Fuel pressure regualator

[turbosneezinCB7]

I have a 1993 accord lx I've been working on to get it back on the road, I don't plan on doing any mods to it until I get it running perfect. I had a rough idle and I bleed the cooling system and it is running better, but it still runs like there is a vacuum leak. I pull of the vacuum line that goes to the fuel pressure regulator and it didn't run like there was a leak anymore, my question is does this mean my fuel pressure regulator is bad? if so can I just buy a new diaphragm and rebuild this part or is it necessary to buy a new one. thank you #staydirty

[1813mdw]

Does that vacuum line smell like gas?

[johnl]

I had a rough idle and I bleed the cooling system and it is running better, but it still runs like there is a vacuum leak. I pull of the vacuum line that goes to the fuel pressure regulator and it didn't run like there was a leak anymore, my question is does this mean my fuel pressure regulator is bad?

Did you block the orifice leading into the manifold? If not then you would have a leak at the open orifice, and if the engine idled better with such a leak then it would suggest that the base idle speed was too low. If so, then it would also suggest that the IACV isn't compensating for the base idle being too low.

The hose at the FPR allows the regulator to keep the pressure at X psi over manifold pressure, which will be less than atmospheric (i.e. a partial vacuum). As manifold pressure varies so too does the absolute fuel pressure, but not the pressure relative to manifold pressure. If you disconnect the vacuum hose to the FPR, then the FPR will 'think' that manifold pressure is higher than it really is and the absolute pressure will increase, which will cause the injectors to inject more fuel (i.e. the AFR will richen up). The ECU should respond to this according to what the O2 sensor is telling it, and lean the AFR, but it might not be instant and the idle may become rough, at least until the ECU compensates. Even if the ECU does compensate in closed loop, the AFR would likely still be significantly rich in open loop.

If the FPR is leaking then raw unregulated fuel can leak into the manifold through the hose, and the ECU probably won't be able to lean off the AFR enough to account for this. Any hint of fuel in the FPR hose and the FPR should be replaced.

Of course the FPR may be an innocent party and you may just have a vacuum leak (or other issue) that you haven't found yet...

if so can I just buy a new diaphragm and rebuild this part or is it necessary to buy a new one. thank you #staydirty

My understanding is that the FPR is a sealed unit and can't be repaired. A new one shouldn't be hugely expensive (though I've never priced one, they rarely fail, so second hand would be the cheapest and reasonable option).

[RyanD]

i just looked on Rockauto and the FPR runs between $40 and $100.

I agree with John however, they rarely fail. at least on here iv only heard of one instance on here

[turbosneezinCB7]

thanks john

thanks for the info john, when you mean by the orifice leading to the manifold do you mean the open that the vacuum line leads to? im confused. a

Did you block the orifice leading into the manifold? If not then you would have a leak at the open orifice, and if the engine idled better with such a leak then it would suggest that the base idle speed was too low. If so, then it would also suggest that the IACV isn't compensating for the base idle being too low

also I spoke to my A Tech teacher about this and he told me that when you take the vacuum line off the FPR there is more pressure going in, so if the car ran better than it did before I pulled off the vacuum line would that mean i could have a clogged fuel filter restricting gas flow? or maybe a pump on its way to going bad?

[johnl]

thanks for the info john, when you mean by the orifice leading to the manifold do you mean the open that the vacuum line leads to? im confused.

Fuel pressure regulators are connected to the inlet manifold with a small diameter rubber tube. Nothing really passes through this tube other than changes in pressure, i.e. when the pressure in the manifold changes this is 'communicated' to the FPR via the tube. The orifice I mean is the metal spigot on the manifold to which the rubber tube attaches, if this is open then an uncontrolled quantity of air will enter the manifold and cause idle speed to increase. An improvement in idle 'quality' could be caused by a rise in the idle speed. When you pull that tube off just block the spigot with your finger and see what happens. If the idle speed remains more or less correct but smooths out, then you might have a fuel pressure problem.

also I spoke to my A Tech teacher about this and he told me that when you take the vacuum line off the FPR there is more pressure going in, so if the car ran better than it did before I pulled off the vacuum line would that mean i could have a clogged fuel filter restricting gas flow? or maybe a pump on its way to going bad?

Disconnecting the hose will increase fuel pressure, assuming the pressure isn't already at the max the pump can provide. This might allow the engine to idle better assuming the pressure were low with the hose connected. It could also cause the pressure to over-shoot the other way, cause AFR to change from too lean to too rich. Either way, in closed loop the ECU ought to be able to compensate (in some degree) according to the O2 sensor data it's receiving.

Either a clogged filter or bad pump will mean less fuel pressure. The ECU should be able to compensate for this to some degree (in closed loop) by increasing the injector pulse width (i.e. enrich the AFR). The biggest affects of disconnecting the FPR hose will be to increase the air inducted and to increase the fuel pressure. An increase in inducted air will increase the idle speed and tend to lean out the AFR. In closed loop the ECU should detect a rich or lean AFR caused by raised fuel pressure or uncontrolled air, and narrow or widen the injector pulse width in response. How accurately / consistently it will do this is hard (for me) to say, but I'm certain this situation is far from ideal (especially in open loop).

