Verifying Fuel Pressure on '71 Pontiac 455HO Quadrajet

[olemansmith]

I am trying to verify fuel pressure supplied from the mechanical pump to the carb. Built &Installed a 3/8" temporary fuel line with a 0-15psi Mr. Gasket (cheap $20 Non Liquid Filled) gauge. It is hard lined at the pump & carb. Rubber fuel line connected between gauge and hard lines. Gauge is mounted on the branch of the 3/8" brass tee. No leaks exist (thankfully). My problem is the pump generates so much pulse I can't read the gauge. It is fluctuating at a very high rate between 4-ish & 11-ish psi. That's a 7.5 psi average. But I don't want to bet on it.

Questions or possible causes? I'm just guessing here.
Do I simply need a snubber, or a good liquid filled gauge? I didn't think I would need the liquid filled gauge just to briefly test pressure.
Is the pump pressure so high it is driving the needle off it's seat? Car idles rough due to cam, but does not overfill or flood.
Is my pump too weak to totally fill and maintain proper bowl/float height? Would that answer why it is so erratic?

All help is very much appreciated.
Eddie

[Cliff Ruggles]

If the pump is working correctly it should be bouncing the gauge that much nor should it be spiking the pressure to 11 psi.....Cliff

[jwilson645]

Run down to the parts store and pick up an Actron vacuum/pressure tester. Think I paid about $20 for mine. It is a much better gauge for this.  So far I haven't found any of the small gauges (cheap) that don't bounce.

http://shop.advanceautoparts.com/p/actron-vacuum-and-pressure-test-kit-cp7803/9090006-P

My OE replacement fuel pump was pegging the needle at 10psi and I put in a Summit fuel pressure regulator to turn it down.

https://www.summitracing.com/parts/sum-g3032b/overview/

[olemansmith]

Thanks Cliff.
Yeah the needle really bounces a lot, I mean a LOT! It's a blur. I went back  out to the garage after I posted this yesterday and started the car and took a long needle nosed vice grip and adjusted it to where it was just pinching the fuel hose closed between the pump and the gauge. The moment I pinched off the hose the gauge would read steady 4psi, sometimes 5psi, but mostly 4psi. It was just a quick "snapshot of the pressure" as fuel was being consumed the pressure dropped. I held the engine @ about 1500 rpm hoping the pulse would lessen or equalize or cancel out, the needle remained a blur. I did it several times with no improvement.

I think I answered my question about whether the bowl was full & the float/needle valve was operating. I don't think there would have been any pressure if the float/needle valve was not operating correctly. However, the pump must be just barely supplying the absolute minimum to keep it full at idle. Fuel level is probably not sufficient for anything above cruising speed RPM.   

[olemansmith]

Thanks Cliff & J Wilson,

Replaced old fuel pump with a new Carter M6122 (Large Canister). That fixed it. I now have 7.5 psi, with very little fluctuation on the gauge.  I realize that is a little higher than stock pressure, but I had installed the .135" needle & seat Cliff sent when he sent me all the other parts to re-calibrate my idle circuit.

In case anybody else driving a GM is having problems...
M6122 is Carter's GM Muscle Car replacement pump. They made exact replacements for the AC units on 70-71 Firebirds many years ago. The new ones will work but you will have to change a few things. I had to re-route the supply & the return vapor hoses. They are 90 degrees from  the original position. Not a show stopper, just had to isolate the fuel tank lines so when the existing hoses were removed I didn't take a bath in gas. The steel carb to pump line used to be on a 45 degree angle from the mount. Now it's  90 degrees. You'll have to tweak (twist) your steel line to get it to fit. Not too bad of a job. I'm just glad Carter is still making parts for all of us to keep our cars running. I can live with a few changes.

Thanks again for your comments.

[Cliff Ruggles]

Good information on the pump still being available and that it corrected the pressure fluctuation issues.

We get a LOT of complaints to the shop these days from folks who have just purchased new mechanical fuel pumps that are not working correctly and causing flooding.  Not uncommon for folks to report seeing 12 psi or more from some of these "new" pumps.

I suspect that most or all of these pumps are imported.....Cliff

[olemansmith]

I suspect this one is offshore too Cliff. Carter used to be proud enough to have their name embossed on the side of the pump. The one I got (and I'm glad it works) has nothing any where except some tiny little production numbers stamped on the bolt up flange to identify what it is. Other than that, it is a "slickity no name". I read a dozen or more reviews on the Summit site with guys sending them back because they have too much pressure. I had to bite the bullet and get one knowing I may have to add a regulator. What are you going to do? Can't buy an old stock pump that was made long before ethanol gas was here. Things change, I keep the tank as close to full as I can before I park it. I started using Joe Gibbs Driven Carb Defender fuel treatment a good while back.. I haven't saw any degradation to any fuel components. Anybody else doing this?