Code P0300, random misfire diagnosis

ERIC ALTEN

MEMBER
2003 CHEVROLET IMPALA
3.4L
6 CYL
2WD
AUTOMATIC
102,000 MILES

So, I have replaced the spark plugs, wires, fuel filter, and now I was thinking about replacing an injector because I did the stethoscope test, and one wasn’t clicking as loudly as the others. It was clicking though. However, when I turn it on, I’ve realized I do have symptoms that could point to the EGR valve but I’m not totally sure. I’m also wondering if it’s obviously something else that I don’t see or know. When I start the car, there is a rough idle, it struggles for power right after ignition and is tougher than it is after giving it a little gas. There is also a bit of grey or white smoke from the exhaust at ignition, but it stops. I did notice a slight gas smell as well but that could have also been when I was disconnecting hoses to get to the bolts to remove the exhaust manifold to get to the fuel rail and eventually replace the injector I was looking at. I also have a newer GM electronic EGR valve. So, I’m wondering if I can test if this one is bad before buying a new one and testing it that way or where should I be focusing?

8 Replies

STEVE W.

MECHANIC
12,972 POSTS

Okay, a P0300 is a random multiple misfire. So, it is something that affects more than a single cylinder. The first thing would be to look at the live misfire data and see which cylinders are showing misfires. Then think about which items affect those cylinders. 99% of the time it won't be plugs or wires (unless something damaged them like oil leaks or rodent damage) If the vehicle has a single coil that could affect it, as could a bad twin output coil if the misfires were on the cylinders thar coil served. Not likely injector related unless it is something affecting all of them, a single injector would instead set a P0301-P0312 single cylinder misfire.

Most of the time it will be caused by either a fuel supply problem like low or very high pressure. Or some type of air intake issue like a manifold gasket or vacuum leak or another system that allows air into the system. In your case the EGR is possible and likely given the symptoms you state, plus they like to fail and usually stick open. The easy way to test them is using a scan tool to open/close it and see what effect it has and if it actually does work. For the average DIYer that can be difficult as most don't have a scan tool that has bi-directional controls to do that. Without that swapping the EGR is about the only test you have as it is a PWM controlled item with position feedback that isn't easy to monitor without a scope. Attached are the directions to change it. If you wanted to see if it was the issue there is a test you could try, unbolt the EGR and use some thick foil to block the port and bolt it back on. If the problem goes away, you have an answer. Just don't drive it that way or it will cause problems.

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Sunday, March 27th, 2022 AT 11:21 PM

ERIC ALTEN

MEMBER
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Okay, very much appreciated. I did some research before getting this and found if I had a power probe, I could send 12v and see if it will actuate but I don’t. So, I swapped out the EGR valve today before getting this also lol and it did help but I do still have a major loss in power and a rough idle. On startup, I’m not getting any smoke from the tailpipe, and it doesn’t start nearly as rough. It was a lot smoother starting and then going into lower rpms to idle, it would begin to tremble and shake in a rough idle again. I was able to give it gas and rev the rpm’s but after letting off, it felt like it was going to stall but didn’t. After letting it idle for several minutes, the same code came back, and the engine light was flashing instead of solid. I was getting that intermittently when replacing the plugs and wires going from solid to blinking depending on if it was a cold start or if I had been working on it. I’ve had this car for about two years daily, not driving very far often. It sat outside for several years before I bought it, I had mice chew the wires to the ECU partially and I repaired that. I know mice can cause problems anywhere but I drove it for several months after dealing with that with no issues, so I don’t think that related. I replaced my fuel filter as well because it seemed like a fuel issue. I’m still able to start and idle so it can’t be my fuel pump. It does seem starved for fuel. I’ve checked my vacuum lines and air intake for holes or cracks and can’t find any. After replacing the EGR valve and with my current symptoms now, I'm not sure where to look. Any ideas? I guess maybe a manifold leak. I was driving it about hour away before this happened and kind of pushing it, driving fast on the freeways since the v6 in the sedan picks up. Could that have caused a manifold leak possibly? Wouldn’t a compression test be able to test that?

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Sunday, March 27th, 2022 AT 11:37 PM

STEVE W.

MECHANIC
12,972 POSTS

Still could be the pump, start and idle uses a small amount of fuel, the higher the rpms the more fuel it uses, if the pump is putting out low pressure or volume it could cause it to run bad at higher RPMs when it needs more fuel. The power probe won't actually actuate it, they use a pulsed signal that rotates the armature in the valve and the position sensor counts a step and stops, if it gets say 6 pulses of one polarity it rotates 6 steps, then it could get 6 in reverse polarity and it moves 6 steps the opposite way. That is why you need a scan tool for that.
If you were driving it and this started, was it an instant problem, like it started shaking and flashing check engine light or was it a gradual thing where it started getting worse over a few miles at speed?
Checking for a vacuum leak isn't hard, use a can of carburetor cleaner to spray around the manifold and lines. Block off the line to the brake booster as well. If you find nothing, then I would test the pump.

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

Check both pressure and flow rate. You need 52-59 PSI key on engine off. Flow should be about a quart in 15 seconds.

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Monday, March 28th, 2022 AT 12:47 AM

ERIC ALTEN

MEMBER
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Okay. So, I did the soap method since I’ve seen it’s a little safer down their line for rubber parts. I could find any vacuum leaks and I sprayed at the base of the head gaskets and the engine didn’t idle any faster. I tested the fuel pressure regulator from the Shrader valve on the fuel rail and got 60PSI right when I turned it to the on position but immediately after the pressure drops to zero. I didn’t even bother trying to start or load it because I think that’s an indication of a weak pump, clogged fuel filter or line, correct? I’m still getting a random misfire p0300 though so do I have multiple issues, or do you think the misfire is caused by the lack of fuel? It’s also idling fine now, not rough at all unless I give it gas which makes sense with the fuel pump. I still have the test kit and can test the fuel pressure with the engine running and also loaded if you think I still should. I attached a video of the psi gauge as well as when I had it idling without the fuel pressure gauge attached. You can see me rev the engine from the throttle body and a small amount of smoke coming from the new EGR valve I put in. You should also be able to hear how it struggles after giving it some gas.

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Friday, April 1st, 2022 AT 6:18 PM

STEVE W.

MECHANIC
12,972 POSTS

If it dropped instantly then the issue is the check valve in the pump or more likely a leak internal to the fuel tank. I have seen cracked lines between the pump and external connections do that. Both will cause low fuel pressure because the fuel pump can't maintain pressure. From the testing I would say to pull the pump assembly out and replace it. That pump can be accessed through the trunk area, remove the trim panel that is behind the seat, and you will see the access panel to the pump. Remove the nuts and use a bar to pry it up, they usually goop it pretty good to seal it in place. Blow all the crud away and spray it down with some cleaner and let it dry. Remove the harness connector, then the fuel lines disconnect. Now release the retainer ring and the pump should pop up. Pull it and check the white accordion line between it and the external pressure connection for a crack or missing clamp. Both are possible. At one time you could get the pipe from a dealer, but I believe they are off the shelves now. To install the replacement just reverse the process, spray the inside of the rubber ring with a shot of silicone so the pump will seat easily and replace the ring. Reconnect and enjoy your car.

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