I'd be somewhat surprised if the FPR is the source of your problem, but can't say it isn't. If you don't know when the fuel filter was last replaced, now is the time to do it in any case. Then check fuel pressure and flow at the rail.

[turbosneezinCB7]

Fuel pressure regulators are connected to the inlet manifold with a small diameter rubber tube. Nothing really passes through this tube other than changes in pressure, i.e. when the pressure in the manifold changes this is 'communicated' to the FPR via the tube. The orifice I mean is the metal spigot on the manifold to which the rubber tube attaches, if this is open then an uncontrolled quantity of air will enter the manifold and cause idle speed to increase. An improvement in idle 'quality' could be caused by a rise in the idle speed. When you pull that tube off just block the spigot with your finger and see what happens. If the idle speed remains more or less correct but smooths out, then you might have a fuel pressure problem.



Disconnecting the hose will increase fuel pressure, assuming the pressure isn't already at the max the pump can provide. This might allow the engine to idle better assuming the pressure were low with the hose connected. It could also cause the pressure to over-shoot the other way, cause AFR to change from too lean to too rich. Either way, in closed loop the ECU ought to be able to compensate (in some degree) according to the O2 sensor data it's receiving.

Either a clogged filter or bad pump will mean less fuel pressure. The ECU should be able to compensate for this to some degree (in closed loop) by increasing the injector pulse width (i.e. enrich the AFR). The biggest affects of disconnecting the FPR hose will be to increase the air inducted and to increase the fuel pressure. An increase in inducted air will increase the idle speed and tend to lean out the AFR. In closed loop the ECU should detect a rich or lean AFR caused by raised fuel pressure or uncontrolled air, and narrow or widen the injector pulse width in response. How accurately / consistently it will do this is hard (for me) to say, but I'm certain this situation is far from ideal (especially in open loop).

I'd be somewhat surprised if the FPR is the source of your problem, but can't say it isn't. If you don't know when the fuel filter was last replaced, now is the time to do it in any case. Then check fuel pressure and flow at the rail.

Thank you very much for your detailed response, I have come to believe that the FPR is not where my problem is.. it is definitely an air leak, because i took the line that leads from FPR to the nip off from the manifold and put my finger on top of the orifice and when i do so it runs as if there is a leak, but when i remove my finger the idle speed increases and runs smooth......... one other problem I am experiencing is a rev limiting affect maybe the ecu has put the car in limp mode so i cant rev past 3k, but when i short the ecu connector under the dash i can rev much higher but not enough to even get it into redline.

[johnl]

.. it is definitely an air leak, because i took the line that leads from FPR to the nip off from the manifold and put my finger on top of the orifice and when i do so it runs as if there is a leak, but when i remove my finger the idle speed increases and runs smooth.....

"nip"?

When you create an air leak by taking off one of the vacuum lines (say), then the idle rpm should increase because more air will be entering the manifold (by-passing the throttle body). So that's as expected. But, it runs "as if there is a leak" when this 'leak' is closed off, so I assume you mean it runs more roughly at a lower rpm.

What idle speeds are we talking about? The correct idle should be 650 to 750 rpm. The idle will be 'rough' if it's much below that, which doesn't necessarily mean the roughness is caused by an induction leak, it's just that at very low rpm the valve timing causes erratic changes in the quantity and quality of air entering each cylinder and thus the accuracy of the AFR on individual induction strokes.

At very low idle rpm enough exhaust gas 'reverses' and flows backwards into the cylinder to significantly dilute the O2 content of the ingested air, to the point that poor combustion and erratic misfiring occurs (the more 'adventurous' the cam grind the more of a problem this is, hence the need for higher idle rpm with 'sports' and racing cam grinds).

What we need to figure out is whether the idle is smooth at the correct idle speed. If it is, but isn't at lower rpm than that then it's not necessarily a problem.

.... one other problem I am experiencing is a rev limiting affect maybe the ecu has put the car in limp mode so i cant rev past 3k, but when i short the ecu connector under the dash i can rev much higher but not enough to even get it into redline.

May or may not be related. I'd check the air filter to start with, I doubt it's causing your problem, but easy to eliminate from suspicion.

I don't know how the limp mode works, or even if these cars actually have a limp mode (I've never encountered such a condition, so never researched it). It's a bit strange that the engine runs better in 'diagnostic' mode than not. My suspicion tends to lean a little toward the O2 sensor. The ECU ignores some of the sensors when in diagnostic mode, so perhaps it's fuelling from hard maps and not adjusting AFR when the plug is 'jumped'?

If so then that would result in a richish AFR (in diagnostic mode), but possibly not as rich as can occur with a defective O2 sensor falsely reporting a lean AFR (which would cause the ECU to keep enriching the AFR, possibly to the point of poor idle and restricted rpm / power??). I'm just guessing here. Any other signs that might point toward a rich AFR? Maybe try disconnecting the O2 sensor connector and see if that affects how the engine runs.

With the O2 sensor disconnected the ECU will default to hard maps in it's memory, and the AFR will tend to be fairly rich, but as above maybe not as grossly rich as a faulty O2 sensor can cause.

[Chrisnick]

http://www.ericthecarguy.com/kunena/...s-hunting-idle

Good luck! That should be the end-all to solving a poor idling Honda. If not, there's other fish to fry.