Hello Everyone ! Hope someone can help!
I live in Ontario Canada and have the wonderful joy of having to take my 88 GTA for an emission testing in order to have her plated. In the first test she only failed on the Hydrocarbon parts per million; the garage tweaked the timing and got the score down to an almost pass. Unfortunately $300.00 and 4 hours later the head mechanic came to me and said they had no idea why she was failing, everything is working the way it should; but when they hold it at 38 km for the test (car has to be held between 38-40 km for 1 min) after about 30 seconds the motor starts to slightly surge and the hydrocarbons go off the scale. We have tried the oxygen sensor, the plugs are perfect and are going to replace the EGR valve tonight. Other than that we are stumped. There are no trouble codes; she runs fantastic, tonnes of power etc. I have to take her again tomorrow morning; hopefully someone has an idea!

 

The EGR valve has nothing to do with hydrocarbons.
EGR reduces NOX at part throttle cruise. The recycled exhaust gasses reduce peak combustion temp by deluting the Air/fuel mix a little, reducing the formulation of NOX in the exhaust.

You have unburned fuel in the exhaust. Change the oil.
check the pcv hose for oil. Check the temperature of the cat while doing the test. If its not up to temp, its not working. (Infrared temp gun)
The operational temp that needs to be achieved slips me right now. Someone will know. Inspect the ignition wires, cap and rotor. Inspect the spark plugs for resistance. Do a ignition diagnostic. Looking for random misfireing at part throttle/load.
Check the fuel vapour canistor and purge system. This system purges stored fuel vapour into the intake manifold right about the time of the test mode. If the canistor is full (saturated) or the lines to the tank or tank vent are plugged you're going to get raw fuel sucked into the motor.

 

Thank you for the info about the EGR. The ignition should be sound; we gave it a complete tune up prior to taking it for the test. We did change the oil; but can change it again. The only inclination that there is an ignition breakdown is that when the car is held @ approx 1100 rpm (38 km) about 30 seconds into the test the engine starts to surge; that's when the hydrocarbons go off the scale. Suggestions on what could be causing this? According to the mechanic, this is the only spot that this happens; they tried to get this to happen at different rpm levels and nothing.

 

check the canistor purge system. Check for a broken valve spring coil. (happened to me)

 

Hydrocarbons will always shoot to the moon as soon as you get misfires. If the motor is audibly "breaking up" then you're definitely getting misfires and that's what's blowing up the HC reading.

Bad plugs, wires, cap, rotor or coil will also cause some nasty HC increases. You can get higher HC even before you notice a "miss" or stumble. As soon as you can hear it though, it's definitely gonna be belching some nasty HCs out the tailpipe.

Find out why and fix it. Given that it runs fine for 30 seconds and then starts to surge it's likely to be something that the ECM relies on for information that's lying to it. Bad sensor, electrical connections or similar.

Fix the stumble and you will fix the HC problem, guaranteed.

 

The fuel system purges at the same rpm as the emission test is done right? Would a failure in this system cause the engine to surge? Any ideas what we should be looking for? Cracked vacumn lines???

 

Approximatly, yes , just disconnect the canister purge and retest. If the test is suddenly a lot different, you've found the cause. Don;t assume just cause you "gave it a tuneup" that the ignition system is up to snuff. The last part you replace on a car should always be suspect when a problem sudenly comes up. Some new parts are faulty. Have a close look.