: Engine misfire at certain times of the day - 2003 STS



crezzy2k1
07-10-12, 01:34 PM
Okay this thing has me stumped. It has only been happening for 3 days.
I drive 14 miles to work each day, there and back. Car starts fine first thing no problems.
I then drive to lunch which is about 2 miles each way. Car starts fine no issues.
Now after lunch i come back to the car start it up and all is well for about 2-5 minutes, then in light traffic it will develop a nasty misfire (it won't even rev higher than 2000rpm), the SES light will begin to flash, i get on to an open stretch of road and it calms a little but still light mis. By the time i get back to work the mis-fire is over and the engine is running normally again. The flashing SES has stopped. I shut the car off and go back to work.
I come out of work about 4 hours later, start her up, have a solid SES light and P0300 (mis-fire) code Current (the only code)

I start the car the next day in the morning and all is well and the Current P0300 goes into history, i then restart the car after lunch and back to the mis-firing.

This is driving me nuts!!!

What i have done so far is pull all the plugs, investigate them all of them where fine apart from one which looked fine apart from from the base of the thread which was wet, the electrode was to be expected though. I have since swapped that plug and its boot to the cylinder next to it to see if i can rule out the plug and ignition cassette.
Last time the plugs where changed was 57,000 miles ago and they all look good.
Something i should mention is that i have had P0420 (catalyst low efficency) come up twice in the past 40 days the last time it came up was last week. The car seems to run fine when not mis-firing.

Do you guys think that a clogged or partially clogged cat could be causing the engine to strain and cause a mis-fire?

CadillacLuke24
07-10-12, 02:59 PM
To me, the misfire and the low cat efficiency point to a raw fuel source in your exhaust. Either the plugs are bad, which is debatable, or you have a fuel delivery issue. The clogged cat MAY be a possibility, but I'm not sure. Keep in mind, I'm still learning myself, just my 2 cents, but that's what it sounds like.

Do whatever's cheapest. Check the cat, listen to your injectors, and check the fuel pressure regulator. Then go from there.

crezzy2k1
07-10-12, 03:31 PM
Thanks for the input CadillacLuke24. I have checked the injectors with the screwdriver to ear test, all of them are clicking along nicely. I have checked the regulator and see no problem with fuel in there or in the vacuum line. What i haven't checked yet in the fuel filter, i do have a new one lying around that i have been putting off installing it so I'll put that in and see if anything improves.

ELDO GUY
07-10-12, 03:41 PM
Just had this problem about a month ago Drove me nuts car wouldnt throw a code until it was real bad. Then it would go away for a while them come back again . My guess is the ICM . Mine had a intermintint problem I replaced it with one I had from a old coil pack I got luckly and guessed the right coil pack to attack from looking at the plugs. Look under (WOT PROBLEM and PO300) a lot of good ideas there and help from the forum.

crezzy2k1
07-10-12, 03:48 PM
Thanks ELDO GUY i read your thread yesterday, (i always do a bit of homework on these here before i post, so much info on here!) i was thinking about getting myself over to the nearest junkyard and picking up a coil cassette, but i'm not gonna be able to do that until the weekend. Did your plugs show any signs of problems while the ICM was bad?

crezzy2k1
07-10-12, 06:09 PM
Just got home from work. Car started and immediately started mis-firing at idle, i gave here a little gas and she sounds and smells terrible. I let the car warm up a little before i took the highway home, once on the highway the SES flashed on the onramp then went to a steady SES for the rest of the trip, once the car was upto speed there was again no issues. When slowing down for a stop light i no longer had the mis-fire.

I pulled into the garage and let the car idle a bit, no mis-firing. I pulled the codes and the only one is P0300 History, i then shut her off then restarted a minute later, i noticed this time the idle was about 650 as opposed to about 750 when i pulled into the garage, not sure what the computer is doing properly compensating for the mis-fire. I'll let her cool down for a bit then i'll go out there and start her again and see if she mis-fires, if she does i'll pull the plugs again and see whats going on.

While i was in the garage i gave the Cat a little tap with a broom handle (sophisticated test, i know ;-) i can definitely here rattling in there so I'm pretty sure its had it.

ELDO GUY
07-10-12, 08:54 PM
I would go to the junk yard and try and find one that looks good. The date of mfg is stamped on the back so id go for the newest i could find. Some of my plugs looked good and some not so much I think. I posted pictures of the plugs in that thread. If you swap out icm's make sure it all comes out some times a. Small connector stays in there if you miss it youll really have a problem.

