Cranks, starts, then dies unless manually choking it

S

Scum

Greenie N00B Member
Greenie Member
2007 Mazdaspeed3
Mileage: ~60k (JDM about 3mo ago)
Location: Idaho
Concern:
My son called and said he couldn't get it to run right. He's up at college and without tools/space to work on it. So I just towed it home for us to work on over Thanksgiving break. Meaning I'm under a time limit (need fixed by Sunday). When it first got home, it fired up and ran just fine for about 3 short trips. Then started acting up again. Now, it will start up, run for about 3 seconds then die. When it runs, it sounds fine but puffs some black smoke. If we do it repeatedly (fire up over and over), it slowly runs a little longer. After about 5 min of trying this, I was able to get it to stay running (horribly) by constantly feathering the throttle so we could pull it into the garage. After that, it ran fine idling. Turned it off and let it sit and then we were back to square one (run for 3 seconds).

What I've Tried (with no luck):
  • Swapped HPFP from the original engine. Made no difference.
  • I pulled the MAF and cleaned it (was super dirty). Made no difference. Replaced it just in case and no difference.
  • Pulled the sparkplugs. They were very black. Replaced them. No difference.
  • Unplugged the EGR. Not part of troubleshooting - just want to disable it to see if it clears up the black puff on start and plugs run cleaner.

My Biggest Clue:
  • Last night after replacing the plugs, it FINALLY threw a code. Codes were EGR and IAT Sensor related.
  • On a whim, while trying to get it to start, I manually choked the throttle by holding my hand over the intake. If any of you remember working on carb cars, it's a way to get them to pull in more fuel when cold starting. This WORKED. Fired up and I was able to keep it running indefinitely AS LONG AS I held my hand over the intake. As soon as I pulled away, it died.
  • I should add, it's cold here but not crazy cold. Getting down into 20s.

So Now What?:
  • So, obviously, my first step will be to check/replace the IAT sensor. My hope is the ECU is reading a totally different temp so isn't properly 'choking' for a cold start. However, I honestly doubt this is the issue. The IAT code didn't trip until after many days of doodling. Also, I have a very hard time believing that it would so far off to prevent it from running - even after I was able to force it to run for a min.
  • I can't help feel like it's a fuel issue. My Fuel pressures looked good (Internal FP was 70-80, HPFP was ~450 at idle and bumped up to >1k when we blip the throttle before it dies).
Some Extra Info:
  • We think it has a tune on it. We got it at auction so the tuner is gone but there's some COBB badged stuff on it (intake) and is cat deleted. No way to tell for sure though.
  • As mentioned, we swapped out for a JDM Engine this summer. We had problems running at first because the HPFP wasn't working. We finally (thanks to folks here), ran a new ground directly to it and that fixed it. As a preventative, we also ran grounds to the throttle and a few other spots on the Engine, Frame and Trans. We've checked all these and they're good and clean.
 
If it was tuned and was parted out, it should have a stock intake on it now; this would cause it to run rich though, so is unlikely to be the issue. If the intake is aftermarket, pull the filter off and check to make sure the airflow straightener is in there properly. I've seen an aftermarket one dislodge and cause all sorts of hell. You can leave the filter off for testing.

Also the IAT sensor is part of the MAF, so you'll be getting one of those unless you meant the BAT sensor, which is separate and installed into the intake manifold.

As for the symptoms, putting your hand over the throttle body with the intake disconnected is kind of a no go, since the MAF is used to measure air intake and with that connected to turbo it's reading whatever the hell it wants to.

Do the following:
1. Reconnect everything, but unplug the MAF. Does it run better? If yes, you either have a vacuum leak or a tune that isn't for the intake on the car (also pic of engine bay can help identify immediate issues)

2. If unplugging the MAF helps, double check every single hose connection and gasketed seal going from the head to the air filter on the intake. Include under the intake manifold (under the car is best for this).

3. Get some kind of logging device (assuming you have one and not just a code reader) and log everything you can on one of these starts; we can generally identify issues pretty quickly with a datalog.

Gluck and godspeed.
 
Thanks for the response.

