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EG Final Year
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Discussion Starter #1
I thought I would post this here seeing if anyone could help me with diagnosing why my motor is creating high NoX readings. This is on a D15B7 motor with about 170,000KM or roughly a bit over 105,000 miles.

I've already passed emissions recently and It wasn't fun. Since the motor runs somewhat fine I thought I would look into possible causes of the high NoX reading.

I had a new cat installed. Checked the timing and it was set properly, the car doesn't overheat and I don't have any smoking or coolant burning problems.

Compression is solid all the way across and the motor burns almost no oil or so little that I can't tell.

1. Could a bubble in my cooling system perhaps cause the motor to run hot but not overheat ever?
2. Should I maybe replace my O2 sensor? Although I get 550-650km a tank.
3. My idle does fluctuate slightly and everything has been reverted to bone stock on the car intake and header.

I'm leaning towards an air bubble hopefully or the O2 sensor.

Thanks in advance I'm heading back to school this Tuesday and just wan't to make sure my car is 100% reliable since I would hate to be stranded before a test or something.
 

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Brokedick Millionaire
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40,237 Posts
In Ohio, they didn't check 95 and back for NOx. Because with better burning and more efficient NOx goes up.

Next thing you know CO2 is be labelled a bad emission, therefore you can't exhale.
 

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EG Final Year
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Discussion Starter #3 (Edited)
I'm worried about passing my next emissions test since It would be done within a year from when I just did it. I ended up passing on my 4th attempt which was advancing the dizzy and running the engine completely cold I had 625 and the limit was 632 I believe.

Never ever have I had a single issue with this motor passing. Everything would be way under the limit only happened this time around.

I can find the full results for tomorrow morning if need be. Bad O2 or a slow one causing the ecu to run lean therefore increasing the flame front temps? I have a AEM wideband but the stock header I would have to install a bung to hook it up and I really don't feel like doing that right now.
 

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Ferio inspired
90' Civic sedan
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2,441 Posts
do you have a lot of carbon buildup on the pistons? these are like coals, and can preignite fuel to cause an advanced timing like combustion.
 

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EG Final Year
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Discussion Starter #5
I would have no idea about the amount of carbon build up. I'm actually thinking of seafoaming the motor this weekend perhaps and I'm going to check for any exhaust leaks.
 

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98 civic ex coupe
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938 Posts
What are your other readings like? In CT local shops do the emissions testing so I have some experience with diagnosing these problems. Here we check for Nox, HC and CO. Do you just having a running test on a dyno at a set speed like 25mph or do you have a two speed idle test. Some states or country's test not only cruising emissions but also idle and say 2500rpm.
 

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Adia's Daddy
2004 Honda Element
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3,364 Posts
What kind of converter do you have and what condition is it in?

Reason I as is even though we're not a Drive Clean facility at my dealership, we do go to training to be able to diagnose some emission-related failures. Case in point...A friend of mine came in one time with his '91 Accord. It was the stock engine with the basic intake, header and exhaust bolt-ons and he could not pass on NoX. The car was given to me since I knew him (and the other guys are too uber conservative and uptight to work on "modified" cars:greddy:) so I told him that since we didn't have a 5-gas analyzer, I'd have to basically go step by step from easiest/cheapest to most expensive.


  1. Cleaned out the EGR ports (your engine doesn't have EGR) since he had a new valve installed elsewhere and they didn't clean them. Tested and failed.
  2. Had him run Seafoam through his engine. Tested better readings but still failed.
  3. Replaced his O2 sensor. Tested and failed again.
  4. Last step was his converter. He took it somewhere to have it replaced with an OEM-style and fitment and it passed.
What kind of catalytic converter do you have and where is it located in the exhaust system? My friend had an aftermarket cat and it was a very small and short one which the exhaust shop positioned closer to the mid-pipe and not the front pipe.

With emissions, you want to get the converter hot otherwise it doesn't work near as well. I have a 1995 Civic which has the same engine as yours and it also doesn't burn, leak or consume any fluids whatsoever. Cooling system is bled properly and always topped up. It has 244,xxx hard-driven kms on it and when it came time for it's emissions test, it had an AEM CAI (it now has an AEM short ram) and it still has the DC 4-2-1 header and ESPELIR JGT500 cat-back exhaust system on it when I tested it a few years ago. Prior to that the car sat for a couple years and was only started and back out of the garage to let run for a bit.

Timing was stock and before my test I took it out for a GOOD HARD run for about a half hour at highway speeds. Then I drove it normally before I brought it in for it's test. Passed with flying colours.

I would perhaps try SeaFoam then do your O2 sensor and see how it goes. If not then look to your converter.
 

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Discussion Starter #8
The cat was a new magnaflow I'm going to double check the length and inlet/outlet diameters. It's located after the downpipe and slightly further back behind the oil pan. I watched as they cut my old one out and welded this one in place.

I'm going to try and seafoam the motor as I've never done it before on this engine. Look for another O2 and contact the exhaust shop in regards to if the installed the wrong cat for my car.
 

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Adia's Daddy
2004 Honda Element
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The cat was a new magnaflow I'm going to double check the length and inlet/outlet diameters. It's located after the downpipe and slightly further back behind the oil pan. I watched as they cut my old one out and welded this one in place.

I'm going to try and seafoam the motor as I've never done it before on this engine. Look for another O2 and contact the exhaust shop in regards to if the installed the wrong cat for my car.
The car i diagnosed was an older Accord and the cat was noticeably and significantly smaller than the OEM one it replaced. You could even see the section in the exhaust tunnel in the underbody where the OEM converter would have resided and the difference was easily visible.

