99 old alero. car runs ruff misfires will only go bout 50mph but hard to get to that. dies if you idle or stop. i replaced plugs boots starter coil packs fuel pump regulator. any ideas? oh i had the exhaust ck its fine and the catillac converter was straight piped. ty so much.

Replace spark plugs and look at coils. Sounds like a misfire at idle. Car is trying to keep idle high enough with 1 or 2 cylinders misfiring. Car compensates then revs up as the cylinders catch and run again. These cars often have bad coils. Scan car and look for codes 301 through 306. Replace all plugs and if misfire comes back then replace the coils on the cylinders that occasionally misfire. Look at boots and spring wires on coils and replace coils that have bad boots.

I's a bad distributor. Get a Spectrum NGS 60. The cam sensor is going bad. A lot of 99-00 models have this issue.

replace no 2 injector as plugs and stuff are seldom the reason for misfires
dirty injectors or injector controls are generally the cause

Wow that was alot of parts and money.
Is code 300 the only one in the computer ?
Have you checked fuel pressure with a gauge ?

do a compression test

For engine misfiring problem check the help links mentioned below. Click the link below to go through the troubleshooting procedure :---- Engine misses and truck jumps? Hard to down shift? Issue with high gear only? http://technoanswers.blogspot.in/2012/02/engine-miss-and-truck-jumps-hard-to.html

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Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Engine Misfire codes for car?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html
Engine Misfire codes?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html

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Misfire on cylinder 4?

http://schematicsdiagram.blogspot.com/2011/12/misfire-on-cylinder-4.html

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Oil on spark plugs and car misfires?

http://technoanswers.blogspot.in/2012/02/oil-on-spark-plugs-and-car-misfires.html

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This will help.

Thanks.

You replaced the spar4kplugs but how about the plug boots, these boots are like sparkplug wires and they short out sometimes , with over 100,000 miles i would replace them, also you may have a bad coil . the coil is a complete unit its 4coils in one and may be misfiring, the other problem maybe a dirty throtlebody or bad idle air control valve, you said it runs rough at idle, i would first take it to have the computer scanned to see if there are any stored codes, the computer is always yesting all systems and when it sees a problem it stores a code, this code will tell you what is failing, many autopart stores will scan it at no charge, also if it sees the problem a certain about of times it will turn on the engine light, if you get it scanned and give me the codes i will be able to help you further. for example they may scan it and get a code that reads po302- cyl 2 misfire. this means cylinder 2 is misfiring and then you need to see why its misfiring, it may be due to a plugboot or coil or injector. i would scan it first. hope this helps. good luck.

May need to also replace the coil tower which is the white piece under that metal cover that the spark plug boot attach to. They usually crack and give misfires.

COULD BE GROUND PROBLEMS OR THE TACH GOING BAD.EVEN THOUGH CAR SEEMS TO BE RUNNING GOOD IDLING GOOD.SOME TIMES A SPARK PLUG MISFIRE.CAUSING A LITTLE ROUGH IDLING CAUSING TACH NEEDLE TO GO CRAZY.I SEEN ALTERNATOR WITH VOLTAGE REGULATOR GOING BAD CAUSE A POWER SURGE CAUSING HEAD LIGHTS TO DIM A LITTLE THEN BRIGHT WHILE ENGINE IDLING.THIS WILL CAUSE TACH METER NEEDLE BOUNCE AROUND ALSO.

I'm sure you have installed a fuel pressure gauge and monitored the fuel pressure while the problem is occurring?Another diagnostic tool which is easy to do,is while the problem is occurring,unplug one sensor at a time beginning with the Mass Air Flow first,.if it uses one.Many times,not always,depending on the sensor,if the sensor is taken out of the picture as far as what the PCM is seeing,like unplugging the MAF,the PCM will substitute the missing sensor output with a generic value that will allow the engine to at least run,and if unplugging for example the MAF,the engine idle speed returns to normal,you know you're onto the problem.You didn't mention if the engine ran ok other than the idle problem.As far as checking for vacuum leaks,i use something like brake cleaner or WD 40,more of a liquid,and it does sound like a vacuum leak.Plastic intake manifolds do develop nearly invisible cracks that open up more with heat.Revsisit the possibility.And as for the PCM,it is possible,but more likely a mechanical problem.Lastly,remove an O2 sensor or two before the catalytic convertor,run the engine and see if anything changes.Cheap way to verify the catalytic convertor is flowing and not restricting engine airflow,as i have seen bad cats cause misfire codes and strange problems.Good luck.