Code PO300

[Bentlerpride]

I need Help!

A couple weeks ago as I was parking my 2001 Pontiac Bonneville SLE and before I was able to shut it off it started running really rough. I have not had any issues with the car since I purchased it four years ago.

I got a tester and got code PO300. I looked on youtube and found all kinds of answers about so many possible issues. I have done all of the following to it and it still runs rough when in gear and is getting only about 40 miles to a TANK.

Someone said it could be as simple as a bad battery so I had the battery tested and they also tested the alternator and said both were fine.

I replaced the spark plugs and wires.
I changed the oil.
I replaced the coil packs
I replaced the EGR Valve
I replaced the CO2 Sensor 1 Bank 1 because that is the code from DEQ (I took it there to see if they had different codes)
I replaced the thermostat
I replaced the radiator cap
Still running poorly

Today I did the Fuel injectors and I cleared the codes before starting it up. It ran fine until I shifted gears and then the check engine light came back on and code PO300 again.

I am at a loss here. Spending so much on this car and no results.

Does anyone have any suggestions. I am desperate.

[redfury]

Have you looked at the MAF sensor. Sometimes a simple cleaning will take care of the problem. P0300 is a random misfire, which means that all cylinders are reporting a misfire of some sort. Generally speaking, this will eliminate a problem with the coils, plugs and wires because a full system failure all at once is extremely unlikely. Your culprit lies in something that can affect every cylinder equally. This includes the Coolant temp sensor located under the thermostat...when they go bad they will report the temp to the computer as full cold...like -35F which will cause the computer to run full rich. The MAF ( mass air flow sensor ) reads the incoming air flow and the sensors at times get dirty which causes the sensor to not read the incoming air charge/flow properly. Cleaning can help, but in certain cases, replacement is necessary ( mine for example, would run for a while and then stall and die and restart right away, but cough and sputter under a load. A replacement MAF solved my problem, though I did have a MAF code pop.
the MAP sensor can cause a problem, but it's very unlikely to be the issue. Fuel pressure regulator and fuel pump issues can cause problems as well.

A problem with the crank position sensor can also cause issues, because the computer cannot determine where Top Dead Center is and it needs this information to adjust spark timing.

Considering the problem "just happened", it's likely that you had something fail, or begin to fail enough to cause problems. Spark plugs, wires and coils ( all three ) generally don't just "fail". Sensors will suddenly fail. Fuel pumps can suddenly fail. A fuel pressure regulator can just fail, though it's less likely to cause problems suddenly ( more likely to cause extended cranking to start due to loss of fuel pressure since it last ran ).

Based off of your last run with the car, it's entirely possible that you may have an exposed wire that is shorting out when the engine load changes and causes enough movement in the engine bay. Get a flashlight and really look at the wiring, particularly the wiring going to the crank sensor. Look for chaffing, move the wires if you can. It wouldn't be a terrible idea to pull connetors off and look for signs of corrosion as well.

If you can, a scanner with live data running may help you narrow down your search. Watch spark advance and MAF readings at steady RPM's. Look at your Short and long term fuel trim readings. You want these to be in the low single digits ideally..either side of zero ( positive or negative....adding fuel or subtracting fuel ).

[96 SSEi]

hi, I would be sure the plugs stayed at a .060 inch gap- its tight in the back and could throw gap off if you're rough

unplug the MAF and see if it runs better........

[sseilmnop]

I code reader that displays a misfire table is real handy since thyou did the shotgun approach and it didn't fix it.

My experience says misfires at idle are always vacuum leaks! A vacuum fitting on the lower manifold or upper intake is likely cracked. These cars are old enough now that even if not cracked, it will be after pull them off to check them.

On the other side, misfires under power are almost always coils and wires!

[crash93ssei]

40 miles on a tank from full to empty, 2 MPG? I can't even see anything other then a broken fuel line leaking like crazy causing that kind of mileage, I don't even think the engine would run at all with that much fuel and if it did it would very quickly dilute and destroy your oil. Your catalytic converter would be destroyed as well with that much raw fuel being dumped in.

Check for leaks in the fuel lines.

[96 SSEi]

do you hear an exhaust leak when you drive? is there loss of power and maybe it gets hotter than usual?
if so, it may be a bad or clogged cat restricting flow and causing missfires.

[MKMike]

Is this an SE as your post says or an SSEi like your signature says?
If it's an SSEi...There is a vacuum line that runs from the MAP sensor at the rear of the engine, connects to a tee beneath the supercharger, to the fuel pressure regulator and then to the boost control solenoid.
Over time, the rubber fittings break and create a vacuum leak and non-working MAP sensor and fuel pressure regulator.
If the fuel pressure regulator doesn't have vacuum, the result is too high fuel pressure and very high fuel consumption.

