View Full Version : fuel pressure

07-29-2009, 04:34 PM
If i were to test the fuel pressure on the fuel rail, what reading should i get at idle on a 94? Should i also test at certain RPM's and or boost levels? Does anyone have a chart for those reference points?


07-29-2009, 04:55 PM
with the vac line on the reg disconnected you should be seeing 39.5 psi. With the vac line connected you'll see slightly less...maybe around 33 psi. Under boost you'll see a 1:1 ratio of boost to additional fuel pressure over your static setting (39.5). At 10psi boost you should be seeing 49.5psi, etc etc...

07-29-2009, 04:59 PM
Thanks...appreciate it!

07-31-2009, 09:56 AM
I tested the fuel pressure last night at the fuel rail...The gauge read ~30.5 PSI...that seems a little low...Is that low enough to cause a driveability/idle/loss of power condition?

First thing I am going to do is change the fuel filter....

07-31-2009, 10:02 AM
is that with the vac line on the regulator?

If so, that's an acceptable reading at idle. Assuming you're idling at 20 inhg, that's roughly 9.82 psi. Subtract that from your static pressure of 39.5 and you get 29.68psi.

For instance, I have my fuel pressure set to 43psi static, and I idle around 13 inhg, which puts my idling pressure around 36psi.

07-31-2009, 10:06 AM
yes, sorry, forgot to include that factoid....i wasn't able to log on last night, it looked like i was in, but couldn't post, so i tried typing the message about 1/2 dozen times last nightm, and tried to say that I toughed nothing else, i.e. vacuum to regulator.

ugh, i was almost hoping you were going to say it was a little low, and just a fuel filter...I guess i will have to test some sensors.


07-31-2009, 10:07 AM
yes, idling at 20in hg. sounds like there is a formula where you are coming up with the 29.xx, etc.....What is it if i may ask?

07-31-2009, 10:08 AM
what's your vac at idle? Keep in mind, this is only fuel pressure at one instance. During driving, a clogged filter could be causing issues with fuel supply. Could also be a clogged pump sock in the tank. Explain your problem in more detail.

07-31-2009, 10:10 AM
there is no indication of a vacuum leak, the gauge is steady at idle. its not like it doesn't idle at all, just sometimes it gets a rough spot....

07-31-2009, 10:11 AM
inches of mercury is just a pressure term. I converted inches of mercury to psi. 1 inch of mercury is equal to 0.49109778 psi.

07-31-2009, 10:14 AM
Car just didn't seem to run 'perfect..' Then CEL light came on. I thought it was from installing headers, even though O2 sensors were re-installed.

the CEL turned out to be a cam sensor, i replaced it, reset, and no light since.

In another thread, i was adviced to check all of the sensors for temp, BAP, etc.

Car just doesn't seem to make the power that i was used to with stock or just slightly modded SC in the past. I think it may still have a reg air filter, i need to put my K&N in.....

it will idle, then stumble, and RPM will go low, just shy of where it would want to stall...

I did wires and plugs last summer, but car probably barely has 500 miles since then. I cleaned the MAF as well with teh appropriate spray cleaner too.