Fuel pressure problems

[1984grandnat]

I've been having some problems with fuel pressure lately, and need a little assistance. Here is my fuel system setup: Walbro pump with hotwire kit (less than a year old), Accufab adjustable fuel pressure regulator (a little over a year old), 50 lb injectors from Kenne Bell, and a Kenne Bell hood-mount pressure gauge. Here is what happens: when I first turn the key on, the pump runs for about 3 seconds (which is normal). During this few seconds, fuel pressure is at about 35 psi. As soon as the pump shuts off, pressure drops to about 30 psi. After I start the engine (and the pump comes back on), fuel pressure goes back up to 35. When I step on the gas, fuel pressure DROPS from 35 to 32 or so (instead of going UP). When I let off the gas, the pressure levels off at 35. While driving the car, the fuel pressure does not fluctuate much, even when in boost. I have tried following the "Fuel System Check Up" as posted at Gnttype.org, but this does not tell me much, and doesn't really seem to apply to my specific problem. The drivability is pretty poor, and the car sputters when boost rises above about 7 or 8 psi (running lean, I am guessing). Any ideas?

Thanks

 

[gofstbuick]

what kind of chip? is your chip made for 50lbs injectors?

 

[ChrisCairns]

Either your regulator ain't working or the signal vac/boost line is plugged or leaking.

And 35 psi sounds way too low. Check with your chip guy to see what fp he recommends.

 

[1984grandnat]

It is a custom chip burned for 50 lb Lucas injectors. I haven't considered the chip, since this seems to be a mechanical problem with pressure itself. I have been using several chips lately, and have had the same problem with all of them.

I am beginning to think that maybe it's just a clogged filter, since it has been about 6 months since replacing it. Either that, or I guess I'll have to drop the tank Just trying to avoid this if at all possible.

 

[Steve Wood]

Did I understand you to say that the the fuel pressure does not rise one pound for each pound of boost?

In otherwise, if the fp with the vacuum hose off is 40#, then (with the hose back on, of course, if you run 12# of boost, the fuel pressure should be at 52# when the boost is at 12#.

If this is not happening, you have a problem...regulator, pump, or plugged up fuel filter.

When you set the fuel pressure with the hose off, and then put the hose on, the pressure will drop due to the vacuum in the hose. If you accelerate and vacuum drops, the pressure will rise back toward your hose off setting. When it goes into boost, the fp will rise above your hose off setting. Again, it should rise one pound for each pound of boost as compared to your hose off fp.

 

[1984grandnat]

I understand that this is supposed to work this way, but in my case, it isn't. When I set the regulator at 35 psi (hose off), I connect the hose and the pressure doesn't drop as much as it should (maybe only 3 or 4 psi with 15 to 20" vacuum). When I step on the gas, the pressure doesn't rise like it is supposed to, actually the pressure DROPS to about 30 psi. When I get the car into boost (10 psi or so), the pressure rises back to about 35 psi, but never goes as high as it should. It's like the regulator is acting very lazy.

I also noticed that when I set the fuel pressure at a higher static level (say 55 psi), the regulator seems to work almost perfectly, showing about 45 psi at idle, and about 65 psi at 10 psi boost. That would be okay, but I can't run 55 psi static pressure on the street, because that makes it run WAYY too rich on the bottom end (complete with poor spool up). This is why I would like to decrease the FP for street use (since I don't plan to run over 15 psi boost on the street anyway).

Would a clogged filter really cause problems such as these? I am replacing the filter later this afternoon, so we'll see what happens then.

 

[1984grandnat]

I forgot to mention that while I was doing some pressure testing earlier today, I crimped off the return hose, and turned on the ignition. The fuel pressure shot up to about 100 psi, and didn't really drop much when I shut the key off, so I think maybe the pump is working pretty well?

 

[Steve Wood]

As someone said above, 35# is awfully low for hose off pressure...most end up around 42-45 if the chip is cut right.

It would seem that something is amiss with your regulator. Perhaps it is possible that 35# is outside the range? I have never had mine below 38.

If it is too rich at 40-42, you have a chip problem, I would think. If the regulator does not work properly at 40-42, then you also have a regulator problem. Sounds like it is sticking to me.

At any rate, stay out of the boost until you get it working correctly or you will be changing headgaskets soon.

