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addermk2 said:

time to bring it to a garage.

If you had a Moates Quarterhorse, you could do the diag yourself.

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Micheal Nadeau said:

I am a technician, and work in a shop......

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Go Micheal!

I have this mental picture of addermk2 taking his foot out of his mouth...
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Micheal Nadeau said:

I am a technician, and work in a shop......

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Let me rephrase my statement... Maybe you should bring it to a shop who employs people with a clue.

jrichker said:

Go Micheal!



why would I put my foot in my mouth? If you cant diag a lean problem, what kind of tech can you really be?

Must do mostly quick-lube.


Have you verified voltage to or from the sensors? Have you done any voltage backprobing at all?

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Infact, I know I could.

Owner and operator of 64bit Tuning Concepts, ASE certified technician, Nissan Certified, Toyota Certified, Ford Certified, SCT certified, and Moates trained.


As I said before... If you had a Quarterhorse, you'd be able to diagnose your problem quickly.

Good work, I am glad you found the culprit.
I am fond of saying that "Persistence is often of greater value than genius".

It was definitely something a Motes Quarterhorse would not have found. .

jrichker said:

Good work, I am glad you found the culprit.
I am fond of saying that "Persistence is often of greater value than genius".

It was definitely something a Motes Quarterhorse would not have found. .

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actually, it would have told him WHEN he was going lean... which would have brought him around to what he just guessed at.


Although his post isnt clear, I'm going to assume you tested your fuel pressure with the vac line disconnected, then ran the RPM's up? Because doing such a test with the vac line still on the regulator can give you false results.

Some more on the subject of fuel pressure...

How the fuel pressure regulator works

Revised 29-Aug-2013 to make the pressure/vacuum relationship clearer.

The fuel pressure regulator in 5.0 pushrod Mustangs is a shunt regulator that works in parallel with the fuel injection system. The regulator bypasses fuel back to the tank to maintain a constant 39 PSI to the injector tips. A constant pressure insures that the computer will always have the same flow rate to base its calculations on.

The 39 PSI pressure is measured at 29.92 inches of atmospheric pressure to get the proper flow rate. But the pressure inside the intake manifold may be higher or lower than the atmospheric pressure outside the intake manifold. These differences would cause the flow rate to change and mess up the computer’s air/fuel calculations.

As the vacuum inside the intake manifold increases, the effective pressure at the injector tips increases. Conversely, as vacuum inside the manifold decreases, the effective pressure at the injector tip decreases.

Some math to illustrate the effect:
39 PSI at 20” of vacuum inside the manifold works out to be 49 PSI,
since the 20 “ vacuum/2 = 10 PSI that you add to the base fuel pressure.
That gives you 49 PSI at the injector tip.

39 PSI at 5” of vacuum inside the manifold works out to be 41.5 PSI,
Since 5” vacuum/2 = 2.5 PSI that you add to the base fuel pressure
That gives you 41.5 PSI at the injector tip

39 PSI with 10 lbs of boost inside the manifold works out to be 29 PSI.
That gives you 29 PSI at the injector tip

That reduces the flow rate and explains the need for higher pressures on engines with pressurized induction.

Since intake manifold vacuum and pressure plays havoc with the pressure at the injector tips, what has to be done to get it back in the magic 39 PSI range? That’s where vacuum applied to the back side of the fuel pressure regulator comes in. Remember this: unless you have some really poorly designed or trick plumbing, vacuum is the same throughout the engine’s vacuum system.

Apply 20” of vacuum to the back of the regulator and the 49 PSI pump pressure with 20” of vacuum at the injector tips drops to 39 PSI.

Apply 5” of vacuum to the back of the regulator and the 41.5 PSI pump pressure with 5” of vacuum at the injector tips drops to 39 PSI.

Here’s another side effect: apply 10 PSI boost pressure to the back of the regulator and the normal 39 PSI at the injector tips increases to 49 PSI. That overcomes the 10 PSI in the intake manifold to give you 39 PSI at the injector tips. Pretty clever of these engineers to use intake manifold vacuum and pressure that way.

Simply stated, intake manifold vacuum adds to the effective fuel pressure at the injector tips. Apply the same vacuum to the back side of the fuel pressure regulator, and everything balances out. Add pressure to the intake manifold and the effective fuel pressure at the injector tip decreases. Apply the same pressure to the back side of the fuel pressure regulator, and everything balances out.

Now you know why to disconnect the vacuum when making fuel pressure measurements.

Micheal Nadeau said:

I tested it both ways, and testing it with the vacuum line on is a real world test. It's the pressure the vehicle would actually see. When I accelerate and the vacuum drops, the pressure should increase. Not by a lot, but should increase. It shouldn't drop 6-8 psi. Anything that had the ability to datalog would have told me when I was going lean, which would have led me to where I am a lot sooner. My intention is to get a Motes set up here in the next couple of weeks (tied money up in a vehicle I am flipping)

All that aside, how is it this became some sort of ****ing contest? Your ability vs mine? Had I had access to data on this thing, I would have figured this out a while ago. I, unfortunately, don't have several hundred bucks to toss at a Motes considering I have spent the last year fixing the previous owners massive amounts of screw up.

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Accelerating with a load, and simply running the RPMs up at a stand still are two completely different things. So TBH, unless you were holding a manifold vac reference gauge next to your FP gauge while doing said test... you may have seen skewed results.

Taking the vac off completely and THEN running up the RPMs is the only real sure fire way to make sure that is your problem... unless, you could see the FP gauge from inside the car while driving.

Glad you figured it out. I was leaning towards it being the PIP. I hate it when I think I fixed something and it still doesn't run correctly. Nothing more frustrating to me. I know plenty of people who love those "mysterys". I'm not one of those people. I have no patience with that crap. Good job on the perseverance

Micheal Nadeau said:

At idle I have 14 inhg vacuum. When I pull the RPM's up to 3000, I have 11 inhg.

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Good info. Would have been nice to include it before.