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Failed smog, high HC levels

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Old 11-30-13, 10:57 PM   #1
nthach
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Default Failed smog, high HC levels

So my old reliable LS failed smog due to high HC levels. Went to a very good smog shop near my house, and we talked cars while my car was being smogged. It failed ASM 5015, passed(barely) ASM2025.

The 15MPH test registered 77PPM of HC, max is 71PPM. NOx was 15. 25MPH was at 52PPM of HC, just on the cusp. NOx was at 7. EGR seems to work fine. I changed the spark plugs, a few of the old ones were gray. I know earlier this year, I drove 5-6 miles on 4 cylinders due to a bad coil one on the right distributor dumping raw fuel into the cats. I told the smog guy this and he thinks its a possibility but the car is idling fine. He thinks one or more injectors are flaky or bum O2 sensors - I have universal Walker sensors installed from a few years back. PCV valve was working at last check. What else should I check?
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Old 12-01-13, 02:32 AM   #2
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Re-check the EGR valve and all that connect to it. When I've encountered high NO readings on other cars it eventually leads to some portion of the EGR is blocked/inoperative. That will also drop the HC readings to passing. Of course you'll have to re-check other important systems like the upstream O2 sensors.

If the car is dumping fuel (poor MPG) check the ECT and the vacuum lines to the intake plenum.
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Old 12-01-13, 06:20 AM   #3
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Quote:
I have universal Walker sensors installed from a few years back.
Use genuine Denso OEM sensors - they are cheap at amazon or rockauto, not a good plan to use no-name O2 sensor or any other aftermarket fuel injection part for that matter!

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PCV valve was working at last check. What else should I check?
The PCV valve costs like $5 - get a new one, it helps your car last longer, but that is probably not the cause of this failure


when was the engine coolant temp sensor last changed with a real Toyota one?

how you done a meticulous check for vacuum leaks?

what year is your car and have you downloaded/scanned the diagnostic codes? (there is not always a CEL lit)
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Old 12-01-13, 07:36 AM   #4
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damn LScowboyLS you are EVERYWHERE. I've been a member for a short time and have been pouring over the threads for the LS400. In almost every one where there is significant progress or in need of an actual answer, there you are being a boss and giving real feedback/solutions to an issue. Thank you.

As for the OP, I would start buy doing EXACTLY what LS said. Ruling out any and all fuel related components would be the first thing I would do.
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Old 12-01-13, 08:07 AM   #5
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I agree with RC40 recommendation to have a close look at the EGR system as well

other common causes of high HC besides vacuum leaks that I already mentioned are:

● carboned up plugs, bad caps or rotors or coils, or wires, with wires being more common than you would think. (if you have a 98 or newer, then obviously we can rules out plug wires, caps, and rotors since you don't have those.

● timing set incorrectly (though timing fine-tuning is controlled by the ECU) - like if someone intentionally fooled with the primary timing, so check it at the crank just to be sure with a timing light.

● a messed up catalytic converter is possible, but not likely, LS400 cats are the most bulletproof I have ever seen on a car. They take a lot of abuse/overheating
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Old 12-01-13, 08:19 AM   #6
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My NOx readings are well below the max level allowed for California, so the EGR is working. I replaced the ECT sensor with OEM a few months ago. I'll do a vacuum leak check and probe the O2s today. I suspect the cats are in OK shape, otherwise wouldn't the NOx and HC readings be off the scales too?

Car is a 91.

Last edited by nthach; 12-01-13 at 08:34 AM.
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Old 12-01-13, 10:25 AM   #7
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yes, EGR issues are normally associated with high NOx rather than high HC, however our EGR systems have had some issues, so it never hurts to have a look (for example as the source of a vacuum leak)

are your CO levels were high or low?
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Old 12-01-13, 11:49 AM   #8
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CO was within passing levels.
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Old 12-01-13, 11:50 AM   #9
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Could a cracked EGR tube also cause high HC levels due to air being sucked in?
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Old 12-02-13, 08:38 AM   #10
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So, I probed the O2 sensors with a DMM and the voltage readings were between .2-.4VAC at idle with TE1/E1 bridged, at 2-3K RPM it bumped up to .3-.4VAC. Each measurement was taken between VF1/VF2 and E1. I took readings off OX1/OX2 and E1 and the sensor values fluctated but didn't go anywhere above .5V.

I'm replacing those with OEM Denso, I did notice a drop in MPG. Spark plugs are brand new Denso platinums, I pulled those after a 20 mile drive and they look clean. The old ones had light gray deposits.
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Old 12-02-13, 08:40 AM   #11
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I had and still do have a cracked egr tube. But I failed emissions, slapped two new cats on, and passed with LOW numbers. Only ones over SPEC was HC though. The previous owner on mine drove the car an unknown miles with a busted rotor too.. once that was fixed car drive fine. As stated use denso sensors. Can get yhen for cheap at a parts warehouse. Just get the oem plug style though.. I'd be weary of the solder connections throwing off the ~1v outputs.
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Old 12-02-13, 09:01 AM   #12
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Quote:
Originally Posted by nthach View Post
So, I probed the O2 sensors with a DMM and the voltage readings were between .2-.4VAC at idle with TE1/E1 bridged, at 2-3K RPM it bumped up to .3-.4VAC. Each measurement was taken between VF1/VF2 and E1. I took readings off OX1/OX2 and E1 and the sensor values fluctated but didn't go anywhere above .5V.

I'm replacing those with OEM Denso, I did notice a drop in MPG. Spark plugs are brand new Denso platinums, I pulled those after a 20 mile drive and they look clean. The old ones had light gray deposits.
have you done the tests shown here and I still am not clear if you have dumped the codes as shown here

if your car is running bad enough to have high HC and you get NO DTC codes, then it may be time to look at the ECU caps as shown here
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Old 12-02-13, 10:20 AM   #13
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Quote:
Originally Posted by LScowboyLS View Post
have you done the tests shown here and I still am not clear if you have dumped the codes as shown here

if your car is running bad enough to have high HC and you get NO DTC codes, then it may be time to look at the ECU caps as shown here
Did that, but I'll put the car into diag mode tonight.

I know you mention bad caps, but I don't have the soldering skills to replace caps on a PCB.

/edit - I live near a "hacker" space and one friend is a member there. I can ask him to see if he knows someone good at PCB repair and I'll compensate the person fairly.

Last edited by nthach; 12-02-13 at 10:29 AM.
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Old 12-04-13, 09:22 AM   #14
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Old 12-04-13, 11:28 AM   #15
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Quote:
Originally Posted by nthach View Post
yes, it appears that your HC is quite high, while your CO and NOx are just fine

awaiting the results of the diagnostic code dump!

once you have dumped the codes, disconnect negative battery for a few hours to clear them and then drive the car around town and then dump the codes again, we need to see which codes come back as well as the ones that have been there.
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