So I have a 2004 Tahoe flex fuel model with just starting, last night, rough idle, stumble, stall issue with no CEL, won’t rev or delayed rev. Started last night, drove, then turned off, comes during restart. Found and repaired cracked PCV connections, cleaned MAF and looked for loose connections and cracks in all engine bay/vacuum lines and fuel connectors. Used scan tool, here are the stft and ltft when it’s idling normal. I know I’m showing rich ltft, looking for ideas of what to check next.
2004 Tahoe 5.3 flex fuel intermittent hot rough idle and stall
- Thread starter Eboz
- Start date
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LTFTs show it ADDING fuel, so it's been running lean. The STFTs are okay, but the ECM has learned to add fuel before, perhaps due to the cracked PCV connections.
Throttle body clean?
How old are the O2 sensors, and are they OE, not aftermarket?
Do the fuel trims come down at 1800 or 2000 rpm?
Fuel composition -- what percentage of ethanol is in the fuel in the tank (E10?), and what does the scanner say the fuel alcohol content is? Are those values pretty close?
At hot idle, what does the MAF report in g/sec (not lb/min, please)?
Throttle body clean?
How old are the O2 sensors, and are they OE, not aftermarket?
Do the fuel trims come down at 1800 or 2000 rpm?
Fuel composition -- what percentage of ethanol is in the fuel in the tank (E10?), and what does the scanner say the fuel alcohol content is? Are those values pretty close?
At hot idle, what does the MAF report in g/sec (not lb/min, please)?
throttle body cleaned in November
Don’t know the age of the o2 sensors, can’t make out brand markings, look pretty old
Stft go to slightly negative when reved and held and Ltft raise slightly. This morning with warm engine ltft are 8.6 and 7.0 and stft are between 0 and 3.9 when idling.
Fuel here is E10, my scanner doesn’t have a fuel content option.
MAF is between between 4.4 and 4.9 g/sec at hot idle.
So far can’t recreate problem this morning while idling. Going to take it for a drive and report back.
Drove a short distance, turned engine off, went to restart instantly died. Restart again died. 3rd restart running rough. MAF is at 1.58g/s regardless of throttle position. Have to hold gas pedal for a few seconds to get a consistent rev. MAF readings don’t change. Truck stalled again. Letting it cool down for an hour before restarting. Video of sensors while running rough before stalling.
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Update, pretty sure it’s the MAF, I disconnected it while it was sputtering and it ran better, when I reconnected it I started getting good flow numbers from it again. But another question my second o2 sensor on each side is showing 99.2% indicating maxed out rich and voltage for both is showing steady at .730- .740. Is this indicating a need for replacement?
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The downstream O2 values will be what you have, so they seem to be okay; their voltage should be steady. It's the upstream ones that can wreak havoc with trims, especially if they're old. If you get to the point of changing the upstream ones, do yourself a favor and buy GM OE ones from a trusted vendor like Rock Auto, the dealer, or a brick and mortar auto parts store. "Genuine" ones on eBay or Amazon may be fake, and work worse than what you have.
First of all, though, I'd suggest replacing the MAF with a GM OE one (again, from a trusted parts store) and see if that improves things.
First of all, though, I'd suggest replacing the MAF with a GM OE one (again, from a trusted parts store) and see if that improves things.
Currently looking for one that isn’t 300.00+. Rock autos listed one has a Delphi logo, so does AutoZone, Napa, was the OE a Delphi unit?
I believe that mine is OE and it's a Delphi, 25168491.
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