Hello all,

I just had my 1993 Z28 emissions tested and didn't have much success. A bit of info about the car: It has a newly rebuilt LT1 (simple .030 over rebuild...ground the crank, new rods, pistons, rings, valvetrain...) and new O2 sensors. The old exhaust seemed to be in good condition, so I reused it (a mechanic examined the cat and said it still had catalyst in it and appeared to be okay). A potential issue is that I had to use '94 exhaust manifolds. The car has the '94 EGR plumbing and the AIR system is removed. The computer throws no codes.

When I had the car tested, I let it warm up for 30 minutes and it still failed miserably. It passed all the idle testing, but here are the results for the 2300 RPM running test:

HC (ppm) Reading 39 with limit 58
CO (%) Reading 0.65 with limit 0.32
NO (ppm) Reading 772 with limit 435

I expected to fail the HCs, but that's the only area I passed. What are the possible root causes of this result? I would think that if the motor was rich, the HC would be high. CO does suggest incomplete combustion though. Might one of my O2 sensors not be functional? With the EGR setup, I thought my NOx would be okay. The valve isn't stuck open...might it be stuck closed? My catalytic convertor is old, will replacing it help with the CO and NOx? Thanks for the advice.

 

With all the readings being high like yours it usually points to the converter. You could have them do a pre converter and post converter readings to find its efficiency. I assume all tune up parts were replaced when the engine was done. Also pull up on the EGR diaphram to make sure its not plugged up at idle it should pretty much kill the engine.

 

The EGR I will check. What's the best way to check it (i.e. where do I 'pull')? I was thinking of hooking it up to a vacuum pump and trying to open it that way. ALL the tune-up parts were replaced--new Optispark, new plugs, new wires, new filters, all new vacuum lines...the motor seems to run like a champ. I'm really suspecting the cat...I just want to make sure I've covered everything.

 

The vacuum pump will work fine. The valve senses backpressure and vaires the openning even if theres 20" of Hg put to the valve. By hand you can reach under the valve and lift the diaphram up. Check out shbox.com for a full write up on EGR valves.

 

Might want to adjust the temp you are running at. I'm pretty sure as temp goes up, so does the NOx. Just a suggestion, I don't know for sure.

 

That's true about NOx. It forms at higher temperatures. The EGR recirculating the exhaust gases serves to lower the temperature...that's the part it plays in NOx reduction. I currently run a 180 degree thermostat and the engine shows no signs of overheating. I also have a new rad, water pump and all applicable hoses installed. Should I be running a 160 degree t-stat? I've heard some people haveing emissions issues if the engine doesn't run hot enough.

 

I'm pretty sure stock came with a 205 or something like that. So, if you are at 180 then there must be something going on. I don't think the LT1 will throw a code for EGR flow. My Ford truck had a P switch on it and it threw a code for that. Are you sure you have vac at the EGR? Another thing is the feed line to the EGR from the exhaust might be plugged up.

I have the factory service manual with the troubleshooting chart. If you need it I can scan it in for you.

Also, do you have 2 way or 3 way CATS on F-bodys?

 

I checked the EGR valve manually today. It's not stuck (the engine bogs when I open it at idle). The EGR pipe I just bought from the dealership last week, so it's definitely not plugged. I do have vacuum to the EGR valve...I don't know if it actually opens during operation though. How can I check that? I'd be really interested to see those service manual pages if you get the chance to scan them.

About the cats, I'm not sure about the 2-way/3-way...what's the difference? I'm likely going to buy a new catalytic convertor, perhaps a complete catback exhaust tomorrow and see if that helps.

 

Just went through all this same garbage on my 94:

NOx is caused by high cylinder temps. The only thing that get's rid of it is the EGR and the cat. Test the EGR with the vaccum pump and the car off. The EGR should hold vaccum...if it doesn't it's hosed. Start the car and test again. Should bog and nearly die. When/if you replace it, make sure to clean the carbon out of the manifold and the EGR hose from the manifold.

Now, for the bad news...your car is so far over on NOx that no EGR valve in the world is going to fix it. My car had a completely dead EGR valve on my 1st pass and the numbers only got about 10% better with a new EGR.

If you are running an aftermarket program, ditch it for stock for the test. The program I was using only opened the EGR 12% for some reason according to my datalogger

Throw on a new cat and you will pass almost gauranteed. A fresh cat can cover up some remarkably bad 1st test numbers. Make sure to put 100 miles or so on before you go for the test to burn the manufactuing oil out of the cat or you will fail right away. Also make sure you have the stock 180 degree t-stat when you test so the cat gets to the right temp.

BTW...our cars use a 3 way cat...3 way indicates how many catalysts they have. IE they reduce, NOx CO2 and HC.

Chad-

 

Great advice boozrcruzr! My EGR valve opens with a vacuum pump and the engine makes more than enough vacuum. I also run the 180 degree thermostat and a stock PCM tune...which leads to one cause for failure: The cat. I replaced the cat this week and, after seeing the catalyst in the new converter, I've realized there was next to none left in the old one. We'll see how the next emissions test goes...but not until I fix my leaking water pump driveshaft seal . One would like to think that a newly rebuilt motor wouldn't be plauged by oil leaks, but such is not the case. I also, unfortunately, can't do the 100 miles to clear the manufacturer's oil from the cat because the car is not street legal until it passes the emissions testing (I just bought it and can't even get plates for it until it passes). I'll have to run it in the driveway, but I don't like running it for a long period of time unloaded. Any guess as to how long I should run it for?

 

Well, I've got the oil leak fixed and the motor all back together...still no passing the emissions testing because now it barely runs. Bring on the troubleshooting!

 

After a new cat, new O2 sensors again (Bosch O2 sensors suck), a new EGR valve and several optispark re-installations, the car passed the emissions test with flying colors (and no AIR system). After 6 months of work, it's such an awesome feeling to actually drive the car. Wahooey!