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Discussion Starter · #1 ·
Hi All, we've been working on our 1995 Corolla (1.6L, base model, auto-transmission) with a failed NOX emission test. Anyway, I have been learning a LOT about the EGR and related components. In our case the EGR wouldn't pull much of a vacuum until we got it off the car. We found the pencil / needle valve pretty coated with carbon. We also removed the EGR tube (it was the easiest way for us to get the EGR off the car with the tools we had). Anyway, we discovered the EGR tube was completely blocked (no light at could be seen from one end of the tube to the other). Had to use a metal coat hanger to clean out the tube of carbon. I don't know if this would cause our car to fail the Nitrous Oxide test at 15 MPH - but it passed at 25 mph but maybe people here might know if that matters. Also, I know there is a way to test the vacuum switching valve (with a vacuum and a 12 volt power source) but I am wondering if there is a way to test the vacuum modulator that is out of the car? I think I read somewhere that you plug 1 one of the 3 ports and then blow into one of the others and that you should feel the air coming out of the third port, but I am not sure. Anyway, they are only about $20 bucks so I can get one pretty easy, but I would like to know how to test it. I also want to say thanks to everyone here for all the help and advice already given - I DO appreciate it. Beyond that I want to say I hope everyone has a great Thanksgiving!!!
 

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The simulated load test is designed to fail. How high is your nox reading at 2500 versus 3500? How’s it possible that it passed the 25mph load test and failed the 15mph load test?
 

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I don't know if this would cause our car to fail the Nitrous Oxide test at 15 MPH - but it passed at 25 mph but maybe people here might know if that matters.
It might be that the engine is operating more efficiently at the 25 MPH.
 
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The tests in the repair manual for the modulator don't test if the internal diaphragm is broken which is usually what goes wrong. Get a tight fitting hose for the bigger bottom port and pull a small vacuum, like 10mmHg. If the diaphragm is ruptured it won't hold vacuum. If it's good it will hold vacuum.

The diaphragm is rubber, so even if it's not ruptured it may have become less flexible, so just because it holds vacuum doesn't mean the modulator is working like new.

The tests on the top ports seem to be more designed to see if the filter or ports are clogged. You can pull the top cap off and check that the small filter is clean.
 

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Discussion Starter · #7 ·
Thanks! We have confirmed that there wasn't a gasket on either the EGR valve where it meets the intake manifold, and not on the EGR pipe/tube where it meets the engine. Since neither were available locally & I have had to order them both, maybe the previous owner didn't want to wait and decided to just do without. But like I said, the car literally runs great. Heck, I didn't even know there was a problem until the smog tech said it failed the NOx test at 15 mph. Now, I'm not a mechanic, so my understanding is limited on this emissions stuff, but I am wondering how the car could pass the NOx test at 25 mph, but fail at 15 mph?? Isn't the engine running cooler at 15 mph? And isn't the whole point of the EGR system to lower the temp of the exhaust gasses by recirculating the exhaust back into the intake to reduce the temp? It would seem to me that the car would fail at the 25 mph test. Anyone know?
 

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Thanks! We have confirmed that there wasn't a gasket on either the EGR valve where it meets the intake manifold, and not on the EGR pipe/tube where it meets the engine. Since neither were available locally & I have had to order them both, maybe the previous owner didn't want to wait and decided to just do without. But like I said, the car literally runs great. Heck, I didn't even know there was a problem until the smog tech said it failed the NOx test at 15 mph. Now, I'm not a mechanic, so my understanding is limited on this emissions stuff, but I am wondering how the car could pass the NOx test at 25 mph, but fail at 15 mph?? Isn't the engine running cooler at 15 mph? And isn't the whole point of the EGR system to lower the temp of the exhaust gasses by recirculating the exhaust back into the intake to reduce the temp? It would seem to me that the car would fail at the 25 mph test. Anyone know?
Good luck on your retest. I bet it passes with flying colors after you cleaned it and added the gasket.
 

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At 25mph during the test it may be in a higher gear and lower RPM than at 15mph.

The recirculated exhaust gases are inert so when they mix with the air and fuel it dilutes the mixture and slows the combustion down. There is still the same amount of heat generated it just lowers the peak temperature which is when NOx gases are created.

If the EGR system isn't working correctly you may get pinging during acceleration, but if your engine has a knock sensor the computer will adjust the ignition timing to stop the pinging, but this could reduce performance or gas mileage.
 
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Discussion Starter · #10 ·
Well, I will let you know how the car does at it's retest, once the gasket gets in to the local Toyota dealer (delayed probably due to what's going on in the world). Since we have a 1995, we don't have the OBD2 connector, but we never even got a 'Check Engine' light, so that's why I was surprised the car had any problems with emissions. The car runs great, no hesitation, no pinging. Other than the NOx test failure, the only issue we have is the little leak in the radiator by the neck, but that seems to be pretty common with old plastic radiators. Once this car passes smog, i am thinking about replacing the radiator with an 'all-aluminum' radiator. Anyone have any thoughts or recommendations about that?
 

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I wouldn't bother with aluminum. The plastic lasts long enough. When I did the radiator in my old '93 Corolla, O'Reilly's wanted like $90 or $100 or more, but Rock Auto had the OEM Denso for like $55 plus shipping! I went with the Denso
 

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Another thing that can cause higher peak combustion temperatures and thus more NOx is if the ignition timing is too advanced. On 93-95 models the base timing needs to be set correctly using a timing light. If it were a couple degrees too advanced it could increase the NOx a bit, but you might experience pinging if that were the case, so maybe something else is to blame, like the catalytic converter if the EGR system is working correctly.

Note that late 95 models don't have an adjustable distributor where the ignition timing needs to be set.
 

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Discussion Starter · #14 ·
Thanks Dr Z, one of the two things I asked the kid at the mechanic shop was to check the timing (as well as if there was a good vacuum). The kid said that the timing was spot on but I am tempted to borrow a timing light to double check. It's hard for my old eyes to see, especially in low light, so I am wondering if I can mark the timing mark(s) with something that will really show under a timing light. Any suggestions. If not I will get my son who can see a LOT better than I can.
 

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You can mark the notch on the pulley and marks on the timing cover with white paint and it will make it easier to see with the timing light. Also, doing it close to dawn or dusk or in a darker garage will make it easier, but you don't want it too dark with the moving belts and pulleys for safety reasons.
 
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