Engine pops or misses above 4000 rpm P0404

[jrprattjr]

I have a 2004 GTP 189,000 miles, no mods. Recently when I accelerate hard the engine totally bogs down at about 4500 rpm and I have to ease off. It doesn't act like the rev limiter, it pops or booms like it is misfiring. There were no misfire codes however. It runs strong up to the "miss" and runs, idles and appears to get the same mileage as before this problem. Autozone scanned it and I got a P0420, which I've had for years, and a P0404 and P1404 which just appeared. The EGR doesn't look carboned up and the pintle moves easily. I had a supercharger belt let go a week ago, is it possible it knock a wire or hose loose? Any suggestions, PLEASE

[olopezm]

Hi there,

I would say your biggest concern is the p0420 catalyst system efficiency which points to a plugged catalytic converter; that also seems almost confirmed because of the bogging down at 4500 RPM, since there is a big blockage the exhaust gases cannot properly escape creating backpressure and chocking the engine.

I believe the EGR thing could be related to the elevated temperature caused by the catalytic converter being plugged or the stovepipe between the EGR valve and exhaust pipe blocked too (maybe carbon buildup). In any case, make sure the wiring to each component is OK and connections are properly seated.

Oscar.

[Tech II]

It's possible it is a blockage in the cat.....also possible it's forcing unmetered exhaust gases into the intake, via the EGR, due to back pressure....

One way to check, is create an exhaust leak BEFORE the cat(create a gap at the ex pipe and ex manifold, or remove the O2 sensor before the cat), and then drive the car(noisy, but a quick test)......if the misfire is gone and power returns, you know the cat is the problem...

Another indicator of a blocked/partially blocked cat is this:

With car at normal operating temp, from a dead start, lay into the accel pedal.....if the car bogs down and doesn't even shift.....stop.....now from a dead stop, LIGHTLY accel......if it slowly gets up to speed and shifts, it's a plugged cat....

[jrprattjr]

Originally Posted by jrprattjr

I have a 2004 GTP 189,000 miles, no mods. Recently when I accelerate hard the engine totally bogs down at about 4500 rpm and I have to ease off. It doesn't act like the rev limiter, it pops or booms like it is misfiring. There were no misfire codes however. It runs strong up to the "miss" and runs, idles and appears to get the same mileage as before this problem. Autozone scanned it and I got a P0420, which I've had for years, and a P0404 and P1404 which just appeared. The EGR doesn't look carboned up and the pintle moves easily. I had a supercharger belt let go a week ago, is it possible it knock a wire or hose loose? Any suggestions, PLEASE

Replaced catalytic converter with ZZP 3" downpipe and cat and problem solved. Engine runs and accelerates better than ever. Gradual cat failure, added back pressure, I didn't realize how sluggish the response had gotten.

[Tech II]

You can get away with that in Michigan....