sudden P0300 on otherwise good running truck

[Kran]

99 tahoe. p0300 multiple random misfire after leaving car show and hitting the highway. noticed motor had abnormally low power, barely able to get to 65mph in 3rd. CEL started flashing then stayed solid. afterwards, torque converter wouldn’t go into lockup, like some kind of limp mode. pulled over off the highway, cleared the code, and it immediately drove perfectly fine for the two hour ride home. ideas? plugs, wires, distro cap/rotor, icm, and coil all done about 15k ago. i’ve never had drivability issues in my 7 years of ownership

 

[Schurkey]

If the code doesn't come back...forget it ever happened.

Does your code reader/scan tool have the ability to show individual cylinder misfire codes, or individual cylinder misfire counts?

 

[Kran]

If the code doesn't come back...forget it ever happened.

Does your code reader/scan tool have the ability to show individual cylinder misfire codes, or individual cylinder misfire counts?

thanks. i will have to double check on that, thanks for the idea

 

[Road Trip]

99 tahoe. p0300 multiple random misfire after leaving car show and hitting the highway. noticed motor had abnormally low power, barely able to get to 65mph in 3rd. CEL started flashing then stayed solid. afterwards, torque converter wouldn’t go into lockup, like some kind of limp mode. pulled over off the highway, cleared the code, and it immediately drove perfectly fine for the two hour ride home. ideas? plugs, wires, distro cap/rotor, icm, and coil all done about 15k ago. i’ve never had drivability issues in my 7 years of ownership

Given the clean track record of your Tahoe, it's possible for the engine to be the victim instead of the perpetrator.

Q1: Had you refilled the tank recently?
Q2: If so, was this at your normal (known-good) gas station, or was it at a station you've never used before?
Q3: Did you happen to notice if the big tanker happened to be in the process of refilling the station's underground tanks when you pulled up?

OK, to put a little context around why I'm asking these questions centers around the weight of gasoline (~6 lbs/gallon) vs water (~8 lbs/gallon).

To illustrate my point, let's assume for a moment that when you refilled your tank in your perfectly running car, the pump shows that
you bought exactly 20 gallons of fuel & charged you accordingly. But in reality you got 19 gallons + 15 cups of gas, AND 1 cup of
incombustible water. And you drive off, full tank, all smiles, no worries. Since water & gas don't mix, and you are driving around normally,
all the fuel sloshing around in the tank is keeping the water dispersed into tiny droplets.

And as long as this happens, it's quite possible that you will never feel a thing, for the ratio is such that you bought 319 cups of gas to 1
cup of water. Best case scenario if you get a cup of water in your gas is when this happens you just happen to be on a road trip, and you proceed
to drive out the entire tank before stopping again. There's an excellent chance that if any misfires do occur, because of the way that the water is
being metered over the time it takes to use up the tank, that the misfire count won't exceed the threshold and no CEL will be lit. (!)

****

OK, after that big windup, now we can better appreciate the worst-case scenario. You buy that same tank of gas, and shortly thereafter you
park the car for a few hours. (Like, when you attend a car show.) Or, you fill the tank on the way home & then park the vehicle overnight.

Now, that 1 cup of water that's dispersed into literally millions of individual droplets, thanks to it's specific gravity, it will all end up dropping to the
bottom of the tank & reconstituting itself as a single cup of concentrated misfire. :0)

In this scenario, you get back in the car after some time has passed, the car starts fine (thanks to the gas already in transit between
the pump in the tank & the injectors) ...and then you hit the slug of water a short time later, and you get the exact symptoms that you described.

By the way, the flashing check engine light is for when the engine is running so poorly that the computer is trying to warn the
driver that the cats are at risk of being damaged if driven this way for long enough. And a high ratio of water/gas will certainly
make any engine misfire/run that rough. And once the water/gas ratio starts to improve, the flashing light will go back to a steady on.

And once you were back to pure gas, your clearing of the codes allowed the transmission to take itself out of self-protection mode.

****

All of the above is why Schurkey recommended that you ignore it if it doesn't return, for in this case the engine was the victim, not
the perpetrator. Water in the gas is not nearly as common an issue as it used to be, but even so, it does occasionally happen.

Hope this helps --

 

[Kran]

Given the clean track record of your Tahoe, it's possible for the engine to be the victim instead of the perpetrator.

