View Full Version : P0300 Code goes off at high rpm.

11-19-11, 02:29 AM
Hello, every one.

My 01 Deville has the P0300 (Random Misfire Code) with no other codes present. I've replaced the common things like coil packs, spark plugs, and plug boots. As well as the Fuel Pressure Regulator last year. And i just recently put on a new EGR Valve, and the light actually went out for awhile while driving. Then restarted the car and BOOM the light is back on.

The light flashes when driving, but if you bring the car up too high rpm like 4000rpm the light will go off. Sometimes you can smell fuel burning inside the car, I've tried Techron to clean the fuel system also. As well as the fuel pressure, it was 44psi, i checked it a couple more times and it dropped a little.

I listened to the injectors and they all seem ticking and working. What would make the misfire go away when pushing the engine real hard? it does not miss when driving, just at idle a little bit.

I had the front valve cover replaced recently, which is after i put the newer spark plugs in. Maybe the oil in the plugs was causing the misfire and it fouled out the front plugs?
I plan on checking them tomorrow.

Any other suggestions or tips is greatly appreciated.

11-19-11, 09:12 AM
Did you replace the coil packs and modules as one piece or did you buy coil packs and switch your old modules onto the new coil packs? Also check your fuel pressure...If thats good I would suspect one of the injectors...If you go to a mechanic that has a rather decent scanner he should be able to bring up the misfire data and see which cylinder(s) have been misfiring.. When my coil pack went on my 2000 Deville it was throwing a P300 code through the cars self check OBD as well as a generic pep boys scanner..But once I connected my Snap On Brick scanner it showed me which cylinders were misfiring ( there were 2 on the same coil pack misfiring) I then replaced the coil pack/ module assembly and have been good since..

11-19-11, 10:51 AM
Sometimes you can smell fuel burning inside the car
Exhaust smell or raw fuel smell?

11-19-11, 07:46 PM
I replaced the coil packs as one piece. Checked the fuel pressure, it was 44psi. Checked a couple other times and it dropped a little bit. What are some good scanners to use that aren't real expensive?
Even though i do not get others codes, would the plenum still be ripped? I've sprayed and sprayed it but no change in idle. What is the best way to check the injectors? I've used a stethoscope to listen to them and they all seem ticking.

And i would say it's more of a raw fuel smell, it's not real heavy but you can smell it some. Cadillac want's almost 200$ for a diagnostic.

11-19-11, 08:57 PM
What is the best way to check the injectors?
Other than a stethoscope, a noid light is the only other way, but it sounds like they are working fine.

What are some good scanners to use that aren't real expensive?
A "good" scanner is expensive.

Even though i do not get others codes, would the plenum still be ripped?
Did you flex it near the bottom while spraying it? Have you cleaned the TB?

And i would say it's more of a raw fuel smell,
Might double check the FPR and the installation.

11-20-11, 02:42 AM
I did clean the TB with some TB Cleaner and a toothbrush and rag. But i was planning on putting on new intake gaskets, the plenum, and a new fuel filter. Maybe i should go ahead and put new plugs and boots on the front also since had a bad valve cover on the front that was leaking oil and shorting the alternator out.

What are the chances it being the CAT Converter? I just don't understand how i can put the car in 1st gear and get it up too 4000RPM and the light goes off but right after it comes right back.

11-20-11, 12:16 PM
I'd say chances of it being the CAT are slim.

11-23-11, 03:29 AM
Okay i was driving tonight, decided to punch it and when i was up at around 80+mph there was a rotten egg smell. When the car warms up you can really smell it

11-23-11, 11:47 AM
I seem to recall out old Guru saying that was an indication of a rich condition.

11-24-11, 08:47 AM
Would you suggest the fuel injectors are not working properly? I have been getting around 15mpg with mostly highway driving.

11-24-11, 12:14 PM
Injectors are pretty reliable. They are the last thing I would suspect.

11-24-11, 01:39 PM
FPR............... ? (Yes, it was replaced, but check the lower O-ring for cuts and do the "gas in the nipple or vacuum line" checks again.)

With "mostly highway driving" your gas mileage should be about 21 - 24 mpg. Open road, long-term.......26.5 mpg. Maybe 27, all on 87 octane.

11-25-11, 03:55 AM
The FPR i replaced it with was a generic brand from Autozone. I think ill try and get a ACDELCO one and try that. The car is running rich, but why don't i get any other codes to help me pinpoint what is causing this? I see other people have at least multiple codes.

11-25-11, 11:02 AM
You'll most likely find that the "AC Delco" boxed FPR is made by Bosch in Germany, so the same can be found at a NAPA, Rippy Cadillac >>>>, or www.rockauto.com (http://www.rockauto.com)

(Same for the fuel injectors.)

Look closely................

11-27-11, 08:15 PM
UPDATE: I unplugged the MAF sensor, the P0300 code goes away, but then i get a P0102 (MAF low circuit) code. Went and bought a new one, but still when i plug it in i get the P0300 code and with it unplugged i get the MAF Sensor code. Checked the voltage and i am getting 12v so it is getting power.

With it unplugged the SES Light does not come on, but the P0102 code is there. For some reason with it plugged in it's causing a misfire and making it run rich. May be there is a leak in the intake duct?

11-27-11, 09:53 PM
I think the '96 MAF is bolted to the TB, correct? If so, any leak upstream of the MAF sensor is irrelevant. Your problem may be downstream of the MAF sensor, (loose TB bolts, intake bolts, PCV connection).

11-27-11, 10:13 PM
This is on my 01 Deville. Can i get the car inspected with the p0102 code displayed but with no SES light on? The car runs just fine and I've drove it quite some time and no light is coming on. But you check the codes and it is there.

