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Discussion Starter #1
I have a 2004 Mazda3 with about 305,000 miles on it. I know that the car is due for replacement soon, but given that I don’t know what’s going on with my job because of the lockdown, I’d like to avoid buying one right now.

Over the past few months, I noticed that it was idling rough/hesitating. Shortly after that, it started stalling at idle or when stopping at red lights, but would always start right back up, although sometimes I had to hold down the gas pedal to get it to start. This doesn’t happen until the car has been driven a few minutes and the engine is warmed up. Now that the weather is warmer, I’ve noticed the stalling issue at idle is much more constant with the AC running. I’m able to keep the car running if I increase the throttle a little bit. When the engine is under load (while driving), it seems to run fine.

About a month ago, I went to start it and it almost didn’t start. It took a few tries and a lot of cranking to get it started. It had never done that before, and hadn’t done it since, so I didn’t worry about it.

However, last Friday, I went to start it and it wouldn’t start. It felt like it wanted to, but just kept cranking. At times, I think I even heard some minor sputtering from the exhaust like a minor backfire, but I can’t be sure. I was getting spark, and also noticed fuel spraying from the injector holes when I was checking spark. I don’t know if the amount of fuel was correct, but it was getting fuel. I tried spraying brake cleaner into the throttle body and it still wouldn’t start. I tried to start it several times that day, and it never started.

The next day, I sprayed some more brake cleaner into the throttle body, and after a few minutes of trying, I was able to get it to start up. Ever since then, it starts up fine every time now without having to spray anything into the throttle.

I’ve already cleaned the throttle body, reset the KAM, and did a relearn. It made no difference. I checked the values for the MAF, MAP, fuel trims and O2 sensors. All looked good to me, but I’m not a professional mechanic by any means, so maybe I’m missing something.

I feel like I can rule out a timing, compression, or clogged exhaust issue because those wouldn’t have fixed themselves after a day. I know I had spark when it wasn’t starting, so I feel like it has to be an issue with the fuel delivery. Could the throttle itself be going bad? How can I test that? Any suggestions are appreciated.
 

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Discussion Starter #3
I will pickup a bottle next time I'm out.

That being said, I did some more testing today and am leaning towards the throttle body being bad. I found a different app for my scan tool that provides values for the throttle position. At idle, the throttle was only commanded at 3% but was opened 13% - 14%. With the engine off, it showed it was opened 12%, and at full throttle (engine off), it only showed about 88%.

With the engine off and the key in the run position, I can hear an electronic whine coming from the throttle body. Upon closer inspection, it seems like computer is trying to close the throttle body even though it shouldn't need to. It continues to whine like this until you remove the key. Not sure if it whines when running, the sound of the engine is too loud and overpowers it.

My guess is, when at idle, the computer thinks the throttle is open more than it should be and is closing it completely, cutting off air to the engine and causing a stall. Does that sound plausible?
 

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Although I'm not familiar with how to interpret the 'commanded' PID values, the other numbers look typical of what I've seen from other vehicles. And IMO an audible whining noise is more likely to be coming from a serp belt pulley, instead of the tiny throttle motor. A mechanic's stethoscope might be able to pinpoint the source of the noise, but that doesn't always work either.

I'm not saying your vehicle can't have a bad TB, only that it could be other things as well. I'd want to try the Techron first and see if there is any noticeable improvement. One other diagnostic thing you can do is let it stall out at home, and then immediately pull all of the spark plugs. It might help figure this out by seeing if the plugs are wet (too much fuel), or completely dry (not enough fuel). However, saying the above assumes it's definitely a fuel delivery issue, and even that has not been solidly established yet either, because the stalling could also be caused by losing spark. Fuel seems to be the more likely suspect though, and perhaps examining the plugs will help figure it out.
 

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I'll pass this on to you on the off chance that it might help you. My wife's 04 has 288k miles on it. Last year while on a trip alone, the engine stopped running at the end of an off ramp and wouldn't restart. Fortunately, it was a small town, and a Chevy dealer was just down the street. They saw her stranded there and came down to see if they could help. They tested the car and determined that she needed a throttle body. Nobody in town had one, and they offered to r & r the throttle body and clean it to see if that would get her going again, which they did, and she continued on her trip. We use Techron from time to time, and I do believe it's a good product. However, if carbon deposits on the throttle body are significant enough that the butterfly isn't closing to where it should be for idle, or isn't moving freely through its range, it will have to be cleaned. We are running Techron through it on a more regular basis, but we still have not replaced the throttle body, and it's working fine.
 
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