 

[1984grandnat]

I just rebuilt the regulator today, because I thought that was the problem (unfortunately, it wasn't). I can't really build any boost right now anyway (I can't see over 10 psi, no matter where the W/G is set...the engine seems to sputter), so I guess that is a good thing for now.

 

[Steve Wood]

if the fuel pressure is not increasing, the engine is short of fuel and detonating or cutting out from lack of fuel...

Or, the coil is bad or such....

 

[1984grandnat]

Checked the coil the other day, but seems okay (testing over 12k ohms on the towers). Just changed fuel filter...still doesn't help. Double-checked voltage at the pump - seems fine. Maybe my Walbro pump went bad after only 9 months...? I guess anything's possible, huh? I went ahead and turned the pressure up at the regulator (to about 42 psi static), and seems like it is working better now, but why doesn't it work properly at lower pressures? I checked with Accufab, and the tech said that this regulator should operate properly throughout the entire range (from as low as 10 psi). The pressure is still dropping right after the pump shuts off (when the key is on). It builds to about 42 psi, but then when the pump quits, the pressure drops to about 35 or so. This tells me that maybe the pump is bad (maybe the check valve in the pump)? Hate to say it, but I'm just not impressed with this pump if it's gonna do this every 9 months.

 

[azgn]

set the line off fuel pressure according to the specs on your chip....I have never seen FP spec'd at less than 42, line off...

most everything else you describe is normal, except the fuel pressure not tracking boost....once the engine is out of vacuum, pressure should rise lb for lb.....check all vacuum lines, particularly the line to the FP regulator.....for example, the steel tube off the vac block to the regulator developed a leak on my car....try a direct vac hose line to the regulator

 

[Steve Wood]

Originally posted by 1984grandnat
builds to about 42 psi, but then when the pump quits, the pressure drops to about 35 or so. This tells me that maybe the pump is bad (maybe the check valve in the pump)? Hate to say it, but I'm just not impressed with this pump if it's gonna do this every 9 months.

It is hard to say..the checkvalve could be leaking back a bit...should not really hurt anything.

If you have access to an air compressor or some air pressure, you can check the regulator by pumping 15# of air pressure into the port and seeing if your fuel pressure rises 15#. I have always been suspicious of Accufab regulators but I have one on one of my cars and it seems to work okay.

The other test for your pump would be to turn the regulator up to about 60# of fp and then put a hose on the fuel rail and see how long it takes to fill a gallon jug with gas. By putting a lot of fuel pressure on the pump, it strains it to deliver volume...if it will fill a gallon in about a minute, it should be working fine.

 

[Chuck Leeper]

Reg test

Put a vac/press. hand pump on the vac. line nipple on the reg. Start the car and let it idle. Add a psi or two at a time and watch the gauge. If it does not increase from the line off PSI as you add pressure w/ the pump, the reg is most likely FUBAR.
I doubt the pump is the problem as it is capable of maintaining a 100 PSI, [even with the flow being low]
The fact you lose some PSI when the key is turned off does not mean the pump is bad. However a fast loss of all the PSI is indication of several possible problems, none of them are showing up here.

 

[1984grandnat]

I wish I could tell you what fuel pressure should be set at for my chip, but it is a custom-burned chip (from a friend of mine), so there is no "manufacturer" to give recommended pressure ratings (we are on our own, in other words). The car seems to run alot better since I turned the static press up to 42, and the regulator seems to be working pretty well now, with the higher pressures. Maybe it just won't work at pressures below about 40...? It still has a little knock and sputter at about 8 to 10 psi boost...I may be getting close to the limits of 93 octane and no intercooler (I can't remember what timing is set at with this chip, but maybe that is affecting it also).

 

[Racetronix]

Perhaps it is the signal to the regulator that is the problem?
Are you sure the regulator is seeing boost?

I have seen some cars where people have rearranged the rubber vacuum lines on the intake and plumbed the regulator's signal feed into the wrong location. There are one-way check valves on some of the vacuum hoses that will not allow boost to enter into certain locations. Does the signal line to your regulator run directly into the top of the throttle body? If so you might want to make sure the line is clean and perhaps place a pressure gauge on the signal line to make sure the regulator is actually seeing boost.

Jack
Racetronix