Failed emissions

[LEMHEAD16]

QofDoom failed emissions today Kind of at a lose where to start on it. Changed the oil last night, checked for codes with ECUtalk and got nothing. Car runs like a champ.

Passed all tests except HC at idle. 324 was my number 220 is max allowable.

I pulled a plug and it looked fine.

Anybody got ideas?

[kevindanielk]

My Q failed emissions once because of a large hole in a vacuum hose(the short elbow hose directly under the throttle body). Shop said a lean condition would cause it to fail.

[maxnix]

EGR?

[hk20000]

Did you run the car hot enough before you went in?

HC at idle sounds like rich mixture, like someone said already, EGR?

[BadQ45t]

When you get an older car that isn't passing smog you have to find a shop that has all the smog equipment and the ability to diagnose problems. I live in California and LA and SF Bay Area both have tough smog laws. I have a 84' 300ZX that I take back to LA for tests and they can run a "dry run" test, figure out the issue, re-run the test (while NOT connected to the DMV). You hope it is the EGR and not Cats, but that is pretty far out of whack on the HC's so probably nothing simple...I will now drive my car all the way back to LA if I have a non-pass on the Z, but in the last 15 years I have replaced 02 sensors, EGR x2, and cat converter.

Finding it on your own is going to be very tought and the HC being that high is very bad. If I recall when I had my 97' last smogged that number was sub 100, so your's is really out of whack.

[Unnatural1]

What are the other numbers? CO, O2 and NOx specifically?

The EGR is not the issue as it does nothing to control HC's. HC's are raw unburned fuel. If the other numbers are high (but obviously still under the limit) I would suspect that you're in need of catalytic converters. A rich condition will also cause excessive HC's but you will generally see high CO numbers too. Also, you will typically notice the plugs have a heavy layer of carbon resulting in a black appearing electrode.

The car is running like a champ? Engine is smooth, no misfires?

[LEMHEAD16]

Car runs great.

All other numbers are in the passing zone by large margins. It's just Idle HC thats killing me. 235,000 miles on the car so I wouldn't doubt that the cats are bad. It does have a full custom exhaust that deleted the EGR, but I don't think that is what is causing it.

[Q451990]

It's just Idle HC thats killing me. 235,000 miles on the car so I wouldn't doubt that the cats are bad. It does have a full custom exhaust that deleted the EGR, but I don't think that is what is causing it.

I wouldn't think the deleted EGR is part of the equation, since it doesn't open at idle anyway... Has it passed emissions with this setup in the past?

Heath

[Haitian_King]

Hey Lemhead16 -

I also just failed emissions in NJ for HC during Idle & High Idle. I think my problem is that I left the ECU in Mad Max mode and forgot to change to stock before the test. I can't redo the inspection at a State facility since they require documentation of repairs. My repair would be pushing a button...Anywho, I hope you can get to the bottom of your issue.

[Q451990]

Could show them a receipt for a set of oxygen sensors (that you didn't actually install) from a parts store - then return the sensors?

Heath

[GSUjinih]

I might try running 89 octane fuel instead of premium. I doubt in factory trim these cars are taking full advantage of that extra octane (93 octane in GA) and having a slightly incomplete burn, but not enough to show up on the plugs. Also, they sell this stuff at the parts store that they say you will pass emissions or your money back!

[Q451990]

I might try running 89 octane fuel instead of premium. I doubt in factory trim these cars are taking full advantage of that extra octane (93 octane in GA) and having a slightly incomplete burn, but not enough to show up on the plugs. Also, they sell this stuff at the parts store that they say you will pass emissions or your money back!

The Q is actually suffering with today's 93 octane crap gas vs. what was available in 1988 when it was designed. Burning a lower octane fuel will cause detonation, and when that's detected by the knock sensors - put the ECU in a "safe mode" in terms of the timing advance - causing it to enrich the fuel mixture. So unfortunately that would make a rich condition worse...

This brings up another thought... if the knock sensors are not functioning, the ECU is already using the "safe" timing map, so that might be something to check.

Heath

[Rex]

Car runs great.

All other numbers are in the passing zone by large margins. It's just Idle HC thats killing me. 235,000 miles on the car so I wouldn't doubt that the cats are bad. It does have a full custom exhaust that deleted the EGR, but I don't think that is what is causing it.

Cats are inexpensive aftermarket units. I bought the car from rswicki with no cats, I had inexpensive ones added when I sold it to MoD, as he's in an emissions inspection area and needed to pass both sniff and visual.

Look for another set of $100 cats.

