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P0300 on a 2001 Cavalier 2.2

I’m not having much luck with this so figured I might as well ask here. I am on the trip from hell over these 2 months. When I left the car was fine except for a clutch pedal catch-point problem. About half-way through the trip, the fuel pump started to make a buzzing humming noise and 1 day after I arrived at my destination 800 miles from home, it seemed to pretty much quit. Managed to get the car started, drove it with stuttering and lurching to the nearest repair shot, paid them $600 for a new pump and fuel filter and then - the buzzing humming noise was gone, but the stuttering, lurching was not only still there, but has become worse. So after about 2 weeks of test driving it, yesterday it got way worse and threw a p0300 code. That doesn’t help me since I already knew about the general misfire and what I really need is to nail down the most likely cause so I don’t just start replacing everything. I changed the air filter and cleaned the throttle body yesterday. I don’t have any of my tools or anything with me so I don’t know whether to just pay some shop to tell me more detailed diagnostic info or what. But I doubt it will make it back home as poorly as it is running now. My assumption was that the fuel injectors must be clogged but I really just don’t know. Could all my problems have been from bad gas on the trip? What should I be looking for as the most likely? I am assuming the place didn’t put in a bad fuel pump to replace the original pump.

Any suggestions? The plugs were just replaced 2 years ago. Oh - and one more clue - the car idles fine. You wouldn’t know there is a problem until your basically asking the most power of the engine and for me that is accelerating in 2nd and 3rd gears. The more I hit the gas, the worse the problem. If it is just one cylinder, it would be cheaper for me to deal with that cylinder’s injector, plug, etc…but with this p0300, I am not even to that point yet and can’t just replace everything.

Makes me wonder if there’s a plugged cat. Try removing oxygen sensor to make an exhaust leak.

This car has two fuel filters, one mounted to the fuel pump (the “sock”) and the other is an inline filter mounted behind the fuel tank. Did they change them both?

Thanks for the replies. Yes, new fuel pump module/assembly with the whole deal - which I assume has the new sock, and a new in-line filter which I have verified is on there.

When I had the whole air intake mess apart yesterday - which I really did just to gauge how long it was going to take for half-way point of getting the fuel rail, injectors and pressure regulator and other assorted sensors down in there - I didn’t see or smell any gas. What I was hoping to do is to take the suffering car out for a 20 mile wasting-gasoline drive around in circles every day until I got to just under 1/4 tank - then add a bottle of cleaner and fill up with (hopefully good) gas to see if that helped. I don’t know what a dying fuel pressure regulator would cause, but I would think what I am seeing would be close. It would be a big coincidence for anything (compression, spark, air) to also fail at exactly the same time the fuel pump died. That’s why the fuel system still has my attention for than anything else.

do you have . . . or can you borrow . . . a fuel pressure gauge?

I believe your fuel rail has a test port

Yes, there is a fuel pressure test port but it’s not mounted directly on the fuel rail. It’s mounted on the plastic fuel line itself just before it connects to the fuel rail. It’s very easy to spot about midway along the side of the engine that faces the firewall. It’s got a light green cap on it. The pressure should be 41 to 47 PSI.

I happen to own virtually the same car/engine. I just went outside to look at it. Brrr, it’s cold outside!

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What I have = 1999 Cav Z24 - a car that finally loves my pocketbook & fun to drive w/o speeding tickets, only flaws were Chevy batteries (now MAXX @ Walmart) & auto window motors, now on 3rd set.

The only thing similar it experienced to what you described =
2015 September surges - scary, just it to “Honest Abe’s Repairs” whew bucking and trying to die out - Mr. Toad’s Wild Ride

THE FIX $374
3-part TBI Cleaner
Clean Throttle Body
I.A.C. Valve BWD 28975 $120 … my suspect & easy to change self right up front & center

Please leave us … the rest of this story!

So the CEL went out but the problem is worse than ever. Rough engine with extreme rpm fluctuations and just crappy sick engine running at all speeds, in all gears, in all rpm ranges. Worst when it starts up in the morning and worst in 2nd and 3rd gears while accelerating. I think I am going to have to take it somewhere to have a diagnostic. Inspected and cleaned throttle body, replaced air filter, new tank of gas, oil change, and of course the fuel pump and fuel filter are new.

So from most likely to least likely - what are the top 10-15 things that would cause this? I know there are about 10 or so sensors and the injectors and pressure regulator on that list, but what else and in what order? I am about to drive 800 miles back home into zero degree temps and sure with I had more confidence that this problem was not as serious as I am afraid of - especially since I already spent $600 on the fuel pump and filter and am now broke as a result of that.

If I had this problem on my Corolla, and there was no obvious visual problem, and the engine air filter and fuel filter were known good – as is your case – and an intake manifold vacuum measures was normal looking, I’d hook up a fuel pressure gauge and drive it while watching the fuel pressure. The computer has no way to know the fuel pressure is off, and if it is off the computer will inject too much gas or too little gas, and this symptom can be the result.

A shop with the proper Chevy scan tool should be able to do quite a bit with that too, like a fuel trim test and at a least a basic ignition system test.