Hesitation issue, car running poorly p0300, random misfire

2008 PONTIAC GRAND PRIX
164,000 MILES • 3.8L • V6 • AUTOMATIC
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BIKERMAN002
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Hesitation problem at 2500-3000 rpm cannot get much higher than that when driving, gas mileage has dropped. I have just replaced the spark plugs and wires. Still a p0300 code random misfire. Cleaned the mass air flow sensors before that also had a ignition control module code replaced that. There is also a p455 or 445 forgot emissions leak or says check gas cap my main concern is getting the full horsepower back. I am thinking maybe a coil pack. Maybe cleaning the throddle body I have cleaner have not cleaner. I have also put fuel injection cleaner too not luck or change. I have not tested the fuel pressure yet, maybe fuel filter or I don't think it's showing symptoms of fuel pump but idles fine and steady let me.know What you think
Update, well fuel filter is in the tank with the fuel pump so don't think it's that. Replaced the purge valve, the check gas cap signal not been coming up, bought one coil pack tried switching out the new coil pack with the old ones , one at a time and after clearing the codes, still no luck, hesitation, car running pretty poorly now, even with these new plugs, it's like the problem slowly getting worse, air filter and that area is air right. If I.floor it, just wasting gas, no.power at that rpm level. No check engine light right now after clearing the code 300, if I.floor it engine light will flash sometimes and then usually the check engine light will come up and stay on. Possibly a catalyst converter going bad, watch's some videos maybe I.can loosen it or clean it and test out, also check the fuel injectors doing the screwdriver test noise at idle speed seems ok but not while reviving. Idle and starts ups are smooth and ok.
May 14, 2015 at 12:50 AM
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HMAC300
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it's probably low fuel pressure 300 codes are really hard to diagnose and best left for a pro to do because any sensor can cause it. shold be 56-62psi key on only. it should hold for at least 5 minutes within 10 psi if not the pump is bad.
May 14, 2015 at 7:34 AM
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BIKERMAN002
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Tested fuel pressure today it is fine, what other problems move onto next
May 14, 2015 at 3:32 PM
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HMAC300
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if the light flashs you need ot have a pro use his scanner to see i f he can pick up what is causing the flash. the code you think is 455 there isn't one but there is a code 446 which may be a fuel tank pressure sensor due to a blocked evap canister. In this case the system should be smoke tested to find where the blockage is or if it's a bad ftp sensor which is in the fuel tank.
May 14, 2015 at 4:34 PM
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BIKERMAN002
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It was a clogged catalyst converter! New one in, MPG up! Also added restore compression additive, my MPG is at 30, before was 22-23 and under 20 when converter was going bad, replace your plugs, clean MAF sensor and throttle body, plugs wires, your car with perform so well
Jul 15, 2015 at 11:23 AM
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LAKEYSHA
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I have an 2007 poniac grand prix and it has a misfire in cylinder 1 I changed fuel injector ,spark plugs and ignition coils still having the problem what could be wrong your question...
Aug 26, 2019 at 11:57 AM (Merged)
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PROTECH1980
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Could be low compression on that cylinder due to cam, valve, or spring, Do a compression/running compression test. Could be the mass air flow sensor, change it and I it should be fine.
Aug 26, 2019 at 11:57 AM (Merged)
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LAKEYSHA
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Ok had my wires check outcome was the wires were good i was told it could be the computer problem could that be possible
Aug 26, 2019 at 11:57 AM (Merged)
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BOBBY THURMAN
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Supercharged. I am a tech that is lost on this one. New plugs, wires, coils ICM, good fuel pressure. Car runs great until warm. drives great until you put load on it between 2500 to 3000 rpm. Car bucks and sputters ck engine light starts to flash. Let off pedal car will run great. Code 300 misfire. Watching on scanner cylinders three and six hit under load. Swapped coils still three and six. Car leans out. Don't want to hurt motor. Could it be possible two injectors having problem or PCM. Please help poorboyzdreamz.
Aug 26, 2019 at 11:57 AM (Merged)
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STRAILER
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Hey Bobby,

A couple things come to mind, first is fuel pressure, I would check that fist, here is a guide,

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Next is the mass air flow sensor. If that is not giving the correct reading it will make the engine lean and it might not throw a code to tell you its off.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Please run some tests and get back to us so we can continue helping you.