[GSUjinih]

I might try running 89 octane fuel instead of premium. I doubt in factory trim these cars are taking full advantage of that extra octane (93 octane in GA) and having a slightly incomplete burn, but not enough to show up on the plugs. Also, they sell this stuff at the parts store that they say you will pass emissions or your money back!

The Q is actually suffering with today's 93 octane crap gas vs. what was available in 1988 when it was designed. Burning a lower octane fuel will cause detonation, and when that's detected by the knock sensors - put the ECU in a "safe mode" in terms of the timing advance - causing it to enrich the fuel mixture. So unfortunately that would make a rich condition worse...

This brings up another thought... if the knock sensors are not functioning, the ECU is already using the "safe" timing map, so that might be something to check.

Heath

Not true, at lower throttle and RPM you do not need this extra octane. I was averaging 22MPG with 50/50 mixed driving on 93 octane fuel. I have been using 89 octane fuel for the last few tanks (due to the crazy price hike from midgrade to premium) and actual saw gas mileage go up to 23MPG. This was using the same gas station at the same pump, even at about the same time of day with nearly identical trips. I will log my ignition timing and check to see if it is backing off at all.

Also I am pretty sure all knock sensors do is retard the timing, it does not change the pulse width on the fuel injectors. However you will get less power/fuel ratio with this retarded timing. So this does not cause the car to run richer, but you might have to use more throttle for the same acceleration which would increase the pulse width and use more fuel.

[BadQ45t]

Plenty of people have tried to use lower octane gas with Q's through the years. It never ends well, but given your cars advanced age who really gives a crap anymore. No way the car runs better on lower octane, but you can keep thinking that

[GSUjinih]

I did not say that it ran better, just that with my conservative driving style, that my gas mileage was slightly increased. With WOT and high RPM you will need the high octane, but for how I drive most of the time I really do not need it. Also, pump numbers is an AVERAGE of two values. So, an 89 rating may have the same exact RON number or better as 93 octane, but of course the MON numbers will be different. If your ECU can not advance you timing anymore than with a higher octane fuel you can get a reduction of efficiency compared to an octane of fuel that allow just enough for maximum advance. Extra octane not use can lead to carbon build up also.

My 1993 240SX coupe averages 31-33MPG with mixed driving using 87 octane fuel and I have got as high as 36MPG on a highway trip. On a hot day I can occasionally feel the timing back off some, but not too bad. The car still runs the 1/8th mile as good as published magazine times with just 87 octane fuel though they say premium is recommended too. However, if I blasted my car up to redline every shift, I would probably be running premium all the time.

[Haitian_King]

So the extra $.10-.20 you save by using 89 octane is worth that extra 1mpg? Since I never know exactly when I'll need to stomp the pedal & go WOT (which happens a lot), I'll keep going with 93. (94 when I can get it.)

*edit*
BTW, I passed emissions the 2nd time around. You know, just in case anyone was wondering...

[GSUjinih]

Around here there is more than a 10 cent difference between midgrade and premium, sometimes up to 40 cents difference. My advice was actually for the OP.

[GSUjinih]

I might try running 89 octane fuel instead of premium. I doubt in factory trim these cars are taking full advantage of that extra octane (93 octane in GA) and having a slightly incomplete burn, but not enough to show up on the plugs. Also, they sell this stuff at the parts store that they say you will pass emissions or your money back!

The Q is actually suffering with today's 93 octane crap gas vs. what was available in 1988 when it was designed. Burning a lower octane fuel will cause detonation, and when that's detected by the knock sensors - put the ECU in a "safe mode" in terms of the timing advance - causing it to enrich the fuel mixture. So unfortunately that would make a rich condition worse...


Heath

If you think 93 octane is crap gas then you must be getting your gas from the wrong station! Do they not sell JWT and NICO ECUs that bump up the timing and run totally fine off 91 octane in most conditions? I still am not sold on the car enriching the fuel mixture based on knock sensor input.

[Haitian_King]

Around here there is more than a 10 cent difference between midgrade and premium, sometimes up to 40 cents difference. My advice was actually for the OP.

Really? Damn! Around my way 87 is $3.17 (today) & 92/3 is $3.43. The extra $.26 is totally worth the peace of mind I get . I don't even miss it.

[GSUjinih]

There is a gas station called Loves where midgrade is only 10 cents more and premium is 10 cents more than that. However one of the guys at work who goes there got a check engine light and had some sensors go out on his car. LOL He drove around illegally for a while since he could not get his car to pass emissions until he replaced several sensors. So yes there are some places that do not charge too much for premium, but not sure if it is worth it if it has a bunch of water and other crap in it! I imagine with our sensitive injectors, you should always use a higher quality gas like Chevron.