My inspection is ending next month, and without one the car will be towed away from my complex. The engine is very quiet, has all the power, idles just fine, runs way better than 80% of these other trash cars i see on the road all the time.

11-27-11, 10:21 PM
Good question. You WILL automatically fail if the light is on. They won't even bother testing it. That said, P0102 is emission related and I believe they plug into the OBD system so I would suspect it will show up and you'd fail. I am surprised that you have no SES light (unless the code is in History).

11-27-11, 10:33 PM
The code is current. And no SES light is coming on at all. But with the MAF Sensor plugged in i get the P0300 code and the SES light at the same time like the PCM cannot detect what exactly is causing it. This is very confusing to me why it would do this.

Maybe the PCM is faulty? I never had a car drive so great and it can't even pass an inspection. I've done so much work on the car and neighbors around me just says "Cadillac's suck" "Northstars are horrible engines" and it's quite embarrassing.

11-27-11, 10:39 PM
Did you replace the MAF sensor with a quality unit or one from Autozone?

11-27-11, 10:51 PM
I got it from Autozone. The brand is Cardone it was 150$ but i was able to get a discount and get it for 86$.

Rockauto sells them also but i didn't want to wait for it to be shipped. Maybe i should take one off another Cadillac and try it?

11-27-11, 10:55 PM
Wouldn't hurt if you can. That would condemn or rule out the MAF sensor.

Normally I don't like Autozone Don'tlast parts, but this apparently is not one of them.

11-28-11, 12:04 AM
I will shop around and see if i can find a decent one from a wrecked Caddy and update from there.

11-28-11, 04:05 PM
Now the MAF code is gone, and a P0113 (Intake Air Temperature) code is coming up. Where is the IAT sensor located?

11-28-11, 04:39 PM
On the 2001 Northstar the IAT is the tiny black bead at the edge of the MAF bore - a thermistor with its own 2 pins in the 5-pin MAF connector...........so it's integral and not separately replaceable.

11-28-11, 08:28 PM
Okay yeah i just took a look at it and it's right. I think i may have a short in my MAF sensor wiring somewhere that's causing the misfire when plugged in. The casing of wires were touching the bottom of the engine and may have burned them a little but it's hard to tell with little room to see. But then again i am getting power to it.

Unless i do need a OEM MAF sensor.

11-29-11, 09:10 AM
Take a hard look at the aluminum rectangle on the side of the MAF connector body. Is there a stamping - or evidence of a stamping - that is DELPHI ? If so, that's the OEM MAF.

When I was doing the experiments with the JET Performance so-called "performance MAF" if you removed the JET sticker you could see where they had milled off the word "DELPHI" - I used dye penetrant on the aluminum and the word came back........Cardone remanufactures parts, so you never know.................

11-30-11, 01:33 AM
I installed a aftermarket one branded CARDONE. I never really purchased anything with that brand, but i figure it wouldn't hurt. Say if the plenum boot does have a vacuum leak, unplugging the MAF Sensor won't detect how much air flow is traveling in the engine so maybe that's why the misfire goes away? With it plugged in it's detecting the vacuum leak?

But by leaving it unplugged the MAF code will come up so that's no help.

12-03-11, 10:17 AM
UPDATE: Had the car connected to a good scanner, and it is missing on cylinders 2, 3, and 5, 8, i had the PCM reprogrammed also.

12-03-11, 10:34 AM
Here's a possible ignition or injector timing clue: 2,3,5, and 8 are each exactly one cylinder apart in the firing order....... 1-2-7-3-4-5-6-8

The left (radiator side) bank is 2-4-6-8; the right (firewall side) is 1-3-5-7.

1 3 5 7

2 4 6 8

12-06-11, 10:13 AM
The car has been at a buddy of mines shop for about a week now, and they cannot find a thing wrong with it. The MAF sensor he said is reading proper voltage, and they used a pressurized smoke machine to look for vacuum leaks and could not find anything.

The car drives and runs smooth, just a little tiny miss. He said he's never seen a car miss on 4 cylinders and still run like it's perfect. I don't know what to check or suspect next

12-06-11, 01:42 PM
So it isn't a vacuum leak and probably not the MAF in the first place.........read the first 10 words in Post #30.

Loss of 4 cylinders is anything BUT a "tiny miss".............and your earlier statement about the 15 mpg highway is a clue.

Post #29 - PCM programmed.............done by a GM dealer with a Tech-II or it was NOT programmed.

12-13-11, 06:32 AM
On the cylinders that are misfiring, I would pull the plugs again and check them for proper gap and possible cracks. The spark energy is taking the path of least resistance, therefore it's not going to the tip of the plug at low speed operation causing the misfire.

The misfire is dumping raw unburned fuel into the exhaust system that will cause the rotten egg smell and possible heat up the converter to the point that is starts to glow red.

I would also check the wiring going to the coils for secure connections or broken wires. Check also for possible vacuum leaks and loose ground wires.

Good luck

01-06-16, 12:35 PM
Trying to bring this thread back from the grave, I am having the exact same problem what was the solution here? I can view misfire data and it's showing me misfires on 2,3,5,8 only but mostly on 5 and 8 at idle. I have fixed several vacuum leaks, done a tune up, etc. Car drives well but the light is flashing so I can't pass emissions test. The exhaust does smell like gas but i'm unsure if that's due to being stuck in open loop fault. Also my bank 1 sensor 2 reads very low all the time. Any help is appreciated!

01-07-16, 01:17 AM
Severe misfire is present and the cylinder is dumping raw fuel in the exhaust system killing your CAT. What have you done so far to rectify the issue? Spark plugs replaced with AC Delco #41-987?