GM P0442

Been seeing a reoccurring issue with evap systems on many different GM products and when smoke testing the system it shows no leaks. I had one customer with a 2007 Chevy Colorado with an intermittent P0442 I believe, did a purge/seal test, brought it up to a good vacuum and sealed the system, it held a vacuum which should mean its sealed right? I did some further research and there are certain conditions that must be met and a particular procedure the pcm uses to identify if it has a small evap. leak. Now this issue was put on hold for a little while until the other day. The customer returned the other day for unrelated services and I decided to take another look at the components of her evap. system. I check the fuel tank pressure sensor koeo, cap off, and it read between 1.45-1.47v and about -0.01kPa. And I checked a known good and it reads the same. If I go to a baro sensor it reads 100kPa. What am I not understanding correctly?

I don't think you're misunderstanding anything, necessarily. Only that, IMO, the Purge/Seal test is not a perfect indication that there are no leaks.

It's a crappy situation to be in. :silly: I can give you a couple suggestions? One would be to repeatedly cycle the purge and vent valves with the smoke machine flowing. Make both seal consistently. You can sometimes catch a purge valve sticking open just enough to cause a small leak code, but not enough to cause a P0496.

The other would be to apply vacuum with a hand pump to both valves and check for sealing. The idea is that one or the other may leak vacuum (which the system uses to check for leaks) and not pressure (which the smoke machine creates). Don't get stupid with it - a few "hg is plenty.

Your check of the FTP voltage is definitely a good idea. :B Obviously, make sure that your reading also changes when vacuum is applied. You can get false leaks codes from an FTP that doesn't change from 1.5V, or a purge valve that's stuck closed and not flowing.

Tyler wrote: I don't think you're misunderstanding anything, necessarily. Only that, IMO, the Purge/Seal test is not a perfect indication that there are no leaks.

It's a crappy situation to be in. :silly: I can give you a couple suggestions? One would be to repeatedly cycle the purge and vent valves with the smoke machine flowing. Make both seal consistently. You can sometimes catch a purge valve sticking open just enough to cause a small leak code, but not enough to cause a P0496.

The other would be to apply vacuum with a hand pump to both valves and check for sealing. The idea is that one or the other may leak vacuum (which the system uses to check for leaks) and not pressure (which the smoke machine creates). Don't get stupid with it - a few "hg is plenty.

Your check of the FTP voltage is definitely a good idea. :B Obviously, make sure that your reading also changes when vacuum is applied. You can get false leaks codes from an FTP that doesn't change from 1.5V, or a purge valve that's stuck closed and not flowing.

Yes I need to get a vacuum pump. But that still confuses me. Why womt the ftps read barometric pressure. How do you test a ftps?

tonybushmin wrote: But that still confuses me. Why womt the ftps read barometric pressure. How do you test a ftps?

You mean the difference between the Baro sensor reading 100 kpa and the FTP reading 0 kpa? That's because the Baro reading is in pressure absolute, and the FTP is in pressure gauge. On the absolute scale, zero is a perfect vacuum. The gauge scale has zero at ambient pressure. The data PIDs show different values, but they're really the same reading.

The test of an FTP sensor would be to compare it's reading to a rational situation. As in, if the FTP says there's 10" H2O in the tank, but the fuel cap is off. :lol: This isn't rationally possible. Or, the FTP says 0" H2O, but the purge is wide open and the fuel trims are pegged out rich. You know rationally that flow is taking place. Make sense?

thats some master tips right there! thanks