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Help with an '01 STS misfire P0300


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Driving to the airport last weekend started to feel a stumble going uphill on the interstate, sat back and waited for the SES to start flashing and sure enough PCM 0300 misfire.

Turned around and drove it back got the plant truck and off to FL for a nice week.

Started working on it this morning so far have checked:

lifted fuel rail and checked for leakby with key on -none found
Inspected injectors and connectors, found one on cylinder 4 that a mouse had chewed(this was along time ago) so resoldered a new wire in.

All injectors ohmed out @12.1ohms.

Pulled front coil pack and all plugs and boots were clean.

Rear coil pack boots and all plugs except #2 cylinder was wet(gas wet) and appeared to not be igniting.

Cylinder test at first few bumps was 30-40, but I could hear leak, tightened and got 145-150. Air tested cylinder and saw nothing in the coolant reservoir. About got myself in a pickle as the plug adapter came off the hose down bore and socket wall size was too thick - good old super glue on the hose threads waited 20mins and it backed out.

Another thing I checked by using some long swabs was the cylinder top #4 clean #2 a lot of black gum oil/gas smell. I know when carbon on piston gets hot it can cause misfires

I replaced the rear coil bank as it was still under warranty from my tune up back in September(exact same 0300 misfire and same cylinder had wet plug). Kicking myself for not moving the plug to a different cylinder - the plugs ohmed out ~4.16kohm

Well got it back together and no dice - still missing. The cat doesn't smell nearly as bad as it did last fall.

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Started pulling injector connectors one at a time - pulled 0202, 0204, 0206, and 0300 on 2,4,6 cylinders.

#8 0300 code went away and pulled 0208.

So I'm going to the store for a plug, boot, and injector. Will replace injector first as its the easiest, could swap them I suppose to see if it follows but the on/off of parts is getting old fast.

As I was browsing parts online noticed they sell the injector connector, they don't have it in store - anyone ever soldered a new one in? wondering how long the leads are as I would replace my repair from yesterday on cylinder #4. These injectors gate pulse the ground (as I understand) and don't want any funky resistance to cause future issues

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Clarify above post(s) - I'm working on the rear bank cylinder #7 - sorry for the confusion

Replaced injector first changed nothing.

Pulled coil pack and tested cylinder ~145psi, changed plug and boot -still no fix.

Just to ensure I didn't disturb anything and create another problem I pulled the injector to #7 again and the misfire 0300 code goes away????

I was browsing other posts around this issue and there was a diagram posted with the crank position sensors feeding the ICM/coils, and one fellow that tried a whole bunch of stuff and showed back up under a new handle saying the CPS fixed his problem.

I had mine replaced back around 2004(35k) under the TSB in warranty.

Any ideas? I'll probably replace the CPS's or take it to the shop

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What are you using to pull codes? There really is no such thing as a p0301-308 on a 2001. Just p0300...you then use a Tech 2 to figure out which one is misfiring. See attached pic. Some tools may be using reverse engineering to make up the miss codes..

The front and rear coils are identical...except for color. I would swap the coils and see if the problem moves with the coil.

post-2-0-57257800-1393187069_thumb.jpg

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I'm just pulling the codes from the DIC. The 0202,4,6 etc came when I disconnected each injector - I didn't look them up but assume its open injector circuit or something along those lines. They were not there before I started testing.

The only code I'm getting with everything hooked up is 0300.

I put a screwdriver on each injector and am hearing a clicking noise on each.

Going to do a vacuum leak check and PCM connector inspection

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Ok so I swapped the coils still had the 0300. Removed injector plug from #7 and the 0300 code clears I get 0207. The vibration feels less prevalent but is still there

With injector plug pulled from #1(same coil) the 0300 stays and I get a 0201.

Tested for intake leak with both propane and carb cleaner and have no rpm difference - even spraying up underneath from the passenger side of the intake.

The PCM connections looked factory new.

Tested voltage at each injector connection with key on and get 12V on the positive.

I don't have my o-scope at the house to see if the injector negative is gating properly I assume a scanning tool will see this as well.

Starting to wonder if the PCM isn't the culprit - any troubleshooting tips for it?

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Ok so I swapped the coils still had the 0300. Removed injector plug from #7 and the 0300 code clears I get 0207. The vibration feels less prevalent but is still there

With injector plug pulled from #1(same coil) the 0300 stays and I get a 0201.

Tested for intake leak with both propane and carb cleaner and have no rpm difference - even spraying up underneath from the passenger side of the intake.

The PCM connections looked factory new.

Tested voltage at each injector connection with key on and get 12V on the positive.

I don't have my o-scope at the house to see if the injector negative is gating properly I assume a scanning tool will see this as well.

Starting to wonder if the PCM isn't the culprit - any troubleshooting tips for it?

The PCM is a very durable unit. I would not suspect it as the cause of the misfire.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I cannot think of any way...to make a p0300 diagnosis any more difficult.

Sure, the miss will still be there if you swap the coils. The idea is to see if the miss has now changed cylinders. The only real accurate way to do this is to look at the missfire graph on a Tech 2. See picture posted above.

