ZZ3Astro

Hi everyone, I'm trying to narrow down the source of my fuel delivery issues. Something has been bothering me since getting my fuel pressure gauge. My fuel pressure is reading lower than I think it is supposed to be. I was having a problem with the fuel pressure dropping when I get above 11 psi of boost, which I narrowed down to a leak on the rubber line between the fuel pump (walbro 255) and hard line in the tank. I replaced that and it helped tremendously with fueling, but today at the drag strip I realized I am once again running short on fuel.

Here is what I am seeing with various conditions:

Engine off, fuel pump running: 42-43 psi.

Engine off, fuel pump running, return line blocked off: 125+ psi

Engine idling, reference line DISCONNECTED: 32-35 psi

Engine idling, reference line hooked up: 23-26 psi (drop matches vacuum)

Engine off, fuel pump running, reference line fed with 20 psi: 62-63 psi

Engine at 15 psi boost: 30-45 psi (big problems here!)


So my main question is in regards to what fuel pressure you have seen on a stock TPI, idling, with the vacuum reference line disconnected. Is it 42 psi or is it in the 30's like mine?

Thanks for any help!

ZZ3Astro

Another test result from just now..

I pinched the return line with vice grips to achieve 85 psi idling. Leaving the vice grips on, when I go into full boost, it still drops into the 30's. The interesting thing here is it has no problem holding 85 psi at idle so that seems to implicate the regulator, assuming that I SHOULD be seeing 42 psi at idle with the reference line disconnected.

mechanic58

How much horsepower are you making? You may have a fuel line size issue. It may not even necessarily be the bulk length of the line...there could be some fittings somewhere in the line that have a reduced orifice size that is creating a restriction. Have you always had this problem or is this something that just developed? Always remember too - with a constant speed pump of any sort, when flow goes up (high fuel demand) pressure will drop proportionately. It can be shown on a curve. If you do have a restrictive orifice somewhere in the line, cavitation will occur at that point under high fuel demand situations, causing a pressure drop down stream. Would be an interesting experiment if you could tie in a pressure gauge at the pump discharge at the fuel tank so you could compare the readings to what you're getting at the rail. This would tell you if you have a restriction problem. Edit: This reading comparison would have to be done under a load, like in a situation when you would normally see this pressure drop.

mechanic58

Just thought of something else....it would also be interesting to know whether or not you're getting any return fuel to the tank during these low pressure indications. If you are...then you might have a regulator problem. Under boost, when your fuel pressure drops substantially due to demand...one would think that the regulator would pinch off the flow to the return.

ZZ3Astro

I agree, seeing the pressure in back would be useful as would knowing what kind of return flow is occurring. But that is a complicated measurment to take as it will require modifying the hard lines and I'd rather leave them untouched if I can. I have to start with the idle fuel pressure though. Perhaps the injectors pulsing are messing with the regulator in such a way it is unable to maintain as high of a pressure as it should. I dunno because no one has been able to confirm to me if what I'm seeing at idle is correct.

Horsepower will be about 600 when everything is working right. Currently it seems to be making only around 425-450 based on the mph at the track. Well within stock fuel lines on these cars (as is the 600 I plan on haivng). I've already swapped regulators, a wix fuel filter, hard wired the pump power to the battery and replaced the walbro with a brand new one. Each of these changes slightly improved fueling but not enough to fix the problem.

It's really bugging me that the pressure is so low at idle, but pinching the return line produces more pressure than I could ever use.

ZZ3Astro

One other thought concerning the injectors.. my friend has a nearly identical setup in an 89 corvette. The one significant difference is he has sequential injectors while mine are batch fired. So each impulse of the injectors on mine will cause a greater hit on the fuel pressure than his. Perhaps that is how he gets away with using a stock regulator and perhaps an aftermarket regulator would be better with my batch fire setup. Then again, plenty of people have batch fired combos running a stock reg and big injectors (65's) around here, as far as I know.

mechanic58

This is true. However if you are really making 600hp I can almost guarantee you that you have a flow restriction problem. You need a bigger fuel lines to support this much horsepower.

mechanic58

You keep hanging up on pressure...forget that. What you need to be concerned with is flow. Your pressure is dropping at the rail because you're starving for fuel because of a flow restriction. That fuel pump you have should support the power you're making with the correct size fuel lines. It just doesn't have enough *** to push the required amount of fuel through the factory stock lines on that car at the flow rate your engine demands. It would be possible....but the pump would have to be able to make a lot more pressure at the discharge to do it in order to overcome the static loss through the soda straw lines.

mechanic58

This is because the manifold vacuum is high...which tells the regulator there is low demand for fuel - so it returns at max flow to the tank, hence your lower line pressure. When you're seeing the pressure drop to those same levels (or below) under high boost situations its not happening because the regulator is returning...its happening because your fuel rail is cavitating.

Now I understand why you're baffled.

ZZ3Astro

I understand that the manifold vacuum lowers the fuel pressure via the reference line. But what I'm saying is when I leave the reference line disconnected, and start the engine, the pressure is dropping from 42 down to the low 30's while idling. If I then connect the reference line, it drops down even more into the 20's at idle. Should it drop from 42 to 32 just from the injectors firing or should the regulator be able to maintain the 42 psi (while the reference line is not hooked up)?

Are you saying it is normal for the fuel to cavitate in the rails at high boost? Or what is the solution if it is not? Something isn't right because I have 65 lb/hr injectors running at 100% duty cycle and I'm not even making 500 hp. In order for the injectors to function properly they must have ~42 psi more pressure than the manifold pressure. My problem has been that the pressure falls to within 15 psi of manifold pressure. This results in less flow than they are rated for and poor atomization. It causes both a lean condition and a slow burn which is overheating the manifolds and turbo.

