Lacks power and misfire

KEVINO77

MEMBER
1999 CHEVROLET TAHOE
5.7L
V8
4WD
AUTOMATIC
230,000 MILES

Starts and idles fine (possible slight misfire) accelerates "okay" to 20 mph and/or shifting to third (auto) then the miss is very noticeable and lacks power.
I liken this to an older carburetor with vacuum advance disconnected and won't accelerate.
But, I can cruise at 30 - 80 and feel something from shudder (low RPM) to buzzing (high RPM) from this misfire.
No code thrown. Just had tested for License and passed.
Replaced plugs, cap, rotor, cam position sensor.
Was running fine and after jaunt down a dirt road. Now this. I checked for loose connectors, wires, hoses. None found.

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
42,861 POSTS

Hello,

It sounds like you have a weak fuel pump or a partially plugged catalytic converter. here are two guides to help us see whats going on:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

and

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Please run down these guides and report back.

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Saturday, May 4th, 2019 AT 11:43 AM

KEVINO77

MEMBER
2 POSTS

I will check the fuel pressure(pump replaced 1 year ago) and catalytic converter (a few years older)/ I believe my pressure regulator is under the manifold unlike the photo in the link?
A couple things I failed to mention is can't detect a misfire at all when in gear (stopped)and throttling up, or unloaded out of gear holding revs at various points. Also, 500 to 2,500 rpm first gear moving not noticeable second, third very noticeable. At first had thought maybe I stirred up sediment in the tank, plugged the filter and was not getting fuel until the above test.

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Monday, May 6th, 2019 AT 8:31 AM

KEN L

MASTER CERTIFIED MECHANIC
42,861 POSTS

When replacing the fuel pump and ignition components did you use AC Delco? It sounds like a weak pump and still a misfire due to a cranked spark plug? Please check the pressure and get back to me. Can you please shoot a quick video with your phone so we can see what's going on, that would be great. You can upload it here with your response.

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Monday, May 6th, 2019 AT 2:32 PM