Exorcism for a 1999 GMC Yukon ? Help !

[eknsar01]

Hi There. I am a new member to any discussion group so if I do something wrong please let me know ! I have a 1999 Yukon SLT 4WD with 230,000 miles on the clock. I have had it since it had about 45,000 so I know its history.The recent problem is this: if it sits more than overnight it becomes VERY hard to start, a problem that has never occurred before. Usually 5 sec crank and is running. Now spends 5 min or so cranking, backfiring through the engine , trying to start.Once it starts, runs fine and will work fine for the rest of the day, starting & stopping. Good battery, ignition seems fine, seems to be a fuel problem but nothing seems to work. Cleaned fuel line,new fuel filter,fuel pump,checked pressure regulator, nothing seems to work. My mechanic ( at a chain auto repair shop) who has worked on it for about 15 years is stumped. The only thing that has come up is the oxygen sensors are now out in both banks.
I use this truck as a member of a volunteer Search & Rescue Unit located here in North Alabama, and I need something to work. Lots of good history in this truck and I don't want to give up. Anyone have any Ideas ? Thanks for listening. Ed

 

[Zipper369]

Welcome to the Tahoe forums! This is the first I've been back on here in a while, but everytime I asked questions on here I got pretty good answers. Hopefully you find this place as helpful and useful as I have. Anyway, on to your question...

I'd start with the O2 sensors if you already know they're bad. Mine went out around 190k miles and it had a hard time starting and idling. Also, have you tried turning the ignition on for a few seconds to let the fuel pump build pressure before trying to start it? Just a thought.

 

[eknsar01]

Thanks for responding. The O2 sensors showed good when this started - only after about 6 weeks of fooling with this did they come up bad (confirmed by having 3 different shops testing !)
I also tried the turn-on-and-wait trick, but did not seem to make any difference. Will replace the sensors and see if that helps. I am going to leave it with the shop ( with strict instructions NOT to try to start if for 5 days and see what happens. It would be nice if something would just break permanently , so we can fix it.
Thanks again for the suggestions.