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failed smog test - taylorandkylie - 02-14-2011 08:23 PM

My 89 Voyager has failed it's smog test. On the first test, it passed all emission requirements, but failed the pressure test of the EVAP system with a drop of .070 with a max of .040. I found a couple of bad hoses that I thought would take care of the problem, and got it to a .053, but still not to the .040 where I need to be so I am getting the system smoke tested to find the leak. More concerning to me though is the fact that I failed on NOx on the second test at 786 with a max of 613. On the first test the NOx were at 407, well within passing, but the HC was at 118 with a max of 123. I had heard that if you make sure the catalytic converter is good and hot that it may bring the HC down, so I made sure I drove at least 10 miles on the freeway before the second test, and the HC were down to 91 from the 118 on the first test. The guy at the smog shop said that the hotter the converter gets, the higher the NOx will get and recommended that I not drive as far when I bring it back in. Anyone have any thoughts on why such a big jump in the NOx? I forgot to mention that the first test was Saturday afternoon, and the second was Monday morning.

RE: failed smog test - Rupe - 02-14-2011 11:32 PM

You didn't mention which engine you have or what state you live in, which is a big variable for emissions testing. That said, a tired cat can be woken up a bit if this engine had adjustable timing. A few dergees retarded will clean up the HC and the NOx a few points without changing the CO part of the tail pipe test. Of course some old vac lines (leaking?) will also raise the HC and Nox so look there first.

On the evap part of the test we need to know if they tested the gas cap, which is what most states do unless there's a code in the ECM showing another problem. A bad cap is very common and a cheap fix if that is all you need. The canister purge valve is another common problem that is often overlooked.

Let us know how you make out.

RE: failed smog test - taylorandkylie - 02-15-2011 01:12 AM


Thanks for helping me out.

The van has a V6 3.0L engine and, unfortunately, we live in California (a smog nightmare). I guess I just do not understand how it passed without any problem on the NOx on one day and then failed so badly two days later. I will look for vacuum leaks for sure, but don't see how it changed so much in such a short period of time. Is there any truth to what the guy at the smog shop told me, in that with the engine being hotter during the second test than the first that it could cause the NOx to be higher?

On the evap part, it failed when they pressurized from the filler neck through the evap canister. They use their own cap with a fitting to allow them to pressurize the system, so I do not think that it is the cap. I had to replace the fuel pump a few months ago, and am thinking that it could be leaking around the seal for the pump or one of the rubber lines on top of the tank?

Any other insight will be greatly appreciated.

RE: failed smog test - Rupe - 02-15-2011 11:14 AM

Never having gone through a Calif. smog test myself I am not familiar with their routine other than knowing they don't miss too much. However, we are still back to basics and if everything is right then you should pass. I would double check your recent work on that fuel pump and the various clamps / hoses, do a visual inspection on any steel lines along the floor boards for rust, and possibly do a smoke test if you can't pin it down yourself.

Something else that came to mind on your tail pipe test is a hot exhaust is actually better because the cat works better when hot. OTOH, if the engine was running hot from that blast down the freeway perhaps the ECM was doing odd things to compensate. Take a close look at engine temp, t-stat, radiator issues, and vac lines again. Old vac lines can be soft (and collapse with the heat) or cracked and leak. This can change things dramatically. A partial restriction in the exhaust can also cause a hot engine and make you pull your hair out.

Keep us posted on what you find as it may help others.