Fueling Issue Questions
 

Okay, so I've been chasing these symptoms for several months now, and need some help.

After the car has been on for at least an hour, my fuel pressure stops being stable, as in short sudden drops. The longer I go, the less stable and often drops during acceleration. When I start it up fresh, it's rock solid and everything is fine, and it's fine for at least 45 minutes or so.The FPR gauge I have on the rail is rock solid until it starts getting dippy (jumpy but downward).

Fuel pump, right? I've been through 3 of them. First one failed, and I took it to R Speed (local Miata shop) and asked them to replace it since I couldn't get to the pump due to a stripped screw. They did that (OEM Denso unit), and the new one fails in the same way. Then I swapped fuel filters and bought an entire pump unit from treasure coast Miata since my current one had been hacked up, only a bit but I wanted to rule it out. Same symptoms.

I'm currently running a VVT swapped car with an MS3. I have a Radium fuel rail and afpr. I wouldn't think these symptoms would be the fpr, but at this point I'm lost. My next step is to get another fuel pressure test kit and check at the pump to see if there's a pressure difference between the rail and pump, because if there is I could be burning through pumps, but I'm not sure how there could be. I've replaced the fuel filter and I'm still seeing issues.

I also have half a mind to replace my aftermarket afpr, but that doesn't really make sense to me with the symptoms. Wouldn't a faulty afpr not work well for an hour? Another thing, and I don't know if this is relevant, is that my fuel lines only have pressure when the fuel pump is on. I thought there was a check valve in the pump and they are supposed to hold pressure. Is that a sign of a bad fpr? I've looked for leaks and don't see any, and I did follow the lines all the way from the tank to the fuel filter to the engine bay to the rail.

What other troubleshooting can I do? What other data should I get?

Do you have and na or an nb chassis?
It sounds to me like you are having issues regulating fuel pressure. I am going to take a guess that you have an NA chassis but I might be wrong.
NA has return style fueling with a fuel pressure regulator at the fuel rail. Have you tried using a stock fuel pressure regulator in place of your AFPR? You shouldn't need an adjustable one and you have a MS3.

I have an NA chassis and a return style fuel system. I have an NA8 FPR I can try, but I have doubts that the fpr failing would lead to the symptoms I have. The FPR doesn't have any electronic parts, and I'm not sure how usage would cause it to start failing like this. If the FPR was bad, wouldn't it be bad from the start? Current symptoms are everything is A-OK until about an hour or so.

Swap the FP and the main relays out for fresh one and see if it improves. When relays get hot they get flaky and they are a known point of trouble on Miatas.

Relays plural? I'm familiar with the fuel relay, and I've swapped that out already. Is there a way I can check it with a voltmeter to see if it's good like volts at the fuel pump after an hour? Is there another relay I should swap?

The main relay by the passenger fender, I can't recall the actual name on the lid of the fuse box *edit "FUEL INJ". It will show adequate voltage but won't flow enough amperage when the points wear/corrode/get carbon-ed up. The resistance can change as it heats up.

Then there's the fuel pump relay above the clutch pedal

Thanks! This one I did not know about. I've changed the main relay already, and will look into this.

Is this the relay I need to replace? I don't want to start messing with things without knowing what I'm doing as making wiring worse would not be good. The label is LA10 - DC12V10A, which is definitely a relay. A quick google says that this might be the horn relay, but I don't really see anything else.


https://cimg5.ibsrv.net/gimg/www.mia...faf4c205fb.jpg
https://cimg7.ibsrv.net/gimg/www.mia...b2bd29309a.jpg
https://cimg8.ibsrv.net/gimg/www.mia...8e79e9afc4.jpg

What year is your car

Disconnect it and see if the horn or the fuel pump stops working.
Most relays can be pulled during operation without damaging ECU or wiring.
Sometimes it will create codes in an OEM computer. You will have to clear them later.
Want to be super safe? Turn car off or disco battery before removing relay.
Not really necessary but...

Often a single relay part number is used for many circuits in a car.
A normal swap for me is the compressor relay (AC) and the horn relay. They are commonly the same puppy and I can replace a suspected bad AC relay with thousands of "cycles" on it with a relay with less than 200 cycles on it.
I test both relays with the horn.
So much better than selling the customer a new relay. No down time.
A Miata FP relay may not follow this trend but if it does change it with a similar relay that has had limited use and is already on the car.

I use factory relays ONLY. The aftermarket stuff is all Chinese now and they suck.
A used factory relay is better than a brand new Chinese one.

Chassis is a '91. I'll remove it and see if I can still hear pump noise.

You could just check the wire colors and see what it is.

You could also touch the relay with your left hand and turn the key with your right and feel the click if it is fuel.

Thanks. I'll double check that the next time I can. I'm currently out of town. As an update, I swapped the fpr, and that didn't resolve it so at this point, it's gotta be electrical.

So an update: that (the picture) was not the fuel pump relay. The fuel pump relay was closer to the driver's door above the interior fuse box.

Swapped that out, and a new (again) fuel pump, still not resolved. At this point, I have replaced: fuel pump (multiple times), fuel filter, fuel pressure regulator, main relay, and fuel pump relay. At this point, my thought is that it's got to be wiring since I've done everything else. I might have to pull everything apart and start chasing wires. Anyway, I'm kind of at a loss. This is going on a backburner for a while, and I'm definitely out of my depth.

I had this same garbage happen. Check the ground that is on the inside of the tub near the driver front side of the fuel tank hump. Take the screw out, sand down to metal, star washer, reinstall.

I would love it if this is the problem, because that's something I think I can do. Do you have any pictures or a diagram? Is this underneath the carpeting on the interior shelf near the fuel pump, or in the trunk? Can I get to this ground easily? Is it underneath the car?

I think you would probably have to access it from the trunk. It would be under the carpet. I know I have a picture of it circled in my build thread somewhere. I think it was after I got the full cage?

EDIT: Post 83 in my build thread has a picture of the exact one I'm talking about.

Wait, is that a ground on the fuel tank itself? How do you even get to that? The only covering I have to open up is for the fuel pump. It looks like you've removed metal on your car to see that.