return valving in the Carter pump, thus fuel flows backwards easily through the gerotor gears when the pump stops.

Apart from that, I get 5 psi at all times. First thing I would check in your shoes would be the gauge.
After that - look for a vacuum in the tank.

HTH

cobra427mnsi · #5 (**permalink**) 05-29-2003, 03:47 AM

jmille

Try removing the gas cap, while the engine is running, to see if that affects the pressure guage. If you are getting a vacuum in the tank from a plugged (or non existing) vent, removing the gas cap will prove it. If there is no change, follow the other suggestions given.

Paul

Cobra20646 · #6 (**permalink**) 05-29-2003, 04:25 AM

Jerry,

I had the identical problem..... and as Chuck (cwmcobra) said, it turned out to be the gauge. Tried two liquid gauges and both did the samething, even going to zero when it got real hot under the hood!! Now using a "regular" one and pressure steady 6 pounds from OE pump at all underhood temps. Also bleeds down in a few seconds after engine shut off too, so don't think you have a problem..... other than that gauge!

- Jim -

Roscoe · #7 (**permalink**) 05-29-2003, 06:55 AM

Chuck and Jerry are correct, BG tech told me the same thing about liquid filled gauges. They also will fluctuate due to the temp of the liquid. You will get different readings when the engine is hot/cold due to the liquid.

However, the best place to start is the pump. Get a vacuum/fuel pressure test gauge and hook it directly to the fuel pump and see if you are putting out the pressure the pump is rated for, usually around 12 psi or so. Then check it at the pressure regulator to make sure that is adjusted properly. Then check it at the carb.

You can get a vacuum/fuel pressure test gauge at any auto parts store and it is not expensive. It is great for tuning the car.

Roscoe

mr0077 · #8 (**permalink**) 05-29-2003, 07:12 AM

Shouldn't have a pressure regulator on a mechanical pump...But a good idea to put a Tee in the line and compare the test gage with the installed pressure gage. I hadn't heard the problem with the temps and the liquid-filled gages, but it makes sense. Wonder why they sell them for this use? If you really did have low pressure, the car would run poorly at/near WOT through the gears as the fuel level in the bowls dropped.

Roscoe · #9 (**permalink**) 05-29-2003, 07:17 AM

OOPS, missed that part on the mechanical pump. You should still check the pressure at the pump and at the carb (after the filter).

Ken: Liquid filled is ok if you keep them away from the engine and not mount them on the log.

Roscoe

jmille · #10 (**permalink**) 05-29-2003, 04:54 PM

Thanks for the advise.

Should be pretty easy to trouble shoot this weekend. Will be easy to determine if its my liquid filled gage. I had never heard of that one but it makes sense if the gage body is not vented. That will be pretty easy to check out. I will either heat the gage when I first start the engine and see how it reads. Or I will wrap the hot gage in a cold towel and see if the reading changes. Replacing the gage is an easy fix.

Will report the findings.

Jerry

Turk · #11 (**permalink**) 05-29-2003, 08:51 PM

Do you have the fuel coming all the way into the cockpit to the gauge, or do you have a fuel pressure isloator( that is normally filled with antifreeze) and sits on the engine side of the firewall?

TURK
If you have an isolator, you may want to make sure there are no air pockets in it..

SSSnake · #12 (**permalink**) 05-30-2003, 03:48 AM

Summit sells a Barry Grant pressue gauge that is not affected by heat. We had the same problem in our car. Once we installed the new gauge, it reads 7 psi all the time.

Glenn

jmille · #13 (**permalink**) 05-31-2003, 06:19 AM

Went out this morning started her up. Perfect 7.5 psi fuel pressure reading. Took my trusty heat gun and began blowing hot air on the gage. Within a minute the gage read 2 psi. Wraped gage in a cold towel and were quickly back to normal. Pulled gage drained glycerin and left the plug out so the gage was vented. Started the engine 7 to 8 psi. No change when we heat the gage. Needle is a little jumpy without the dampening that the glycerin provided, but it reads right at all temps.

Thanks for the tip. I would have looked at a lot of other things before the liquid filled gage.

Jerry Miller