: Multiple mechanics, no solutions, cold engine random misfires P0300



Dead_Cow
03-10-09, 10:55 PM
Hey everyone. My 2006 STS4 (V8) has this symptom, and everything seems to point towards the ECM, but I thought I'd post here before I buy one and have it swapped. I do realize that this could also be a air temp sensor thing... but I'm unsure of why the problem manifests the way it does.

I've taken it to 3 different mechanics (one being a stealership), and no one can find what's wrong. Here's the symptoms:

From COLD start (totally cold) the engine random misfires. It's really bad. Not just an occassional random misfire. It will continue to do this (but gradually slow) this until the engine gets to FULL operating temperature... and I mean complete. Not a notch below full operating temp. On occasion, this throws a P0300 (random misfire) code.

After this has been doing this for a little while (one time I drove it for a few weeks with it doing it between mechanics) it will start rough idling when hot. I took the intake assembly apart and cleaned the throttle body. That fixed the rough idle (when HOT) entirely - no change to the random misfires. Once the engine is HOT though, STILL get random misfires, but probably what would be considered normal (is it normal to have a few random misfires.. I mean like 1 or 2 per cycle?) :confused:

Here's what's been done:


The car's been put on a scope and the misfires monitored. One time a mechanic swore it was only occurring on cylinder 5 and that it was probably a bad plug, so we swapped the plugs, and the problem did not go away. Then just for safe measure, I put all new plugs in. The problem did not go away.
another mechanic had heard of fuel injectors clogging on this model, so I swapped an injector with the supposed misfiring cylinder with a supposed good one (I say supposed because the misfires are truly random, the whole "bad cylinder" idea was just a figment of imagination). again, no change.
I've put a new fuel filter in, replaced the air filter, all the routine stuff.
The fuel pressure has been checked, it's in spec.
The spark power has been check, it's in spec.
The vacuum system has been checked. There's no leaks.
The MAS has been eliminated as the culprit


Here's what I've garnered off this site (things others have claimed fixed their issues):


It's not a loose intake plenum.
It's not EVAP.

The DEALER suggested the oil was low (it wasn't REALLY low, just not up to the top of the FULL line). Get that. That was their diagnosis - and while I understand the sensitivity of these engines, it would NOT make the engine sputter like this in cold weather. EVEN IN THAT CASE... I did as told and not only added oil, but fully replaced the oil/filter.

If anyone has any suggestions, I'm open to them. The mechanic I have the car at now is especially tenacious, and takes not being able to find the problem personally.

Things you should know:


I'm very mechanically inclined, so feel free to pelt me with acronyms and explicit instructions.
My car only has 75k miles on it.
My car (again) is an STS4 premium with V8 and NAV (I know it probably doesn't matter)
My car is BONE STOCK.

If anyone here thinks it's the ECM, and knows what's involved with programming one, and any shortcuts, I'd like to hear that too, as well as any tips you can offer. Thanks!

EChas3
03-10-09, 11:20 PM
Any way you could have bad fuel?

Dead_Cow
03-10-09, 11:30 PM
Any way you could have bad fuel?
Thought of that when it started happening. Filled new fuel, added some drygas. No change. New fuel at next fill up added some fuel line cleaner. No change.

05+CTS+V
03-11-09, 03:55 PM
You mentioned changing plugs, but I didnt' see anything about changing wires. Have you tried different plug wires or tested the current wires?

WillySTS
03-11-09, 05:10 PM
You mentioned changing plugs, but I didnt' see anything about changing wires. Have you tried different plug wires or tested the current wires?

All current Caddies are "coil on plug", except Escalades. No plug wires on this one.

You may not want to hear this but, have them check the cooling system for hydrocarbons...you may have this misfire due to coolant in the cylinders,hence a headgasket leaking. When warm, it seals up until it gets much worse. Also while the misfire is ocurring is there white smoke or steam from the exhaust?

ifmracin
03-11-09, 06:05 PM
Mine was running kinda rough the other morning when I started it and then it tossed a code.
My tech scanned it as a cam timing sensor, he checked the sensor and said it was ok, and that it probably just messed up once.

