: Fuel pressure at idle/WOT?



eldorado1
09-28-04, 11:05 PM
I think my FPR/pump may be on the fritz. I need to know what readings are normal at say, 800rpm (idle) and WOT(6000rpm) I suspect fuel flow is slowing in the upper rpms, and I don't know if it's because the fpr isn't doing it's job, or the fuel pump is weak. Either way, I need to know those pressures, I can't seem to find them listed in my service manual. But it seems as though 40-50psi at idle is normal, but doesn't mention anything more than that. If that's incorrect, please let me know. Thanks!! :thumbsup:

Eldyfig
09-28-04, 11:39 PM
In a North*, 40-50 is normal.

eldorado1
09-28-04, 11:56 PM
In a North*, 40-50 is normal.

at idle and wot? shouldn't it rise like 5-10psi at wot?

Eldyfig
09-29-04, 12:05 AM
Engine specs in my manual do not say idle or under throttle.

eldorado1
09-29-04, 12:25 AM
Engine specs in my manual do not say idle or under throttle.

Don't you find that weird? These manuals only give 1/2 the information!!! I swear! Maybe I need to call my local caddy service guys?

Ranger
09-29-04, 12:51 PM
It should remain relatively constant. That's what the FPR is for. If there is a big drop when you go WOT, then the pump is going bad.

kcnewell
09-29-04, 01:16 PM
The fuel pressure control system is set up to maintain a constant pressure drop across the fuel injector - from the fuel rail to the tip of the injector. Since the tip of the injector is exposed to manifold vacuum, the fuel pressure must change slightly in the rail as the manfold vacuum changes to keep the pressure drop across the injector constant. That is why there's a vacuum line attached to the fuel pressure regulator and that is what the fuel pressure regulator does.

When checking the fuel pressure there is basically a value that you will see at idle and a value that should be a few PSI higher at WOT. In normal driving you will see the fuel pressure change by a few PSI as you operate the throttle and change the engine vacuum. That's normal. When you go to WOT the fuel pressure should go to a fixed value and stay there regardless of the RPM or load on the engine. You can see this at idle if you watch the fuel pressure while idling and then pull the vacuum line off the fuel pressure regulator. This simulates WOT and the fuel pressure will change accordingly by a few PSI - not much but a little.

The most important thing to do is to put the fuel pressure gauge on the fuel rail so that you can drive the car with the gauge in place and see it. Just route the hose out from under the hood and duct tape the gauge to the hood where it's visible. Drive the car and accelerate at WOT up thru a 1-2 WOT upshift and watch the fuel pressure carefully. It should jump up a few PSI when you snap the throttle to WOT and then stay constant as the engine RPM increases up to 6500rpm at the shift point. If the pressure drops as the RPM increases or drops when the engine stumbles or something then I would suspect a fuel supply problem - possibly with the pump or something.

eldorado1
09-29-04, 03:10 PM
Thanks for the explanation! Makes sense to me now... Probably helps the development team with creating fuel maps if the injector pressure drop (and then flow rates) are constant... Now just watch - I'll put the gauge on, and jar the fuel rail just enough to cause a pinhole leak, and it'll burn to the ground. :rolleyes2

eldorado1
09-29-04, 05:56 PM
Yep, 90% sure it's the fuel pump. New filter, idles at 37PSI, with the vacuum line disconnected, it rises to 42psi... At WOT, it drops to 26psi, (and is trying to destroy my engine.) :rant2:

kcnewell
09-30-04, 01:07 AM
Yeah. I'd drop the tank and check the pump. Remember to check the wires going to the pump for any signs of melting due to excess current draw as the pump was failing.