It's an aftermarket intake. Have already pulled the filter. I don't see an airflow straightener. Not even sure what to look for there. When I was choking it, I had pulled the filter (MAF and intake still installed). So it was still reading flow from MAF.

Yes, I meant IAT. Didn't realize at first that it was part of the MAF. We measured MAF resistance for the IAT and it was way off for the temp (which aligns with the codes). So, just replaced with an OEM one. No difference. Start and dies after 1-3 seconds. Except one time it ran for maybe 5-6 (giving us false hope).

I will try kicking it over with MAF unplugged and report back.

Could throttle itself be causing an issue like this? I've checked plugs and wiring but haven't pulled it. Just trying to think what else to chase after next.

I have an OBD2 hooked up to forscan or torque pro. It can do some logging. I'll fiddle with that next.
 
Unlikely a throttle issue, as gasoline engines are air-metered and throttled, so the amount of fuel going in is dictated by how much air goes in. Conversely, diesels are fuel metered/throttled as their throttle bodies basically don't exist (except on newer models but that's a whole different thing) so engine RPM and the like is controlled by the injectors directly (making them more efficient).

What size intake is it? Basically any larger intake *needs* some kind of flow straightener in order to operate properly; I recommend the honeycomb from Treadstone Performance (just crush it to fit at the edges, don't cut it. You want it tight in there).

I'm almost certain it will start and run properly with the MAF unplugged now based on what you said about covering for the test, but that certainty can only be validated or denied via logs.
 
Well, I'll be. We unplugged the MAF. It died immediately on 1st try. 2nd try, it ran fine.

I assume by " a tune that isn't for the intake on the car" you're referring to the cold-air intake, not the intake manifold. The intake is the same as came with the car and has never been an issue even after the engine swap. After the swap, he drove for a while and then up to college (300 miles) and a couple weeks of driving around there before it acted up.

We'll start digging into the vacuum hoses to see if something popped off or came loose.

20211126_170049.jpg
 
One of the things I don't like about the Cobb intake is that there's virtually no distance between the MAF and the filter, so it's possible that the filter being dirty on one side could skew flow badly enough to affect trims and the like, especially since you said there's no airflow straightener in it (there should be though, even the stock intake has one). That said, if it has the same issue with the filter off then can't do much about that, but I do recommend putting the longest cone filter on there you can get away with for a number of reasons, including *slightly* more top end.

And yeah, changing the MAF housing (intake) is one of the few things you absolutely have to tune for; changing manifolds shouldn't matter unless you start making airflow past what your tune is calibrated for (so you'd need a retune if AFRs go wonky because you've never had the MAF flow that much air before).
 
Awesome. Really appreciate the help here. After we get the vacuum leak figured out we'll start digging into filter options and how we can get that honeycomb to fit. What you're saying makes sense.

I'll also say, I'm very unhappy with the air filter itself. The original MAF was caked with dirt. We live down a dirt road so some is inevitable but not like this. So, I'll be looking for a better filter - open to recommendations. I know better than to go k&n or any other oiled type. I've had good luck with s&b in the past.

Few min into digging into it and he found the main intake tube at the back connecting to the turbo is stupid loose. Like barely on. Obviously when doing the swap we missed tightening a clamp and it must have held on good enough to get some good miles on it before wiggling loose. Letting him monkey around in that tight spot to get it on and we'll test and report back. Really hope that's it.
 
I use an AEM dry flow; as I live in the desert, running a much larger filter has another benefit: extended filter life.
 
Is it definitely black smoke or perhaps dark grey? What does it smell like? Maybe it’s running too rich? In that case, I’d suggest checking the throttle body/connector.
 
Got everything put back together. Took 2-3 tries of dying right away before it finally fired up and held. Had a little bit of misfire for a bit and then smoothed out. Letting it idle for a while as I recall reading somewhere that's good for ECU relearn (not sure it's necessary but doing it anyway). After 20, we'll shut her down and confirm it fires up consistently. Then try again tomorrow morning with a cold start.

It was definitely a black puff. I'm hoping the bad maf and vacuum leak combo were contributing to running too rich and giving us that black. Time will tell.
 
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