If it's a high-flow cat, that might be your problem. I know guys who got the Magnaflow converters and failed emissions even though it was supposed to pass. Honestly, I would try to source a good used OEM cat if you can and just install it to be tested, then swap it out for your Magnaflow after. High-flow converters claim to be able to pass emissions but as I said I knew someone who failed with a new one.

NoX is due to high combustion temps and since your engine doesn't have an EGR but is small and has a properly functioning cooling system, once you eliminate carbon buildup and the O2 sensor from the possible cause equation, I'd put money on your converter being the culprit.

Just make make sure you Sea Foam with your old O2 sensor then install the new one after.

P.S. I still have the original converter on my EG
 

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EG Final Year
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Discussion Starter #10
I still got my original as well ;) didn't let those bastards take it.

Quick question the bleeer screw for my coolant I want to use it to make sure I have absolutely as little air in my cooling system as possible.

My overflow fills up to the Max line when the car is warm and I've driven it and after it cools it goes back to the Min line. I probably have a good amount of air in my system wouldn't I?

So should I use the bleeder screw warm up the car and attach a hose ontop to a canister to catch the fluid from the bleeder?
 

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In Ohio, they didn't check 95 and back for NOx. Because with better burning and more efficient NOx goes up.

Next thing you know CO2 is be labelled a bad emission, therefore you can't exhale.
thats pretty interesting, its like the govt want you to have inefficient vehicles...by haveing inspection that tests for low numbers instead of high?

but then my next question is how harmful is the NOx?
 

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thats pretty interesting, its like the govt want you to have inefficient vehicles...by haveing inspection that tests for low numbers instead of high?

but then my next question is how harmful is the NOx?
NOx causes acid rain. To what extent, I don't know.
 

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Adia's Daddy
2004 Honda Element
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3,364 Posts
I still got my original as well ;) didn't let those bastards take it.

Quick question the bleeer screw for my coolant I want to use it to make sure I have absolutely as little air in my cooling system as possible.

My overflow fills up to the Max line when the car is warm and I've driven it and after it cools it goes back to the Min line. I probably have a good amount of air in my system wouldn't I?

So should I use the bleeder screw warm up the car and attach a hose ontop to a canister to catch the fluid from the bleeder?
To bled your cooling system, this is what we do at our work on most vehicles and this is also what Honda recommends in many of the service manuals.


  1. With the engine and coolant cold, remove the radiator cap
  2. Top up the radiator until it is just below the filler neck
  3. Remove the overflow hose from the reservoir
  4. Top up the reservoir to the MAX line
  5. Turn your HVAC temperature dial or lever to full HOT (any air trapped in your heater core will be circulated once you need to use your heat in cold weather)
  6. Turn your climate control fan speed completely OFF.
  7. Start the engine
  8. Wait until the engine reaches operating temperature. This is the point at which the thermostat opens fully and the radiator fan cycles twice. Now you can get the engine to operating temp one of two ways...A) if you have the time, just find something to do and let it idle until the fan cycles twice, or B) if you are on a schedule simply hold the engine RPMs between 2000-3000 until the fans cycle twice.
  9. Once the fan has cycled the second time, top up the radiator to the filler neck as required.
  10. When the coolant level remains the same in the radiator, re-install the overflow tube into the reservoir and then reinstall the radiator cap.
This is the same process that Honda recommends in many of the service manuals and should bleed pretty much all the air in your cooling system. The bleeder screw can be opened while you fill the radiator up but if I remember correctly the filler neck is the highest point in the cooling system so bubbles will naturally want to rise to the highest point. But when the engine is cold you can crack open the bleeder screw and fill the radiator until coolant comes out of it steady then close it and perform the bleed procedure above.

If bleeding turns up no problems and you Sea Foam and replace your O2 sensor, I'd be willing to bet your OEM cat would help you pass :bigok:.
 

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Adia's Daddy
2004 Honda Element
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3,364 Posts
parking on an incline helps too, esp. with 88-91's.......

Bleeder screw for what??????
There's a nipple on the top of the inlet where the upper radiator hose meets the cylinder head. You crack that when filling up the radiator after a coolant flush or when the engine coolant passages have been opened, during a water pump change for example. Theoretically when the coolant comes out of it, the level in the engine is full.

The '98-'02 Accords have the same bleeder nipple on the backside of the cylinder head by the thermostat housing. When we do timing belt and waterpump jobs on those years of Accords at work, the bleeder is left open while filling up the radiator. When coolant starts to pour out of the bleeder, the coolant has filled the water jackets. Just to be safe, I still do the rad cap off and let the engine idle until the fans come on method to ensure there are no air pockets in the system.

On '01-'05 Civics, we take out the ECT sensor in the cylinder head by the airbox and fill the rad until coolant comes out of the opening. Then we replace it, top up the rad and do the cap-off bleed procedure.

Honda has a Shop Talk article out on gurgling in the heater cores of '01-'05 Civics and they say to jack up the front right side of the car and use a CV boot in the filler neck of the rad and run the engine. Apparently air gets trapped easier in the 7th generation model Civics but the ECT sensor trick and the idle bleed has worked well for us.
 

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Adia's Daddy
2004 Honda Element
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3,364 Posts
My 95 Civic gurgles in the heater core.......but that is from a cracked head.....AGAIN!
Ouch :pinch:.

We had an '01 Civic come in with an overheating problem and coolant being pushed into the reservoir with air in the rad.

When the tech pulled the head to do the gasket, turns out a cylinder wall was cracked almost all the way down :wacko:.
 
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