Bought a 1999 GMC Sierra. PO says that he took it to the dealer and they diagnosed evap. Not that I don't believe him or the dealer, but before I tear into the dash how do you know? Is it process of elimination that there is a leak and dye doesn't show anywhere, so therefore it must be buried in the dash?
If there is a leak, it will be greasy from thr refrigerant oil. Hard to see, though.
Dye in a leaking evaporator usually runs out of the A/C box drain on the firewall.
Is the AC blowing warm or cold? If it's still cold then the refridgerent hasn't leaked enough yet to even see.
There are a couple ways to check for AC leak. I'm not sure what tools you have access to but there are special tools for the job. The local parts guys usually have a kit to buy or tools you can borrow. Some of those DIY recharge cans have dye in them.
You can recharge the system and add dye again. If the evaporator is leaking the dye will eventually run out of the drain but it may take a few cycles for the condensation to "wash" the dye out of the drain so you can see it. Look for an oily residue on all of the AC components. More often than not you can spot a leak that way.
The other most common way is to use an electronic leak detector. Again you'll have to have the system charged. If you suspect the evaporator you stick the probe in the dash vents and evaporator drain to see if it detects a leak. I personally don't trust the detectors that much. Any moving air can give a false reading and send you chasing leaks that aren't there.
My FD is leaking in the evaporator, and yeah, it pees green dye out of the drain hose. Didn't take many cycles to do it, either, although you do need to run the AC for a while to generate enough water.
Evaporators are really hard to "see". I have seen them externally plugged solid with dye slime but never any dye out the evaporator drain.
If you turn the A/C on and the air coming out of the vents smells like Silly Putty, that's a sure clue. Silly Putty has a similar chemical smell to common R-134a refrigerant oils.
Silly putty!!!! I had never put those two together but you are 100% right!
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