: 2000 DTS P0300 code

08-07-09, 07:45 PM
Our 2000 Cadillac DTS was running really rough off and on with no acceration. Took to Cadillac Dealer and they did tune-up, replaced P0134 Code Heated Oxygen Senor Circuit Bank and flushed all fuel injectors at cost of $725. Next day, back to way it was. Next replaced P0440, evap vent solenoid at cost of $154. Got home, took car out again, runs like it did before. After reading forum, I checked Codes it says PCM P0300, faulty crankshaft position sensor. So upset they haven't fixed it correctly the first time. Have to take back and tell them that may have been the problem. Anyone else had this replaced?
Also, history of codes never says the two they replaced.

08-07-09, 08:42 PM
Wait a minute. P0300 is a misfire code. Do a search for plenum and check it. That may well be your problem.

08-07-09, 08:49 PM
Thanks Ranger. But may I ask what is a Plenum? and, could that have been the problem from the beginning?

08-07-09, 09:44 PM
The plenum is the rubber coupler between the TB and the manifold. Yes, it could well have been the problem all along. A search should turn up many threads about it. It's easy to check.

08-08-09, 09:40 PM
I had the P0300 code...I had the spark plugs checked and replaced with the correct AC's...still had the code...took it in for a Coolant Exchange and an ATF Exchange at the GM Dealer, reset my light and code hasn't come back...its pretty strange...

Mark C
08-09-09, 12:29 AM
P0300 is a misfire code, injectors, plugs or a bad coil cassette (and /or ICM on the cassette) are about all the will cause it unless you had the engine apart and got a cam installed a tooth or two out of phase. If you replaced your plugs its more than likely one of the 2 coil cassettes. Dealer is throwing parts at it unless there are some other codes. O2 sensors do not cause a misfire, neither does the Evap solenoid. Unless its also running lean (P0171 0r 0174) its probably not your plenum which would cause a vacuum leak if it was torn and you'd have a high idle.

08-09-09, 12:40 AM
A p0300 is a random misfire code. if you can find out which cylinders are misfiring that will tell you much more about the problem. the enhanced GM software will tell you which cylinders are misfiring. if you don't have them your dealer does. My 2000 Eldorado was misfiring on the #4 giving me the dtc of P0304.and since it was the only one and misfiring every power stroke it meant it was most liking electric not fuel .so first I changed the plugs and sure enough the 4 plug was not pearly white like the rest .But it still misfired.So I checked the boots. they were all good so then I switched the coil packs and the misfire went to cylinder 5. So I replaced the coil pack on bank 1 and fixed the problem . Now it runs like a top. hope this helps.

08-09-09, 08:47 AM
its probably not your plenum which would cause a vacuum leak if it was torn and you'd have a high idle.
That would be the logical line of thought Mark and IS what I used to think until I have "seen" ripped plenums cause a misfire here. Don't rule it out.

08-09-09, 11:48 AM
Thanks guys for the info., but the dealer did replace the 02 sensor and when taken back again, they found plugs 2 & 7 misfiring so they switched a couple leads on the coil pack, was okay until the next time started up the car and it still ran rough again like nothing was fixed. So far, it's $900.00 and still the same. Taking it back Monday morning, and afraid of more money and the problem not being fixed or a guessing game costing lots replacing parts and labour. They are a Cadillac dealership. Getting really really frustrated. Guess I will ask them to check the plenum, should I ask them about the coil pack on bank 1?

08-09-09, 02:09 PM
There are no wires to switch on the coil cassettes. There are no plug wires, period, on a 2000+ Northstar.

In your 2000 the even cylinders are the front bank, odd is the rear, and naturally, the hardest cassette to remove.

YOU remove the engine beauty cover - 2 10mm nuts and it lifts and slips off - and check the integrity of the two ICM (Ignition Control Module) connectors at the inner end of each coil cassette. Then check the PCV vacuum lines to the (front) / from (rear) cam cover nipples. Start the engine. Take a can of starter fluid or choke cleaner and shoot a squirt down under the hose clamp screw at the throttlebody/intake manifild rubber coupling plenum). Any idle speed change is a vacuum leak. Use a cheap stethoscope from Walgreen, modified with a 2-foot piece of vacuum hose (blister pack from any parts house) to listen to each fuel injector for a steady, rhythmic "tick-tick-tick-tick". While the engine is running, pull the vacuum line for the FPR (Fuel Pressure regulator) off the nipple at the regulator - any raw fuel in there ? If so, not good. Easy $40 replacement - the regulator has 2 O-rings and is held in by a hairpin clip. 10 minutes.

08-09-09, 02:28 PM
Thanks for that info. My husband and I are not mechanically inclined so we will have to take it back to the dealership that has been working on it. we did note when we were going through the codes that the engine did sort of kicked in while idling and then it ran better after that, but then when we take it for a run, it goes back into the rough engine almost stalling type, put your foot on the gas and it doesn't kick in or go faster, just slow.

09-29-09, 04:44 PM
I'm having the same problem....I just finished replacing the plenum and started it up and went for the "cross your fingers test ride" and it wont get up to speed and idles rough. I used my code scanner and it read P0304 Cyl 4 Misfire Detected. I checked all of the fuel injectors and they looked seated, I sprayed some carb and choke cleaner to look for vacume leaks, and plugs etc that I didn't plug back in...It appears to be fine?

Any Ideas?

Long Island, NY

09-29-09, 05:56 PM
:sneaky: At least #4 is on the front bank..........

Did you use new intake manifold gaskets and oil the lower injector O-rings before dropping in the fuel rail ? Sometimes those lower rings roll and crimp and allow a vacuum leak. Listen at the power steering end of the intake manifold - in the recess - for a vacuum leak at either the PCV hose connection or the manifold overpressure valve. #4 fuel injector electrical connector OK ? Does #4 (and all others) give off a nice, steady "tick-tick-tick-tick" at idle ? (Use a stethoscope with rubber tube probe for this). Double check the connector and alignment of the MAF in the intake piping.

03-26-13, 10:13 PM
Harbor freight has a mechanics stethoscope for 2.99, works great.