If it has got condensation in the engine, it can cause the oil to emulsify - whitish goo or lumps.
That clogs up oilways, especially thing like the ultra fine ones that feed hydraulic valve lifters.
Give it an oil & filter change, using a dose of "Engine flush" oil additive with the engine hot, a few minutes before draining the oil - just follow the instructions on the stuff.
That should give you the best chance of it running smoothly.
Not bad gas - I have bought a car that sat for more than a year and the gas was fine. In the days before sealed fuel systems gasoline would lose volatile components in a few weeks, but gasoline in a modern vehicle fuel tank lasts years. Ehanol is not an issue unless the gasoline was wet when it was put away.
As GMdrivingMOPARguy says, it doesn't sound like fuel at all. I believe all engines in all Tahoes use hydraulic valve lifters, and that is my first thought - collapsed lifters, which create a clatter rather than a thud. You can tell with a mechanic's stethoscope. If the sound comes from the top of the engine rather than the bottom it is the lifters.
Mr hyde is the closest here.. ever since the addition of ethanol in fuel it doesnt last much longer than a few months.. and since the truck sat you need to run the old fuel out and replace with new stuff of higher octane. Now not knowing how you prepped the car for storage you may have water in the fuel and or soon enough oil leaking from bad seals so run the truck for a bit drive it around for a few days and put good fuel in it and keep running it till the new stuff is all thats running and then sell it quick Im willing to bet if it sits for to long it will start leaking oil
When a car has been standing as long as that, you need to remove the spark plugs and squirt a little engine oil in each port, otherwise the pistons are going to run dry until the engine oil in the sump is fully circulated. This could be a possibility why it is knocking now.
Gasoline expires after a few months. The additives congeal. Water is attracted to gas so it become watered down. You need to get rid of the old gas and get some good gas in the tank.
If by knock you mean a ping or "spark knock" then yes I would expect rotten gas to cause that, but if by knock you mean a physical "clunk" then you have a bigger issue on your hands than bad gas.
More than likely the *anti-knock sensor under the intake manifold needs to be replaced. Until you have it changed try 93 octane fuel.
if water got in your gas, water and oil dont mix so if water got in your gas, then got in your engine could cause some damage to internals
I parked my tahoe with no engine noise approximately a year ago. No engine issues at all. Now, about a year later I decided I want to sell it so I start it up and the engine is knocking like a *****. This could be from bad gas according to a few friends. Any help is appreciated. The issue of bad gas could mean a world of difference in the price I get for my vehicle
