Total cub mechanic just got my pressure tester from ebay today. Car primes to 40 psi, starts slow. Idles at 33 psi shut car off pressure will hold at 33 until ign is turned back on then goes to 40,and holds. Start up back to 33. Taking off vac line @ regulator it will go to 40 psi and hold. Pressing throttle will get the gauge to hit 40 but it will mostly hover @ 35 and below. I will need to get a longer hose to tape the gauge to my windshield and try it under load. Car has been starting slow, not right away like it did in the past. I also have been getting backfire @ high RPM and High speed (110+) Thought it might be a fuel pressure problem. I also get a random EGR DTC sometimes, which doesnt seem to affect the engine other than going into closed loop, shut it off then start it back up.. SES gone and back to normal. I have replaced plugs and injectors. I dont have a DATA reader yet to show you guys what it is doing. The car runs ok other than startup and WOT like I have said. Fuel pump, Regulator or something else? Just need to know where to go next, I have replaced all sensors and have a new MAF albeit a Granatelli. Fuel filter is new. OPTI and wires are original with a new MSD cap and rotor. (90K mile CA car) I am hoping it is the fuel pump.. is 33psi too low,??? what do you think?

 

Pressure is suppose to be between 41-47psi once primed, then it's suppose to drop anywhere from 3-10psi on start up. So to me it seems normal at 33. It goes to 40 with ignition on again because the pump is priming it again. I'm not sure though on what effects removing the vac line off the reg should do to the pressure...

 

The vacuum line drops the pressure in the rails, so that the DIFFERENTIAL pressure between the rails and the inside of the intake manifold is held as close to 43.5psi as possible. Pull the vacuum line off when its idling, and that's when you should see 41-47psi on the rails. Put the vacuum line back on, and the regulator drops the pressure in proportion to intake manifold vacuum, to control the DIFFERENTIAL pressure.

With a stock cam, intake vacuum can be 20"Hg, and that's equivalant to about negative 10psi.

40psi without the vacuum line while idling, is just below the GM spec. Could be a faulty fuel pressure regulator, could be a failing pump, or low voltage to the pump.

 

Learning my way around this thing pretty quick.... So Fred with a 20"hg equaling negative 10 psi that would be why 33 psi could be considered ok. Anyway..Thanks guys, just ordered a new regulator. I will try that first.

 

It ususally doesn't drop 10psi, probably more like 6 - 8psi. But measure the pressure at idle with the vacuum line off. If you only get 40psi, the pressure is marginal. At a 40psi differential pressure the injectors flow about 4% less than they are supposed to. And if the pressure is marginal at idle, you definitely have to check the pressure under max RPM/max load conditions.