36 replies to this topic

[Dexter Racing]

Hi, I think I am having some kind of fuel issue although not entirely sure. I'm running the adjustable fuel pressure regulator in the trunk of my 2003 sm and at idle I have it set at about 57-58 psi. The car runs rich on the afr (12.7 ish), but it takes huge leaps in fuel pressure change to get much of a change in the afr. Watching the fuel pressure during a dyno run, the pressure begins to drop all the way down to about 49-50 psi at around 5 thousand rpm. Is this drop in pressure normal? The car makes great torque, but bad power numbers... just trying to figure out how to squeeze that last 5 hp the car is missing. I'm not really sure what would make to have this drop but still maintain a rich afr (being measured right behind the manifold). Any ideas??

[MPR22]

Hi, I think I am having some kind of fuel issue although not entirely sure. I'm running the adjustable fuel pressure regulator in the trunk of my 2003 sm and at idle I have it set at about 57-58 psi. The car runs rich on the afr (12.7 ish), but it takes huge leaps in fuel pressure change to get much of a change in the afr. Watching the fuel pressure during a dyno run, the pressure begins to drop all the way down to about 49-50 psi at around 5 thousand rpm. Is this drop in pressure normal? The car makes great torque, but bad power numbers... just trying to figure out how to squeeze that last 5 hp the car is missing. I'm not really sure what would make to have this drop but still maintain a rich afr (being measured right behind the manifold). Any ideas??

You have a failing fuel pump.  Your fuel pressure should not fluctuate that much.  Maybe 1-2 psi as things heat up.  

[Michael Novak]

Your too lean.... and it seems like a little bit too large of a drop. 

 

12.7 at what rpm? 

[Dexter Racing]

ok, so the fuel pump is only able to give the 50 ish psi at the high flow rate which means even if I change the fuel pressure it "leaks" down to the 50 psi at high rpm which is why I'm not able to mess with the afr... looks like I better get one of those on order!!

[chris haldeman]

Replace entire assembly. The 01 up pumps are known too have pin holes in the tree assembly. These pin holes cause weird issues

[Steve Scheifler]

Replace entire assembly. The 01 up pumps are known too have pin holes in the tree assembly. These pin holes cause weird issues

Great information! I've got a finicky 01 with (I assume) the original assembly. Time to eliminate one suspect.

Have you tried any aftermarket ones or only direct from Mazda?

[Jim Drago]

Steve
What Chris is talking about is the tree, not the pump. I dont think they sell those aftermarket? We have had this issue as well
Jim

[RazerX]

So...

 

I have a 99, after I turn the car off, the system holds pressure sometimes only for a few hours and sometimes for days.  It this an early indicator of imminent failure or potential intermittent operation?  Meaning should i just replace it?  Or is it fine and just a faulty check valve.  

 

Yes, the pump can hold the set pressure on the dyno during the pulls...

[FTodaro]

So...

 

I have a 99, after I turn the car off, the system holds pressure sometimes only for a few hours and sometimes for days.  It this an early indicator of imminent failure or potential intermittent operation?  Meaning should i just replace it?  Or is it fine and just a faulty check valve.  

 

Yes, the pump can hold the set pressure on the dyno during the pulls...

We can let the pros answer but from my experience pumps hold pressure, ranging from a few min. to something less than 60 min.  Part of that can depend on the regulator also. I monitor fuel pressure in the car and you should be concerned with dropping pressure under a load.

[Steve Scheifler]

I know Jim, but I have seen for many cars the complete unit aftermarket, such as Eclipse. Thought maybe a similar source exists for Miata.

[Jim Drago]

I know Jim, but I have seen for many cars the complete unit aftermarket, such as Eclipse. Thought maybe a similar source exists for Miata.

could be, I have not seen though

[Jamz14]

So...

 

I have a 99, after I turn the car off, the system holds pressure sometimes only for a few hours and sometimes for days.  It this an early indicator of imminent failure or potential intermittent operation?  Meaning should i just replace it?  Or is it fine and just a faulty check valve.  

 

Yes, the pump can hold the set pressure on the dyno during the pulls...

Razer,

 

Please take this with big grain of salt. I AM NOT AN EXPERT.

 

When my pressure bleeds down on the fuel rail ( I have a 97 with the pressure gauge in the engine compartment ) within a short time ( a day or less, even multiple days ), I start to worry. I have lost engines because the car goes lean due to leaking injector insulators and o-rings. If I see it leaking down I replace the o-rings and insulators immediately. If it continues to leak down I service or replace the injectors. If it is still leaking down I replace the pressure gauge and rubber fuel lines going to the rail and gauge. So far these have always fixed the issue and I haven't had to go deeper yet.

 

But in my opinion, the pressure should be 70 % retained after a day of not running. In my case, this means holding about 25 lbs at all times when the car isn't running minimum. Typical seems to be about holding 30 lbs after a day.

 

I am an amateur mechanic. If the pro mechanics have a different opinion, I would disregard what I am saying and go with what they recommend.

[Bench Racer]

The 01 up pumps are known too have pin holes in the tree assembly. These pin holes cause weird issues

Help, tree assembly???

 

[MPR22]

Help, tree assembly???

The metal assembly that the fuel pump mounts to.  

[Johnny D]

Did you want to mention the return line mod ?

J~

[Steve Scheifler]

I'm with you Jamz, except that the insulators don't hold pressure. An an injector with the drips can also cause that cylinder to be rich and not burn cleanly.

[Steve Scheifler]

Did you want to mention the return line mod ?
J~

??

[Johnny D]

Run the return over the sock/strainer

J~

[Jamz14]

I'm with you Jamz, except that the insulators don't hold pressure. An an injector with the drips can also cause that cylinder to be rich and not burn cleanly.

LOL, I knew someone was going to mention that. True. Just in the habit of replacing all the rubber parts once I have gone through the trouble of removing the rail.

[chris haldeman]

I'll add I've never seen an nb hold pressure for long. Aka drops at shut off and soon after it is 0
And I've seen a few 😜