Best, Ken

Aug 26, 2019 at 11:57 AM (Merged)
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CHRISINGERSOLL
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At idle #6 cylinder is missing. If you raise the RPM's up by 800, it comes back in. Does the timing work through the throttle body position censor. the Plug, and injector has been checked(all good).????
Aug 26, 2019 at 11:57 AM (Merged)
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CARADIODOC
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Try swapping two injectors to see if the misfire switches to the other cylinder. GM has had a problem with injectors that weren't flow-matched. You didn't list the mileage so I can't make any judgments on that, but at higher mileages, the mismatched injectors start to cause some cylinders to run a little lean, sometimes enough to cause intermittent misfires. If swapping injectors makes a different cylinder misfire, install a complete set of flow-matched rebuilt injectors.
Aug 26, 2019 at 11:57 AM (Merged)
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PALEX
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I'm getting coded for injector 2 malfunction.. misfire number 2 cylinder..getting good ground pulse on all injectors but dim ground pulse on number 2
Aug 26, 2019 at 11:57 AM (Merged)
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SATURNTECH9
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Have you checked to make sure all the power feeds to the injectors are all the same voltage? Also that there are no damaged wires to the injectors around the intake manifold area?
Aug 26, 2019 at 11:57 AM (Merged)
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PALEX
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I have checked all the wires.. have not checked voltage
Aug 26, 2019 at 11:57 AM (Merged)
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SATURNTECH9
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If the light flashes dimmer then the other 3 then i would check the voltage on the ppwer feed side.
Aug 26, 2019 at 11:58 AM (Merged)
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SATURNTECH9
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Also if you have a analog multi meter you might be able to measure the ground pulse to see if its less then 2 ohms to ground.
Aug 26, 2019 at 11:58 AM (Merged)
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NEWMOMMY0212
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I still have a cylinder 6 misfire after i have changed sparkplugs and wires also ignition coil, fuel injector for said cylinder, and fuel filter to make sure that it was getting enough gas to the engine. I don't know what else to do I love my car and would hate to give it up.

What else is there to change?
Aug 26, 2019 at 11:58 AM (Merged)
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PALEX
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I checked the voltage on injectors.. all the same.. also getting a code for EGR flow bank A low voltage
Aug 26, 2019 at 11:58 AM (Merged)
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MHPAUTOS
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Have you done a compression test on that cyl and also check for vacuum leaks around that cylinder induction.
Aug 26, 2019 at 11:58 AM (Merged)
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SATURNTECH9
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Have you measured the resistance of each injector yet? Lets worry about the fuel injector code first then move on to your egr code.
Aug 26, 2019 at 11:58 AM (Merged)
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PALEX
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Yes I have checked res. On injectors..all the same....I was told to replace pigtail on #2 injector and had a spare new injector. So just went ahead and replaced #2 injector also... still no change...
Aug 26, 2019 at 11:58 AM (Merged)
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SATURNTECH9
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If the number 2 injector connector makes the noid light flash much dimmmer the others there has to a problem with the wires somewhere or the ecm itself.
Aug 26, 2019 at 11:58 AM (Merged)
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MRWOOD6053
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02 grandprix. I put intake gaskets on after the car had been overheated and it skips while pulling a random multiple cylinder misfire and i think heads are warped is problem but wanted another opinion, please and thanks.
Aug 26, 2019 at 11:58 AM (Merged)
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CRZYDRVR
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I think you are on the right track with the heads
Aug 26, 2019 at 11:58 AM (Merged)
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MRWOOD6053
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Well i really thank you for your advice and maybe next time i will have account set up and will gladly donate to those who help. thanks again!
Aug 26, 2019 at 11:58 AM (Merged)
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SHORTBUS9999
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Engine Mechanical problem
2002 Pontiac Grand Prix 6 cyl Front Wheel Drive
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computer code is saying multipel cylinder missfire what do you think it could be and how can i fix it
Aug 26, 2019 at 11:58 AM (Merged)
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BLACKOP555
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those are the hardest to track, first i would start with ignitions system.

