: high hc on smog test/ evap connections

silver duke
03-19-10, 01:57 AM
Hi everyone, My 93 sts just failed the local emissions test on the hc loaded portion.My sts has been off the road for about a year now(just replaced trans).It has 125,000 miles. replaced wires(delco) coil packs ,plugs, front o2 ,All previously. It failed by a fairly small amount(0.50g/km max allowable and sts read 0.50) and the co and nox were fine. There are no codes and the car seems to run completely fine. The cat convertor has about 15,000 miles on it (aftermarket). I am suspecting the evap hoses at the canister may be mixed up. If anyone could check theirs and get back to me it would be greatly appreciated. On my car the larger port marked "tank" is connected to a purge valve with engine vacuum port on top.The canister port marked "purge" is just connected to hose (seems like vent hose to throttle body. The "air" port seems to be correct as it is the only large hose of that diameter.Thanks in advance all, Any help or photos of evap connections is appreciated greatly.

03-19-10, 01:11 PM
HC test, as I understand it, measures the unburned hydrocarbons going out the tailpipe - not in the fuel delivery system. I'm sure you will get a lot of other suggestions, but if your HC is high, I think that means that either you are running too rich, or you could also be running lean due to a vacuum leak, thereby causing a lean miss condition. I have read a lot of threads that include a reference to the fuel pressure regulator causing problems. Maybe you should do a search for that or FPR?

03-19-10, 06:04 PM
If you are running rich, the FPR would be a good suspect.

03-20-10, 07:10 PM
From Smogtips.com

3. Lean Fuel Mixture - Any condition which will cause unmetered air to enter the intake manifold, and ultimately the combustion chambers, will cause high hydrocarbons (HC). This condition is called a lean miss-fire. Such faults as vacuum leaks and gasket leaks will cause lean fuel/air mixtures. Broken, disconnected or misrouted vacuum hoses will do the same. It is also important to note that many engine components rely on engine vacuum for proper operation. If any of these components are defective, externally or internally, they may cause large vacuum leaks as well. A good example of such a component is your vehicle's power brake booster.

Check all vacuum hoses and gaskets for vacuum leaks. Next time you go to visit your friendly smog check station, be sure to drive the car at least 30 minutes and drive it like you stole it (full throttle / 6500 rpm's). You might also want to try the Seafoam trick.

03-20-10, 07:18 PM
If you can post pictures of the EVAP system connections and a nice snapshot of the diagram I might be able to help confirm what you have going on there.

HC under load should be unburned fuel.
It could be a weak cat if you are close to the limit, or it could be something like excess fuel vapors from the EVAP system if it is plumbed wrong.

Can you post all your readings?

silver duke
03-20-10, 08:29 PM
Thanks guys, I am leaning towards a vaccum leak as well. I am going to rod out the egr mouse holes and replace the plenum gaskets and check for vacc leaks. Maybe there is a leak there . I will take a photo of the evap connects and post. thanks for all your help.

03-21-10, 10:01 AM
I'd still have to know more about the test.
Vacuum leaks would create a lean condition and should drive up CO not HC.
Also if this was on the dyno, what speed was it at?
Vacuum leaks will barely affect fuel control under a load but it will screw fuel trims up at idle.
Cleaning the EGR ports will help if you have a NOx problem but it doesn't affect CO or HC.
I am not sure about the "plenum gasket" you speak of on a 93... this is a 4.9 right?

Let us know how it goes.