You can do some really good diagnostics with a basic scan tool, a fuel pressure gauge and a vacuum gauge.
With fuel trims you can check for all sorts of things, including vacuum leaks.
A vacuum gauge can clue you in to the general condition inside the engine, as well as determining whether the exhaust is restricted or not.
A fuel pressure gauge is the only certain way to know if fuel pressure is too high.

See these links for details :
http://www.easterncatalytic.com/educati ... stic-tool/
http://www.freeasestudyguides.com/engin ... -test.html
http://www.gregsengine.com/using-a-vacuum-gauge.html

MAF sensor is also a frequent offender--though my car got 6 to 7 mpg when it failed ;-)

[Bentlerpride]

Ok, so now I have pulled the MAF and looked inside with a magnifiying glass and a light and it is shiny and looks like new. Problem still exists. Now the car runs really ruff at idle and when you first step on the gas. Once the car is over about 10mph it runs like a charm.

Park = rough idle
Reverse = Rough idle
Neutral or Drive = Really rough idle. the longer you sit at a light the worse it gets. The car actually starts to shake until you get moving again.

My Bad = Bonneville is an SLE

[MKMike]

What is the DEQ that you mentioned earlier?

First, understand what the P0300 code is telling you.
It means that the computer has determined that there are multiple cylinders misfiring.
Often this is caused by a vacuum leak but a bad ignition control module, weak fuel pump, clogged fuel filter and bad Mass Air Flow sensor can all do this.

Vacuum leaks can be caused by stuck open PCV valve or simple broken vacuum hose or fitting or even a bad gasket on the upper intake manifold (aka a plenum).

The cheapest and easiest way to diagnose it to look carefully at every vacuum fitting.
These often break in ways that aren't obvious, such as splitting on the bottom where you can't see without an inspection mirror.

In the absence of something obvious, such as a broken vacuum hose, it is less expensive to diagnose the issue.
To do this, you'll have to borrow, buy or otherwise get hold of some tools.
There are stores which will lend tools for free, when you leave a deposit for the cost of the tool.
Autozone is one and I know there are others.

If you read the links I gave earlier, you will understand how much information you will receive from a basic scan tool , a fuel pressure gauge and a vacuum gauge so you can actually diagnose the issue.
A vacuum reading can actually indicate whether the engine has an internal problem or not.

Be aware that the 3.8 liter engine is susceptible to problems with the lower and upper intake manifold gaskets failing and the upper intake manifold itself(aka plenum) failing.
Both issues are typically remedied at the same time by replacing the plenum and the lower intake manifold gaskets.
Often this is first noticed as a loss of coolant, with more severe issues arising later.

Even if you would prefer to continue replacing parts without diagnosing, there are so many possible causes that it becomes expensive to do them one by one.
As mentined before the ignition control module, the fuel pump, fuel pressure regulator, Intake Air Temperature sensor and the MAF sensor are all amongst the possibilities.

It truly doesn't matter how clean a part looks.
Electronic parts can fail electronically while looking ok.

You can edit your profile so that people know you're talking about an SLE whenever you post, which has a different engine than the SSEI.

[Bentlerpride]

hi, I would be sure the plugs stayed at a .060 inch gap- its tight in the back and could throw gap off if you're rough

unplug the MAF and see if it runs better........

Plug gap is correct on all. Yes PO300 is the only code I am getting.

[Bentlerpride]

hi, I would be sure the plugs stayed at a .060 inch gap- its tight in the back and could throw gap off if you're rough

unplug the MAF and see if it runs better........

Plug gap is correct on all. Yes PO300 is the only code I am getting.

I unplugged the MAF and there is no change in how it runs.

[Bentlerpride]

Found a couple hoses that were cracked but it still runs rough at idle and at take off. Most of the time there is a very strong gas smell. Sometimes when I start the car, it almost seems like it is flooding itself now but yet sometimes while sitting at a light it seems like it is starving for gas. Still getting PO300

[MKMike]

Without doing any testing, all you can do is change suspect parts.
All we know now is that there are multiple cylinders misfiring. Diagnostics involves testing to eliminate possible causes.
As mentioned earlier, the ignition control module, the fuel pump, fuel pressure regulator, Intake Air Temperature sensor and the MAF sensor are all amongst the possibilities.
Borrow test tools from Advanceauto or similar if you'd like to pinpoint the problem.
Post test results and we can help.