Thanks for any input! Someone out there must have checked their rail pressure while idling with the stock regulator and reference line disconnected!! Pressure please?

antoine

pressure with vac line off should be higher

antoine

And I think its not a good ideal to run injectors at 100% duty cycle and you may want to look into a aeromotive 340 lph intank pump

mechanic58

No, it is not normal for the rails to cavitate during high boost...however it sounds like that is what is happening to you. The reason it is happening is because the engine is sucking more fuel than is being provided to the fuel rail. There may be a number of different reasons why that is, but I suspect the main culprit is a flow restriction in your fuel system, e.g. too small of a line or possibly a reduced orifice size at a fitting.

Edit: This may be true if in fact your regulator is working properly. If it isn't, then this scenario could be false....or it could still be partially true. My guts tell me though that 600hp needs bigger than stock dia fuel lines.

mechanic58

I can't explain why its doing what its doing at idle with the vac line disconnected. That is not a normal indication based on the experiences I have had. The fuel pressure should be at its highest with the engine idling and the vac line disconnected from the regulator. Only time it would be higher would be if the engine was off and the fuel pump was running. You may have a bad regulator.

ZZ3Astro

Thanks guys. I'm starting to wonder if this low idle fuel pressure has something to do with pulsation in the line from the large injectors, especially being in batch fire vs sequential. That said, I experimented with different settings for the number of injector pulses per cycle and it had no appreciable effect on the idle fuel pressure, or the top end problem either.

As for the high boost fueling issues, I'm just at a loss. I have to think though that it is related to the idle fuel pressure somehow. That points towards the regulator, but I've tested the regulator extensively as above and it passes every test perfectly.

I feel like I'm going to replace this pump and end up with the same results. That would suck, but at least I'd know for sure I have a pump more than capable of supporting my hp plans.

mechanic58

Welp, I don't have anymore input. Good luck.

ZZ3Astro

So today I was driving the car to get the tank low on fuel. Stopped by a friend's shop and we did some testing on a stock TPI corvette. The fuel pressure did not stray from the regulator spring pressure of about 42 psi while the reference line was disconnected. Once hooked up it dropped about 7-8 psi as expected.

He then gave me an Aeromotive FPR to try out. I had AN fittings installed on the return line up front and put the regulator in between the engine and tank. Yes the stock regulator was still hooked up, with the Aeromotive backing it up. I set the fuel pressure to 44 and hooked up the reference line. It dropped 7-8 psi and maintained about 36-37 psi. I drove the car around a bit and got into boost. Fuel pressure falls still. On the way home I got caught at a red light. I noticed the idle fuel pressure had fallen to 29-32 psi!!! I pulled over and disconnected the reference line from the external regulator. The fuel pressure rose about 6 psi. It would not maintain the 44 spring pressure. I then shut off the engine and turned the key back on. It primed to 44 psi on the mark. I restarted with the reference line hooked up and it was right back to 29-32 psi. Too low. Next I got into boost and it had no problem rising into the low 40's before quickly dropping back down and leaning.

Makes no sense at all. When I got home I pinched off the return line and it immediately primed to 104 psi. Baffled!

TTOP350

iTrader: (8)

What are the odds of having a bad fuel psi gauge?
sounds like your pump may a volume problem??

ninetyone

iTrader: (2)

Mine is at stock 43.5 idling with vacuum disconnected and plugged (simulates WOT)

ZZ3Astro

Ok guys... clue me in if I'm doing this the wrong way:

Flow testing a fuel pump.. place regulator on pressure side of pump. Set it to whatever pressure (say 60 psi). Block off the outlet and measure the fuel that comes out of the return line for a set amount of time. Is this a sufficient way of measuring pumping capacity at a given pressure?

If so, my pump is performing almost exactly to spec on this page http://www.autoperformanceengineerin...l/fpspecs.html

We actually had the regulator set to 60 psi but during the test it stayed at 69-70 psi. The setup wasn't ideal by any means.. because the rubber hose shop put some oddball size flare fittings on, I couldn't put the regulator directly on the main feed line. Instead I had to run the fuel through the rails and factory regulator, down the return line to the wheel well. I installed the aeromotive regulator there and measured the fuel coming out of it for 34 seconds. A battery charger was connected at the alternator to simulate the engine running.

It delivered 1.81 qts in 34 seconds. That is 0.053235 qts per second or 191.64 quarts per hour. That converts to 181.36 liters per hour ir 47.91 gallons per hour.

Expected flow from a Walbro 255 LPH at 70 psi @ 13.5 volts is 50 gallons per hour.

Considering the expected loss of 5-6 psi from pump to front of the car, not to mention going through two regulators and the fuel rails, this number is phenomenal!

47 gallons per hour equates to 290 pounds per hour. That is 483 hp at .6 bsfc and that's at 70 psi. I should be at 57 psi right now. At 60 psi the pump should support 563 hp - well above a car running 7.99!

And my fuel pressure just 10 minutes before this test was 39 psi with 16 psi of boost. Static fuel pressure during the prime a perfect 42 every time (using stock regulator only).

Continued baffledness!

fullsize fun

one thing to consider is checking the voltage going to the pump...are you getting a constant 13-14 volts with the engine running. Low voltage will affect flow.

84'B.A.T.A.

iTrader: (2)

I have seen a few people I know have had similar probs with dual regs. I would have to say that is most likely your problem my .02. The people I know personally have solved there probs by switching to 1 reg.