1bad94z28

Hi Guys,

I have a 94 Z28 and it is Smog time here in California, I failed for high NOX only, it is in the 1100 ppm range when it needs to be less than 450 ish. I have to do test only on the dyno, and it fails at both 15 and 25 MPH. It has passed smog multiple times before and is kept in tip top shape. For the other Smog test parameters it passes with flying colors, NOX is holding me back!

Now for the info, the Air pump flat out does not turn on, and I get an intermitent DTC code 29. I can pull the relay and jump the terminals and it turns on. The relay is good, the fuse is good as well. The car has new O2's, new cat, new oil, clean air filter, new opti, new plugs and wires, and a new battery. I pulled EGR off motor and cleaned it and the passage ways, and the motor is kept in great shape as stated earlier. My performance and gas mileage have not decreased and the car runs great!

I used the search function and cannot find anything close to my Air Pump problem.

Any suggestions/ Help is appreciated!

Happy New Year!!

RamAir95TA

Any mods?

1bad94z28

Yes, K&N CAI, Flowmaster exhaust, Edelbrock shorty headers, LT4 knock module, all this stuff has been on the car for 9 years and the car has been smogged 4 times and no probs before. I am thinking the Air pump is not putting O2 in the exhaust and this is creating high NOX? What do you think? I could jump the pump and smog it, but I want to fix it right.

Thanks for the help!

RamAir95TA

The AIR pump only runs for 3 or 4 minutes on cold start and then is not used, so that is not your problem.

What were your CO, HC, and CO2 readings? They can help pinpoint your issue.

1bad94z28

At 15 MPH
RPM 1544, CO2 14.5, 02% 0.5, HC 14, CO .01, NOX 1180

At 25 MPH
RPM 1504, CO2 14.6, HC 9, CO .00, NOX 1082

Any other info you need please let me know.

I was reading in my Haynes Manual that the pump runs and shuts off above 2850 RPMs, is this incorrect?

Injuneer

As already explained.... the AIR pump runs at startup to help the cat heat up faster. It serves no other purpose. Its not like the 3rd Gens, where the AIR pump flows into the exhaust manifolds at startup, than then switches to the connection on the cat, between the reducing and oxidizing beds. The 4th Gens use a monolithic cat, that does not need AIR injection. AIR has NOTHING to do with NOx in the 4th Gens.

Either your engine is running exceptionally lean, exceptionally hot, or the EGR is not actually working.

Have you verified the vacuum lines for the EGR system are not leaking? Have you verified the EGR vacuum solenoid is opening when commanded? Have you verified that the EGR valve actually opens when the vacuum solenoid actuates? At idle, have you tried to force the EGR valve open manually, to verify there is actually EGR flow when the valve opens (the engine will stumble if there is flow at idle)?

1bad94z28

Thank you for the help on the AIR pump part of this, the EGR lines are good, and when I manually push the diaphram the engine stumbles and then smooths out as the diaphram expands back out, I will have to get the scanner to check if the egr is being commanded and opening as it should.
Anything else I should check in the mean time?

Thank you for the help guys!!

Injuneer

We just ran through this with someone, and we finally figured out his PCM had been reprogrammed to prevent the EGR system from activating. He reset to stock, and sorted out the problem. Any chance your PCM has been reprogrammed?

1bad94z28

Not to my knowledge, under my ownership I have never had the PCM tuned or adjusted/ modified at all. As far as I know it is running the stock program. Also I have passed smog many times before and never been close to failing, I believe after reasearching my problem with your guys help it will be something that has worn out or degraded with age.

Thank you again for the help!

I will follow up on this post when I get any new info!

1bad94z28

One more question guys, I will check my Air Pump for operation on cold start up, if it does not work then, will my cat still reach its optimum temperture to be most efficient, and clean the exhaust as much as possible? Also long term if my Air pump does not work will this damage my cat? Just asking as this will effect my Smog readings, thank you!

Guest47904

The air pump will not damage the cat by itself. It merely gets it to working temperature faster. So the cat will work correctly eventually once it reaches full temperature. Which would have been the case on a dyno.

As was probably listed before, the causes of hi NOx are lean AFR, poor operating EGR, bad cat, excessive carbon in comb. chamber, misfires and a few other small issues too.

If the EGR was not functioning properly, we would expect a code for that. Since the exhaust response is monitored for the effect of the EGR.

If the engine has ever been running lean bad enough. It could have damaged the cat. You have replaced the cat. But the engine could still be running lean. It would be a good idea to have the injectors and fuel rails cleaned on the engine with an injector cleaner such as the one made by 3M. It is connected to the fuel rail and the engine runs only on the can of injector cleaner. Many independent shops have the hardware to do this. Once the can of cleaner runs out, the engine shuts down and you're fuel system is clean.

I also did not see where you replaced the fuel filter lately. Good idea to do so as well.

As far as combustion chamber deposits are concerned. This is the finest cleaner there is. http://www.bgprod.com/products/fuelair.html.

Since it relatively inexpensive to do both the fuel system and combustion chamber. That's my starting point.

Injuneer

In the 94/OBD-I (and in OBD-II), EGR flow is verified by looking at the effect on MAP when cycling the EGR valve. The OBD-I test is extremely insensitive, with maybe 3/4 of the people who delete EGR never getting a code for no flow.

1bad94z28

Thank you guys! I will look into these!

chevyss_98

100% sure that your EGR isnt even partially working with those readings, and your air pump is set not to come on.


Someone has turned them off in the PCM.

trust me, ive seen peoples readings here in Canada with them off

Very simple to do, Ive already done it to mine plus some other things.

If you can find someone with some programming software + cable (or just buy them) then you can fix this yourself without much trouble

I use TunerCat myself plus the ALDL cable.

1bad94z28

Thanks for the input. I just find your idea hard to believe since I have smogged this car 4 times before without any issues. I will look into this thank you for the help!