2000 Chevrolet Malibu

Engine Performance problem
2000 Chevy Malibu 6 cyl Front Wheel Drive Automatic 110000 miles

This is a 2000 Chevy Malibu, 3.1 engine, aprox. 110,000 miles.

In the last 3 months I have done:
- replace fuel filter
- replace fuel pump
- replace lower intake gaskets
- re-wired IAT plug
- replaced IAT sensor

I am still getting OBD codes. Most of the time I am getting " Cylinder misfire on 6" and 'Injector Malfunction on 5.&Quot; These are pretty intermitent and when I don't have them, the car runs great. When I do, it runs terrible and skips and bucks horrendously.

Sometimes, if I can get the rpms up, it smoothes out. Also, occaisonaly when it gets really bad, I get two more errors -- PO410 - Secondary Air Injection System Malfunction and PO412 - secondary air injection system switching valve a circuit malfunction.

I took it to a shop first. He said the misfire was due to a vacumn leak on the upper plenum. I then took it to the dealer. Eventually, he gave me the same diagnosis (after I told him what the first guy said) along with that I needed to change the injector on 5.

I did change the upper gaskets, and no improvement. I also checked the injector connections on 5 and made sure it was seated correctly. No improvement there.

I am skeptical on that diagnosis for two reasons. First, I couldn't find any vacumn leak when I sprayed the area with carb cleaner, and clearly replacing the gaskets didn't work. Second, I am hard pressed to believe that two unrelated problems (misfire and injector malfunction) keep happening at the exact same time over and over. I never get one without the other.

Just to remove variables, I did also replace the plug, plug wire, and swapped coils -- no improvement.

Any ideas on what could be happening here or what to try and test?
March 12, 2010.

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