Fuel Fuel Psi Problem?

Discussion in 'SN95 4.6L Mustang Tech' started by adamv7010, May 27, 2014.


  1. adamv7010

    adamv7010 Member

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    I've been having some rough idle conditions on my 03. I initially thought it was due to the cracked intake manifold, which I'm still not completely convinced it didn't kill a couple cops by saturating them. At any rate I've since replaced the intake with an frpp replacement.

    I cleaned the maf and that smoothed the idle somewhat for a short period of time. I decided to see what fuel psi looked like, with a mechanical gauge on the test port it idles at 28-30 psi. During cranking it drops to around 20.

    During idle, the fpr reads 40psi via an sct and my genesys.

    Does that sound right? I read somewhere that fords spec was 28-50 psi or something. That's a really broad range. Ideas?
  2. Bob Hughes

    Bob Hughes Member

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    At idle the rail will read app 28-30 die to engine vacuum compensation from frps. As throttle increases vacuum decrease more psi at rail port. Ideal psi is 40.
  3. wmburns

    wmburns SN Certified Technician

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    +1 on above. It depends on HOW the measurement was taken.

    An external gauge is not adjusted for the intake vacuum.

    Where as the FRPS is adjusted for intake vacuum. Hence why the PCM shows the fuel pressure at 40 PSI while the external gauge show 32 PSI.

    Try this for proof. Disconnect and plug the FRPS intake vacuum reference line. The PCM and external gauge should now read the same. Don't drive for long periods with the intake vacuum reference line disconnected.
  4. adamv7010

    adamv7010 Member

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    And it did when I pulled the vac line off.

    I just wanted to rule out a fuel issue with the rough idle. I really think its a cop or two
  5. Bob Hughes

    Bob Hughes Member

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    Normally if a coil goes you'll get a misfite code from that bad cylinder. Check around for any vacuum leaks. You mentions having replaced manifold maybe a connection was missed or damaged
  6. adamv7010

    adamv7010 Member

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    Everywhere I read talks about how bad mustangs are for not setting a code.

    No vacuum leaks. The rough idle was present before I changed the manifold. 2/8 coils had water standing in the bore.
  7. Bob Hughes

    Bob Hughes Member

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    I've had 2 go on me. One when I bought the car and the other I accidentally got wet with the hose. Both times ecu immediately threw a code. But if it was there before it's possible. Could do the old unplug trick listen for. Change in the idle. Do one at a time. Or there may be a way to test them but i don't know how
  8. adamv7010

    adamv7010 Member

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    I have unplugged them one at a time it did stumble on each unplug. I'm still not convinced one or two aren't breaking down under heat.

    It smooths out when the ac is on at idle.
  9. Bob Hughes

    Bob Hughes Member

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    That would lead me to think it's more of an iac or tps issue. Bevause the a/c triggers the iac to introduce more air into the inlet to counteract the load from the compressor
  10. Rusty67

    Rusty67 Well-Known Member

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    There is a set of Ford self tests in there called TIDs which check for missfire (among other things). If you can pull that data you will know if your coils are missfiring. These will not cause a CEL to come on unless they break a certain threshold. I had an issue with COP #8 in my Mach 1 and it all but refused to set a CEL but the TID for missfire on #8 told me exactly what I needed to know to fix the problem.

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