Ranger
07-10-12, 09:30 PM
then in light traffic it will develop a nasty misfire (it won't even rev higher than 2000rpm)

While i was in the garage i gave the Cat a little tap with a broom handle (sophisticated test, i know ;-) i can definitely here rattling in there so I'm pretty sure its had it.
I was leaning towards the ICM or ignition cassette, but that CAT does sound highly suspicious.

CadillacLuke24
07-11-12, 01:51 AM
I read a sign in the Midas here that if the CAT went bad, it's an indicator that something else is amiss. For the knowledge files, can a CAT just get old and go out?

dkozloski
07-11-12, 03:48 AM
Get a spraybottle of water. When the engine is running rough and misfiring as badly as it will, spray the exhaust manifolds with water and you'll be able to tell pretty quickly which cylinder/s are misfiring.

crezzy2k1
07-11-12, 09:42 AM
Quick update:

Last night i went back to the car after it had been shut down for about 2 hours, started her up no mis-fire. The SES was on with P0300 in History, i shut her off.

I had a new fuel filter so i decided to go about putting that in just incase, i relieved the fuel pressure from the schrader valve in the engine bay (lots of pressure!) then went about trying to get the old fuel filter out, what a PITA it is to get that nut off!, anyway i got it out it looked fine. Put the new one, stated her up and she was running fine. Decided to take her onto the back roads for a few WOT's, again no issues at all. Filled her up with BP 87, then bought her home. All this time the SES was on.

Got out there this morning and stated her up for the trip to work, again no issues. SES went out about 1/2 mile down the road.
Got to work and she was running fine.

Would the ignition cassette/ICM really be that erratic?

crezzy2k1
07-11-12, 09:44 AM
Get a spraybottle of water. When the engine is running rough and misfiring as badly as it will, spray the exhaust manifolds with water and you'll be able to tell pretty quickly which cylinder/s are misfiring.

Bit tight in there to even spray the manifolds. Plus it hasn't ever mis-fired when i have been at home so even checking the plugs doesn't do much good.

crezzy2k1
07-11-12, 09:48 AM
I read a sign in the Midas here that if the CAT went bad, it's an indicator that something else is amiss. For the knowledge files, can a CAT just get old and go out?

You're right on that one. But i think the CAT has just decided to quit. 9 years/120000 miles, car originally came from New Jersey so perhaps lots of traffic has killed it. That being said this is the first car i have ever owned with a CAT that is on its way out.

Ranger
07-11-12, 11:35 AM
Would the ignition cassette/ICM really be that erratic?
It's possible that as it heats up the heat affects something. I've had a starter fail only when it was hot. Contacts expand and break contact.

Everything you ever wanted to know about CAT's.
http://www.aa1car.com/library/converter.htm

CadillacLuke24
07-11-12, 02:47 PM
Thanks Ranger! More info for the knowledge bank :thumbsup:

ELDO GUY
07-11-12, 03:34 PM
Im with ranger I think Its a heat related thing that causes some type of short in the coilpack/ ICM system . I had a cat problem on a 82 eldo and it was way different it was clogged up and limited the speed to about 45 MPH.

crezzy2k1
07-11-12, 06:38 PM
It's possible that as it heats up the heat affects something. I've had a starter fail only when it was hot. Contacts expand and break contact.

Everything you ever wanted to know about CAT's.
http://www.aa1car.com/library/converter.htm

What a great bit of information :)

crezzy2k1
07-11-12, 06:44 PM
Update:

Didn't go to lunch today in the car and had no issues whatsoever this morning when starting out and no issues when returning home, i did have about a 20 minute stop round the brother in laws and had no issues when i restarted which is usually when it'll mis-fire.

I will update as the situation changes, thanks everyone for your input.

crezzy2k1
07-12-12, 08:36 PM
Quick Update:

Started the car this morning and had no issues on the way to work.
Started the car at lunch time and it immediately started mis-firing, when we came back out and started it still was. The SES will now flash when driving along with it mis-firing and will stop and go solid as speed is increased above 30mph or so.

Well i started the car when i left work and initially it was okay while idling and then started missing again. It was then missing at practically every stop light until i got on the highway, when i pulled up to a stop light the car seemed fine then just started missing again, it was still missing when i got home which is a first!

