I have an 06 sierra 1500 5.3l motor. I've been having issues with the tighten fuel cap message. I've owned it for 2 years. I replaced the canister vent solenoid a little over a year ago due to a P0455 code and it fixed the problem. Recently I got a P0442 code and the fuel cap message. I replaced the purge valve but, it didn't fix the issue. I took it to a shop and he found a leak on the top of the fuel tank. The fuel pump top was rusted and had a pin hole. I replaced the pump. Still getting the message but, no code. The mechanic did another leak test and a smoke test. No leaks found. He did 3 different evap tests with his scan tool. No issues found. On the way home from the shop I got the fuel cap message again. I checked for codes, even though there was no SES light. It showed a pending P0455. I figured the pending code might have been set during testing. I cleared the code and also disconnected the battery for about 30 minutes. What do I need to look at if the message or code returns? This has me and the mechanic baffled. Thanks!

 

I would look at the backside of the filler neck to make sure that it has not rusted out. It could be that when stationary and not running, the welded joint is sealed. However, when in motion, and over various road surfaces, the assembly might shift just enough to leak. I have heard of people putting uV dye into the mix for the smoke generator and then using a black light to look for tough to find leaks.

 

for most vehicles a tighten gas cap message occurs do to a large leak or perceived large leak.
the truck sees the fuel level change and performs a large leak test...it fails so it assumes you left the cap off.

for evap leaks the first thing they should do is flow test the system. this involves the use of a smoke test machine but instead of looking for smoke you are monitoring the machine flow meter. if the flow meters zeros after the system is filled with smoke...there is no leak... the fault is in what the system uses to determine leak/ no leak...ie in your case the fuel tank pressure sensor.
so i can flow test the system in 5 minutes and know i don't have to look for leaks or play with smoke. I need to open the purge valve koer, and monitor for a change in the fuel tank pressure sensor.

 

Thank you for the reply. This is the most definitive answer I've received to my question on several forums. The fuel pump and thus pressure sensor is brand new. (The sensor is in the fuel pump top plate, right?) The message did come back up yesterday after clearing the pending code. If I'm correct and the sensor is part of the new pump assy, could it possibly be a ground/wiring issue? Just a guess but, I'm thinking one side of the sensor is grounded and the other wire goes to the computer?
The mechanic did do the flow test and told me the flow meter read a dead zero, indicating the system is completely sealed. The tests were done with the bed removed so the top of the tank and all tubing and related connections could be inspected while doing the co2 leak test and smoke/flow tests were performed.

 

the ftp sensor is located on the top of the sending unit assembly ie fuel pump. the next step is to verify the sensor readings. with engine off, fuel cap off...the sensor should rear 0"h2o. install the cap, start the engine and open the purge to put vac on the tank.... you should see the reading go negative as vac is applied. if no change...check the purge line to the cannister...is it clear...can you feel vac...if yes conitnue moving back til you get to the tank. if plugged...that is the problem.

 

Thank you! I'll let my mechanic know.

 

@Tech2 What are the chances of the canister being plugged? I've owned the truck for 2 1/2 years and don't top off at the pump but, I have no idea whether or not the previous owners did.

 

for that generation i have seen cannisters spill charcoal pellets into the purge line.

 

Have you (or your mechanic) checked or replaced the gas cap? It’s an easy thing to overlook and assume it couldn’t be the problem, but oftentimes the seal can dry out and not seal properly, even if you tighten it until it clicks.

There’s also a 2 way valve inside the cap that’s supposed to release excess pressure or vacuum as needed. It’s possible the vacuum side of the valve failed which is why it’s passing a smoke test (system under pressure) but keeps setting the check cap message while driving (system under vacuum).

 

I have ran into this issue a couple of times and my next step would be to drop the tank ( not as bad as you would think) plug off all the openings and smoke test it where you can see every thing. The fuel pump module lid on some of these are steel and can have small holes rusted through. also some of the vent tubes are known to break/ crack and may actually blow the smoke where you cant see it. basically you are breaking the system down into sections and smoke/ flow testing them separately.sometimes the flow test can be faster to eliminate a section of the system from suspicion of being the leaker.