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2002 STS stock
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Thank you, submariner, for the in-depth tech info! I was hoping you'd jump in with some gold nuggets...:).

I still have to check fuel pressure and the regulator, then swap injectors around, now that I have more new O-rings.

Parker, I think if I were you, I'd do a cold compression check, just to eliminate other mechanical issues. I'd be worried about gummed-up rings or valve seating issues, or even ... gotta stop before the rabbit-hole gets too deep.

Now I'm off to look into how to back-probe the PCM connectors so I can hook up a scope. And try to find a cheap piece of software that will give me OBDII mode 6 on the Caddy (my current setup with Veepeak and OBD Fusion works fine with the '06 Ford, but leaves me hanging with the STS.
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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86,969 Posts
I've replaced the plugs (with Autolites
For what it's worth, we used to have a GM powertrain engineer that posted here. He told us that the Northstar, for some odd reason did not like plugs other than A/C Delco (or Denso), but especially did not like Autolite. I don't recall him ever going into any great detail as to why, but it would be interesting to see your results after installing the proper A/C Delco plugs. Keep us posted, inquiring minds want to know.
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #83
Thank you, submariner, for the in-depth tech info! I was hoping you'd jump in with some gold nuggets...:).

I still have to check fuel pressure and the regulator, then swap injectors around, now that I have more new O-rings.

Parker, I think if I were you, I'd do a cold compression check, just to eliminate other mechanical issues. I'd be worried about gummed-up rings or valve seating issues, or even ... gotta stop before the rabbit-hole gets too deep.

Now I'm off to look into how to back-probe the PCM connectors so I can hook up a scope. And try to find a cheap piece of software that will give me OBDII mode 6 on the Caddy (my current setup with Veepeak and OBD Fusion works fine with the '06 Ford, but leaves me hanging with the STS.
I’ve already done a cold compression check and all the numbers were about 30-40 psi over minimum lol
 

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2002 STS stock
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I’ve already done a cold compression check and all the numbers were about 30-40 psi over minimum lol
Well, that puts a concrete plug in THAT rabbit hole! Lol

Ranger, that's interesting that something as simple as a sparkplug, supposedly designed for a given heat range and spark size, could have such a big impact on an internal combustion engine. The Ford 4.6s didn't like ACDelcos, but thrived on Autolites...go figure! These are not your lawnmower engines anymore...:-(
 

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2005 Cadillac Deville
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24 Posts
I'm not the sharpest tool in the drawer, but my 2 cents here; Even though we may question their intelligence at times, the engineers that design these cars are pretty smart fellers. They put a certain plug in for a reason. I have found, as a general rule, with a very few exceptions, Chrysler likes Champion, Ford likes Motorcraft or Autolite, Gm likes Ac/Delco.
 

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2002 STS stock
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Yup, sounds like corporate capitalistic favoritism to me...! Heeeheeeeeee...

Well, I think I found the miss problem, but I'm not sure. Did a compression test today, and all cylinders are between 140-160 at 2500' above sea level, which in my book is good. HOWEVER, #1 was 130, which I'll accept, and #7 was 65. So, haven't squirted oil yet to check rings, still looking for a straw to drip it in with.

Looking at the engine compartment, I think it wouldn't be too difficult to remove the rear (right) cam cover to check the valves from top-side. Is that relatively straight-forward, or would I be missing something?
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #87
Emissions loop change occurs at about 160 degrees coolant temp, depending on ambient temperature. Before loop change the O2 sensors are out of the system. AIR operates for X to XX seconds depending on ambient temperature, coolant temperature, time since last run event, time since last start, other inputs. Sometimes it won't run at all.

2002 STS temp gauge - loop change at about 11 o'clock.

View attachment 582516
This is where I’m at now. Honestly don’t know which direction I’m going to go or how far but I’m labeling things as I take them apart so lol
582540
582541
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #88
Yup, sounds like corporate capitalistic favoritism to me...! Heeeheeeeeee...

Well, I think I found the miss problem, but I'm not sure. Did a compression test today, and all cylinders are between 140-160 at 2500' above sea level, which in my book is good. HOWEVER, #1 was 130, which I'll accept, and #7 was 65. So, haven't squirted oil yet to check rings, still looking for a straw to drip it in with.

