Could these fuel pressure numbers cause a misfire?

Could these fuel pressure numbers cause a misfire?

Postby SportsBoy [OP] » June 10th 2021, 8:07pm

I have an intermittent misfire that is usually unnoticeable, but yesterday had me thinking the van was going to stall.

So, time to deal with it!
1997 AWD 141k miles

It has not thrown a code for this, but I see misfires logging on my bluetooth scantool, and it has had p0300 (random misfire), and yesterday p0172 and p0175 (fuel trim rich for both banks) as pending codes - although they never triggered the SES light.

The misfires do seem to be at every cylinder, and happen under different conditions. I'd say it most consistently happens when I take my foot off the gas after accelerating.

Within the last 20k miles, pretty much everything is new.
Coil, plugs, wires, cap, rotor, distributor, map, pcv, vac lines, fuel pump, injectors (upgraded to mpfi), plenum gasket, intake manifold gasket, etc.

Rented a fuel pressure tester kit from the Zone. I checked the numbers, then cleaned the MAF, cleaned the fuel pump ground, and replaced the fuel filter. Got the same readings before and after -and still misfires.

When I prime the fuel pump with key, it hops to 58psi, then drops to 55 when the pump stops.
Over the course of 15 mins it will lose ~10psi slowly.
When the engine is running at idle, it reads 48psi, and goes up a bit with throttle.
When logging misfires on my scanner, there is no jump of the needle or anything obvious on the pressure gauge.

The fuel pump was a new NAPA pump I had installed in 2017.

My factory manual says 60-66psi is spec when priming the pump with the key. I believe the mpfi has a slightly lower value than the system that came with the ''97, and when I did this test after I redid the injectors/intake manifold gasket in early 2018 I'm pretty sure I had the same readings.

Should I be considering the fuel pump as a potential source of this issue? Any suggestions on where to look next?
:ty:
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Re: Could these fuel pressure numbers cause a misfire?

Postby CopperFiremistCopperFiremist is online! » Yesterday, 12:56am

I had this exact same issue on a Van the other day same psi. I was lucky enough to have another Van come in that had a new pump in it and the fuel psi was 60psi running vs 48psi running and would increase a bit with throttle. It also had a P0300.
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Re: Could these fuel pressure numbers cause a misfire?

Postby SportsBoy [OP] » Yesterday, 3:13am

Thanks for the input Copper - so you'd put your money on fuel pump? I luckily have a spare I got on a closeout last year, so I could make it happen.
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Re: Could these fuel pressure numbers cause a misfire?

Postby CopperFiremistCopperFiremist is online! » Yesterday, 3:33am

I like to see about 58-62PSI at idle on these Vans and its not to much work to put one in. Do you remember what brand that NAPA one was? I only put DELPHI OE pumps in.
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Re: Could these fuel pressure numbers cause a misfire?

Postby SportsBoy [OP] » Today, 2:48am

No idea at all what brand unfortunately
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