Don Carter

I was out in the garage this evening trying to diagnose a rough idle on my 86.5 and found that the coil wire had come lose from the coil and was arching. Problem solved. I had also installed the Roger Fuel Pressure gauge to check the fuel pressure, and noticed that my fuel pressure at idle is only 20psi, and goes up to maybe 25psi when I rev it. The car actually has plenty of power, but of course I'd like to get this back up to 55psi where it should be (87 FPR installed with PorKen Chips).

So should I look at a new fuel pump? FPR? or just replace the filter first? I was planning on taking this car to Marble Falls for the first time since 2010, so need to get it sorted soon. I could pull a pump off the 89 to see if that fixes it, but not sure if the pumps are the same.

dr bob

Don--

Before you get to replacing metal parts, spend a few minutes and dollars on the power feed for the fuel pump, and a new fuel pump relay. Dedicated power runs from one of those smaller wires at the battery positive. Clean with a small brass brush to remove any stray stuff and oxidation. Reinstall with the nut snugged firmly. I coat them whole terminal with a little Vaseline. Stan and others recommend Deoxit for cleaning the connections. Swap in a new fuel pump relay while the battery is disconnected.

----

Stranded at the bottom of the driveway with low fuel pressure, a new relay brought everything back to normal. I suggest getting the critical relays replaced before they cause problems. LH, ignition/EZK, Fuel Pump, injectors, X-bus, etc. Relatively inexpensive 53B relays all. Much easier to replace in the comfort convenience and safety of the garage, vs the side of the road somewhere.

Don Carter

bob,

Thanks, I'll take a look at the connections on the wiring for the fuel pump. All the 53 relays were replaced a year or two ago. I do have some Deoxit, but haven't used it yet.

928 at last

And if that doesn't cover it, check your inlet and filter.

Don Carter

Bob,

Thanks for getting me thinking about the power supply to the pump. I cleaned the ground point in the spare tire well today and the pressure went from 25 to 45. Not perfect but nice improvement. The connections didn't look too bad, but did have a slight white haze on them.

jcorenman

Don, with the engine running (and vacuum connected to the fuel pressure regulator), 45 psi is about right. Engine vacuum reduces fuel pressure in order to maintain a constant 55psi differential pressure across the injector.

If you do this test with engine stopped (no vacuum), and fuel pump jumpered, then it should read 55 psi on the gauge.

Don Carter

Jim, thanks that makes sense. I was thinking that I had seen higher pressures when I first installed the S4 FPR, but could be wrong.

FBIII

You could also squeeze off the fuel return line to see if the fpr is bleeding off too much pressure.