Fuel pressure is tied to manifold pressure, so you'd expect to see it rise at WOT (ie, no vacuum) and drop at idle. If you want to know what WOT pressure should be, pull the vacuum line off the FPR. It's purely mechanical.
First step is check your WOT pressure - pull that line off. It should jump up to the low 40s. Usually when regulators fail they fail closed, which means you get 40-ish all the time.
Given what you're describing, I'm thinking about just plain fuel flow: a weak pump or a clogged filter. Could also be a clogged pickup sock, which might happen if the tank's recently been run very, very low.
In reply to Keith Tanner :
Thanks Keith! I just swapped the FPR with the other(unknown condition) one from the spare engine, but there was no change in pressure. I’ll pull the pump & take a look.
Here’s the sock & inside of the tank. There’s some debris, but it’s still at almost 3/8 tank.
That sock looks pretty grimey to me. Im not advocating the parts shotgun approach but I just replaced my pump, denso 9500118 ($70 from rockauto) is a direct fit and comes with the sock. I used the connector pigtail because mine was very corroded but you don't need to.
A new pump is a fairly directed parts shotgun, it's looking pretty likely.
Parts shotgun with a full choke
In reply to Keith Tanner :
I forgot I had the pump assembly from the 91, so installed it(ps- thanks Mazda for making it only a 1/2-job!). It was a known-good running/driving car up until a year ago & appeared to have been well cared for. The pump assembly had a Mazda sticker, so it was likely OE, with under 100k on it. And...
The car kinda/sorta seems better. It definitely isn’t falling on its face 100% of the time when I dip into the throttle. I filled it up with Shell 92 octane & drove about 3-miles. Sometimes when I dip into it, it pulled fine. Sometimes it felt down on power, yet still smooth. A few times it did cut out, but that was only about 1/4 of the times I tried it.
Keep watching that fuel pressure, it's doing weird things.
In reply to Keith Tanner :
The filter was new 1000-miles ago, and I drained the tank while the old filter was off - not that the new one couldn’t be plugged.
Have you encountered problems with collapsed/plugged vent lines, or do you think it’s most likely a fuel-supply issue?
If you're concerned about vent lines, go for a drive with the gas cap off 
I doubt that's it, it wouldn't pull a vacuum immediately.
What's the voltage look like at the pump?
In reply to Keith Tanner :
I actually left it loose after I filled up, but didn’t notice a significant difference. I didn’t check voltage since it seemed fixed after the pump swap in the garage. I’ll have to pull the cover back off & check.
I installed a brand new pump & strainer this afternoon, and it runs worse than it ever has... So now I’m back to two inop vehicles. :(
A few thoughts after sleeping on it:
- I don’t think it’s an issue with the fuel lines. I’m not sure they could collapse since they’re pressurized from the tank all the way to the rail, and for that same reason any crack or hole would spring a leak vs. sucking in air. Correct?
- The fuel filter was replaced 1000-miles ago(and the tank drained too), but I suppose that doesn’t mean it couldn’t be clogged again. Since it’s relatively cheap & easy to change, that seems like the next logical step to me. Yes/No?
- I suppose it could be an injector problem, maybe? I do have a spare set from the non-running engine. Would it be worth swapping them, or should I spend the $$ to have them serviced first?
- I might pick up a new FP gauge. I had to use 5 different fittings to make a T between it & the fuel lines, and it leaks a bit both with/without Teflon tape, so I can’t use it very long while it’s running. I’d like to T into it directly off the pump so I can monitor it while driving.
What else am am I missing?
What's the voltage at the fuel pump?
I agree that it's worth changing out the filter again. They're cheap.
In reply to Keith Tanner :
I didn’t think to check it - the last thing I expected was the problem to get worse after installing the new pump. I did check the terminals in the connector & they didn’t look corroded. I’ll put a meter on it when I pull it back apart.
If I wasn’t such a E36 M3ty berkeleying mechanic...
I picked up the HF fuel injection test kit last week - FYI it seems really nice - and checked pressure at the rail: 20psi at idle, 25psi with the vacuum line plugged to the FPR. I checked it at the tank & it was only 5psi higher.
I replaced the fuel filter, it definitely had some crud in it, but the symptoms were pretty much the same. It started fine, only slightly bogged when I revved it in neutral, but the more I revved it the worse it got.
I pulled the pump assembly back out & noticed the o-ring between the pump & inlet tube was crooked... I pulled apart the other pump to check, and found this piece(with the slot in it) was missing from the one in the car - and it’s what helps center the o-ring.
In reply to Stampie :
From my ~2 mile drive it appears so. I did several pulls to redline & it feels like a Miata. I’m registered for the Nola autox Sunday, so I’ll find out for sure then.
Good. Can't have you with two broken vehicles. Next thing you know your yard would look like mine.
