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2000 deville still misfiring


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My old pump wasnt building pressure so i put a new filter on and i switched wit a used one from an old car i had. key on engine off Pressure goes to 45 then instantly starts droppin? Once i turn key an do again same thing... Car is starting tho. fuel Pressure around 38 while idling. I pulled fuel rail up and looked for leakin injectors.. There was none yet pressure still bled down kinda quick. I also have my cat aloose to keep from messing it up any further. Only codes keep showin is po300 multiple misfires. I need help figuring this out please!runaround.gif

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Its difficult to understand your description....

Anyway...sounds like you put a different fuel filter on. Fuel pressure sounds ok.

As for holding fuel pressure. Altough the service manual suggests that the system should hold pressure at shut down...GM cars rarely hold fuel pressure after shut off.

On the Deville...the onboard diagnostics will only show a po300. You would need a scan tool that can show exactly which cylinder is misfiring. Most scan tools cannot do this. See attached picture of a GM Tech 2 showing a cylinder misfire.

Without a scan tool...you are just guessing.

Could be a coil pack has gone bad. There are 2 different ones. They are identical except for color and can be interchanged. See attached picture.

The hot cat could be from the misfire or fuel dumping in to the exhaust from a bad fuel pressure regulator.

post-2-0-24997500-1402668105_thumb.jpg

post-2-0-13274100-1402668499_thumb.jpg

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no im not losing any coolant... as for the fuel filter i put on a new one and the pump was an old one from a running car i had...now im gettin better pressure. today i went to yard and pull a coil set so i can see what it do.. if not imma hafta borrow my cuzns tech 2 i guess

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I had a p 0300 (misfire) and found that I had 2 coils bad. It would be faster and more direct with the scan but I disconnected each coil one at a time and found that the coil that was disconnected and made no change in the engine vibration was bad....I had to cycle about 2 times to catch it and wished I had a scan....

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On your 2005...each cylinder has it's own coil. So they can be disconnected one at a time.

On the 2000-2003....you do not have that option. The 4 coils for each side are built into one large coil bank unit. See picture above.

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The FPR can leak down fuel pressure. However, if the diaphragm does not leak and you don't have driveability problems, I would leave it alone.

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  • 2 months later...

Hi guys I be been busy and forgot to get back witcha on what my problem was. The issue was water inside each rear spark plug hole...no not coolant...rain water!!! Seems the rubber on that cover over the wiper arms where they overlap in the middle had curled up and let water drip onto coils. If I woulda pulled both coils instead of the "easy one" things would been much easier!!!

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  • 2 months later...

So, what was the diagnosis and solution to the misfire? My 2000 Deville continues to show a misfire code on Cyl #4. Prior to that, I kept getting Crank Case Position Sensore (CPS) code. Changed the CPS's twice in less than six months. Second time I changed them I wised up and installed OEM CPS's. I also changed the boot and the plug and problem went away for about 5 months. Now, misfire Code continues to return. Rather than swapping the coils, since the misfire continues to return on Cy #4, I'm inclined to suspect a bad front coil. Suggestion was made to install a used coil that has been tested and proven to be OK.

Pgh Steve

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