Ok I searched a crap load of threads and found no answer to this. I installed a fuel pressure gauge so I decided to check the pressures in the fuel system. The numbers in parentheses are the Mazda shop manual specs. and page numbers

Fuel hold pressure is 32psi (> 21 after 5 min. F-96)
Fuel line pressure is 38psi (36-38 F-97)
Pressure regulator fuel line pressure is 33psi (28-32 F-104)
Fuel pump relay checks good
Voltage and continuity are good at the connector.

Now here is the fun part. The fuel pump maximum pressure should be between 71-107psi. My stock pump was around 31psi. I put my brother's old stock pump in my car which had about 30,000 less miles on it and I got 52psi. I bought a new Supra TT pump and installed it and got 38psi. Three pumps and I am still almost 20psi low on the highest one. I can't imagine I have three bad fuel pumps. I can find no leaks in the system. The pumps all sound the same while running. Does anyone have any clues what might be happening here. I am stumped.

The only theory we have is that maybe the pump is not getting good voltage under a load. Thoughts?

 

The maximum pressure listed in the shop manual is the pressure you will get if you completely block the output of the pump (i.e. the pressure at which the pump will no longer be pumping anything). You aren't testing that, so you will never see pressure that high.

Your fuel pressure should be about 38 psi + however much boost you are running. So if your boost gauge reads 10 psi, the fuel pressure gauge should read about 48 psi. Under vacuum, the pressure will be below lower than 38 psi.

The idea is to maintain consistent pressure across the injector (which has one end in the fuel rail and the other in the intake manifold). That way, the same amount of fuel will be delivered for a given injector opening time.

The fuel pressure regulator is totally in charge of the fuel pressure. It has a vacuum hose to the intake manifold as a reference signal (so it can lower pressure under vacuum and raise it under boost). However, if your fuel pump is inadequate, the pressure will drop below what the regulator wants. At that point you are totally out of control. Tuning will change with electrical load, etc. It is a total ******* mess. So, you just want a fuel pump that can flow enough fuel while maintaining the pressure the regulator wants. The toughest load is roughly your power peak RPM at maximum boost (assuming reasonable tuning). If it can handle that, you've got enough pump. Tripling the output of the pump would give no benefit -- you just need the pump to be adequate. It is important to have some safety margin, but tripling the required flow would be overkill, for instance.

-Max

 

Has anyone ever measured the pressure drop of the stock 3rd gen fuel lines???

STEPHEN

 

That's the problem. We are blocking off the fuel line. I have a 5/16 bolt blocking the main fuel line with clamps on it. We had leakage once or twice which is immediately indicated on the fuel pressure gauge. All I can get is 38psi on the Supra pump and no visible leakage. As far as I know there is nothig but the fuel filter, which is new, between the pump and my bolt. We did get a high of 52 psi with the one old stock pump. This is what makes this so perplexing. Three very different pressures with three different pumps.

I should add the only real performance mods are an M2 down pipe, HKS super mega flow intake, Apexi N1 exhaust and an M2 Stage II remapped ECU. I am told the Supre pump should be more than adequate for those mods.

edit: added mods.

 

Have you tried changing the fuel filter lately? Sounds like it's clogged.

 

Looks like we posted at about the same time. Yes the fuel filter is new with 0 miles on it. See my previous post.

 

Gotcha.. the page had sat a while before I replied. In light of that, I have no usable input

 

Ok I got me a fuel pressure tester. I hooked it up to the out let on the fuel tank to try to eliminate anything down stream. It still reads 38 psi so the problem has to be limited to the fuel pump or fuel pump assembly. Can anyone think of anything I am leaving out? This is driving me crazy!

 

Are you running the pump at full voltage? Remember that it will run at low voltage if the ECU thinks you are at low engine load. I would check the voltage across the pump while it is running.

Pressure drop will only matter at high flow -- it shouldn't make any difference if you are blocking the line manually for the test. Unless something is almost totally blocking the line, the pressure drop that would normally occur is not going to be present.

-Max

 

Assuming it isn't the voltage issue. Is the pump fully seated? Maybe the holder is a little bent and leaking in the tank.

BTW, these pumps can crap out from age alone (not just mileage). I had a pump on a babied 9 yr. old Miata die on me and there were only 12k mi. on it and the rest of the car. I had an aftermarket fuel pressure reg. and the damn thing would oscillate pressure wildly when I tried to measure max no-flow pressure.

 

Ummm, check you battery and alternator as well. It might seem goofy but since there is no other apparent reason...

 

I believe it's 15psi based on Max's fuel calculator.

 

We are running the pump by placing a jumper in the diagnostics box. My understanding is that it causes the pump to run at full voltage. Also we did the voltage test at the pump connector and it showed full voltage. I supose it could actually be a bad pump but what is the likelyhood of having three bad pumps.

Last night I ran the pump with only the bottom submerged in the fuel tank and could see no leakage from the assembly. The pump appears to be seated well and there is no damage to the pipe leading to the top of the tank.

I am going to put the one from my brother's car back in and see if it will go back to the 52 psi it had before. I'm not sure what this will acomplish but at least it had the higest output. Argh.

