fuel pressure test

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 2:00 am

ok ive still got that dam hesitation in my 92 SS... recap, replaced the plugs, wires, coil, ignitor, knock sensor, MAF, fuel filter, air filter, checked the turbo, gutted the mid pipe. im thinking that its not getting the proper fuel pressure.

when i replaced the fuel filter i turned the ignition on and off 3 times, then it cranked for about 5 seconds, no start, then on off another 3 times and then it had to crank for another 7 seconds before it started.

sometimes is will run perfect for one WOT pull... then a block down the street it runs like shit again with no power. im thinking bad pump or clogged pump sock, so i want to test the pressure. where is the schrader valve for testing? also what are the specs soposted to be?

Thanks,
Shean

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 2:05 am

There is no Schraeder valve for testing fuel pressure. You just need to tee into the hose going from the fuel filter to the fuel rail.

Fuel rail pressure should always be 36.3 psi above manifold pressure. That means at idle it should be somewhere around 26 psi and at full stock boost it should be somewhere around 45 psi.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 2:19 am

ok but how am i soposted to check fuel pressure under boost? sit in my driveway, put it in Drive and load up the engine while my girlfriend reads the gauge?

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 2:22 am

The "right" way would be to use an electronic gauge, I suppose.

If you have to do it ghetto-style, you could brake-torque it like you say, or you could use hose long enough to tape the gauge to your windshield for a quick run.

You can check for pressure without flow by simply applying pressure to the FPR's vacuum nipple using a hand pump.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 2:43 am

ok just did the test, hooked it up after the filter. idle was around 35psi. brake torqued it and it hit 60 psi at 10/11psi of boost.

is that too much pressure?

douglas vincent · Post by **douglas vincent** » Mon Feb 20, 2006 3:58 am

I vote for a bad temp sensor!

And no, thats not too much fuel pressure under boost, just make sure all your lines are good! Thats about what mine jacks up to on boost, maxing out at about 65 psi.

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 4:25 am

That is too much pressure. Something's not right.

At idle it should be around 26 psi, like I said. And at 10 psi it should be around 43 psi.

Do you trust your gauge? What is the fuel pressure right after priming the pump but before cranking the engine?

If those numbers are correct, you may have a blockage somewhere in the fuel return plumbing, or your fuel pressure regulator may be bad.

If you hook up my scan tool I expect you'll find some fairly large negative fuel trims.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 4:52 am

well im trying to get ahold of a laptop to use to read my car, my dad is going to try to get me one for around $75...

unless i can find someone in the seattle area that could help me out with their scanner... i really want this fixed soon, its killing my MPG, and im going down to portland for the torquefreaks dyno march 26th.

also how could i check my fpr or for a blockage in my return line?

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 5:05 am

You could check for a blockage by connecting some 5/16" fuel hose (for purposes of the test, cheap SAE 30R6 hose normally only used on carbureted cars will do) to the FPR's outlet and directing it into a large gasoline-safe container. If fuel pressure is correct in this situation (the container will start to fill with fuel quickly, so just check it with the engine off, or at idle for a few seconds if necessary), there's a blockage in the return lines.

If you do this and pressure remains high, it's probably the FPR that's at fault.

Reaching this hose is rather awkward, especially on turbo models. You might want to instead use the fitting at the middle of the driver's side of the intake manifold and do it from there first (it's the hose that's the same size as the one coming from the filter, but not the one coming from the filter). If pressure is correct, the blockage is downstream of that spot. If not, there may be a blockage between the FPR and that spot.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 5:14 am

ok so to test the fpr, hook up the fuel press tester again like i did before, after the filter. then unhook the return line and hook a hose from that to a gas can on the ground. then what start the car or just turn on the ignition? and if i get fuel going into the can and the pressure returns to normal... 26psi at idle then its after the fpr to the tank? sorry, im just spazzin out trying to figure this out for the last 3 weeks

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 5:22 am

Yes, you have the plumbing right.

First test it just by turning the ignition switch on. The pressure should shoot up to about 36 psi, and a little fuel should come out the return line.

