The 88 gta failed the IM240 dyno emissions test for high HC. First time it failed, I had a .94 against a .80 standard. At this point I did not get a graph of the drive cycle. Second test, I retarded timing to 0 BTDC & disconnected the secondary cooling fan. Problem was, it was 92F outside temp & the car had a full tank of gas. The second time HC climbed to 2.53. Having a friend show this to an emissions instructor, he said the drive cycle graph shows HC to be contantly high, basically a straight horizontal line. This, he said, is a pattern failure for 3rd gens, caused by a leak at the gas tank filler neck allowing the exhaust suction devices to pull in gasoline fumes and record them as HC. All this makes sense, but I don't see any type of leak from the filler neck to gas tank, after pressurizing the tank with compressed air. What I do question is the remote pressure/vaccum relief valve ( a plastic device) that terminates off a rubber vent hose at the left rear brake line. The valve appears OK, but by blowing into it, you can hear the valve poopet fluttering. Does anyone know what the pop off pressure is for this valve? It is not mentioned in the factory shop manual. I hate to buy another valve without knowing if this one is bad. I'm thinking of plugging the vent hose (ie leave this valve out, temporarilay) & cracking open the gas cap until I take the test. If it were 60F out, this would also probably help, along with a close to empty fuel tank. Anyone know the specs on this valve or can offer any suggestions?
Thanks, FJK

 

FJK,

I don't know the relief setting of the tank pressure control valve, but it should be lower than what is required to force any vapors through the EVAP canister. Is the canister plumbed correctly and the unit not saturated?

As a WAG, the pressure probably shouldn't be much over 5-7 PSI, I would think, but that IS just a guess.

 

Well, it is not an issue anymore, because I took it out & plugged the line, & still flunked HC, same as before, a 2.5 against a .80 standard. I think I'm failing by too much to be able to trick my way into lower HC. I am going to try to go after root cause, maybe starting with the AIR system. Any other thoughts out there for constant high HC over the drive cycle. I've got an auxilary fuel pump in series with the stock one. Could excessive fuel pressure cause injector dribble?