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03 Malibu Hard start/rough idle/Dies

New here and couldn't find a discussion covering the craziness we are going through with this car. I'll first say what the symptoms are and then then check list of what we have done and tested.

When starting you have to give it fuel, once it turns over you have to keep the rpm's above 500 (manulally with your foot on the throttle) or it dies. Between 500 and near 1000 rpm's it runs rough but smooths after 1000. But you have to give it fuel to keep it running period.

We are pulling a code, misfire on 6. Which happens to be the same code it has pulled since before we did all of this.......

New heads and gaskets (good compression on all cylinders)
New intake valves
New plugs (good fire to all plugs)
New water pump
New fuel filter
New coils

No vaccume leaks
Ignition systems checks all good
Good compression

Continues to pull only a misfire on 6 code nothing else. We are stumped. Any ideas of what could possibly be the root of our problem would be greatly appreciated. We don't know what else to try.


  • thecardoc3thecardoc3 Posts: 4,989
    One of the toughest things to do when you haven't been totally successful diagnosing and repairing a problem with a car is starting over again, as if you never saw this car before and test completely.

    Now you have a misfire code for cylinder #6. In the freeze frame data, does it show the code setting at idle, or another time? Using a scan tool and watching the live misfire data are any other cylinders showing misfires? How does changing the engine speed affect the counts?
    How does putting the car into gear and while holding the brake, speed the engine up to 1500rpm affect the misfire counts?

    Now do the same while watching the fuel trim percentages, both the long term and short term trims. I need to see them at idle with no load, at 1500 rpm with no load and 2500 rpm with no load. Then I need to see them in gear, and in gear at 1500 rpm.

    Those changing conditons will truly let me see if this is a fuel issue or not. Jjust because you have said at this point that "it isn't a vacuum leak" really doesn't mean that it isn't and as a diagnostic technician I would have to confirm every thing myself or risk falling for a trap and chasing my tail just like you are already doing. The main point here is this only takes about sixty seconds to really confirm if an air/fuel ratio issue is present or not and it's all done with the scan data. Keep in mind maybe it isn't a vacuum leak too lean, maybe you are getting false fuel and that cylinder is too rich. The fuel trim checks will help determine this.

    Once this has been observed, the other reasons for a misfire must get taken into consideration and ruled in or out. You said the compression is OK but how did you actually check the compression and what were the specs?

    Today top techs would do a cranking and a running compression test, and we do it with a pressure transducer and an oscilloscope. This lets us see exactly how an engine is able to pump air. There have been a lot of times where we would see someone say they have spark, fuel, and compression but the cylinder doesn't fire. Well the reality is you not only need compression but you have to have oxygen in the air being compressed. You not only need fuel, you need the right amount. You need spark with sufficient strength, and it has to occur at the right time. All of these things can be proven with the right tools, training, and a solid diagnostic routine.

    Nightmarish failures sometimes occur and without a good testing rotuine you could really change parts forever and keep missing the source of the trouble. Imagine what a worn or out of time exhaust camshaft lobe would do. Combustion gasses from one cylinder would not make it into the exhaust, they would instead be released back into the intake manifold. The cylinder that is attempting to take in a fresh charge of air a that moment wouldn't get the oxygen that the fuel needs and so it misfires. This is a condition normally visible with a vacuum gage. You should see a rythmic bouncing of the needle on the gage if a valve train issue is the cause of the misfire. Again today we use a pressure transducer and a scope to draw out the engines vacuum waveform instead of just a vacuum gage. Not only can we see a cylinder that isn't breathing the same as the rest of them, we can sychronize the scope to the engine and we know which cylinder is casuing the trouble.

    So at this point, you need to startall over again with the testing as if you never saw this car before. Just because something was done doesn't give you any guarantees that it is working OK. It isn't easy to do but it's cheaper than throwing more parts at this. You may need a pro who can diagnose this for you.
  • Very sound advice. Thank you! We did get a bit emotionally invested to say the least lol. Heeded your advice. Stepped back and brought someone else in with fresh eyes to take a look. Got a friend of a friend who works for a dealership to come by and bring some of his fancy testing gear. Ended up being a clogged injector and a stuck IAC. It also ended up not being cylinder 6 but number 3 that was the actual issue. The injector was getting power but not pushing fuel. Anyhow, she's running like like a champ now. Thanks again for the advice!
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