Basic Fuel Pressure Regulator Question. Already searched!
 

OK. From what I've read it seems there are 2 different ideas of what's happening at idle, at the fuel pressure regulator.

Question : is the FPR supposed to be open at idle, allowing the fuel to return to the tank? Partially open? Totally open ?

I keep seeing posts that imply the opposite, since they say low fuel pressure at idle may be caused by faulty fuel pressure regulator allowing gas to return from rail to tank. But isn't the fpr supposed to let the fuel flow back at idle ??? Or is it supposed to only partially let the fuel back?

I am getting only 26 psi at idle and it would be great if a bad fpr was causing it, but I can't get a grip on what the fpr is really supposed to be doing.

When I disconnect the vacuum line to the fpr, no gas comes out, no smell of gas. And nothing happens to the idle or the vacuum reading on the autozone rental tester. Just stays at 26 psi. When revving the engine, gauge just stays at 26 psi also.

Someone said crimp off the return line and see if pressure goes up. Can't see any rubber return line anywhere? Is it under the car near the rear shock?



Clues needed! :thanks:

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Based on your signature you have a TPI? The return line is at the front of the engine. If you look at the two hard lines coming down the frame rail, the smaller of the two is the return. Pinch the rubber hose some and see if the fuel pressure goes up, I am guessing the advice was to verify the rented gauge is working properly and you're not getting an erroneous pressure reading

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Yes, it's TPI. 88 5.7 liter GTA. And someone did suggested verifying the rented gauge is accurate, but not by pinching off the return line.

I've looked around the FPR and fuel rail and I can see steel gas lines, but no rubber lines anywhere. I will look farther along the frame rail for the rubber lines.

Thanks

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OK, I think I may have figured it out. I don't know why no one ever stated it this way in the stuff I read, but.....

The fuel pressure regulator allows gas to return to the tank, at IDLE, only when the fuel pressure is 39 PSI or HIGHER. If the pressure doesn't rise to 39 or higher, the fpr remains closed, to keep the pressure in, since the pressure is supposed to be a MINIMUM of 39 PSI, under ANY condition except off.

Sound right?


And then under wide open throttle, and low vacuum, the fpr is supposed to allow the pressure to go up to around 45 PSI to feed more fuel to the high revving engine which wants more fuel.

So the fpr is supposed to maintain a fuel pressure inside the rail of 39 to 45 PSI, always, depending on conditions. Sound right ?


Thanks

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

That is more or less correct.

THIMK for a minute about fuel flow through an orifice: if your fuel pressure was, say, 44 psi (right about 3 times atmospheric), a certain amount of fuel would flow through the "orifice" consisting of the injector, when the pressure inside the manifold is equal to atmospheric (i.e. at WOT). Now imagine putting a blower or turbo on it, which pressurizes the intake. How much fuel would flow if the pressure inside the manifold was 2x atmospheric? How much if it was 3x atmospheric? (hint on that last: ZERO) Now turn the concept around; how much fuel would flow if the pressure was, let's say, HALF of atmospheric? how much if it was HARD DEEP-SPACE VACUUM? (cruising down the highway, suddenly your foot lifts off the gas, vacuum shoots to near 30 in.)

THAT in a nutshell is what the FPR is for. When the pressure in the manifold is low, i.e. high vacuum, it lowers the fuel pressure, such that the DIFFERENCE in pressure between the fuel side and the intake side of the injector, stays more or less constant.

It's really not so much a matter of "load" and "feed", as it is, the pressure difference across the injector. The ECM controls the proportion of the time (duty cycle) that the injector is open. Having the pressure somewhat automatically regulated means the ECM doesn't have to do any calculations or anything, just to compensate for the change in the pressure the fuel end up being subjected to.

As to how it works:

The fuel pump is constant-volume. No matter what, at all times that it's pumping, it's moving a certain volume of fuel. The FPR is nothing more or less than a diaphragm connected to a valve with a spring behind it, such that when the pressure on the diaphragm exceeds some given amount, it allows the fuel to begin returning to the tank. The spring is opposed by vacuum, such that when vac is high, it takes less fuel pressure to open the valve.

