9 replies to this topic

[DrDomm]

Just trying to troubleshoot a fuel pressure problem.  Without getting into all the details, I'm running 60psi fuel pressure on a 99/00 to make max hp.  Just thinking it may be related to fuel flow, so was going to measure how fast it took to pump out 1 gallon.  Is there a known "spec" for this?  Visually, the stream seems a bit weak, but that isn't very objective.  Has anyone measured this, and is it a useful measurement?

[Bench Racer]

For a 1990 the FSM indicates outlet psi of 64-85. Your 99-00 FSM indicates?

 

Saul Speedwell

 

http://miataturbo.wi...t.com/fuel-pump

 

Scroll to bottom and there is a flow rate in gallons per minute for OEM pumps and aftermarket pumps.

[MPR22]

More data please. I assume you are getting the proper AFR at 60, which is way high for a 99. Watch for pressure drop under load and or during session. That would point to pump or regulator issue. If not, are your injectors flowing properly? Look at plugs and see if a cylinder is running lean. You could be getting proper AFR number but running 3 cylinders rich and one lean.

[Tom Hampton]

Just trying to troubleshoot a fuel pressure problem.  Without getting into all the details, I'm running 60psi fuel pressure on a 99/00 to make max hp.  Just thinking it may be related to fuel flow, so was going to measure how fast it took to pump out 1 gallon.  Is there a known "spec" for this?  Visually, the stream seems a bit weak, but that isn't very objective.  Has anyone measured this, and is it a useful measurement?

 

Through the fuel sample port? No. Its dependent upon actual pressure at sample port, and flow path configuration between the fuel pump and the sample port. It further depends upon the exact sample port you have elected to install (which is really just part of the flow path). I do not think that would be a useful way to troubleshoot your problem. At best it would be a gross indicator that wouldn't really lead you anywhere except to draw the conclusion: "yep, there's a problem somewhere." But, you already know that.

[davew]

Too many variables to do this type of test accurately; battery voltage (fuel pump speed), size of pump out hose, location of pump out hose, are injectors clean, etc.

 

What problem are you trying to cure? Have you double checked you fuel pressure gauge for accuracy? please give more info

[Tom Hampton]

Your fuel pressure sensor is DOWNSTREAM of your regulator, right? Some regulators have multiple ports where a sensor can be connected, and some are on the inlet side, and others are on the outlet side.

Have you tried a different FP sender/guage?

With your FP so far out of whack vice expected values, I'd confirm the real fuel pressure matches your gage before I spent forever chasing a ghost.

[DrDomm]

Oh boy, this is confusing...

 

Let's see, I've changes injectors, so it would be unlikely that both sets are bad.

 

My fuel pressure regulator is in the trunk, so regulates the return.  The analog gauge on the FPR there reads 60, and the digital sensor near the engine reads 60 on my data dash.  Analog gauge into fuel port also read 60 on dyno.  Pretty sure we're around 60.

 

Proper AFR?...not sure.  I'm no expert.  5100-5600 rpm is 13.2.  It's about 12.3 at 6900 rpm.  Is that proper?

 

I do have one cylinder that apparently has a bad exhaust valve (leak down was 25%, hissing out the tailpipe).  I'm working to get that repaired very soon, just assumed my problem of needing too much fuel pressure might be the cause of that, and not a symptom of that.  That's what I'm trying to cure.  I'm down 5-10 hp, that I'm assuming is the exhaust valve.  I don't want to get the head repaired, but not fix the potential cause.  Now, if somehow the leaky exhaust valve can be the cause of needing higher fuel pressure to create more power, then I guess I'm good.  I'd love an explanation, though.

 

I was wondering if low fuel rate could be compensated by increasing the pressure.  But I guess if the pressure at the test port near the engine is adequate, where all the lines are standard, the flow has to be adequate.

 

I appreciate the help.

[BNaumann]

I didn't want to say much as I have done zero investigation into the matter, but I'm seeing the exact same numbers on the 99 I just put together - 60psi on the little gage on the regulator yields ~12.3AFR just before rev limiter in third on an AEM 30-4110 mounted behind the driver. Let me know if you find a "problem."

IIRC it takes 1:30~1:45 to pump a gallon out my test port key on engine off with no apparent back pressure on this gage.

[DrDomm]

Despite all the "variables", mine took 3 minutes to pump 1 gallon.  I can't imagine there are enough differences (typical Schrader valve/tube from Dave W) to make that a normal value.

[Bench Racer]

Per the flow rate chart in post #2, 35 gpm at 60 psi or 1.7 gpm. Similar results to post #8