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I have a 91 tt with 46k mikes clean title. It won't pass CA smog for the idle test. It passes the 2500rpm part but not idle. The idle sounds rough when car is warming up. It's at the mechanic and they think it's either the O2 sensors gone bad or the mass air flow meter sensor. Not sure if this sounds correct but I'm tired of not passing the smog. Any thoughts?
 

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I would start with the basics... pressure test for leaks and datalog the ECU. Trims could be pegged to compensate for whatever issue(s) exist, such as vacuum leak, overrun FPR, lazy oxygen sensor, etc.

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Yeah, you need to start gathering some data to narrow it down. I would think that the O2 sensors or MAF would also affect the 2500 portion, since it s no-load test and the ECU would be in closed loop then as well. If the problem is there before the car is warmed up, it shouldnt be the O2s as they are not being used yet.
 

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I already listed examples of possibilities... proper testing and diagnosis will prevent trial-and-error parts replacement i.e. throwing money at the problem.
 

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Yup, follow advice above to gather more data or else we are just guessing. Also, you didnt mention - what did it fail for? HC? NOx?
 

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Mine was not running rough, 93 stealth base. But if all the smog bits check out, the only thing left is the catilitic. I would look for a gold star smog shop in your area. This means they are a full service garage that has the good rating with the state for being an honest shop. They will check all the smog bits & best bet not to screw you. If you need a CAT apply for CAP ( cal. assistance program) since CAT's aren't cheap, if I remember about 500 big ones.
 

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Mine was not running rough, 93 stealth base. But if all the smog bits check out, the only thing left is the catilitic. I would look for a gold star smog shop in your area. This means they are a full service garage that has the good rating with the state for being an honest shop. They will check all the smog bits & best bet not to screw you. If you need a CAT apply for CAP ( cal. assistance program) since CAT's aren't cheap, if I remember about 500 big ones.
Not sure if it applies to all cars of this year range in CA, but i am required to get STAR test-only stations for my biennial smog checks, 91 VR-4. This means, as the same implies, they are test only. Also, the bolded part above simply is not true lol.

Also, you can get cats for way cheaper than $500...

I would reiterate my post and Alan's post above. gather some more data to narrow down the actual problem. If the car is running rough, you need to figure that out before you suspect the cat might be bad, which is actually quite rare, especially for a car with such low miles (although age can be a factor).
 

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With CAP your cost is $100 + cert, I dought you buy a CAT & install it your self for that. Gold star shops are not test only but are allowed to work on all ages of cars because of their rating. Mines about 2 mi. from me they have been smogging my car for the last 15yrs. My cat was replaced 4 yrs ago. But yes he should fix the miss, the honest smog shop can help with that also
 

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A replacement cat will only get trashed quickly if the current issue(s) is not addressed first - exactly what we are suggesting.

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Exactly the full service smog shop will diagnose all the sensors before suggesting a CAT. The cat is something they can't actually test. If they don't find anything wrong elsewhere they won't charge for the test. They only charge for test if it is sent to dmv. This is something a test only shop can't do, they can only pass or fail, plus they generally charge more than a gold star shop. $ 29.95 compared to $50.00+
 

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The signal wire of the map sensor is going to generate a frequency, so a voltage test isn't going to be very helpful. 3.5v sounds pretty normal. Even if you could measure the Hz I couldn't give you a spec to match it to. The engine is running pig rich and the o2 sensor knows it. i would look for a a fuel leak into the system such as a leaking fuel pressure regulator or look for a reason it's not going into closed loop (such as not getting to normal temperature)
 
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