1993 240sx Manual Failed California Smog from High NOx.

[zer0c123]

I ran a pretest, where the tech used the manual diagnostic mode and it exceed Nox 773 || 493 ppm (measured || max) at 15 mph. This was after a drove from Los Angeles to San Francisco.

The next day I ran 2 gallons of denatured alcohol in about ¼ tank of gas and failed the real test for Nox at 15mph 1225 || 493 ppm (measured || max).

I understand the EGR or a vacuum leak is usually the problem. The weird thing is that I replaced ALL the egr (egr valve, egr-bpt, bpt-tube, egr-pipe, and all gaskets) components back in 2005; however it could be a mechanical failure of one of the components instead of carbon build up.

-I don't believe the CAT is the issue, because I passed all the other gases with low numbers.

-So far I have pushed up on the diaphragm to the egr and the car stumbles and almost dies.

- i checked the 90 degree hose that connects the egr and bpt, which looked fine.

-I pulled the diagnostic codes and code 34 popped up, which is the knock sensor. I am unsure whether a malfunctioning knock sensor can cause high Nox; however, google searches say that a malfunctioning egr can trigger a knock sensor code. Can anyone confirm this?




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[yukon]

yank on the throttle cable and feel if egr diaphragm goes up

[zer0c123]

Yea the valve moves when I mess with the throttle cable.

update on my BPT testing.

I followed the procedure to test the BPT on the 240sx.org tech page

1. On one of the top two ports, attach a vacuum pump
2. Plug the other top port
3. Gently blow into the bottom port

The pressure gauge held constant when I did the above steps. Now here's the thing, I stopped blowing air on the bottom port, but still held 1 of the top ports closed and the other port plugged with the vacuum tester. The instructions said that air should leak out, but once I stopped blowing air, the pressure gauge immediately went to zero. That does not look like "leaking" to me. It somewhat makes sense to me because it's supposed to back pressure, so it should hold a pressure and gently leak. right?

My current BPT is about 5 years old. I have an old BPT that passed the smog test 5 years ago. On the old BPT, using the procedure in the first post, the air leaks out very slowly once I take my mouth out of the bottom port and stop blowing air.

Probably going to look for vacuum leaks and exhaust leaks before the cat now.

[zer0c123]

ALright, I compared both EGR-BPT valves and revved the engine a few times after the car was properly warmed up.

5 year old EGR-BPT-- The egr diaphragm seems restricited. The diaphragm seems to move but the displacement is much; however, I noticed even at higher rpm's it's hard to see the difference in displacement.

Older EGR-BPT - The egr diaphragm moved alot and I could see the difference in displacement at different rpms.

After observing the tests, I put in the older EGR-BPT and went for the smog test. The 15 mph Nox emissions were 154 ppm and the 25 mph was 469 ppm. Overall I passed the emissions portion of the test, but i FAILED functional. They measured my timing, which yielded 9 BTDC?!! I was like wtf!!! The smog tech asked me what the hell did I do to my car. This totally does not make any sense to me. It has only been 3 days since my last smog check and the timing was at 20 BTDC. I NEVER TOUCHED THE DISTRIBUTOR between the 3 days.

I immediately went to a mechanic that I trusted and he checked my timing and confirmed that it was at 9 BTDC. He attempted to advance my timing back to 20, but was only able to do 23 BTDC. He said that my timing was fluctuating when he was aiming the timing light. He said that maybe the bearing in my distributor is bad or maybe my timing chain got stretched since the last smog check. Also he commented that I might have a slight vacuum leak since my 25 mph NOx reading was borderline.

Since he readjusted my timing to 23 BTDC, that probably means my NOx readings are going to increase right? This does not look good for passing...

Any help would be appreciated. A vacuum leak is probably the last thing I can think of.

Here is image of all my smog tests compiled.



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[corn322]

Any mods? What spark plugs are you running?

Can you run it with the heater on full blast? Try to get a little heat out of the engine.

[zer0c123]

No engine mods other than AEM short ram intake. Does running the heater full blast prior to smog help? Also running NGK plugs and wires.

Anyway I finally passed freaking smog today; however I am still worried about the NOx numbers. There has be a freaking vacuum leak somewhere that I can't find.... Luckily, the guys at the test center were kind enough to give me 2 free retests. Also I am still confused how my timing was off by 11 degrees in 3days, where I never touched the distributor.