Any chance that might be part of the issue? (though it didn't throw that sensor code at you)

Dead_Cow
03-11-09, 06:23 PM
All current Caddies are "coil on plug", except Escalades. No plug wires on this one.

You may not want to hear this but, have them check the cooling system for hydrocarbons...you may have this misfire due to coolant in the cylinders,hence a headgasket leaking. When warm, it seals up until it gets much worse. Also while the misfire is ocurring is there white smoke or steam from the exhaust?
...and we checked the coil pack(s). Good spark.

We checked both the misfiring cylinders for hydros in the coolant. Compression is perfect, and the coolant is top notch, and not a bit low at all. Besides, random misfires are happening on both sides.... how likely would BOTH head gaskets going be? esp @ 75,000? I will say I'm entertaining this idea along with the ECM even before this post, but you know, I'd almost RATHER it be the gasket than the ECM.... getting to the heads is pretty damn easy in this car, and the gasket doesn't require programming.


Mine was running kinda rough the other morning when I started it and then it tossed a code.
My tech scanned it as a cam timing sensor, he checked the sensor and said it was ok, and that it probably just messed up once.

Any chance that might be part of the issue? (though it didn't throw that sensor code at you)
I don't believe so.

C&C
03-11-09, 07:21 PM
Sounds like a tough nut to crack but how about a bad coolant temperature sensor. If this is bad (telling the ECM that the engine is already warm) the ECM is going to send the fuel map for a hot engine (closed loop) and that fuel map is going to be way-too lean resulting in a very rough idle.

Dead_Cow
03-11-09, 07:25 PM
Sounds like a tough nut to crack but how about a bad coolant temperature sensor. If this is bad (telling the ECM that the engine is already warm) the ECM is going to send the fuel map for a hot engine (closed loop) and that fuel map is going to be way-too lean resulting in a very rough idle.
I like the sound of this. Good idea. But here's the offset of that... if the coolant temp sensor was bad, wouldn't the heat just start blowing right away? Because the sensor for interior heat works fine... it doesn't come on until the engine is actually warmed up.

If I unplug the sensor and start the car when cold, would the ECM throw a code, but know to get it's temp sense from another sensor at that point? How could I test this without actually putting in another sensor?

Dead_Cow
03-11-09, 09:00 PM
You may not want to hear this but, have them check the cooling system for hydrocarbons...you may have this misfire due to coolant in the cylinders,hence a headgasket leaking. When warm, it seals up until it gets much worse. Also while the misfire is ocurring is there white smoke or steam from the exhaust?
I was thinking about this tonight... if it was the head gaskets, it would seem that the misfire would not be random.. it would be continuously in one or two cylinders where the leak was occurring, so this furthers my idea that it's not the head gasket.

Just thinking outloud.

I do like the other suggestion of the coolant temp sensor, but it would seem that if the car was on a Tech II, that I would see the code being thrown from the sensor. Can the sensor just get "stuck"? Is that possible?

Superjim
03-11-09, 11:43 PM
I had a kinda...similar..issue a few years ago on a car with plug wires.
(I know you have coil packs.)

It ran GREAT when fully warm, but would miss pretty bad on the first start of the day.

Turned out to be a VERY TINY hairline crack in the top of the distributor cap.
As the engine cooled off...condensation was getting in the crack, and upon a cold start...the spark was going to the condensation in the crack, instead of the plugs.

Remember..current will ALWAYS follow the path of least resistance.

You may have a crack in a coil pack.

Texas Jim

Dead_Cow
03-12-09, 01:38 AM
I had a kinda...similar..issue a few years ago on a car with plug wires.
(I know you have coil packs.)