So: at max RPM and max load (WOT: high RPM, high load, low vacuum, manifold absolute pressure near atmospheric which is "high"), lots, probably most, maybe even all (although you don't want that) of the fuel goes through the injector and into the intake. At idle (low RPM, low load, high vacuum aka low manifold absolute pressure) very little fuel goes into the engine and most goes back through the return.

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

Like sofakingdom said, the fuel pump is constant volume in these cars, therefore there is ALWAYS fuel returning to the tank, at idle, at WOT, always. If not you would reach a condition where the fuel pressure would drop and you would have a lean condition.

The fuel lines comes up the LH frame rail, on the inside, along side the brake lines, then goes up to about the front end of the heads, then goes to the hard lines to the intake, if you're looking near the FPR you're not listening, they are at the FRONT of the engine.

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I found them. Front of engine, driver side. Thanks. They have corrugated plastic tubes covering them. Thanks.

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Sofakingdom, Thanks. I've reread your reply several times. Very informative.

New question: Since my fuel pressure is only 26, isn't the FPR supposed to not allow any fuel into the return lines?

I ask because the whole premise of the "if you have low fuel pressure, pinch off the return line and if your fuel pressure goes up, your FPR is allowing gas into the return line when it should not be" is that fuel is supposed to be shut off (or greatly reduced?) from return lines when fuel pressure is low.

At a low pressure of 26 PSI, my FPR may have shut off the flow to the return lines, like it's supposed to, and my car is idling and driving around on 26 PSI, with whatever problems that causes, like bogging and occasional bucking.

In my case, I would expect to see no increase in fuel pressure when I pinch off the return hose, indicating that the FPR has already stopped flow to the return line, as it supposed to, at 26 PSI.

If the pressure increases, I would expect that the FPR is open when it should be closed, and is contributing to the low fuel pressure.


Thoughts ???

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

Interesting thread...sub'd

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

The FPR isn't open/closed hard, it is dynamic. It is an orifice covered with a disc and a spring pushes down on that disc at a determined pressure. When the fuel sees resistance to that flow, the covered orifice, the pressure starts to rise in the system. The pressure rises to match the spring pressure, when the pressure rises above the pressure of the spring, fuel starts to flow past the disc covering the orifice and the fuel returns to the tank. The diaphragm in the FPR helps pull/push against the spring to effectively lower the spring seated pressure. If your pressure isn't changing at all, the spring could be bad inside and/or the diaphragm could be bad (less likely). You can buy a replacement spring/diaphragm.

Fuel should always be leaking past the orifice, just how much is dependent on load, and vacuum. At idle there will be more fuel leaking past since the demand is low, and the pump is constant volume. At WOT there will be much less fuel leaking past the FPR since there is high demand for fuel to leave through the injectors

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That would be the general idea; or at least, very little.

The FPR is on the "far" end of the fuel rail from the pump. The fuel comes into one end of one of them, goes through that one, goes over to the other, all the way through that one too, then if AT THE FAR END there's more than the FPR's set point, it opens and allows fuel to drain back through the return line.

The FPR would pretty much have to be stuck wide open to create such a behavior. Never really heard of it happening that I can recall, but I suppose it could. If the spring broke for example. But I'd put a relatively low probability on it.

Just the same, a FPR is CHEEEEEEEEEEEEEEEEEEEEEEEP and EEEEEEEEEEEEEEEEEEEZZZZZZZZZZZZZY to change. You could give that a try before jumping right into the fuel pump.

OTOH if you only have 26 psi with the return line pinched shut, that pretty much rules out the FPR; dinking with it would most likely be a waste of time & money. Probably best to just bite the bullet and get started on the FP.

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Thanks for the replies !

My theory is that very little if any gas is returning to the tank, at 26 PSI. I have 26 psi without pinching the return line. Haven't actually tried pinching it off yet, since I'm afraid the hose will degrade inside since it is 30 years old, when I pinch it. If I do, I assume the pressure won't go up at all, or just very slowly. I'd really like to know if the FPR is supposed to allow ANY gas through when pressure is below 39 psi.

I'm going to assume my FPR is OK since I don't get a quick bleed down of pressure when I shut it off. It takes a few hours to drop from 26 psi to zero.

