Questions and Answers

Hardbody NOx Q. I'm having an emission problem. My truck is a 1991 Nissan Hardbody (D21), extended cab, V-6, 4 wheel drive, fuel injected, 96,000 miles, manual transmission, P/S, P/B, Cruise.

I have installed Thorley headers, hi-flow cat (one month old), high flow muffler. The oxygen sensor, distributor cap, rotor, wires and plugs are less that a year old.

I failed my California emission test with over 3000 ppm NOx at both 15 and 25 mph. All other measurements are well within standards. I have replaced my EGR and EGR solenoid valve. I also found one vacuum line on the rear of the intake manifold with a hole in it and replaced it. The valve on the rear drivers-side of the intake manifold that allows air to bypass the throttle during cold starts (can't remember the name) would not close all the way and was temporarily modified to allow it to close.

Lifting the EGR manifold by hand causes the engine to slow and stumble (from idle position). Bypassing the EGR solenoid valve has not effect. Blocking off line to evap canister has no effect. EGR will temporarily open when going from no throttle to high throttle and then close as rpms increase (open for approximately one second). The intake manifold seams to have good vacuum, but the two vacuum lines that go to the EGR solenoid valve have almost none. I have checked all EGR vacuum lines for blockage, damage.

I removed my K&N filter and replaced it with a semi-dirty Fram with no effect. I can not find any vacuum leaks. My truck is not equipped with a back-pressure sensor, but I did try to partially obstruct the exhaust coming from the tail pipe with no effect. If the AAC valve was stuck open, would this effect my vacuum leading to the EGR? My truck idles fine and the idle increases when the A/C is turned on or when cold.

When the dealer had my truck on the tester and up to 15 mph, they lifted the EGR using a hand vacuum pump and the NOx dropped down to 167 ppm. My NOx two years ago, would not have passed the new standards for this year. I have already spent over $500.00 and my deadline for the SMOG check is the 28th. PLEASE HELP!!!

Thank you.

A. I think the problem is with the new exhaust system you put on. You have reduced the back pressure so much, there is not enough to properly operate the EGR system. The fact that the open EGR valve, at idle, only stumbles the engine when it should, quite effectively, kill the engine also indicates reduced back pressure. At this point the only way I can see to restore proper EGR function is to remove the custom exhaust and install the stock exhaust.

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