I let her cool off for a while and then went and pulled the front plugs, the whole bank goes form very whitish to very darkish on a sliding scale from the ICM to the end. I had seen this the other day when i pulled them but now its much more obvious. When i pulled the rear bank a couple of days okay all the plugs were pretty much in the same condition.

So now i'm pretty sure its guaranteed to be the coil pack.

What a pain, i'm leaving parked up in the garage tomorrow, just so i don't ruin the CAT any further.

Submariner409
07-13-12, 09:21 AM
Have you pulled any Diagnostic Trouble Codes from the car's built-in code reader/scanner after one of these misfire events ?

I see the 0300 and 0420....................but, for other owners................

The sticky thread ^^^ "How to pull codes" for your OBD2 car, write them all down and whether each is Current or History,...........and

AWH: On-Board Diagnostic Trouble Codes (OBD2 Codes) - OBD2 Cadillac Codes (http://myweb.accessus.net/%7E090/dtcobd2.html)

crezzy2k1
07-13-12, 09:39 AM
Have you pulled any Diagnostic Trouble Codes from the car's built-in code reader/scanner after one of these misfire events ?

I see the 0300 and 0420....................but, for other owners................

The sticky thread ^^^ "How to pull codes" for your OBD2 car, write them all down and whether each is Current or History,...........and

AWH: On-Board Diagnostic Trouble Codes (OBD2 Codes) - OBD2 Cadillac Codes (http://myweb.accessus.net/%7E090/dtcobd2.html)

Hi Sub. Currently the only code is P0300 Current. P0420 has not come up in quite sometime.

Update:

Yesterday i went back down to the garage and for shits and giggles i decided to re-check the injectors. To my amazement one of them (front bank) is not working, i think this was intermittent at first and is now permanent. I plugged the connector from the end injector onto the affected injector and it worked so no issues with the injectors. The wires on the connector look a bit squished so I'm borrowing a multimeter and will check the connector when i get home tonight.
I have no codes for injectors at all just P0300 Current which is now obvious.

If this ends up being the issue and if its just the connector I'm going to relieved :)

crezzy2k1
07-15-12, 01:32 PM
Update

Okay after checking the continuity of the cables for Injector 6 back to the PCM all seems good. I also checked the voltage and all is okay, the injector itself is not at fault.

So with all that checked i started her up and all injectors were clicking away nicely, so i let her idle for 10 minutes or so then i heard the engine change tune and saw it bucking a little. So i checked out Injector 6 and it was no longer clicking, so i took the connector off and checked the voltage and i had +14V or so, so thats good, i then checked the Control cable and it was at a steady 2V or so which is bad i'm pretty sure it should be going up and down. So i then unplugged the injector next to it and tested the Voltage and Control cables, +14V and a variation in voltage as expected from the PCM cable which is correct, so i plugged that connector onto Injector 6 and it started clicking again, while i was there i plugged the connector from injector 6 onto the injector next to it and nothing. So it looks very much like the PCM has decided to call it a day on Injector 6, well after it gets a little warm!

Has this happened to anyone before, and is there anything i should do or check before i go and find a new PCM?

crezzy2k1
07-15-12, 01:36 PM
Just a quick note, i tested continuity back to the PCM after this had happened on Injector 6 connector and it was still good, So this isn't a loose connection, at least not in the wire.

crezzy2k1
07-15-12, 03:54 PM
Update:

I have replaced the ICM with no effect on the injector. This is really pointing at the PCM.

CadillacLuke24
07-16-12, 04:54 AM
Yuck. PCMs are pretty reliable from what I've heard, but I suppose they could fail. Check junk yards. You'll have to have a TECH II at a dealer transmit your current mileage and such over I think.

crezzy2k1
07-16-12, 09:11 AM
Yeah i found a place online that sells them for $160 and also codes them to your specific vehicle. The only thing they can't do is the CASE re-learn which is a tech2 job.

crezzy2k1
08-15-12, 12:14 PM
Update:

This has finally been fixed. It did turn out to be the PCM. Bought a used one off eBay then had a place called 'AutoSelect' perform the reprogramming. All in all cost me $165 to get it all done which considering that the PCM is over $800 at the dealer, was a bargain!

Next repair: The steering shaft!

Oh i should mention that the Cadillac dealer i went to in this area didn't have a clue how to re-program the PCM and told me i needed to bring the entire donor car to get it done even after i explained to him how to do it! I promptly walked out!

CadillacLuke24
08-19-12, 04:08 PM
Glad you got it fixed. Even more glad that you are avoiding the local dealer! They sound like a bunch of morons!