Fuel pressure question

[kilou]

Hi,

I got a new fuel pressure gauge and checked the fuel rail pressure on my 1997 Volvo S40 2.0 petrol (200K miles). This is a normally aspirated engine, not the one available in US.

First I released the pressure from the rail before fitting the gauge. Then I turned the key on ignition II to build up pressure in the rail (did that several times so that the rail gets full). The reading are then as follow:

- Turning the key on ignition II (engine not started): pressure spikes up to about 40 PSI while the fuel pump makes its noise (it has always done that, even the previous pump did that). As soon as the fuel pump noise goes away, fuel pressure drops down to 31 PSI and stay there.

- When starting the engine, with engine idling, the pressure goes up to 38PSI. Raising the RPM makes the pressure go up to about 40-45PSI but when idling it goes back to 38 PSI.

- After shutting off the engine and waiting for 20 minutes, the rail keeps a pressure of 23 PSI. VADIS mentions that during a 20 min period, the fuel pressure shouldn't go below 29PSI but they start from about 45 PSI idling while mine was only 31 PSI...

OK I don't have particular trouble with the car but it has always felt a little sluggish. I have no idea if these fuel pressure reading may be an explanation here. Since the pressure drops by about 8 PSI in a 20 min period, I don't think the fuel regulator is leaking. However I don't understand why the pressure spikes up to 40 PSI while the fuel pump is making the noise and then goes down suddenly to 31 PSI and stay there. Where does these 9 PSI go??? Is this an issue with a return valve somewhere...or still an issue with the fuel pressure regulator?

Thanks for any input!

PS: forgot to mention the fuel filter was renewed 6K miles ago and the fuel pump has 130K miles on it.

 

[aaxb970]

When starting the engine, with engine idling, the pressure goes up to 38PSI. Raising the RPM makes the pressure go up to about 40-45PSI but when idling it goes back to 38 PSI.

Thats the only part I would worry about....the other 9psi would go back to tank in return line....IMHO

 

[kilou]

Hi,

thanks for the reply. In fact I had to disconnect the vacuum line to the pressure regulator before taking the readings. Now the pressure is 46PSI all the time (idle/revving). The pressure drop at idle is normal if the vacuum line is attached.

The only "issue" is that as soon as the pump is deactivated (engine shutdown for instance), the fuel pressure drops immediately by about 10PSI. This is probably caused by a weak check valve in the pump itself. Nothing to worry though as the car always starts fine.

 

[mechanicx]

Originally Posted By: kilou
Hi,

thanks for the reply. In fact I had to disconnect the vacuum line to the pressure regulator before taking the readings. Now the pressure is 46PSI all the time (idle/revving). The pressure drop at idle is normal if the vacuum line is attached.

The only "issue" is that as soon as the pump is deactivated (engine shutdown for instance), the fuel pressure drops immediately by about 10PSI. This is probably caused by a weak check valve in the pump itself. Nothing to worry though as the car always starts fine.


I think your right that it is probably a bad pump check valve bleeding off pressure when the pump stops running. What you can do is first check the pressure regulator vaccuum hose to be sure no fuel is leaking in there. Then carefully pinch off the return line with pliers (they have special fuel line pliers). If the pressure no longer drops then it indicates a bad regulator. If it still drops the fuel pump assembly is indicated.

 

[mechtech2]

The pressure drop when the car is off is not too bad. But it has nothing to do with how the car runs.
Look elsewhere for sluggish performance problems.

 

[mechanicx]

Yeah but I couldn't make heads or tail of whether his running pressure was lower than spec or not. If the fuel pump is losing pressure due to a leak at the pick up or really anywhere in the assembly other than just the drain back valve, the pump pressure/volume might be reduced. Actually the pressure could come up right at engine off and idle but it might lack supply volume at engine load and then drop pressure. It has happened on other cars. I just can't really tell if his pressure was in spec or low.

 

[mechtech2]

The check valve is not in operation when running, as the pressure is way above it's realization point.
It is a separate problem, if it even is one at all.
You pressures otherwise seem great - normal.

 

[mechanicx]

What I'm saying is a pump leak down problem is not necessarily the pump check valve. It can be leaking ahead of the check valve, or just leaking down somewhere else. I've seen it on some models before. It seemed like his idling pressure might have been low, but even if not it doesn't really tell you what the pump will put out in volume at engine load if there is a leak.