Looking at the engine compartment, I think it wouldn't be too difficult to remove the rear (right) cam cover to check the valves from top-side. Is that relatively straight-forward, or would I be missing something?
Something is definitely wrong with 7. Did you do that warm? Fsm says minimum is 140
 

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Thanks for that tidbit, Parker. I'm thinking I'm gonna have to get out of your thread and start my own, with this situation. Obviously not the same problem you're chasing.

Looks like you're almost ready to go on the coolant crossover. I recently had to replace the outlet for the heater core feed, under the EGR. I used the Dorman fitting, with the plastic holder that came with it, with good results. I found that unbolting the brake master cylinder from the booster gave me the rest of the room I was looking for, but still broke up the OEM plastic that had degraded over time...ergo, the Dorman. Plus, it looks like the Dorman has a lot more sealing rubber inside it to wrap around the pipe end; that's why the silicone A little silicone spray lube, and the pipe slid right into its new home.
 

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2003 Cadillac Seville STS
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60 Posts
Discussion Starter #92
Thanks for that tidbit, Parker. I'm thinking I'm gonna have to get out of your thread and start my own, with this situation. Obviously not the same problem you're chasing.

Looks like you're almost ready to go on the coolant crossover. I recently had to replace the outlet for the heater core feed, under the EGR. I used the Dorman fitting, with the plastic holder that came with it, with good results. I found that unbolting the brake master cylinder from the booster gave me the rest of the room I was looking for, but still broke up the OEM plastic that had degraded over time...ergo, the Dorman. Plus, it looks like the Dorman has a lot more sealing rubber inside it to wrap around the pipe end; that's why the silicone A little silicone spray lube, and the pipe slid right into its new home.
No worries you don’t have to get off this thread, Sub and ranger will give you the best info and I doubt anyone else will give you any better tbh. I’m debating if I should attempt at dropping the whole cradle with the motor and spend a grand or two replacing sensors and studding the motor but I’m not sure. All my compression is good I just can’t understand why I can’t figure out the misfire. I must have missed something. Only place the wiring harness looks sketchy is behind the exhaust manifold for the crank sensors the wires are cracked and weak but still worked. And the part that lays on the tranny is toasted down to the wires so I have been working on getting the wiring harness out to make repairs and inspect it.
 

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2002 STS stock
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14 Posts
And the part that lays on the tranny is toasted down to the wires so I have been working on getting the wiring harness out to make repairs and inspect it.
I found my PCM harness injured just beyond the tranny rise or seam, whichever it is. The harness was laying on top of the fill check cap, apparently had been pushed down by the resonator or zip tube or MAF assy. I found three bared, almost broken wires, one a sensor ground (might've been for the MAF?), and a few more with the insulation intact, but very pinched. I don't think any were broken, so I butt-spliced the three with damaged insulation, wrapped it up really good, and rerouted it along the top of the tranny/engine junction going toward the radiator, to avoid those two rises. Hopefully, the close proximity to the engine won't toast it...

Only place the wiring harness looks sketchy is behind the exhaust manifold for the crank sensors the wires are cracked and weak but still worked.
You may have found your problem. The crank sensors detect a difference in time (rotational speed) between combustions, and if the crank slows down between two combustions (lesser power), the assumed misfire is detected by the PCM, whether it is a real miss or not. Weakened/burnt/damaged wires can slow down one of the signals, and if that happens, the PCM detects a time differential, = misfire. Quite probably ramdom (0300). I think I'd get a pigtail or two, splice them in, and connect them up to two new crank sensors (relatively VERY cheap), and see what happens.

BTW, I used to design digital logic PCBs, and became very familiar with time delays through traces (wires), so it's not exactly a shot in the dark!
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #94
I found my PCM harness injured just beyond the tranny rise or seam, whichever it is. The harness was laying on top of the fill check cap, apparently had been pushed down by the resonator or zip tube or MAF assy. I found three bared, almost broken wires, one a sensor ground (might've been for the MAF?), and a few more with the insulation intact, but very pinched. I don't think any were broken, so I butt-spliced the three with damaged insulation, wrapped it up really good, and rerouted it along the top of the tranny/engine junction going toward the radiator, to avoid those two rises. Hopefully, the close proximity to the engine won't toast it...



You may have found your problem. The crank sensors detect a difference in time (rotational speed) between combustions, and if the crank slows down between two combustions (lesser power), the assumed misfire is detected by the PCM, whether it is a real miss or not. Weakened/burnt/damaged wires can slow down one of the signals, and if that happens, the PCM detects a time differential, = misfire. Quite probably ramdom (0300). I think I'd get a pigtail or two, splice them in, and connect them up to two new crank sensors (relatively VERY cheap), and see what happens.

