2000 grand am with SES light on codes as follows p0300 and p0420 ... Random missfires and Catalyst system efficiency below threshold (Bank 1) .... Both up and downstream o2 sensors have been replaced . new spark plugs wires and 1 of 2 coil packs replaced. When I first start it, it has a hard time getting idle right then its fine and drives like s dream. It does it hot or cold. 

Try pulling the Idle Air Control valve off and clean it out  real good with some carb cleaner.....Also make sure it doesn't have a vacum leak somewhere...It's also very possible the passage in the cylinder head for the Exhaust Gas Recirculation valve (EGR) might be starting to get clogged up...In which case can possibly be cleaned out with a coat hanger and carb cleaner (will have to remove the EGR valve to do that)....

Try doing those things before rushing out to buy any new parts......

Edited November 2, 20187 yr by TWO LANE BLACK TOP

I agree with Two Lane. Try to do all your diagnostic work first before throwing new parts at it.

Here is a good diagnostic video for help troubleshooting the p0300 code as well as one example of a 2001 Grand Am.

 

 

Edited November 2, 20187 yr by Frosty

What is up with this guy and why does he just pop up at random places?? Haha. Thanks for the advise ill check them first. Normally I do, do diagnostics first ... Why I'm asking around first. 

I checked the iac so far and it was just nasty and the oring/gasket was ripped to. Idles just a little different haven't got to egr yet.

Egr will throw it's own code and will not affect idle. Does it miss at idle or under load? What do the fuel trims look like? It would be good to clean the mass air flow sensor with the correct cleaner, carb and brake clean can leave a film the will affect the readings. 

 

Ant

This car does not have mass air flow only map and air temp. No idea about fuel trim. Only at idle and usly only at startup for a few mins or untill I get going. Every startup warm or cold. I'm starting to think fuel pump

It kinda feels like b-b-b-b-b-b smooths out b-b-b-b-b smooths out then a little lack of power like running out of fuel then it is fine for the rest of the trip in till restarted. After that if I slow down from highway or stop at red light it misses like every few + or - 100 cly fires or every few secs

Egr will throw it's own code and will not affect idle. Does it miss at idle or under load? What do the fuel trims look like? It would be good to clean the mass air flow sensor with the correct cleaner, carb and brake clean can leave a film the will affect the readings. 

 

Ant

What motor do you have? The 3.4 has a mass air flow in the intake tube, you said you replaced the plug wires the 2.4 does not have wires so I'm confused.

 

Ant

2.4l twin cam. No mass air flow. It kinda has wires (springs in boots) .. it coils are just hide from sight . Yes replaced wire set one bad coil pack and spark plugs. 

Those are known to get cracks in the coil housing you need to turn in over and inspect it for cracks that is your most likely cause. I'm going by memory on this but I'm remembering a fuel pressure regulator on the fuel rail with the car running pull the vacuum hose of and make sure no fuel sprays out if it does replace it. If that is good check the fuel pressure under load, I dont think that is your issue though since under load it would it would miss worse then at idle. 

 

My money in on the coil housing

Ant

One coil pack read 10k ohms one pack reads 6k ohms physically they seem fine. It could be the housing ill have to get one to try it out. The one I have I pulled from a junk car. The one that was on the car didn't read at all out of one channel. The one I have now reads fine seems fine and I could tell someone replaced it cuz it had a sticker on it and was a different color. Doesn't really mean anything tho. Cuz you know it could have a hair line crack and a volt meter doesnt touch 60,000 volts. You know. I see what you mean about the fuel it would make since ... Since more is req to go faster. Thanks 🐜 ant

So the current coils read that much difference? That seems odd, I'll check some specs tomorrow when I get to the shop.

ant

Yes the coil are very different in the readings. The one I replaced that was dead was 30k the new one is 10k the one from pull a parts is 6k

I've been using marvel mystery oil for two tanks now and it doesn't miss a beat. The only other thing I did has been take throttle body off and made it spotless. From what I could tell one of the vac line in the body was plugged the EGR solenoid 

Something else that is definitely worth doing is to pull the injectors out and check is to make sure the O rings on the fuel injectors where they plug into the intake aren't brittle... broken....leaking..ETC..and are in good shape and the injectors fit tight....

Edited November 13, 20187 yr by TWO LANE BLACK TOP

Well I think I found the problem! I filled up the gas tank to the max for the first time finally and then smelled gas like crazy strong. Twas like da hell. Yep there is a hole in top the tank where the fill neck connects. Bet 100% that I'm getting water in there.