It ran GREAT when fully warm, but would miss pretty bad on the first start of the day.

Turned out to be a VERY TINY hairline crack in the top of the distributor cap.
As the engine cooled off...condensation was getting in the crack, and upon a cold start...the spark was going to the condensation in the crack, instead of the plugs.

Remember..current will ALWAYS follow the path of least resistance.

You may have a crack in a coil pack.

Texas Jim
That would not explain random misfires in multiple cylinders, just a single one that had the crack. What I'm looking for will have an overall impact on potentially every cylinder, not just one, and not just two. :)

pjbizjak
03-12-09, 10:55 AM
Is your engine using excessive oil? I had a similar problem with my 2000 sts *star about rough idle at cold startup, then as engine warmed up it went away. It would come up with the misfire code, took it to the dealer and their initial diagnosis was plugs and wires which they replaced, the problem went away but only for a short period of time and then the problem started again. Took it back to the dealer and their second diagnosis was head gasket leaks, however my coolant level never went below the full line??? The estimated repair costs $3000.- $3500. My personal diagnosis, the reason I asked about excessive oil consumption, was that oil was getting into the cylinders when the engine was shut down and at startup causing the misfires and it took awhile for the oil to be 'burned' off. This IMO solved two problems, excessive oil consumption(oil getting into clyinder) and rough idle at start(burning off the oil). I was going through 2 qts of oil per 1500 mi. I decided to trade it in on an 07 STS 1SG. That solved the problem!!!

Angus Young
03-12-09, 11:10 AM
I had similar problems except mine would start misfireing when it was fully warm and only occasionally, it turned out to be one of the relays in the 2nd bank that was not fully seated, these are located in the big fuse box on the passenger side, just something to try, sometimes you have to reseat in order to get a complete connection, or I have had one coil pack go bad and produced the same conditions when cold.

Dead_Cow
03-12-09, 11:11 AM
Is your engine using excessive oil? I had a similar problem with my 2000 sts *star about rough idle at cold startup, then as engine warmed up it went away. It would come up with the misfire code, took it to the dealer and their initial diagnosis was plugs and wires which they replaced, the problem went away but only for a short period of time and then the problem started again. Took it back to the dealer and their second diagnosis was head gasket leaks, however my coolant level never went below the full line??? The estimated repair costs $3000.- $3500. My personal diagnosis, the reason I asked about excessive oil consumption, was that oil was getting into the cylinders when the engine was shut down and at startup causing the misfires and it took awhile for the oil to be 'burned' off. This IMO solved two problems, excessive oil consumption(oil getting into clyinder) and rough idle at start(burning off the oil). I was going through 2 qts of oil per 1500 mi. I decided to trade it in on an 07 STS 1SG. That solved the problem!!!
No excessive oil consumption. At all. In fact, no low fluids at all. When we took the original plugs out, they were all clean as a whistle. A shame to have changed them, to be honest.

EChas3
03-12-09, 12:18 PM
The operating temperature relationship keeps bugging me. What components warm more slowly to fit with this temperature profile?

Does the miss get worse with RPM's?

What's an ECM from an auto-recycler cost?

Just thinking and typing... Good Luck!

Dead_Cow
03-12-09, 12:30 PM
The operating temperature relationship keeps bugging me. What components warm more slowly to fit with this temperature profile?

Does the miss get worse with RPM's?

What's an ECM from an auto-recycler cost?

Just thinking and typing... Good Luck!
For posterity, and engine ECM can be had for $400. it's the programming that is stupidly expensive.

He's an update, the mechanic that was looking at it had his computer setup to read the cylinder fire order as a regular GM engine, not a Northstar. Now that it's setup correctly, the problem seems to be pretty apparent. Misfires are all on the one "side" of the engine, and the coil pack does indeed seem to be the source of the problem. I'm gonna replace the assembly and see if that does the trick. :preach:

EChas3
03-12-09, 12:35 PM
Thanks for the update.