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Weird Missfiring 99 Tahoe Problem, Help Please!?

tomrex

By tomrex
in Ask A GM Technician

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tomrex Enthusiast

tomrex

Posted
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Ok so I'm going to do my best to describe what's happening. The other day, I decided to hose the engine off of my Tahoe. I did so, and to my expectation it started missing and the SES light came on with a P300, random cyl missfire. Cleared the codes, let her run for a while and shut it off for the night. The next day it was worse so I changed the cap, rotor, plugs (ac platinum), wires. It was better at first, but went right back to missing. Pulled the cap off, made sure everything was dry and clean, and put it all back together. Still doing it.

 

My observations.

 

1) I can start it and it will run fine for about 30 seconds and then start stumbling

- with this, i can watch my scanner tell me that fuel trim in bank one starts to fall off

- Before it happens, a slight miss happens twice before the computer shuts off the fuel on that bank

2) If I drive it, it's a dog and runs on 4 or 5 cyls for a few seconds, and then takes off like a rocket when the others suddenly decide to kick in (then the fuel trim goes back to normall).

 

3) If I run it hard for a few minutes, it seems to go away and drives perfect. As soon as I let it idle for a minute afterwards, sometimes 2, it starts to stumble again. If I shut it off and restart it, it happens faster. I ran it for 50 miles on the highway yesterday and I thought it went away, but it's back again.

 

 

I guess what I don't understand is why, when I lay into it it stumbles and then kicks back in like someone turned on the lightswitch for the other 4 cyls. Any advice?? The coil is looking pretty nasty and is fairly cheap, I think i'll replace it today. Also, I am planning on pulling the new plugs and gapping them. The guy at NAPA told me they were pre-gapped, but I think he's full-of-it.

 

Any Help would be appreciated.

 

thanks a million!

 

Tom.

 

BTW - It's only got 76k on it, 4x4, etc...

tomrex Enthusiast

tomrex

Posted
  • Author
Posted

I also forgot to add that compression is great and just did a full SeaFoam douching...no luck. I drove to school today, missed until I stuck it to the floor, chugged for a second and then again, like a light switch, they all came to life. It happened twice and then didn't miss a beat the rest of the way to school...i'm sure it will do it when i get out though.

 

Thanks again.

 

Tom

tomrex Enthusiast

tomrex

Posted
  • Author
Posted

Oh boy oh boy... I think I fixed it. I did some reading and searching on here and was praying to everything and everyone that it wasn't injectors or an intake gasket. I wound up pulling the passenger side (pre-cat) O2 sensor and it seems to run great with short and long term fuel trims consistant. Not a miss, shake, or burp. How come the SES light didn't get an O2 code when it took a crap? Should I leave it off or replace the sensor? Thanks again! Tom.

  • 3 weeks later...
1999 Tahoe 4x4 Enthusiast

1999 Tahoe 4x4

Posted
Posted
Oh boy oh boy... I think I fixed it. I did some reading and searching on here and was praying to everything and everyone that it wasn't injectors or an intake gasket. I wound up pulling the passenger side (pre-cat) O2 sensor and it seems to run great with short and long term fuel trims consistant. Not a miss, shake, or burp. How come the SES light didn't get an O2 code when it took a crap? Should I leave it off or replace the sensor? Thanks again! Tom.

I think it could be that you have a bad connection at the O2 connector. When your O2 is connected and the computer is expecting it to work, it supplies wrong intermittent data. When you disconnect it and consequently the computer seses it obvioulsy disconected, the computer turns the system into open loop system = your truck runs without any emission monitoring on factory preset values. Most vehicles run great like that but that isn't great for your cat.

 

So for my part: my advice check the connection of the O2 sensor. Maybe it is oxidized or something.

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