I have changed my mind. I'll argue some more.

About cars, it's clear I have more experience and skill than you have when it comes to working on them. About language, it's clear I have more skill at getting my ideas across, I am more articulate and I can better answer direct questions.

But when it comes to just talking nonsense, you surpass me. There is no argument there. Your use of language is somewhat stilted. You make the wildest assertions and assumptions. You put words in people's mouths. Your failure or refusal to answer direct, civil questions makes you impossible to work with. I don't know why you do that, but it's not really important. You either don't know, or you do know, but you don't say. Your bluster and evasion, while entertaining, is unhelpful at solving the question at hand. You post bullshit, get called on the bullshit, and ignore it. I would too, had I posted the crap you posted.

I'll give you another chance. What you do with it will be interesting. Will you demonstrate your knowledge and civility and answer the question? Or will you dodge it again, demonstrating your ignorance and hostility? Either way, I'm looking forward to your next post.

tw: Will you please explain how measuring gas mileage will fix a rough idle, as you asserted in post #92?

TW, what outstanding questions? You mentioned hooking it up to a computer and revving it. I don't have the tools to do that. Was there something else?

I drove it a short distance this morning, and it never had a chance to warm up. The idle was fine. But since it's only a problem when it's warmed up, it behaved as expected.

I think the problem is caused by the flint on one of the spark plugs needing replacing.

No, no, no...the flint stones didn't use fuel or spark plugs or oil drums. They just ran along with their feet hanging out of the hole in the bottom of the car.

Many examples were requested. Other patterns may exist but would only be apparent to you.
Number must be provided. That was only what you feel. How many gallons (to at least a tenth of a gallon) per how many miles? And what were those MPG numbers back in better days?
So what does it do when at 800 RPM? 1200 RPM?. When cold. When half warm. When hot at all those RPMs? Cold unfortunately means 800 RPM symptoms are not possible. Apparently it is still doing it hot. But at what temperature on the gauge does it start getting rough? Or does it only get rough when the high idle finally drops to 800 RPM.

Not yet stated is if you even have a tachometer on the dash - an example of information I was waiting for or had to assume was not relevant.

What other techniques were used or tried unsuccessfully to aggravate and diminish roughness? What parameter change when it becomes better or worse?

Previously discussed were the two separate systems. Idle and normal operation. How do symptoms change or exist in both modes?
The dealer connects an onboard computer. Or buy one for something like $150. However you must then learn the technology - which a dealer provides with his better computer for a fee. That onboard computer is how dealers find intermittents - should you need it fixed now. It is an option reserved for the future. But the option is how these type problems can be located.

Have you been flooring it after each restart as requested?
More suggestions. While idling, put your ear near the exhaust. Listen. Does it run smooth? Is there a pattern or sound? How does this also change with the next tank of Shell? How does it change with cold and warm? Examples of details that might quickly identify a problem without hiring an expensive dealer mechanic.

So what else was changed in the few thousand miles before this happened. Change of gas. Change of spark plugs. Anything else. All are example of what was noted early: I am assuming this problem still exists. Because nothing said so definitively. Is it still as bad? Or has it diminished as implied in a later post or with another tank of Shell?

If the idle is rough, just sand it down until it's smooth.

Hey! I'm trying not to be abrasive here.

Fuck. Check engine light is on. Got the codes read at AutoZone, they say:

P0421 - Warm up catalyst system (RH) efficiency below threshold
P0431 - Warm up catalyst system (LH) efficiency below threshold
P0174 - Lean Air/Fuel ratio bank 2

I know it will not pass inspection with this issue. But I just passed in November, so I'm safe there for 11 more months. My question is, is this a dangerous thing to be driving around with, or can it hang out for a few months with no real problems other than continuing to degrade a catalytic converter that is probably going to need replacement either way? I have to drive this thing for about 10 hours of holiday travel in just a couple weeks, and fixing it right now is going to be very painful.

I'm replying based on experience, not on knowledge.

Before panic, try filling the tank with "good" gas... not higher octane, but from a trusted gas station.
Be sure the cap in on tightly, as it should be
Try driving thru this first and maybe second tank of gas to see if the light goes out, and or the codes disappear.

This experience did happen with my grandson's Subaru, and the new gas did the job.

Best of luck...

You're fine.

This thread is instructive I believe (I'm guessing about your rig, other symptoms, etc). But the description seems on target. Short description: your catalytic converters are failing, gradually. This is determined by examining the exhaust flow downstream of the converters, and comparing it to some threshold. Yours seems to have crossed some threshold. It should drive fine.

and by fixing it being painful, you mean that you don't have $2000 for two new converters in your wallet, yeah?

Yeah.

This was what I was initially hoping. However, I have one of those cars where the light does not go off by itself, it must be manually turned off once triggered. And since this has to do with emissions, no shop that does state inspections is allowed to shut it off without a thorough investigation of the problem. One guy did turn it off for me on the sly, and it came back on again a couple days later, right after another refill of the gas tank. I don't think he'd do it again.

Anyone know how to turn off a check engine light?

year make and model?

black electrical tape.

2003 Mazda MPV