Q1: Had you refilled the tank recently?
Q2: If so, was this at your normal (known-good) gas station, or was it at a station you've never used before?
Q3: Did you happen to notice if the big tanker happened to be in the process of refilling the station's underground tanks when you pulled up?

OK, to put a little context around why I'm asking these questions centers around the weight of gasoline (~6 lbs/gallon) vs water (~8 lbs/gallon).

To illustrate my point, let's assume for a moment that when you refilled your tank in your perfectly running car, the pump shows that
you bought exactly 20 gallons of fuel & charged you accordingly. But in reality you got 19 gallons + 15 cups of gas, AND 1 cup of
incombustible water. And you drive off, full tank, all smiles, no worries. Since water & gas don't mix, and you are driving around normally,
all the fuel sloshing around in the tank is keeping the water dispersed into tiny droplets.

And as long as this happens, it's quite possible that you will never feel a thing, for the ratio is such that you bought 319 cups of gas to 1
cup of water. Best case scenario if you get a cup of water in your gas is when this happens you just happen to be on a road trip, and you proceed
to drive out the entire tank before stopping again. There's an excellent chance that if any misfires do occur, because of the way that the water is
being metered over the time it takes to use up the tank, that the misfire count won't exceed the threshold and no CEL will be lit. (!)

****

OK, after that big windup, now we can appreciate the worst-case scenario. You buy that same tank of gas, and shortly thereafter you
park the car for a few hours. (Like, when you attend a car show.) Or, you fill the tank on the way home & then park the vehicle overnight.

Now, that 1 cup of water that's dispersed into literally millions of individual droplets, thanks to it's specific gravity, it will all end up dropping to the
bottom of the tank & reconstituting itself as a single cup of concentrated misfire. :0)

In this scenario, you get back in the car after some time has passed, the car starts fine (thanks to the gas already in transit between
the pump in the tank & the injectors) ...and then you hit the slug of water a short time later, and you get the exact symptoms that you described.

By the way, the flashing check engine light is for when the engine is running so poorly that the computer is trying to warn the
driver that the cats are at risk of being damaged if driven this way for long enough. And a high ratio of water/gas will certainly
make any engine misfire/run that rough. And once the water/gas ratio starts to improve, the flashing light will go back to a steady on.

And once you were back to pure gas, your clearing of the codes allowed the transmission to take itself out of self-protection mode.

****

All of the above is why Schurkey recommended that you ignore it if it doesn't return, for in this case the engine was the victim, not
the perpetrator. Water in the gas is not nearly as common an issue as it used to be, but even so, it does occasionally happen.

Hope this helps --

thanks for typing all that out, much appreciated. drove about 50 miles, filled up i think about 10-15 gal or so at a very busy kwik trip that i've never been to before (no tanker on-site), then drove another 100 miles to the show. the return trip back is when the issue occurred. friend that was following me did note he smelled something awful, shortly before i called him to pull over.

sounds like i'm chalking this up to a freak incident.

 

[Road Trip]

thanks for typing all that out, much appreciated. drove about 50 miles, filled up i think about 10-15 gal or so at a very busy kwik trip that i've never been to before (no tanker on-site), then drove another 100 miles to the show. the return trip back is when the issue occurred. friend that was following me did note he smelled something awful, shortly before i called him to pull over.

sounds like i'm chalking this up to a freak incident.

No problem, my pleasure. I've troubleshot this before several times, and the
pattern is pretty unique...so when I read your original post it just had that feel,
if you know what I mean.

Anyway, the good news is that it sounds like your vehicle's perfect track record for the
past 7 years is still intact. Still a faithful traveling companion, just got a touch of food
poisoning through no fault of either one of you. :0)

You know, a can of dry-gas thrown in during the next refill might be a good idea. (To mop
up any residual.) Or even a decent fuel injection cleaner on the off chance that the foreign
liquid contained any goop/funk.

Safe travels --

 

[Schurkey]

You know, a can of dry-gas thrown in during the next refill might be a good idea. (To mop
up any residual.)

Or fill up with the usual E10 and get ten times as much alcohol.

Or even a decent fuel injection cleaner on the off chance that the foreign
liquid contained any goop/funk.

I'm fond of Chevron Techron Complete Fuel System Cleaner. Works great. I dump a bottle into the gas tank at every oil- or filter-change. (6--13K miles.)