This is a continuation of my previous fuel pressure / injector question.

So the missing part for my fuel pressure gauge came in and I was able to do a few more tests. The first thing I think is that there may be multiple problems here. I both did some specific tests, and just drove around a few days watching the FP gauge. Something that stood out was that the intensity of the rough idle was synchronized with the jittering of the fuel pressure. I uploaded a short video here.

Basically, the fuel pressure needle is jittering back and forth between 30-40psi faster than the eye can see, but when I raise the rpms to 3k the jitter almost completely goes away and the engine is pretty smooth. This isn't consistent, sometimes it's much better and there is very little vibration and almost no needle jitter. It's a very inconsistent problem.

Anyways, here are the measurments with the FP gauge, but they're a little strange and inconsistent quite frankly:

  • Max Pressure Engine Off ( 64-92psi ) 42
  • After 5 Minutes 38
  • After 1 1/2 Hours 20
  • Max Pressure Engine Off Return Line Clamped 74
  • After 5 minutes ( Return Line Clamped ) 20
  • Idle Pressure ( 30 - 36 ) 32
  • Vacuum Disconnected from FPR ( 37 - 45 ) 42

Some of the weird things I noticed over the past few days:

  • Pressure initially goes up to 40 from about 32-24 after shutting off engine
  • Idle Pressure with Vacuum disconnected and Max Pressure Engine Off are both 42

Just to summarize the info from the previous question:

  • Fuel Pump draws 4.35 amps both at idle and during EO Max Pressure test
  • Fuel Pump draws 5.85 amps with the return line clamped
  • The #1 & #4 Injectors where wet after sitting for 36 hours and both those spark plugs had carbon fouling
  • I could very clearly hear the fuel flowing through the rail during the engine off max pressure test
  • All four injectors passed the volume test 56-61ml in 15 seconds
  • Injector spray patterns seemed OK according to Paulster2

Other than the rough idle and hard starting there's no problem, the engine has plenty of power at high rpms and doesn't seem to be misfiring, both of which I think it would have problems with if the fuel pump wasn't providing sufficient pressure or volume.

I did run across this in the Tim Giles Automotive Service book:

A pressure regulator can become defective due to foreign material or a ruined diaphram. The result will be hard starting, poor idle quality, and lack of power.

Like I said, some of this data doesn't seem so consistent, and there may be more than one problem. Currently I've got some refurbished injectors on order.

What do you more experienced guys ( or gals ) think?

  • 1
    If you are not having bleed off problems from the injectors into the cylinders (I think you replaced the injectors, right?) and there aren't any leaks, your pump is allowing pressure to bleed back which it shouldn't. I say this due to pressure loss when clamping off the return line. There's an issue with your pump which may be causing all of your issues, but it may be a good place to start. Since you have a return system, the regulator should be taken out of the mix when you clamp off the return line. Commented Jan 13, 2017 at 14:49
  • @Pᴀᴜʟsᴛᴇʀ2 I ordered injectors, but they haven't arrived yet. I guess I should wait for the injectors to arrive, install them and then see what's going on.... Commented Jan 14, 2017 at 16:03

1 Answer 1


Sounds like your pressure fuel pump is 100%. Pressure regulator could be off. If your above 30psi after 5 mins you are probably OK.

The pressure regulator works by bleeding pressure back to the tank. It sounds like it could be sticking a bit. At full throttle it probably closes all the way. I suspect at idle it should open to its max. Maybe just fluttering.

Compression test?

Also, if possible use a scan tool to verify things like MAF and MAP sensors. I had a MAP sensor fail that was giving me rough, surging idle - until if failed all the way and left car dead. If you have check engine light, examine the stored codes.

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