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Have a misfire code P0300.


PAUL T

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My in-laws have a 2003 Deville that had vacuum leak codes and I suspected the boot between the intake and throttle body. Last night we replaced the boot and while we had the intake out I replaced the intake seals. Put it all back together and I cleared the codes and it was running perfect last night. This morning my father-in law takes it out and all was well, then the Service Engine Light started flashing and it lost power like it was not running on all the cylinders. He limped it to my house and the only code I could find was the P0300 misfire code. I checked all the wiring conections, vacuum hoses, and fuel rail connections and nothing appeared unplugged or out of place. Is there a way to tell which cylinder is causing problems? I can not find my fuel pressure gage at the moment and the FPR was good. Anyone have any ideas on where to look?

Edited by PAUL T
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A laptop-based scan tool (and some tablet-based scan tools) or a technician-quality scan tool will show the miss counts. I believe that the 2003 models also support P030n codes where n is the cylinder number that has more miss counts in a specified running time than a given threshold. When the P0300 code is set, the MIL is turned on. You won't see a P030n code on the DIC or a simple scan tool.

Since you just worked on the throttle body and intake manifold, I would start there. I'm sure that you have already checked the intake manifold bolts and made sure that they are tight, and that the throttle body is assembled and attached to the intake manifold properly, and that all the sensors are plugged in, etc.. The first thing I would look for is a hose that came off, either from the throttle body or pulled loose when you were working on it but fell off later, like a PCV valve pulled out of the cam cover. Check and make sure that the oil filler cap is there, and screwed down tight. Likewise check the oil dipstick and make sure it's all the way down. Make sure that the EVAP hoses weren't crossed; your sudden onset of lean mixture and missing could have happened when the car decided to do an EVAP purge and dumped air instead of gas into the throttle body.

Run all the codes and see what's CURRENT. That will go a long way toward telling what went wrong. If all you see is a P0300, it may just be a fouled plug. An air leak would throw some mixture codes.

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-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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No other codes, only P0300. The only thing I could think of is that maybe one of the new intake gaskets came loose and fell out. We put 4 dabs of permatex on the gaskets to hold them in but one could have gotten loose while we were fitting up the intake. I just hate to tear back into it again.

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If you have a air leak bad enough to give you a P0300, you will have lean mixture codes like P0171 and P0172. Since we don't, things that can cause missing but not throw codes are in the focus. Paul T has already looked at the FPR and is suspicious of fuel pressure. In the first post, Paul T says that he as already looked at the vacuum hoses.

There is one quick trick: smell the exhaust while it is missing. If you smell gas, it's an ignition problem. If you don't smell gas, it's a fuel problem.

An ignition problem that can happen right after a job like this is poor grounds on the ignition coil assemblies or ignition modules. Make sure that they weren't worked loose during the work. Also check the connectors on the modules to the wiring harness and make sure that they are tight.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I have checked the connectors to the coils, the locks for them were lost somewhere before I worked on them. I think it is more on the electrical instead of fuel problem. Jim when you say "ignition coil assembly or ignition modules" are you referring to the same thing?

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Just thinking, can a dirty MAF sensor not through a code but make it misfire? That is the only thing that we took off that could have got dirty while we were fixing the intake/throttle body boot. I am going to check that as soon as I get to where the car is. Does anyone know where the MAF sensor cleaning procedure is?

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The ignition modules snap into the ignition coil assemblies and is held by a couple of retaining bolts. The module is called the "ignition cassette" in the 2003 FSM. When you remove the ignition coil assembly, you remove it after you get the coil assembly off the car. An interesting caution in the section on re-installing the "ignition cassette:"

Important: Be sure the ignition cassette grounding spring and insulator are in place when installing the ignition cassette.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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  • 4 weeks later...

Update: Car runs fine for a little while after I clear the codes then it starts missing again. Only code is P0300. Thinking back on the boot replacement, I remember leaning against the rear Ignition control module plug and hearing a snap. I checked what I could see of the module and did not notice any cracks or problems. I think I will check the module closer and see if I damaged it somehow. I will update after I check it out.

Edited by PAUL T
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  • 2 weeks later...

I have a gas exhaust smell so it is an ignition problem. I am going to change the boots and plugs as soon as I can get to it. The car runs fine until it warms up and then it starts missing. Also p0440 is showing up.

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I have had coil packs go out all of the sudden. A month ago I had a 02 deville that I finished head gaskets on and finished a test drive and parked it in the shop. The car ran perfect, no lights on at all. Customer drove up from KY that next morning and I got in the car and started it and had a hard misfire on cyl #1. Had to swap a coil out.

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Yeah, an old coil can get real insulted by being moved around. Or by the sun coming up.

P0440 is a non-specific EVAP code. If you had a leak you would be getting a P0455, so that's probably not related to the miss. If the EVAP system is full, you can purge the system by driving along over 45 mph at a steady speed for awhile. But it might be best to fix the leak before you subject the cat to that if you can.

The first thing that the FSM says to do is to hook up a scan tool and watch the misfire counters for the cylinders and see if they are incrementing for any one cylinder. If so, you focus on that cylinder, if not you start looking at the wiring harness and such. So the first thing to do is to get the miss counts. I don't know of Autozone will help you with that.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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  • 2 weeks later...

I worked on a cars cooling system and got a 0303 code after I was finished. Not a caddy. I got a different coil and engine runs fine. Coils do die on occasion

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I had a coil completly burn out on my 94 Concours. Smoke rolled out from under the hood. It had caused me problems for 6 months but would not give a specific code to work with. I had a mechanic tell me that he had a caddy coil short out so bad that it fried the wiring to the pcm and the pcm itself. Since I do not know when the spark plugs were changed,I thought I would start there on the 2003 Deville.

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  • 2 months later...

I have a 2001 cadillac seville sls. got it from an auction. has 91000 miles. It keeps throwing pcm p0300 codes, engine light, and idles rough at stop signs and when parked.

the plenum had a air leak. replaced it. some vacuum hoses cracked so i replaced those. there was a cyl 2 misfire code so i looked and there was alittle oil in around the spark plug so i cleaned it out and put all new spark plugs. but when i took the coil pack off it touched the engine while it was still hooked up and made a spark noise and i tested it and two of the things didnt have any juice going to them so i got a new coil pack from the junk yard and put it on. i cleaned the throttle body and egr valve. i put a new fuel filter on. after all that it seemed to run excellent until i drove it half mile when the light came back on and it started idling bad again. checked the fuel pressure regulator and its fine as far as i can tell. the fuel pressure is low though. it says 35 psi. it does jump when i put the throttle down and when i take the tube off of the fuel pressure sensor it jumps up to 40psi or so. when i turn the car to the on possition the fuel pressure tester kinda slowly makes its way to 35psi. What does this mean? i took the fuel pump out to look at it and the filter packet thing on the bottom of the fuel pump looks kinda blackish like maybe its dirty and making bad fuel flow or something. not sure why im getting low fuel pressure but worse of all this p0300 code is killing me. it says that on the dash but autozone's tester was more specific and told me it was p0304 (cyl 4). any ideas? please help. Thank you.

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Are there any other codes? If not, I would do a compression check.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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