I've read through many of the threads on emissions problems here but am looking for some additional insight. My Z28 failed a couple of days ago with MAC headers/(new) MAC Cat, new EGR tube, new NGK TR55 platinum plugs, HPP III program installed (just found it in storage after the test), 160* T-stat and Flowmaster cat-back.

Results are:
25mph 15mph
Stnd. Current Result Stnd. Current Result
HC 114 14 Pass 117 15 Pass
CO 0.63 0.02 Pass 0.65 0.02 Pass
NOx 796 625 Pass 879 1366 FAIL

I know from Injuneer and Shoebox's posts that high NOx is from non-operating EGR, lean A/F, too much spark advance or another cause of high combustion temps. If I go back to the stock tune and replace the EGR (it's old and has been non-functional due to block off plates on header/intake) would that be enough to get the NOx under control? I don't have any scan or tuning software yet so I'm looking for an educated guess/suggestion to get the NOx to pass. I even bought a stock T-stat (195*?) but if everything is good but the NOx with the 160*, I'm thinking it would elevate NOx by increasing engine and therefore combustion temps if I switched to it. I know the HPP III adds some spark advance but have no idea how it affects A/F ratios.

I'm open to suggestions.

 

Stock t'stat for an LT1 is 180*F. Raising the coolant temperature is going in the wrong direction. That's one of the things increases NOx.

With the very low HC and CO, it may be running on the lean side.

 

Thanks Injuneer, it's been a while since I put the 160* t-stat in and confused the stock temp with another vehicle I had. I'm not going to change out the t-stat since doing so would just make the NOx worse with higher temps. I only have the HPP III at this time, no cable or tuning/scanning software yet so I'm not yet able to tweak the A/F ratio.

I'm going to pull the Hypertech tune now that I found the unit. I did have a bad #7 plug wire but don't think it was missing noticeably during the test. It did start misfiring badly on the way home. That's been taken care of and now I'm trying to make sure the EGR is working properly. It will bog at idle when manually depressing the diaphragm on the EGR, but I don't have a vacuum pump to test the diaphragm itself. I disabled the EGR years ago by capping the connections of the EGR tube at the header and intake - I didn't pull the EGR valve and cap that. Just blocked off the source of exhaust gas to the unit.

Do you think disabling the knock sensor (unplugging it) will pull enough timing to cool off the cylinder temps to lower the NOx to pass or will it cause a code that will fail it again? This is the first I've had to deal with emissions testing (just moved to Texas) so I'm trying to hedge my bet for the free re-test.

Thanks
Kevin
Z28nut

 

I don't think pulling timing is going to significantly lower the combustion chamber temperature. That, I beleive, is primarily a function of the A/F ratio and it takes a modification of that to lower the temps and reduce the amount of NOx. Unblock the EGR and I think the NOx will fall quickly into line.

Good luck!

 

bw_hunter - That test was done after I put on the new EGR pipe. I just picked up a vac pump to test the EGR valve and solenoid for proper function and with revert back to the factory tune for the retest. I'm going to get a scan tool but won't have it before the 15 days are up for the retest. Do you think the Hypertech tune is leaning out the A/F ratio? I know it advances the timing some which will affect the combustion temps.