For those struggling to Pass Smog with their 280z

[Ownallday]

As you can read from the title, I wanted to make a quick write up on how I got my Z to pass smog with flying colors and something that can help anyone diagnose why their car isn't. There is not a lot of write up on this either.

I have a 1978 Datsun 280z. I ended up buying a second L28 with N42 block and N47 head (proper block and head for 78 280z). I performed a full rebuild on the engine along with restoring or replacing everything I can on the intake manifold. When it came time to smog the car, It failed due to high CO (Measured 1.84 on 25mph, max is 1.16) and HC was really close to being above the max also and NOx was practically 0 indicating my chambers are running very cold. It surprised me considering it was basically a new engine with new parts. Aside from the engine rebuild, I had new injectors, new AFM, all new sensors on the thermostat, new fuel filter, new hoses and lines, new fuel pump. new air filter, new Spark plugs, new cap and rotor, new spark plug wires, new egr valve and new Carb compliant catalytic converter from magnaflow and fresh 91 octane gas. Timing set at 10 BTDC with idle ~800rpm. Almost all the wiring has been redone with new connectors or cleaned connectors.

The steps I took in diagnosing why the car didn't pass started first with making sure my valves were properly adjusted again, these damn valves like to go out of spec pretty quickly from my experience. Minor improvement was saw after doing a valve adjustment again but it was very minor.

I checked all my spark plugs and saw a near perfect brown color that you want to see from spark plugs. My results indicate my car is running rich but the spark plugs say otherwise. California has very little wiggle room for cars running rich even if just slightly rich.

Tested water temp switch resistance and resistance came back indicating my water temp was fine with ~290 ohms at 280 degrees.

Inspected my Cold start injector to see for leaks when the fuel system was pressurized and also checked to see if it was firing when the engine was warm and saw that the cold start valve was working properly.

Inspected all my injectors for leaks even though they were new. With a boroscope and the fuel system pressurized i stuck the camera into each cylinder and looked for fuel seeping. All my new injectors checked out and were good with no leaks.

Ordered a fuel pressure gauge and found my pressure was right at 30-32psi at idle, a little low from the fsm 36 psi but imo was not a big deal since the car is supposedly getting too much fuel anyways. I saw PSI rise to 36 psi when revving the engine which is normal. To check if the fuel pressure regulator is working I removed the vacuum hose going to the fpr and saw a jump from 32psi to ~38-40psi indicating the fpr is good.

THE SOLUTION FOR MY Z:

Fuel was out of the question for me at this point and decided it was time to tinker with the AFM. I recommend doing this as the last step if all else fails. There are other things you can test such as the tps, afm itself, the ecu and so on! I removed my new AFM and reinstalled an old afm I already had that has already had the box open (didn't want to tinker with my new one). There is a good write up online about how to adjust the afm. There is a white gear (could be a different color depending on your afm) that has a spring on the inside. If your AFM has not been tampered with or has no adjustment marks, I would put some markings so you remember where the teeth originally were incase something gets messed up. This gear can be moved in one direction or another that essentially makes the car run richer or leaner. Do not mess with this without reading up on how to properly adjust it, there is tension on the spring! I WOULD HIGHLY RECOMMEND that you find a smog shop that has a smog tech that is willing to constantly run the car on the dyno and get emission results while constantly adjusting the afm at the same time. This is essentially what we did. We got a baseline results on the dyno, after seeing the results were still rich, we turned the car off and I adjusted the afm a couple teeth leaner, tightened the adjustment screw and re-ran the dyno until it was eventually good and passed with flying colors! You may also need to adjust your idle depending how much afm adjustment you do. After you pass, drive the car for a about 50 miles or so and recheck your spark plugs and make sure you are not running too lean! My spark plugs were still a nice brown indicating everything is where it should be for California emissions and the car itself but I will still be swapping back to my new AFM since these cars like to run slightly richer. Heres my results. Didn't think a Z could have such low CO and HC.

Something I noticed after passing this test was my measured 15mph NOx jumped to 862 (before was at nearly 0) which means that running leaner and reducing the CO and HC will most likely in return cause higher NOx (essentially hot polluted air). You will have to find a fine line when adjusting the AFM so everything is equal. If you choose to adjust the AFM, I think it would be beneficial to have an older one for adjusting just for smog reasons and swapping back to a newer one for daily driving which is what I did, otherwise make your markings so you know where to readjust after smog.

Extra things to test and look out for:

Some other things that you can also check on is the overall state of the engine by doing a compression test. Low compression usually causes an engine to run rich. If you have dying cylinders there is only so much adjusting the afm and everything else can do.

You can add the option of putting a rheostat inline with the water temp switch and essentially trick to water temp switch to run leaner sooner or vise versa depending on the condition of your results. I am not sure if California smog techs will question this or not though during visual inspection.

Change your catalytic converter. Depending on how old your catalytic converter is and how many miles have been put on it, it's possible that it is not performing at it's peak. You can test the Cat by using a temp gun and making sure the cat is hotter than the pipe before and after the cat, would indicate the cat is working properly. However, overtime a catalytic converter does slowly deteriorate and become worse at doing what it's suppose to do.

Spark plug gaps can also play a roll in NOx emissions as the wider the gap the more NOx emissions produced. The smaller the gap can lead to unburnt fuel and cause a richer condition.

Next thing you can check for is the TPS. There are multiple write ups on how to properly adjust the TPS. While it can make a difference to your emissions results, I don't think adjustment will make much of a difference unless yours is severely out of spec.

You can test the EGR valve. There is a write up on how to properly test it.

Change your oil is another thing that gets looked over. Fresh oil usually makes a pretty small change to the results depending how old the oil is.

 

Hopefully for those you struggling, this write up gave you some insight on what you can do to hopefully pass your smog test. Some guys will tell you not to mess with the AFM and I do agree that it should be a last resort. If you have two working AFM's do what I said above and use one just for smog and use the other for daily driving. If most things are working as they should the AFM has the biggest impact on how the car does emissions wise but will also drastically change how the car drives if your car actually is not in good running condition to begin with! Good luck!