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Just to show you how insidious these things can be, I used to have a 1968 Firebird. It would not start when hot. Pontiac V-8's were known for that yet all the usual fixes did not work. It was so bad I had to carry a huge screwdriver to reach under the car and jump the starter with it. Many years later I was doing something completely unrelated and I came across the bulkhead connector behind the fender and under the PS booster. The wire that went to the starter S terminal had only three strands of wire left. It was not good enough to energize the starter when hot. The wire was chewed up right where it went into the bulkhead connector and I never would have found it if I had not removed the fender. This is why mechanics hate electrical problems.
 

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Ok so I bought this 2007 Chevy cobalt Lt 2.2L almost a year ago. The lady I bought it from sister got the converter replaced at a muffler shop about 3 years ago before she passed away. Then it sat in the garage for 2 years. Anyways about a month and a half ago it put out a p0420 code but had absolutely no problems. The next day it hesitated when I hit a bump,like a jerking motion. The rpms went down to like 500 and then right back to 2500 in less than a second. Now it only does that when it's above 75 degrees and it's random. It doesn't happen everytime I hit a bump. I've replaced the spark plugs,cleaned the fuel system out,and cleaned the mass air flow sensor which I'm going to replace this weekend. I was wondering if it might be the coil packs?
Welcome to the wonderful world of "guess the problem" of the Cobalt. We have an '08 LT. We've had it for almost 2 years and for most of those 2 years it's been a spinner board of diagnosing this car no matter what the codes read. The codes kind of point you in the right direction but don't ever discount anything based on those codes. On this car the code can point you directly at a problem, so you fix it only to find out that the fix and/or the code affects something else. We had the speedometer that would move while sitting still, along with the tach that was erratic. We got a reading of a bad/problem ECM so we changed that. That helped for maybe 4 or 5 months. Then we got some kind of general code so we changed the MAF, the throttle body and gasket (throttle body kept getting gunked up and sticking), the crankshaft position sensor (after that is replaced, a relearning procedure has to be done), the camshaft position sensor, and finally the thermostat (got a code on it). The thermostat actually affects the AC (if you get all - - - - - - on the digital dash display, your AC probably won't work but if it comes up with an actual reading, the AC will work). What really helped us was the repair shop we took it to had a technician who used to own a Cobalt and he knew what wires/wiring harness to check for with the erratic speedometer and tach. For every problem you encounter, no matter what the code, you'll find at least a dozen people that have repaired it using a dozen different methods. The thing is, it seems to have a good little engine in it.
 

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Just to show you how insidious these things can be, I used to have a 1968 Firebird. It would not start when hot. Pontiac V-8's were known for that yet all the usual fixes did not work. It was so bad I had to carry a huge screwdriver to reach under the car and jump the starter with it. Many years later I was doing something completely unrelated and I came across the bulkhead connector behind the fender and under the PS booster. The wire that went to the starter S terminal had only three strands of wire left. It was not good enough to energize the starter when hot. The wire was chewed up right where it went into the bulkhead connector and I never would have found it if I had not removed the fender. This is why mechanics hate electrical problems.
My '69 Goat did the same thing - it was the starter solenoid. I was running late for work one morning, had to stop for $5 worth of gas to get to work and the DA at the station (this was way back when full service stations were the norm) made me turn it off. I kept telling him it wouldn't restart until it cooled down but he wouldn't do it otherwise. I told him I'd be using his phone to make a long distance call - and that's exactly what happened. It still ticks me off!
 

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Discussion Starter · #25 ·
Just to show you how insidious these things can be, I used to have a 1968 Firebird. It would not start when hot. Pontiac V-8's were known for that yet all the usual fixes did not work. It was so bad I had to carry a huge screwdriver to reach under the car and jump the starter with it. Many years later I was doing something completely unrelated and I came across the bulkhead connector behind the fender and under the PS booster. The wire that went to the starter S terminal had only three strands of wire left. It was not good enough to energize the starter when hot. The wire was chewed up right where it went into the bulkhead connector and I never would have found it if I had not removed the fender. This is why mechanics hate electrical problems.
Jeez man I really hope I just had a change of luck 😂. So I was driving it earlier to lunch and it did the misfire thing twice, well the 2nd time it did it my rpms went to 0 but everything keeps going just fine all the way to Wendy's. I turned it off and on and it was just fine again even the rpms came back and nothing more happened in my way back to work. Thank God I had my sensor plugged in because when I looked at it, it was putting out a p0336 code. Not I just need to figure out if I can replace my crankshaft position sensor when I get some time to look up videos lol. I don't even know where it is.
 

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Discussion Starter · #27 ·
You were driving with the scan tool plugged in? Beautiful!
Yeah I've been putting it in every time I got in the car haha. Wow we've literally talked about everything they have in this video I can't believe I didn't come across this video with all the search keys I put into YouTube 😂 I'm just extremely glad to have found out what was causing this problem. Idk if I'm gonna be able to fix it myself though just because I'm not too confident in my ability to take it out and actually be able to replace it once it's out lol. But so far every mechanic in Denver is so busy I can't get it done till Thursday so I might just give it a try this weekend lmao. I got an auto store right beside my house and they got the part. I'm sure watching this video will help me also. Thank you so much man!
 

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Try wiggling that little harness first. It may be just a loose connection. I just had to do that on a driver door speaker. The speaker was new but the connector's female pins were not tight enough. I pried them closer with a pick tool and now it works fine.
 

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Discussion Starter · #29 ·
Try wiggling that little harness first. It may be just a loose connection. I just had to do that on a driver door speaker. The speaker was new but the connector's female pins were not tight enough. I pried them closer with a pick tool and now it works fine.
That harness thing is interesting lol I'm gonna check that out although I don't think it's that just by the way it's actually doing what is doing just seems like it's more sudden than that and it's always the same feeling. I definitely don't trust myself to take the starter off I'm probably just gonna have to shell out 200 bucks to play it safe 😭 also probably not a bad idea to get an actual mechanic to check the harness and the wiring to the actual sensor and shit.
 
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