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I have a 1997 Cadillac Seville STS with the 4.6l Northstar V8

Bought the car, and it was running really rough and miss-firing.

Checked the codes, and got a P0101 MAF, P0121 Throttle Position sensor and a P0300 random misfire codes back.

Cleared the codes, drove it got the same codes back.

So here is what I did:

Compression check, all cylinders within 7psi of each other.

Installed new AC/Delco Platnum's and AC/Delco Wires.

Tested all coil packs they are good

Installed new MAF sensor

Installed new Throttle Position Sensor.

Installed new Air and Fuel filters

Fresh Oil Change.

Also checked all injectors, they are all good

Also running 93 Octane Premium Fuel

I poured water on the engine (intake manifold) while it was running to see if I could find a vacumn leak, no luck.

So I cleared the codes, drove it, all 3 errors came back.

Engine sounds like a lobed out big block

Sounds like I hear a solenoid activating right before the error codes return, I noticed that after clearing the codes and starting the car it runs a bit rough at idle but smooths out after 2000 rpms, then a few minutes of running the engine: vacumn drops, starts running worse, then activates the 3 codes.

This scenario is the same for letting it just idle, or revving it up, or driving.

Any ideas would be helpful.

Thomas

Edited by Thomas L Black
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All the intake duct clamps tight? PCV grommet tight? FPR leak?

Check for vacuum leaks again. This time use a volatile spray like Brakleen or such and listen for an RPM increase.

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