Just got back from a long weekend at Lake Powell with my 29 Warlock and had an odd problem with my fuel pressure. My motor is a 1999 merc, 502 with standard Whipple kit (doesn't have the upgraded throttle body)92 hrs on motor, 42 since Whippple install. I'm using a Whipple/Gaffrig fuel pressure guage with electric sender and the standard Aux pump that comes with the kit. At initial start up in the morning when the engine was cold had about 35lbs pressure and stayed there untill I shut down, the next tiime I would start up pressure would drop to about 25 lbs., after that each time I would start the motor the gauge would register about 10 lbs less until it was just barely above 0. The next day it would repeat the cycle all over again, this happend three days in a row, seemed to occur after cruising 20 -30 min or so. I disconected and reconected all the electrical fittings, but that made no difference. Put my hand on the pump and had the wife turn the key when the gauge was reading 15 lbs or less and didn't feel any vibration from it. I played it safe and kept it out of boost all weekend (not a big deal as swells were up to 10', winds 40 -70 mph). Similar thing happend once before in Havasu last month after running for 20 mins or so, next time I started up it registered 15 lbs but returned to 30Lbs after running a couple mins.

Any ideas on what my problem might be? Some type of vapor lock? Any suggestions on how to diagnose?

That is a new one to me. I run a high end Autometer electric and decided to add a 2" calibrated liquid mechanical guage directly to the pressure regulator. If your system does not have a regulator with a 1/8" NPT port you can add it anywhere on the fuel rail. It is accurate +/- 1% and the electic guage reads 2psi less. BTW, I picked up the guage for ~$20 from McMaster Carr.

John,

Where is it that you got that gauge from?

For those interested here is Dustin's reply to my e-mail. Haven't performed the test yet as I'm not sure what "the 6 male fitting" is.

Shane,

Sounds like the pump is taking a poo poo. I need you to take the line from the regulator that routes back to the filter off, plug your finger over the 6 male fitting coming out of the regulator and have someone turn the pump on, this should raise the pressure after 8-10 seconds to 65-70psi. This is called "dead-heading" the pump to see if the internals are ok.

Thanks,
Dustin

My areomotive fuel pumps are set at about 43 psi. FYI I have the 502 whipple setup too.

Here's what happened when I deadheaded the pump. Before I disconected the fuel return line from the regulator I turned the key on to see how much pressure the fuel pump would make and it didn't even turn on = 0psi. I pulled the electrical conection used a tester to check for power with the key on - got nothing. Changed the ground on the tester and checked again, this time got power for 15 sec. after turning on key then heard a click on right side of motor and power went away. Reconected the fuel pump and turned on the key - it ran this time (allthough quite loudly), made 30 psi then shut off after 15 sec. So disconected the fuel line from the regulator, blocked the male fitting, turned on the key and made 50 psi in about 4 seconds and stayed there.

One other curious thing, with the power disconected from the aux fuel pump and all the fuel lines hooked up, when I turned on the key the fuel pressure gauge still read 30 psi. What's up with that?

So do I have an electrical problem? A fuel pump problem? All of the above? Or none of the above?

My engines cut off the fuel power if the engine is not running. I think it is some sort of safety interlock. Try jumpering power directly from a bettery source to the pump. Use a remote starter switch if you have one. (I did) Put your pressure guage on and then read pressure.

P.S. it would not hurt to have a fuse inline too. Be carefull with gasoline fumes too!~!!!

For those interested Here is Dustin's most recent reply. I have a liquid filled guage ordered. I still think there is another problem but will use the new gauge to better diagnose.

Shane,

The Whipple ECU primes the pump for 10 seconds when you turn the key on, the clicking is the relay which is powering the pump. The Whipple ECU flips the relay by grounding it, which then sends the power reference to the pump. The pump runs at all times when the motor is running.

When you turned the key off and it held 30psi, that's because it has a regulator in the system, the regulator is trying to maintain pressure, this tells me that the pump is not allowing fuel to leak quickly through it typically meaning that the internals are fine, your regulator is working. This leads me to the next question, what type of gauge are you using? Is it a Gaffrig electric gauge? These have always been off and should only be used as a reference, please install a mechanical pressure gauge, you can get them at Grainger, McMaster Carr, Orchards, etc. You need a 0-60psi gauge. This actually sounds like a gauge problem, so I would check with a mechanical gauge and run it to where you can see it in the boat, verify the proper pressure which should be between 36-38psi at all times when the motor is running. Dead heading the pump should spike up to 55-65psi on the mechanical gauge, then bleed off to nearly 36-38psi +/-3lbs when the key is off.

Thanks,
Dustin