After reviewing the last 4 test numbers, I must say the numbers fluctuate all over the damn place. I believe the following were the factors affecting the emissions,

-Different EGR-BPT, Driving warm-up time, driving warm-up speed, driving warm-up RPM, gasoline brand, and octane.

Image of smog numbers at the very bottom.

Since rarely ANYONE on this forum tells you what they did to fix smog (closure), I'll type up all the stuff I did for each smog check. For your reference and for mine in case I run into the problem for my next check.

Last Years test:
Tune-up: Oil change, filter, and gas filter.
-5 year old EGR, EGR gasket, EGR-BPT, EGR passage gasket, BPT pipe, BPT hose, EGR pipe.
-Drove a total of 28.8 miles before the test
-added 2 gallons of denatured alcohol to 1/4 tank of gas (chevron supreme 91)
-after adding alcohol about 5 miles on the highway to the smog station

I did no tune-up for the next 4 tests.

Test 1 :Pretest:
-Drove from Los Angeles to San Francisco at fast speed
-stopped at home briefly and then went immediately to test center
-Running Cheveron Supreme 91
-no alcohol added
-Car sat for about 5 minutes with iginition off before test.

Test 2:
-added 2 gallons of denatured alcohol to 1/4 tank of gas (chevron supreme 91)
-Drove back and forth on a mountain road with avg speed of 50 mph for about 30 miles.
-Then drove ~10 miles to test station at highway speeds
-filled up with arco 87 to dilute the alcohol. Didn't want to dilute the alcohol with good gas =/
-Tested immediately upon arrival.

Test 3:
-Still running Arco 87 with the diluted alcohol
-Noticed Knock sensor code from ecu
-Timing unknown to me at the time was set at 9 BTDC for some reason
-Replace the Egr-BPT valve with an old one (see above posts regarding my observations)
-Drove back forth on a mountain road with avg speed of 50 mph for 15 miles
-Then drove ~10 miles to test station at highway speeds
-Tested immediately upon arrival.

Test 4
-Adjusted timing to 23 BTDC
-Knock sensor code from ecu still present
-Still running Arco 87 with the diluted alcohol
-Drove at highway speeds for 21.2 miles to the next city
-Immediately turned around and drove 26.2 miles to the test center at highway speed.
-Car sat in the waiting area with ignition off for about 5 minutes.

Smog numbers



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[corn322]

Figuring out emissions problems is always a blast.

Running the heater at full blast during the inspection might help. NOx is created by high heat in the combustion chamber, like when you're cruising and the ECU tries to stay at 14.7AFR. The heater is just another radiator, so in theory it should move heat out of the engine.

The rest of the numbers look good. Did you check the hoses for the PCV system? But your O2 readings look good, so I don't think there's a lot of unmetered air coming in.

I need to put my EGR stuff back on so I can get a smog check. Keep us updated.

[zer0c123]

Figuring out emissions problems is always a blast.
Running the heater at full blast during the inspection might help. NOx is created by high heat in the combustion chamber, like when you're cruising and the ECU tries to stay at 14.7AFR. The heater is just another radiator, so in theory it should move heat out of the engine.

The rest of the numbers look good. Did you check the hoses for the PCV system? But your O2 readings look good, so I don't think there's a lot of unmetered air coming in.

I need to put my EGR stuff back on so I can get a smog check. Keep us updated.

I haven't PCV system or hoses yet, but do know the PCV valve is a year old oem replacement. I got under my car trying to find a hole in the exhaust before the CAT; however my exhaust system is still stock. Therefore, the extra metal heat shield wrapping the system is still present. There is some deterioration in the heat shield, like cracks, chips, and holes, but the actual piping seems to be fine by my visual inspection.

I need to put the hunt for a vacuum leak on hold, because I'm trying to diagnose why my timing was off my 11 degrees to read 9 BTDC for the test. I'm pretty sure now that this is related to the knock sensor code and me using a full tank of lower octane fuel, Arco 87.

If you have vacuum pump, any chance you can test your EGR-BPT valve. I want to know if yours behaves the same way as what I saw when I tested the 2 BPT's that I have. See this post for my observations

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