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Q: Replaced Fuel Pump on 1989 Firebird on 1990 Pontiac Firebird

I had the fuel pump replaced on my firebird, ran good before pump replacement. Now experiencing major engine stumble and backfiring with new fuel pump. I suspect impurities in fuel line. I'm getting approx 35 psi at fuel distributor which is ok. Need help!! Thanks.
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I replaced the fuel filter 1 month before the fuel pump went out. I think it's possible that it could be clogged as a result of the gas tank being removed and replaced. This is a new experience for me. This helps. Thanks for your feedback!!
Did you replace the fuel filter?
Also, can you verify the quality of the new pump? I'm showing you should get in the 40's in fuel pressure.

Last, backfiring and stumble can definitely be caused by an air intake leak. Check your vacuum lines, intake gaskets, air box and tube.
35 psi seems to be low, unless it's the regulated pressure.
Here are the factory specs:
Ignition ON Engine OFF 40.5 to 47.0 psi
With Regulator Vacuum Applied 3.0 - 10.0 psi Less than Ignition ON Engine OFF
Beside the pressure, the volume is an important factor. The fuel pump should supply 1/2 pint or more of fuel in 15 seconds.
I guess you have the MPI (multi port fuel injectors) instead of the TBI. Are you sure you get the fuel pump for the MPI?
The fuel pump's part# for the MPI: 25116163 (the 8th digit of the VIN is F) meanwhile the TBI fuel pump's part# 25168719 (8th digit is E)

You're guess is right, I have MPI. I'll backtrack and check. I hope my mechanic knows the difference between the "E" and "F" fuel pump because it's a hell of a lot work and expense to get to the fuel pump. BTW the repair manual for the '89 Firebird calls for 34-39 PSI (ref pg 309 chap 13). Thanks for your feedback!!
Yes, 34-39 psi is correct, if it's the REGULATED pressure, with the vacuum hose connected to the fuel pressure regulator. According to the specs 30,5 is the minimum regulated pressure.
Do you know if the pressure was measured with the regulator connected?

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