Hey guys. The last couple times I've driven the Iroc, it seems to stumble or hesitate on hard acceleration occasionally. It doesnt do it everytime but often enough to bother me.

Tonight I got out the fuel pressure guage and checked it. At Idle it's right at 30psi, when ya rev it it'll jump up to 38 or so. According to my haynes manual it should be 34-47PSI... is mine low enough to cause the stumbling or is my problem elsewhere?


The car has 220,000 miles on it, and it may very well be the original fuel pump because I cant find it listed in the previous owners maintenance records (He kept flawless records, every little detail) So I'm guessing its probably not in tip top shape, but I hate to replace something that isnt bad.

 

Comparsion: With my FP gage.
1) With key turned on (engine not started) 36/38 psi.
2) Idle 38/40 psi.
3) 45 psi set for WOT.

Sounds like your way Low.

 

I take the FPR hose off to simulate WOT and should get around 45 psi.

 

Good luck if he does not have a adjustable FPR.

 

Actually, that is a valid test. Disconnect the vacuum hose from the FP regulator and you should have about 42-43 PSI under all conditions (with a stock regulator- adjustable ones it should hold whatever pressure you set it at). The vacuum hose is there to reuce fuel pressure in relation to engine vacuum (idle, part throttle, light loads). DIsconnecting it will simulate WOT where manifold vacuum drops close to zero.

The MAIN thing when testing fuel pressure is that you HAVE TO DO IT UNDER ACTUAL HEAVY LOAD/WOT CONDITIONS. This means mounting the gague somewhere you can see it while you are driving around. I have gagues with longer lines on them that I use for exactly this purpose. I then duck tapte the gauge to the windshield (temporarily!) while I make a few trial runs. Marginal pumps can often keep up just fine when fuel demand is low (idling in your driveway) but run outta flow/pressure when demand is high (WOT/high RPMs).

Extending the line on a fuel pressure gague is not difficult- 3 ft. of high pressure 5/16" fuel line, couple of 5/16" double-male barbed hose fittings and some hose clamps will let you insert extra length into the rather short line that is typically supplied with most fuel pressure gagues. Then you can snake it up past the cowl weatherstripping and duck tape it to the windshield for testing.

 

if your pressure raises when you get on it your pump isn't bad....

 

If you drive it and check pressure readings, what you are trying to do is to see if the pump can keep up under fuel use. However, if under idle conditions, the pressure is way low, I wouldn't try it under WOT conditions because leaning out of a motor is bad. Try it under idle with the hose off first and if that is good then you can see if it will do the same under WOT conditions.

 

would it make a difference on your 1/4 mile or even 1/8 if the FPR was dissconnected and caped off? someone told me that would make it run better...true?

 

Wishmaster,
I am assuming that you mean the vacuum line disconnected that runs to the FPR. It should make no difference in performance to do this since you will have no manifold vacuum when at WOT anyhow. Waste of time IMO.

 

Disconnecting it at the track won't make much difference. It wil only have an effect at idle and part-throttle (as the is nearly no vacuum at WOT anyway,) and the ECM will compensate for the rich condition by reducing the injector pulse width anyway.

 

I'd heard people say to disconnect the FPR, but I assumed that was just for the aftermarket adjustable ones. My 305 is completly stock.

Wheres the vacuum line for the stock FPR?


Thanks for all the help guys

 

Rear pass. side of the plenum, just behind the runner for the #8 cylinder. It's the smaller of the 2. Make sure you cap off the port because TPI doesn't like vacuum leaks at ALL.

 

Your FPR is the round thing like a small hockey puck that is between the 2 fuel rails. Each side of it has a pipe going to the fuel rail.

 

ok, thanks guys, I'll check the fuel pressure again in the morning with the FPR disconnected and let ya know how it goes.