Without a Tech 2...you really are just guessing.

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Thanks for bearing with me guys - I'd love to own a Tech 2 and since It's now fixed and there is no new car payment or hefty shop bill one may be in my future toy tool bag.

So after I went up to the local shop the owner was milling around and wasn't gonna give her scan without leaving it there :angry2: - had to wait for the GF to pick me up on the way home from work.

Drove it home pondering why the heck I never changed the injector/plug/boot on the one cylinder that was wet from my original diagnosis(cylinder #1).....I was out thinking myself

Installed the injector from #7 that I replaced yesterday - no fix, pulled boot and plug - plug was dry(probably from being warmed up now) but not grayish/brown.

Installed plug&boot from #7 and put the coils back in their original position - VIOLA :yupi3ti: starts right up smooth I rev's idles all back to normal.

Thanks for the moral support anyways - I didn't sleep much last night pondering how the heck I was going to fix this or if I was going to end up with a car payment.

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Thanks for bearing with me guys - I'd love to own a Tech 2 and since It's now fixed and there is no new car payment or hefty shop bill one may be in my future toy tool bag.

So after I went up to the local shop the owner was milling around and wasn't gonna give her scan without leaving it there :angry2: - had to wait for the GF to pick me up on the way home from work.

Drove it home pondering why the heck I never changed the injector/plug/boot on the one cylinder that was wet from my original diagnosis(cylinder #1).....I was out thinking myself

Installed the injector from #7 that I replaced yesterday - no fix, pulled boot and plug - plug was dry(probably from being warmed up now) but not grayish/brown.

Installed plug&boot from #7 and put the coils back in their original position - VIOLA :yupi3ti: starts right up smooth I rev's idles all back to normal.

Thanks for the moral support anyways - I didn't sleep much last night pondering how the heck I was going to fix this or if I was going to end up with a car payment

Would you mind giving a condensed recap, and what the fix was? I am somewhat lost on the sequence and what was replaced/repaired.

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Initial diagnosis from when I started working on it Saturday morning:
Pulled both coil banks each boot and plug inspecting and reinstalling as I went. The #1 cylinder plug was wet the remaining were clean burning.

All the ignition components were replaced in Sept '13. The plug and boot both ohmed out approximately the same as the others as I tested them.

While in there tested compression on all cylinders ~140psi and let each hold for 10-15mins with no leak down

The ignition coils were under warranty so I went and exchanged the rear bank coil installed and had the same result so I started to think it was fuel related..

Sunday morning I set out to see if an injector was at fault.

Lifted the rail and checked with key on for leakby and none noted.

Ohmed out all injectors ~12ohms each. I did fix a FI connector to #3 cylinder that had been chewed by a mouse some time ago - buggers made a nest under the intake and chewed through the knock sensor back then ~3yrs ago. Of course that fix made no difference.

Anyways continuing on my fuel injector diversion I started pulling one injector plug at a time to see if the misfire cleared - thinking if the fuel was gone from the cylinder I wouldn't have the misfire...right???

As I worked my way through and pulled #7 the misfire went away so I started to believe I had a bad injector/plug/boot on #7.....

So I picked up an injector plug and boot installed on #7 - no fix

even went so far to track the negative gate pulse wire back to the PCM, while working my way back there I did find a stud that was rubbing on the split in the loom of the harness - though I may have found my culprit but the lead ohmed out under 1ohm - I re-taped it anyways to prevent any future issue.

So I'm off in the weeds troubleshooting without a Tech 2 tester to know what cylinder(s) were cause my problem. I had swapped the coils front to back and same result.

Drove to the shop for a diagnostic that I wasn't getting right away(that chit irritates me) and had the epiphany to put the plug&boot from #7 cylinder into the #1 cylinder - which was originally wet.

First swapped the injector from #7 into #1 cylinder no dice.

Installed the old plug & boot from #7 into #1 and put all the coils back in their original place and it purred like a kitten.

So moral of the story don't troubleshoot a misfire with my ridiculous method.... In reality I should have changed the plug and boot on #1 cylinder to begin with.

Sorry if that is not a "condensed" recap :glare:

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Good to hear it's fixed.

Just FYI....the 2000-2003 Deville/Seville/STS is a little unique in that there is no p0301-308 codes. Just p0300. The car is dependent on a Tech 2 to show you which cylinder is missing.

I think the 2004 cars with the indivdual coils will display a p0305 for example. That certainly helps when your out on the road and dont have your Tech 2 handy..

Also, the solid 4 cylinder COP coil banks make it hard too...I used to carry a tested spare one in my 2001.

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I hear you Logan - I'll be keeping my eyes peeled for a tech 2 diagnostic tool. Should have never thrown the old COP's from the change out back in fall away pretty sure that the front was good at least

You know what's worse with these 4cyl COP's is they put the screws for the ICM underneath so it has to come out anyways - might have been the same on the GF's 2.4 Grand Am but that was a heck of a lot easier to pull.

All I can say is I've become proficient at pulling COP's and plugs now...(more than I'd like to be anyways) a 10mm swivel socket is a man's best friend, 1/4" drive of course with a 6" extension

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