99 C1500 Suburban reeks of gas sometimes

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redfishsc

Tired of fixing lousy engineering.
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My 99 C1500 Sub (5.7) occasionally reeks of gas outside the truck (rarely if ever inside). Not the "about to explode" odor of huge amounts of gas vapor, but definitely noticeable after we park it. It seems to be originating from near the fuel tank but I never see any drips, and I never can sniff out the source of it even with the truck running and me crawling around underneath it.

Also..... maybe/maybe not related....... occasionally this truck "surges" when idling and in drive/reverse. About once a week after cranking the truck (with warm engine, it seems) the RPM's will surge from 400-800 up and down, kinda "bouncy". No obvious vacuum leaks to be found. It's enough that you have to hold the brakes a little harder to keep it from creeping. But, if you push the accelerator at all, surging goes away and the truck runs like a champion. Could this be related?

I know I'm throwing an EVAP code, I'll have to double check what the code is. I'm guessing the EVAP code is related, and I'll update later today/tomorrow once I pull that code.

Until then, anyone ever dealt with this?

Brief history, all the gaskets up front (including fuel rail o-rings) are fairly new. Head gasket, upper/lower intake gaskets, and brand new MFI Injectors, as well as new reman cylinder heads and new Enginetech lifters--- all were done about a year ago. Truck idles as slick as glass and runs great other than what I mentioned above.
 

df2x4

4L60E Destroyer
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I know I'm throwing an EVAP code, I'll have to double check what the code is. I'm guessing the EVAP code is related, and I'll update later today/tomorrow once I pull that code.

Almost certainly related. The issue is that most of those codes (at least in my experience) don't really help you diagnose the issue at all, they just tell you there's a leak. I'm going to guess it's probably a P0455 (gross leak). Considering you say it seems to originate from the rear of the truck my guesses would be vent lines falling apart, hole somewhere in the filler neck, or a hole in the top of the tank itself somewhere. That's all they are without having it smoke tested for leaks though, guesses.

That would be my recommendation, find a shop with a smoke machine and have them check the EVAP system for leaks. Costs around $80 for that diagnostic in my area. Unfortunately I had to have it done more than once on my red truck before my issue showed up. (It was a connector on top of the charcoal canister that vibrated ever so slightly loose once in a while)
 

delta_p

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several years ago on my 96 C1500, I had the charcoal can fracture at connection to the fuel tank. The can is up on the driver side at the front in the engine bay. I saw the pieces on the ground under the truck in the driveway after the truck sitting a few days and assumed something was old or clogged and it blew out because of the heat here in the south and i probably had a full tank sitting. The 96 will drive all day without any evap codes even thought the service engine soon will light (There's no fuel tank pressure sensor on 96). Since i drove it like that for about 6 months, i could smell some gas after driving and parking. Eventually, i put a new can on and connected the lines.

The other connection on the can goes through the purge valve etc. and to the vacuum at the intake manifold. If you have crack in that line you might smell gas and have an idle surge when the purge valve occasionally operates to check for gross and small leaks on the EVAP circuit. It opens and closes periodically under drive for these checks. But it should throw a code too due to the leaks. Again, the 96 was first year OBDII without fuel tank pressure sensor, and it doesn't do all the leak checking that the later models do
 
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