 

Ok I got a fourth fuel pump. This one is the Nippondenso from RX-7 Store. It has the exact same number as the Supre pump. This one gets 43 psi doing the fuel pump max pressure test in the manual. I went out and drove it and at around 8 psi of boost I saw a max of about 46 psi. My gauges record the peaks and those showed 10.2 psi boost and 47.4 psi fuel pressure. It idles at about 33 psi so that is a little lower than I want to see. The weird thing is that it didn't always seem like the boost and fuel pressure numbers corresponded at higher throttle. Sometimes 8+ psi boost and 44 psi fuel and others 4 or 5 boost and 46+ psi fuel. I don't know if it is a lag in the gauges or what the hell is happening. I have no idea what to do now.

 

I know this thread is a couple of days old but I had a similar problem on my car, with the Denso pump as well. I had the car down for a lot of modifications and I could not get it to stay running, or even start most of the time. I tracked it down to a fuel pressure problem, and in fact to the pump resistor in my case. I bypassed the resistor by removing the pump relay in the box at the front of the car (not the one by the shock tower) and jumpering the two poles that deliver full power to the pump with a nice fat wire. I believe when you are facing the front of the car, it is the upper pole on the left, and the lower on the right.. check it with a voltmeter. This allowed me to pressure up the fuel lines before starting the car by turning the key to the 'on' position and solved my trouble completely. This may cause undue wear to the pump prematurely but the Denso pumps are known for being reliable and lasting. I realize this is only a temporary fix for me for right now, I haven't gotten a replacement pump resistor yet.. and who knows if that will fix the problem. My pump resistor tested at .7ohms, right at the upper spectrum of what is listed in the shop manual.. all of my relays checked out so this was the only thing I could figure was causing my trouble. HTH.

Matt

BTW this all happened with a new Denso pump, new -6 fuel lines and fittings, new SX regulator, cleaned injectors, new o-rings, and a new Aeromotive fuel filter.

 

sounds like you are plugging and gaging at the return line, so regualtor is still limiting pressure. gage should be in fuel line from tank, before the reg. just a thought.

 

I definitely have the correct line. I am plugging the gauge in on the pipe that comes out of the fuel tank. I wanted to eliminate anything from there to the engine as my problem. If I had the wrong line there I would have filled my garage with gas.

Matt

We checked the voltage at the wires that plug into the pump assembly an it read almost exactly battery voltage. If it was going to the resistor it should have read 2/3-1/2 of battery voltage.

One interesting thing I thought about is this. When I was running the pump with just the bottom submerged in the fuel, once or twice the sock/filter came out of the fuel and stated sucking air. I saw bubbles shooting down into the fuel. At first not thinking right I thought it was just comming out of the return pipe. It was later I realized the return line was not hooked up. The bubbles had to be comming from the bottom of the pump because it was the only part of it in the fuel.

This seems to indicate fuel is flowing in and out of the pump even with the line plugged. The pump cavitated when the air was sucked in. It seems that if it was pumping with the line plugged and no flow the air should never have reached the actual pump. Any fuel pump experts out there care to enlighten me?

 

Make sure the wires that supply the pump with voltage and current are in good shape. You could have and old, cracking, half coonected connector or something like that. So when you first hook it up you may see 12 volts but once you draw current to make the really out put the full amount you may have an added unwanted resistance in the line causing a huge voltage drop, which would make the pumps less effective. Just a though, since the filter is not clogged and the fuel lines are not blocked ect.

 

When i tested my fuel pump for max pressure, i think its called dead head pressure also. I had a paxton fuel pressure regulator and i had a gause on the regulator, i just put the paper clip in the diagnostic connector and i totally pinched off the return line and the pressure shot up to 100 psi. Have you tried to put everything back together and testing it again? How does your car run, i dont remember you say anything about it and i doubt that you have four bad pumps, two of them being new.

 

The pressure is getting close to what I should see when the car is driving. I saw maximums of 10.2 psi boost and 47.4 psi fuel pressure while driving. Unless I did my math wrong at full 10 psi boost I should see between 48 a 52 psi of fuel pressure. I too can't believe all four pumps are bad but I am following the instructions in the manual exactly and these are the numbers I am getting. I get the exact same numbers whether I plug the fuel line at the engine and read my installed fuel pressure gauge or plug the tester gauge in at the fuel tank.

 

Ok here's the deal. I hooked up the car's fully charged stock battery directly to the fuel pump connector with the pressure gauge hooked up to the fuel tank out let tube. I got the pump to 96 psi. It seems that it is a wiring problem. This fix sounds like it is going to be hard.

 

Well for anyone who is still following my fuel pressure adventure here it is. We talked to Tri Point and they said we had done everything they would have tried. Almost! They said to connect a battery directly to the fuel pump connector on the tank. From my previous post you can see that it worked. They also said that any time they put a high flow fuel pump in they do some special wiring. They take the original power wire and hook it up to a relay. Then take a power line hooked up to a 12V power source (the battery works well) and run that through the relay to the fuel pump. When you start the car it powers the relay which then allows the line from the battery to power the pump. They do all this with a beefier wire. The pump will run at full speed all the time but that is not really a problem. XS Engineering says they do this with a high flow pump too.

With this set up I am seeing 52 psi. at about 9.5 pounds of boost. I need to do some fine tuning on the Profec B I just installed to get to 10 pounds of boost.. According to my calculations that is exactly what I would expect to see in a properly working car. Basically when all is said and done I have my original fuel pump (10 years old and 98,000mi.) an extra Supra TT fuel pump, I can give my brother his old pump back and have spent about a month and a half screwing with this thing. Hopefully this can help anyone else who is having fuel pressure problems.