check the ignition coil/coil packs \
igntion control module.
and see if you have crossfire going on with your cylinders.

get back.
Aug 26, 2019 at 11:58 AM (Merged)
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SHORTBUS9999
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i have checked the coil pack by takeing off the plug wires and it sparks between the 2 posts on each coil pack .could it be the ignition control moduel?
Aug 26, 2019 at 11:58 AM (Merged)
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BLACKOP555
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could be ignition module,

maybe crank/cam shaft sensor error.

if thats not it runa fuel pressure test.
Aug 26, 2019 at 11:58 AM (Merged)
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COPER701
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2001 Pontiac Grand Prix GT 3800. Check engine light came on two weeks ago. It was a EGR valve problem. The vehicle was also missing with heavy acceleration. Changed the EGR valve and the error code disappeared but the problem continued. I found the wire to the 2 cylinder was chewed on by a mouse. replaced the wire with no success. I decided to replace all the plugs and wires. Still had the acceleration problem. My mechanic hooked it up on his computer and it was showing a misfire on cylinder 2. The vehicle acts like it is not getting any gas. Mechanic checked fuel pressure but he only checked it at idle. He checked the spark and it was fine. Fuel filter was also fine. He checked the ingiters with a light tester and they were fine. Any ideas would be great. Also does the fuel pressure test have to also be done while moving?
Aug 26, 2019 at 11:58 AM (Merged)
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SERVICE WRITER
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These motors have a problem with the intake leaking. Is there ANY coolant loss?
Aug 26, 2019 at 11:58 AM (Merged)
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COPER701
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I took it to a Pontiac dealer and they found out that it is a plugged catalytic converter.
Aug 26, 2019 at 11:58 AM (Merged)
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GPRIX713
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Hi. My car, which i bought in May 2008, misfires when I accelerate it and hesitates to accelerate. I know that this indicates that one of the cylinders is not working, and may be the effect of a loose wire, broken spark plug, or an engine leak. I have no leak. The engine light started to blink. I turned off the car, restarted it, and the engine light came on again. What is wrong with my car, how can i fix it, and how much will the repairs cost? Thank you.
Aug 26, 2019 at 11:58 AM (Merged)
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MHPAUTOS
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Hi there,

The first thing to do is get a scan done to see what the code is, also have a basic tune done and have a compression test done as well, often these are just tune related.

mark (mhpautos)
Aug 26, 2019 at 11:58 AM (Merged)
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BENC42
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I have a 99 grand prix gt 3.8 non-supercharged. It has a misfire on cylinder 6 according to a scan tool. But the misfire is bad enough to throw the serpentine belt off. I have replaced the following trying to fix the misfire: plugs, plug wires, fuel injectors, coil packs, ignition control module, good compression with 108 on cyl 6, and no valve leakage, new fuel pump, fuel filter and relay, also has an aftermarket exhaust with a high flow cat converter done about a year before the misfire as well as new heads and gaskets that go with also done about a year before misfire. I am almost giving up on it please help.
Aug 26, 2019 at 11:58 AM (Merged)
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PROAUTOTECH
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so you got #6 on top dead and did a leak down test to ensure the valves were good? What is the compression on the other 5 cylinders?
Aug 26, 2019 at 11:58 AM (Merged)
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BENC42
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yes, hooked shop air to it and no leakage, i only tested three cylinders including #6 and they read around 106, and 105 then the 108 for #6.
Aug 26, 2019 at 11:58 AM (Merged)
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DLBJBB2015
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I was wondering if you found out what was causing the issue you was having, because I'm having the same exact problem with my car. Dont think anything else can be replaced we have replaced everything. Ours is misfire on cylinder 2.
Aug 26, 2019 at 11:58 AM (Merged)
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GRAYDON
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The car has developed misfires that occur at random and tend to go away on their own. I've checked the plugs, and plug wires and they're fine. The MAF sensor is new, and i cant hear a difference in the injectors using a steathoscope. Any help would be greatly appreciated.
Aug 26, 2019 at 11:58 AM (Merged)
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RASMATAZ
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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.
Aug 26, 2019 at 11:58 AM (Merged)