1999 Chevy Monte Carlo P0300 random/multiple cylinder misfi

Tiny
MIKETDJR
  • MEMBER
  • 1999 CHEVROLET MONTE CARLO
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 135,000 MILES
I have a 1999 Monte Carlo with the 3.1l V6. I have recently replaced the head gaskets and of course all of the intake gaskets along the way. I also put fresh plugs in it when it was apart. Once together I had to replace the camshaft position sensor also. It also has a P0300 random/multiple cylinder misfire code. I have replaced the plug wires and ohmed the coils and that all checks good. I have run fuel system cleaner through it. It is buring alot of fuel, 100 miles to a tank of fuel when I had been getting close to 300 to a tank before putting head gaskets in. I have also had a puff of smoke after the throttle is released from about 4000 rpm or more under load. Any ideas to check or parts to go with?
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Friday, January 22nd, 2010 AT 2:49 PM

8 Replies

Tiny
RASMATAZ
  • MEMBER
P0300 random/multiple cylinder misfire-Normlly caused by vacuum leaks and bad fuel

Engine misfiring can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.
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Friday, January 22nd, 2010 AT 2:54 PM
Tiny
MIKETDJR
  • MEMBER
I have been leaning towards the fuel or something in that system, Not sure how bad it effects cars as I am from the small engine world. But the fuel that was in when it was finished with the heads was arond 9 months old. I have run some over the shelf cleaning fluid through it and that has not done anything. I was told by a retired GM mechainic to run a half a quart of ATF with a tank of premium fuel and that should clean better then anything store bought. If that does nothing I will be pulling the injectors soon to look at them.
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Friday, January 22nd, 2010 AT 3:22 PM
Tiny
MIKETDJR
  • MEMBER
Is there a way you would recomend to clean the fuel injectors?
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Friday, January 22nd, 2010 AT 4:37 PM
Tiny
RASMATAZ
  • MEMBER
Fuel Injection system cleaning Options:

Should you clean the injectors in place or remove them and use some type of injector cleaning machine? It depends.

The easiest route is to clean the injectors in place because you do not have to remove them (which can be a real chore on some import engines). Running cleaner through the injectors while the engine is running also removes many of the deposits on the valves and inside the combustion chambers. This eliminates the need for an extra cleaning step if the engine is full of carbon deposits. The job takes only 10 to 15 minutes, and you can usually tell right away if the treatment addressed the problem (engine runs smoother, idle misfire gone, etc.).

When doing the cleaning procedure itself, you must use pressurized equipment to feed the solvent directly into the fuel rail while the engine is running. This means you either have to disable the fuel pump and plug the fuel return line, or install a U-tube so the fuel will recirculate right back to the tank. Disabling the fuel pump can set a fault code on some cars, requiring you to clear the code after the job is done.

Easy as it is, there are some limitations with on-car injector cleaning. One is that badly clogged injectors may not pass enough solvent during a normal cleaning cycle to be thoroughly cleaned. Some baked-on deposits can be very difficult to remove, requiring you to prolong or repeat the cleaning process. And if on-car cleaning does not work? You will have to remove the injectors and try to clean them on an injector cleaning machine - or replace them.

Another limitation with on-car injector cleaning is that you may have to do some additional tests to confirm that the injectors responded well enough to your cleaning efforts. A test drive may be needed to see if the driveability symptoms have been eliminated, or you may have to check emissions to make sure HC and CO levels are back to normal. A power balance test is another way to confirm engine performance and check for weak cylinders (there should be less than a 10% power variation between cylinders). An injector pressure drop test will tell you if the injectors are flowing evenly or not.

There may be some risk to the vehicle's fuel system when using concentrated solvent to clean the injectors in place. Most equipment suppliers say to disconnect and plug the fuel return line so that solvent does not circulate back to the fuel tank. Strong solvents may attack rubber and plastic components in the fuel pump, regulator and fuel lines, creating additional problems that you don't want.
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Friday, January 22nd, 2010 AT 5:07 PM
Tiny
SCOTTY DEE
  • MEMBER
Find a shop that has a MotorVac machine. I have solved more random missfires with this tool than any other.

GM dosen't recommend cleaning fuel injectors. This is the same company that took bailout money from the federal government and the still filed for bankruptcy.

Don't get me started on Dex-Cool. I own nothing but GM but they do and say some very stupid things.
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Saturday, January 30th, 2010 AT 8:48 PM
Tiny
MIKETDJR
  • MEMBER
Ok I finally got around to do something. I havn't checked vaccum system yet, have to find a shop that does it. The car had been burning a lot of fuel, about 3 times as much as usuall. So I replaced the fuel pressure regulator and it seems much better. Runs almost perfectly. But the code did come back. This may sound stupid but could the plugs have fouled from it being flooded and this is still causeing the problem? The plugs only have about 500 miles on them but if it was flooding itself the whole time that is my next guess. I just hate to pay to have someone look at it to find out the plugs are just fouled out.
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Saturday, March 20th, 2010 AT 4:33 PM
Tiny
SCOTTY DEE
  • MEMBER
Pull a couple of spark plugs and see if they're gas fouled. Have you had the lower fuel and upper air induction cleaned yet?
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Saturday, March 20th, 2010 AT 5:29 PM
Tiny
MIKETDJR
  • MEMBER
The plugs looked great. I tested the fuel pressure, around 36 psi which is low right? Replaced the fuel filter, it was nasty. Got it to 70mph for the first time in a long time but it still has the misfire. The code is there and I can feel the misfire on acceleration. I have not had the intakes cleaned, how would that be done? They were off maybe 6 months ago when installing the head gaskets and of course I blew air through them. I also checked for air leaks around gasket surfaces by spraying carb cleaner on them while running it and I did not find anything. Almost to the point to bringing it in, just don't have the cash to do it.
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Sunday, March 21st, 2010 AT 2:29 PM

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