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Am I getting scammed at the smog check place?


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Hello, I have a 1995 Corolla (1.6L, 150,000 miles)) and it failed the CA emission smog test (Failed the Nitrous Oxide/NO test at 15 MPH/Passed the NO test at 25 MPH). Smog tech said I should have run the car for at least a 1/2 hour before bringing it in to be smogged so everything was HOT (oxygen sensors, catalytic converter) because the car just barely failed. Put in some better gas & drove it an hour at 60 mph and drove back to the test place (kept the engine running until the guy took the car into to be tested. Now the guy said it failed again, but his time because the EGR valve failed?? He said the diaphragm in the EGR isn't working. Sounds plausible except that the car runs PERFECT!!! And unless I am correct, if the EGR diaphragm really wasn't holding vacuum, the car would run like crap (it runs great) - or am I wrong. And I would assume that the car would fail by a LOT, and not almost pass. Is this possible or is the guy scamming me? Or maybe he has no clue. The NOx reading at 15 MPH MAX is 504PPM, my car 635 PPM. And if the EGR was really bad, wouldn't it fail the NOx test at 25 MPH as well? (My car passed at 25 mph) Anyone know?

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hmm.. tricky.. you're talking about the performance-killers that Australians unbolt and throw away the moment they buy a Corolla.  No smog testing here.

He just said the EGR wasn't working, so it may be 'not working' by not opening even if the diaphragm is fine. So if it jammed shut the car will go better than ever!  ..and I'm sure your one car in umteen million emitting more smog than they like won't kill the planet!

If it is stuck open I expect you'd notice a poor idle, but if its stuck shut it might not make enough difference to see.  That might be why it fails at low speed and not higher up, usually at low speed the vacuum would open the EGR to let exhaust back into the intake, but the lack of vac at more normal speeds would see it closed.

Here's a generic sort of trouble-shooting.

Given the different types of EGR valves, it is always best to follow the troubleshooting procedures detailed in the service manual, however, there are a few generic steps that can help to pinpoint diagnosis:

  • Read any fault codes on electronically controlled EGR valves using a diagnostic tool.
  • Check that all vacuum lines and electrical connections are connected and positioned correctly.
  • Use a vacuum gauge to check the vacuum supply hose for vacuum at 2000 to 2500 rpm. No vacuum at normal operating temperatures would suggest a loose hose, a blocked or faulty ported vacuum switch or solenoid or a faulty vacuum amplifier/pump.
  • Check the vacuum solenoid while engine is running. On electronically controlled EGR valves, activate the solenoid with a scan tool and check the vacuum at end of pipe. If the solenoid does not open when energized, is stuck in the open or closed position or has a corroded electrical connection, loose wire or bad ground, EGR operation will be affected. Identify the root cause before replacing.
  • If possible, check the movement of the valve stem at 1500 to 2000 rpm. The valve stem should move if the valve is functioning correctly – if not, and there’s vacuum, there’s a fault.
  • Apply vacuum directly to the EGR valve using either a hand vacuum pump or scan tool depending on the type of EGR valve. If there is no change in idle quality, then either the EGR valve is faulty or the passages are completely restricted. If the engine idles rough or stalls, the problem is being caused by a malfunctioning control system.
  • Remove the EGR valve and check for carbon build up. Where possible, remove any carbon, being careful not to contaminate the diaphragm. 
  • Inspect the EGR passageway in the manifold for clogging and clean if required. 

But a good one is apparently to reduce the ignition timing a couple of degrees. See here-


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Your car is likely to have a load more emissions stuff than most corollas,being California and all. Dont think my wifes 2001 corolla had EGR. 

EGR is pretty interesting as it injects a percentage of exhaust gas as an inert gas (that you don't need to add fuel to). so essentially you get the same amount of "air" but can use less fuel to get the same correct air fuel ratio. (whilst filling your intake with exhaust shit haha)

If the EGR is blocked the car will still run fine, but i suspect will run a tad lean, which is probably why your NOx is playing up. NOx and SOx changes with air fuel ratio, and its fairly sensitive, small changes in AFR makes your NOx and SOx change dramatically. but still appear to run "fine".  

Its not unreasonable to me that the NOx is out at low RPMs (15mph) as the percentage of exhaust gas is higher than at 25mph. but i dunno, just theorising. 

Unfortunately with emissions tests your only hope is to get all the sensors and equipment working as per oem specs. investigate the EGR and fix it properly. then move onto the rest.


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Thanks everybody! I will see if I see any leaks/cracks or loose vacuum lines, and then before really getting into it, I may just go to another pas/don't pay smog shop. After that, I can start with checking the timing. Hope it's nont the actual EGR unit - about $250 to replace (just for the part)

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