Testing evap valve

[KVacek]

My wife's 2005 Yukon Denali 6.0 had the evap codes maybe 5 tears ago. The lines were all good, and one by one I replaced the valve at the canister, the gas cap, and finally the valve on top of the engine - the easy one to get to. That fixed it for years.

Now she has a P0442 MIL - Small evap system leak. I tested the vacuum holding of the evap valve on top of the engine first, engine off, no key. It only holds an inch or two of vacuum, so I concluded it's bad and ordered another AC Delco (GM 12581282 - Delco 214-1105). Came overnight and since it's raining I decided to test it before installing it. If I don't hold my finger on the bottom (goes into the engine) side, it leaks pretty much the same as the one already on the truck. Checked my hookup, checked my MityVac, etc. I can pull and hold good vacuum on the MityVac and my hose. Attach the valve and it leaks.

One video mentioned that they always test them and have had bad ones from new. It's a real Delco, original part number, as far as I know.

Is it likely it's bad from new?

 

[TJ Baker]

My wife's 2005 Yukon Denali 6.0 had the evap codes maybe 5 tears ago. The lines were all good, and one by one I replaced the valve at the canister, the gas cap, and finally the valve on top of the engine - the easy one to get to. That fixed it for years.

Now she has a P0442 MIL - Small evap system leak. I tested the vacuum holding of the evap valve on top of the engine first, engine off, no key. It only holds an inch or two of vacuum, so I concluded it's bad and ordered another AC Delco (GM 12581282 - Delco 214-1105). Came overnight and since it's raining I decided to test it before installing it. If I don't hold my finger on the bottom (goes into the engine) side, it leaks pretty much the same as the one already on the truck. Checked my hookup, checked my MityVac, etc. I can pull and hold good vacuum on the MityVac and my hose. Attach the valve and it leaks.

One video mentioned that they always test them and have had bad ones from new. It's a real Delco, original part number, as far as I know.

Is it likely it's bad from new?

While it sounds like it is leaking, the purge valve (one at the engine) leaking would not set a small leak code. It would set a code for purge flow while not commanded. A leaking purge valve would make the Evap test vacuum rise, not fall.

 

[S33k3r]

I think some of the AC Delco branded parts are made overseas. IIRC, AC Delco was sold some time ago. Here's the articl;e that caught my attention -- it may/may not be applicable here.

https://gmauthority.com/blog/2021/03/acdelco-facing-lawsuit-over-made-in-usa-claim-on-packaging/

And new stuff can always be bad from the factory, or damaged during shipping.

Good luck, in any case.

 

[KVacek]

Replacement coming. Thanks !!!

 

[KVacek]

Well darn. And I don't really mean DARN.
New one - this one does not look to be a resealed package, but just like the first one I got and the one in the truck, it only holds about 5-6" of vacuum. If I put my finger over the bottom of the valve it'll hold 25", so I'm sure it's not my MityVac or the connection.

Do they really hold 15" or more like the guys in the videos say? First vehicle in years where I haven't bought the factory service publications, and since we've had the truck 13 years I'm not really interested in spending a couple hundred dollars for the Helm books at this late date. Getting kinda rusty.

 

[Bill 1960]

My understanding is that valve seals in the opposite direction to what you described testing it with your finger blocking the engine side. After all, the intake manifold is the vacuum source. The valve closes to prevent excessive engine vacuum from maintaining a constant vacuum on the fuel tank.

I believe it would be properly tested with a vacuum pump applied to the engine side and determine if it seals the intake vacuum from the line to the rear.

 

[KVacek]

The YouTube geniuses were wrong. The engine needs to be running. I went out and tested three more valves at parts stores and only one held some vacuum - 10" or so. I came back, put the vacuum pump onto the valve, and started the engine - it holds 25'.

Its not a check valve, so should seal either way. We're not talking big pressures here. All the testing I've seen and read about is onto the top connection that leads to the fuel tank vent hose, but again it shouldn't matter. And indeed the valve when energized does seal that way.

What I meant about covering the other side with my finger isn't how I tested it - just proof that the pump and hose are OK and there's no leak in the body of the valve. It should seal and does without any finger on the bottom.

For now (it's been rainy and predicted to rain all week) I reset the code. We'll see how long it takes to reappear. Next time it does I get to crawl around under the tank and check the valve down there and the canister and the line to the front. And then when all that's OK it'll be the fuel cap, which should actually show a different code.