Question: Mazda Millenia Fails Inspection
Dear Vince, how are you? My name is Frank and I have a emission problem with a 1995 Mazda Millenia 2.5 liter It's failing on the hydrocarbon (HC) & oxides of nitrogen (NOx). It passes on the curb idle test but when the car is put on the dynamometer running at 40 kph it's putting out around 180 ppm HC (limit 58) and NOx 1,300 ppm (limit 400). As for the carbon monoxide (CO) 0.03%, it's almost none existent. Why would a car fail HC on a dyno test but pass on a curb idle test? O2 sensor reading indicates same voltage at idle and while drive around 40 kph. Now I have went through a great deal of trying to fix the problem so let me explain.
I have changed 2 catalytic converter so far but only little change in the readings, my car has 4 O2 sensors 2 in the intake manifold just as the gas escape the engine and 2 at the bottom of the engine, I replaced the 2 at the top. I put the car on a scanner and look at the O2's reading both 2 on the top are responding very quickly and voltage is constantly changing between low and high, as for the bottom O2 it's reading high between 0.6v-0.9v.
Now I believe the secondary 02's do not set fuel trim because I can see it on my scanner. Now the emission test say I have unburned gas coming out the exhaust therefore less oxygen is present which should give a high voltage by the 02. Now the 02 is going to tell the ECM to cut back on fuel, make the mixture more lean. Now why would the 02 at the bottom still be reading between 0.9v 0.6v which would indicate a rich mixture.
Now lets look at what might cause high NOx, I know EGR is what reduce this so I apply vacuum to EGR valve engine stalls. When this happens does it means that you open the EGR valve therefore exhaust gas is recirculation back into the intake manifold to be re-burn making the engine stall? Also I hook up a vacuum gauge to the vacuum line that goes to it and drive the car to see if it's getting vacuum to open it, turns out to be fine.
Now they say NOx is caused when temperate rise over 2,500° in the combustion chamber. Well if am right with the testing of the EGR system what else can cause this temperate to increase? My ignition timing is perfect which is base timing 10° btdc. I would like to know if NOx is a result of high HC? If I can just find out what's causing 1 of these it would be a great success.
I didn't check fuel pressure and my purge valve canister I read somewhere this can be dumping too much gas into the intake which can cause high HC and if your fuel pressure is too high. If you recommend me to please let me know.
As for all other readings on the scanner, coolant temp, mass air flow, air temp, seems to be reading fine to me. From a cold start with the scanner hook up it would show vehicle in open loop but after 5 min goes into close loop so ECM looks good to me plus at 40 kph vehicle is still in closed loop. O2 sensors is making proper adjustment now why would it be causing high HC?
Another thing I would like to add is this is a used engine. I had put in which came from Japan, the guy who sold it to me said these engine can't pass emissions in Japan so I don't know if it's a internal problem inside the engine. As for my stock engine it was giving me the same high NOx taking into consideration it had oil going to the coolant, coolant going to the oil so I changed it not only to a engine that fail NOx but also HC. I am fed up of wasting time and money, I have even tried methanol in my gas with no success.
A friend told me put a banana in your engine oil which is potassium but am thinking that suppose to help the carbon monoxide which am passing. I also read a wrong mixture of coolant can cause air pockets and create heat which I doubt my coolant mixture is wrong. If there is any more information you would like to know please email me, as for now I leave it up to you Vince Ciulla. You have the experience and if you can't figure it out I still say thank you for your time.
Answer:
I shared that "banana in the oil" tip with the other guys in the shop and while no one ever heard of it we did get a good laugh out of it. My secretary even asked me if I had a banana in my pocket or was I just glad to see her.
I boiled down this letter to one simple question, what is causing the high HC and NOx emission readings. First let's make sure we understand what Carbon Monoxide (CO), Hydrocarbons (HC) and Oxides of Nitrogen (NOx) are.
CO is created when there is more air than fuel entering the combustion chamber, the engine is running lean. Some of the things that can cause an engine to run lean are vacuum leaks, clogged fuel injectors and low fuel pressure.
HC is produced when there is more fuel than air entering the combustion chamber, the engine is running rich. Some of the things that can cause a rich condition are high fuel pressure or injectors not closing completely.
NOx is created because air is about 79% nitrogen and about 20% oxygen. The remaining 1% is made up of other trace gases. High combustion chamber temperatures cause NOx to form since high temperatures are required for the nitrogen atoms to join with the oxygen atoms. To lower the combustion chamber temperatures we take some of the exhaust gas and recirculate it through the engine to lower combustion temperatures. That is the job of the Exhaust Gas Recirculation system.
From what you tell me it seems the EGR system is working properly. When you open it at idle the engine stalls and it is getting vacuum at speed so we can presume it's opening at the right time. So in this case I have to believe if we get the HC readings down the NOx readings will go down as well.
Here are some things to check for.
- Engine: Do a wet/dry test compression test to make sure you have good compression. You should have about 175 - 200 psi and within about 10% of each cylinder.
- Air Intake System: Check for air leaks after the Mass Air Flow Sensor.
- Fuel System: Check the fuel pressure. The fuel pressure should be between 30 -038 psi with the vacuum line connected and 39 - 45 psi with the vacuum line disconnected. Also do an injector leak test.
As for the HO2S, all four HO2S should range from .1v to .9v. If the readings are steady then it could be an indication of a bad HO2S. Here are the HO2S Specifications.
The fact that it is a Japanese market engine may be the problem. Japanese market engines are not designed to meet US emission, much less California, standards so there is a possibility that this engine will never pass the emission test.