If it's higher, either there's a blockage between the FPR and where you unhooked the line, or the FPR is bad.

If it's correct, then there's a blockage between the tank and where you unhooked the line.

If you're feeling daring and have a friend with a fire extinguisher nearby you can double-check your diagnosis by starting the engine and check the pressure at idle (which should be roughly 26 psi) also. While you're doing this, fuel will pour continuously out the return line, at a rate around a few tablespoons per second.

Before you unhook the return line, get the answer to my question about what the fuel pressure is right after you turn the ignition switch on and let the pump prime but before you start the car. If it's not higher than 36 psi there's more to this mystery.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 5:32 am

ok looks as if im going outside right now to check it with the ignition on not running.

now if it is lower then 36psi could it be a clogged filter sock on the fuel pump?

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 5:36 am

Yes, it could.

But I'd be surprised, since the pressure is too high when running.

douglas vincent · Post by **douglas vincent** » Mon Feb 20, 2006 5:44 am

DID you check the temp sensor yet? When mine went bad it would be good for days, then bad, then good, then bad.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 5:49 am

douglas vincent wrote:DID you check the temp sensor yet? When mine went bad it would be good for days, then bad, then good, then bad.

its not one day its fine then another bad, its every day all day with a few exceptions rarly that it will run perfect, then back to crap. and i highly doubt that it is the temp sensor, because that would really only effect cold start which its fine on cold start.

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 5:53 am

I'm not saying the temperature sensor isn't bad, but his fuel pressures are way off, Doug.

douglas vincent · Post by **douglas vincent** » Mon Feb 20, 2006 5:53 am

Not true, if its bad it can affect the vehicle all the time. At least it did with me, pain in the major ass! It would barely run, even if it had been driven for a full hour!

douglas vincent · Post by **douglas vincent** » Mon Feb 20, 2006 5:58 am

Not argueing, but my fuel pressure is 45 at idle (3.25 bar) and 62 psi (4.25 bar) on boost. But I kinda got a whacky rrfpr setup since I still havent done the bleeder set up you (Vicash) gave me instructions for, for the 12-1 rrfpr.

douglas vincent · Post by **douglas vincent** » Mon Feb 20, 2006 5:59 am

Although I must admit that maybe this is why I am having some issues with the Perfect Power.... But I doubt it. That would be a SIMPLE fix!

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 6:02 am

Yeah, your car's basically a bunch of hacks in a very careful balance. With a mostly stock car, there are specifications for things like fuel pressure, and when they're not met it's an indication something's wrong. I don't see how it's relevant what you've set your car's fuel pressure to be.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 6:11 am

ok when the pump kicked on it hit about 45psi... i did it a few times to make sure, but it didn't kick on every time i turned on the key to the on position from putting the key in the ignition, turning it on then turning it off and removing the key, it did it the first time when i first hooked up the gauge. then after i think the 4th time it kicked the pump on again and it 45 or so psi, every other time it sat at 35psi.

vrg3 · Post by **vrg3** » Mon Feb 20, 2006 3:35 pm

Hm. That does kind of sound like a blockage, though I don't quite understand why sometimes the pressure came out corret. Did you do the hose-and-gas-can test?

And what are you using to measure pressure?

NICO · Post by **NICO** » Mon Feb 20, 2006 4:03 pm

just a quick question, im haveing the same problem as The Scooby.

i also have checked or changed everything, if this temp sensor goes how does the car feel in driving it?

i have a lm1 and my car drives fine with a little gas at wot it bogs and leans out while boging.

The Scooby i dont thing its your fuel pump when fuel pumps go they just die. as i was reading your post i have the same problem and it just started yesteday.

The Scooby · Post by **The Scooby** » Mon Feb 20, 2006 9:48 pm

im using a acron fuel pressure tester. and no i haven't had time to check the line into the can.

The Scooby · Post by **The Scooby** » Tue Feb 21, 2006 2:41 am

VRG3... anything else i should give a try... or do you know what is wrong...?

legacycentral bbs

A forum for 89-94 BC-BF(BJ) Legacy Owners and Fans

fuel pressure test

fuel pressure test