Timing and NOx

[ijames]

Several months ago we discussed timing and emissions but of course, now I need some specific advice. I have an 88 Chevy K1500 truck with 5.0 L V8, TBI, and I failed emissions on the IM240 dyno. HC and CO are great (reading/limit of .3/2.4 and 8/80 respectively) but NOx was 4.1 on a limit of 3. The EGR is working (new valve and everything tests okay), new cat (NAPA, with about 3000 miles on it), but I failed . We did find that the distributor was set at 6 deg BTDC instead of at TDC with the ecm lead unplugged, and corrected that, but does anyone have a good idea of how much that will improve the NOx? I don't feel like paying for another test without some confidence that that was all it needed. Joe? Jim? Any input? Thanks in advance.

 

[gofstbuick]

Was the EGR valve working and passages clear before the first test? I just finished replacing an EGR valve (and intake gasket, felpro 1200 to factory) on my GN and NOx dropped from 582 and 286 to 77 and 22 for emissions test type 5015 and 2525. Did you install a GM EGR valve or aftermarket? I noticed that the non-factory PCV and EGR valves seem to affect emissions on I/M240 type test. I have seen timing changes make NOx fall, because the mixture may richen slightly, but cannot recall by how much.

 

[ijames]

Yes, the egr was working and passages clear. I gave the short version - I actually failed back in December at my first test. HC and CO about the same, NOx about 5. We tested the egr valve and it would not stay open with the engine running using a mity vac to open it. Engine off it was fine (and using a scan tool to cycle it with the key on engine off showed the control valve to be okay as well), but pull a vacuum to open the valve and then crank the engine and the valve sucked shut. We replaced it with an ac/delco valve, and looked down into the manifold when it was off and the passages looked okay. I have no idea about the pcv valve, who made it or how long it's been on. I was confident that the egr valve was the problem and so was really surprised when I failed the second test (my free retest). Now I have to pay to get tested again, which is why I'd like an idea about whether just retarding the timing back to where it was supposed to be will be enough. I can believe the chamber will be a good bit cooler with the lower timing, so the NOx shoujld be better, but I guess I just want some reassurance that it will drop 20% before forking over my $14 .

 

[gofstbuick]

I was also thinking, that if you can enrichen the mixture a little (not a whole lot), you NOx numbers will fall. How are the piston tops, maybe a top engine clean may be due. This may clean enough off the piston tops to lower compression and lower NOx.