Other interesting notes :

When I hit the purge valve on the tester, the pressure drops to zero immediately, car keeps idling, and no gas comes out of the drain tube. That's at 26 psi. Once, actually the psi went up to 30 right after I replaced the fuel filter, and a slow dribble of gas came out the tube. Fuel filter I removed is clean inside though. Cut it open and looked. Next day, PSI went back to 26, and no flow out the tube. Seems like 26 PSI ought to be enough to push fuel out. But, if the pressure drops to zero, maybe not. Takes about 4 minutes to build back up to 26 psi.

I think the reason removing the vac line to the FPR does nothing to idle or pressure, is that the diaphragm just stays closed due to low fuel pressure.

It seems this car can run on very little fuel pressure (at least it can idle)
since the vacuum is sucking the fuel out of the injectors.

As a last ditch thing, I'm going to buy a Harbor Freight pressure tester, just to see I still get 26 PSI, like the Autozone rental says I am.

Fuel pump hasn't made the usual buzz/groan at key on, for about 2 years now. Maybe it's just time for a new one. Noooooooo!!!!!

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

If I was reading this tread I would be as confused as this guy too with the contradictions from sofakingdom second post there.

Read what I have re-quoted first. At idle ALMOST ALL THE FUEL is returning to the gas tank. probably like ~95% of if. The fuel pressure at idle is almost irrelevant to the actual amount of fuel being returned to the tank, as long as the fuel pressure is lower than the relief valve in the pump, it is going to essentially flow the same volume of fuel at 5psi as 26 and 43. So even though 26 is low for idle, you should still have a LOT of fuel going through the FPR, the pump HAS to pump fuel or it will burn up, and since it is pumping it, it has to go somewhere, if it's not going through the injectors, it has to go back to the tank. The whole point in pinching the return line is to see if the pump can actually meet the pressure demand from being old, not necessarily to check the regulator.


If you're only getting a dribble out of the tester purge, I suspect you have something blocking the system somewhere else, or you have a pump that can't keep with demand, the pressure in the system should come up to 26, whatever, almost instantly when you "key on" or close that purge on the tester. If you don't hear it at all, I think you just answered the question

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

Not sure how that's a "theory", but...

At 26 psi, essentially NONE should be going back to the tank. The regulator should be closed off, allowing the constant-volume pump to build up pressure in front of it. That's what SHOULD BE happening, assuming the reg is working right.

If OTOH the regulator IS allowing fuel into the return line, then that COULD explain why the FP is only 26 psi.

Quick test for that is, pinch off the return line. One of 2 things will happen: your FP will shoot up to the 60s or something; or, nothing at all. If the pressure skyrockets, the FPR is bad; if nothing happens, the FP is bad.

Break that word down into its 3 component parts: 3 letters, 1 letter, 2 letters. It's short for "make an _ _ _ out of _ and _ _". DON'T EVER DO THAT. Always establish the facts by factual means. In this case, TEST the FPR, by pinching off the line. If the hose fails, it's because it's ... READY TO FAIL ... anyway, and instead of failing right now while you're already working on your fuel system and you're in a mode of repairing things, it'll instead fail in some highly inconvenient place when a member of the other sex is in your car and you're trying to make a good impression. (NOT) Do what you need to do and accept the consequences.

Not sure where there are any "contradictions" in anything I've said. If there are please point them out.

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

Re-reading your response a few times, I think we are looking at it from different angles and I was the one confused.

I think you are assuming the FPR is working correctly and the pump isn't. So in your view, the FPR spring pressure is stopping flow and the pump isn't building enough pressure?
I am looking at it that the pump is working properly and the FPR is somehow at fault, and it allowing fuel to flow past at the wrong pressure, so there would be fuel going past the FPR, only allowing 26psi.

Re: Basic Fuel Pressure Regulator Question. Already searched!
 

Right, I agree with that: as I see it, those are the 2 possibilities. (a) The FPR is already stopping flow to the return line like it should but the pump either isn't moving its design constant-volume fuel flow, or simply can't pump against a head of pressure; or (b) the pump is moving plenty of fuel and is otherwise OK, but the FPR is open when it shouldn't be and allowing fuel to return starting at too low pressure. I'm not really "assuming" either. However I'm inclined to guess that a bad fuel pump is more likely than the FPR failing that particular way, since their more usual failure mode is for the diaphragm to break and allow fuel into the vacuum line. See my signature. ;)

Pinching the return line will establish which possibility is the true fact and put an end to idle speculation.