Sara93Z28

I have a 93 Z28 camaro with approximately 230K miles on it. I had to replace the fuel pump recently. After that I experienced extreme backfire through the exhaust. Then the car showed a bunch of codes. The muffler was also blown out. Before fixing the muffler, the engine coded for EGR valve signaling incorrectly, Oxygen sensor, QDM failure, and air pump. I replaced IAC motor, oxygen sensor and air pump fuse. Then I replaced the muffler. ALL CODES DISAPPEARED. BUT, backfire still occurred, but only under hard acceleration. I tried replacing the ignition coil, and as soon as my car got to op temp it misfired and backfired like crazy. I put the old one back on and it seemed to work fine until the car just shut off, with no backfire. Now it turns but wont start... I have had low oil pressure when it idles at op temp, and now it will turn but not start. I was wondering if this could be a timing issue... please let me know if you can help...

shoebox

Probably the opti. Any trouble codes, now? Do the diagnostic sticky at the top of the forum.

Sara93Z28

I had the opti replaced within the past 3 years, by the dealer. I am not showing any codes.

Injuneer

Sounds like it might be running rich. Did you check the fuel pressure after you installed the new fuel pump?

shoebox

You should still do the opti diagnosis test. Some troubleshooting steps need to be done to track it down.

SuperSportSS

Its your opti. My 1993 z28 did the same thing when i was finished with my cam swap, but it didnt blow up my muffler. Make sure you are getting good spark also. And press your schrauder valve in and see if fuel shoots out after you have cycled the key, if it does then check your spark. No spark ill bet your opti is bad.

Sara93Z28

thanks for the help, it's finally nice enough outside to get under the hood. I'll do the opti diagnostics today. Injuneer, How exactly would I check the fuel pressure?

Injuneer

I've answered this a few times, so here's a copy & paste:

Fuel pressure:

-turn key to run to prime pump, it will shut off after a couple seconds. Should reach greater than 40psi at shut off, and not lose pressure rapidly when the pump shuts off. Rapid blead down of pressure will cause hard starting, but typically it's due to a bad check valve in the fuel pump, which will not affect the fuel system once the engine starts. Could also be due to leaking injectors, faulty fuel pressure regulator, leaking lines, including the flex hose in the tank.

-at idle - vacuum line off, you want 43.5psi (3 bar). GM spec says anything between 41-47psi is OK. Hold finger over vacuum line to prevent a vacuum leak while testing.

-continuing at idle - reattach vacuum line. Pressure will drop proportional to intake manifold vacuum. With a stock cam, and 8psi drop wouldn't be unreasonable. With a more aggressive cam, the pressure will drop less.

-tape gauge to outside of windsheld, take it out on the road, wide open throttle above 5,000 RPM and pressure should not drop below 40psi.

When you are driving at part throttle, the fuel pressure will increase as you press on the throttle, and intake manifold drops. When you close throttle, and intake manifold vacuum increases, fuel pressure will drop. Point is, it's supposed to fluctuate with throttle position.

The fuel pressure test gauge connects to the Schrader valve. Location, courtesy of Shoebox:

http://shbox.com/1/fp_reg.jpg