BTW, I used to design digital logic PCBs, and became very familiar with time delays through traces (wires), so it's not exactly a shot in the dark!
Oh okay so the exact spot as mine across the tranny next to the full tube. Mine was like tightly pulled down my short wires or something you could barely move it. It was definitely rubbing and all the plastic cover tube thing was burnt and crispy but haven’t been able to get the whole harness off to look. I was thinking about taking everything off and putting better heat protector on it. And oh okay sounds good man! The crank connectors were still a b**** to get to even with the radiator and such out... but the wires seemed very stiff and frail. I’m not holding my breath that those were the issue but I can hope and I’ll replace the wire. All the wires down there seem a little weak but mostly the crank sensors. I’ll put new high quality heat guards on the new wires. Have you found anything else that I should look at while I have it this far ripped apart? I’m not really sure what I’m looking for anymore haha
 

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and I’ll replace the wire
Yeah, I would look for a routing path that stays away from any heat source, like the exhaust, the AIR pipe, such like that, too. If you have to extend the wires, now would be the best time to do that. Or, later, if you have problems with them, I can loan you my wire stretcher. Works as well as muffler bearings! Lol

Have you found anything else that I should look at while I have it this far ripped apart?
Unfortunately, no. At least not related to the RMS (Random Misfire Syndrome). However, in your photos, I did notice that your alternator looks just like mine...all caked up with oily debris (probably a mix of dripped oil and general road dirt, with a small amount of tire rubber). I don't know if I should be concerned with it, because my charging system appears to be working fine. But I'm afraid of the limited cooling airflow through the body and over the diodes. I just can't understand why automotive engineers would place a piece of electrical hardware in such a vulnerable position. I'm afraid to clean it off in-situ, for fear that I'd cause it to burn out. I guess I'll just tell my buddy not to drive through any deep puddles...

It was definitely rubbing and all the plastic cover tube thing was burnt and crispy but haven’t been able to get the whole harness off to look.
Was that big harness melted up on the part that goes to the PCM connectors, like mine was? I was able to pull back the plastic tube (can't think of the appropriate name now - damn senility!) plenty far enough to separate the wire bundle and inspect each wire. I'd recommend you do that, just to make sure something didn't get hidden in all the jostling. After all, the coil and injector feeds, and a bunch of the sensors, do use that super-highway.
 

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2003 Cadillac Seville STS
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60 Posts
Discussion Starter #96
Yeah, I would look for a routing path that stays away from any heat source, like the exhaust, the AIR pipe, such like that, too. If you have to extend the wires, now would be the best time to do that. Or, later, if you have problems with them, I can loan you my wire stretcher. Works as well as muffler bearings! Lol



Unfortunately, no. At least not related to the RMS (Random Misfire Syndrome). However, in your photos, I did notice that your alternator looks just like mine...all caked up with oily debris (probably a mix of dripped oil and general road dirt, with a small amount of tire rubber). I don't know if I should be concerned with it, because my charging system appears to be working fine. But I'm afraid of the limited cooling airflow through the body and over the diodes. I just can't understand why automotive engineers would place a piece of electrical hardware in such a vulnerable position. I'm afraid to clean it off in-situ, for fear that I'd cause it to burn out. I guess I'll just tell my buddy not to drive through any deep puddles...



Was that big harness melted up on the part that goes to the PCM connectors, like mine was? I was able to pull back the plastic tube (can't think of the appropriate name now - damn senility!) plenty far enough to separate the wire bundle and inspect each wire. I'd recommend you do that, just to make sure something didn't get hidden in all the jostling. After all, the coil and injector feeds, and a bunch of the sensors, do use that super-highway.
Yea how much would you charge for a month with the wire stretcher lmao. Yes the alternator looks terrible and there is probably a awesome half an inch of the caked crap right above it too. Looks like it’s mostly from the valve cover... I’ll have to deep clean that. If it destroys the alternator so be it I can’t stand looking at that tbh lol. The big raceway that lays across the tranny next to the fill cap was so tightly pulled down across the tranny I had to unhook a bunch of sensors to be able to pull it back. The part in the middle that splits in multiple directions had what looked to be electrical tape or something and it’s all crispy and gross. Wondering if underneath it made it through any insulation. Do you know where G106 is the main pcm ground? I found theee ground wires on the lower from of the engine but they don’t look big enough to be the main pcm ground.
 
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