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Discussion Starter · #1 ·
06 LS 2.2 Automatic completely stock 209,000 miles.
Parts replaced and diagnostics.
Plugs, boots, coil, and ICM replaced. Compression 150 across the board. Fuel pump replaced good pressure.
Timing marks all lined up.
Injectors all good.
So I have a random misfire that used to come and go but it's apparent 30 seconds after startup everytime now. Car starts good then after a minute it makes a air wooshing/gurgling sounds and starts missing. Can hear hissing when the brake is pressed and the car almost stalls on stopping unless the AC is on it never stalls and misfire is much lighter with ac on. Possibly brake booster system vacuum leak?
 

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Sounds like a vacuum leak. If it’s happening mostly on cold startup then I would maybe suspect the intake gaskets need replacing. I had to do that to mine recently.
 

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Discussion Starter · #3 ·
Sounds like a vacuum leak. If it’s happening mostly on cold startup then I would maybe suspect the intake gaskets need replacing. I had to do that to mine recently.
Thank you for the timely reply. I'll investigate vacuum system further.
 

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did I heard power brake booster leaks vacuum? did You check the vacuum manafold for a missing or broken/ cracked cap/ plug?
That one will make the car really unhappy - that's a big vacuum leak.
 

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That one will make the car really unhappy - that's a big vacuum leak.
is there a simple way to check the booster diaphragm for vaccum leak? The only reason I suspected it was occasionally stalling while braking and light air sound when pressing the pedal
 

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Discussion Starter · #8 ·
Update. Found no for sure vacuum leaks but I found a part that changed idle when sprayed. I believe it's a fuel pressure regulator? It has two hoses going in and has a green valve on it. The black hoses feel like they are surging air/fuel inside. The idle sounds like shit when I spray around there with wd40
 

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Do they look damaged or anything? That sounds pretty definitive to me.
 

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Discussion Starter · #11 ·
Update. I have a nice obd2 reader but don't really know what I'm looking at as far as live data. The fuel trims are good.
Replaced that part that had air leak and it sounds better but still misfiring, current codes are p0340 and p0300. I had Map and Maf codes but I think that's because I unhooked the MAF while running to check if it was giving input. Could it be the crank shaft position sensor or is the misfire just interfering with it? Also my alternator output is a little high 15.18 volts. Stays in the 15 range consistently
 

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Can A bad alternator cause misfiring this severe? The alternator belt is pretty worn I'm going to replace that and Im also getting 15mpg. Can't remember if I noted that my throttle will sometimes stick at 1800 if I rev it around 2k
 

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When I start my Cobalt the voltage is 14.4, usually goes to 13.1-13.4 volts after it’s running for a few minutes.

A few months ago my battery went bad, luckily I caught it right before it went out. Didn’t give much warning either, just suddenly began starting really slowly. When I checked voltage, it was going nuts, 15-17 volts. In my case the battery was old, and had a dead cell. It would discharge to about 10.4 volts when it would not start the car without a charger. But while running the alternator was way over charging.

This also happened on my old Lincoln Town Car. I kind of jumped the gun on that one. I had the money at the time and replaced the old battery and alternator at the same time. Although the alternator was going bad, had a bad rectifier and was causing the lights to pulse. Probably was cooking the battery also. I think I had to replace the starter around the same time, no relation. But starts like a champ now.

Now, as far as alternator causing the misfire. I don’t know that one. I would say it isn’t out of question. When the bad battery caused my alternator to over charge, the transmission shifting really hard. Slamming into gears. Has not done that since replacing the battery. It made the car really go haywire. So I suppose yes, electrical surge or out of range can cause weird things on the Cobalt since it’s all electronically controlled.
 
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