2004 3.8 #1 cylinder misfire

**BigToyz** · 03-21-2020

Im struggling guys and need your help. I am diagnosing a 2004 3.8 for a young lady. Its got a constant #1 cyl misfire. Its got a new plug, wire, and injector, electic plug in was noid lt tested, (good) and its got 125 psi compression. It has excellent spark running with the #1 wire attached to a spare plug. I changed the plug wires on the coil because its a wasted spark and it still is only #1 misfire.
I havent checked fuel pressure which I will do monday but........what else could there be? Its got fuel, I verified its got spark, compression??????
Thanks for ANY suggestions.

**Fivefingerdeathpunch** · 03-21-2020

Did you try moving the coils around to see if the misfire follows the coil?

Another possible cause is the ICM under the coils.

**BigToyz** · 03-22-2020

Originally Posted by Fivefingerdeathpunch

Did you try moving the coils around to see if the misfire follows the coil?

Another possible cause is the ICM under the coils.

No, I didnt try switching coils. I figured if that side on the coil was bad, when I switched the 2 plug wires, it would follow to that cylinder. And I did pull #1 plug wire and ran the engine with a spare plug snapped in and it has a great spark. As much as I wanted to believe it was a spark issue, it sure seems to be working correctly. How does the ICM control the coil? Can it change the timing or just provide spark?

**BillBoost37** · 03-22-2020

I'd redo the compression test. 125psi feels low to me. I would usually see 170-180 across all cylinders in a good engine.

And I'd always do the test wrong by leaving in all the plugs except the one I was testing based on being lazy. Unhook icm and crank a little.

**BigToyz** · 03-22-2020

That is how I tested the compression because I was thinking this was a dead cylinder. All plugs were i except #1 and it read 125 psi. But that is enough for combustion. And thats what is throwing me off. Timing cant be off for just 1 cyl. It has gas, spark and compression.

**BrandonHall10** · 03-22-2020

Low hanging fruit for a 3800 misfire is a spark issue. Like stated earlier... Move the coil associated with the misfire. If the misfire moves it's *probably* a coil. If the misfire doesn't move it's *probably* an ICM.

Part 1 -How to Diagnose Misfire Codes P0300-P0306 (GM 3.8L)

Part 1 -How to Test Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L)

**BillBoost37** · 03-23-2020

Did you test another cylinder for comparison and get the same 125psi?

**BigToyz** · 03-23-2020

I didnt check other compression. I figured with 125 that was enough to atleast indicate it was trying. This is a dead cylinder. I will try swapping coils around tonight. I will also report back what I find out for future info.
Thanks guys!

**BillBoost37** · 03-23-2020

The way the pcm reads misfires is based on info from the crank sensor (to know which cylinder) and the knock sensors detecting something being off. Say for example if your compression is low, that could give you a misfire code.

While you are working on it, if you checked say cylinder 3 compression and it comes up as 180ish, that would point you to maybe a chipped piston or something keeping you from getting good combustion in #1.

Is this an NA or SC engine?

**BigToyz** · 03-24-2020

I swapped the coil out with a good one and it stayed right there in #1 cyl. I am going to do a fuel press test tonight although it has the new injector. I am starting to think it might be the ICM board although....This is a wasted spark system correct? If when I swapped #1 and #4 plug wires on the towers, and it was the tower/side for #1 coil that was bad, shouldnt it have moved to #4 cyl?
I will try doing a compression test again. If it is at 125psi and the rest are at 180 psi, would that be enough of a difference to missfire that hard. Last night, under 1000rpm it barely stayed running.
Thanks

**BigToyz** · 03-24-2020

It is not a SC engine. If like billboost said, if a low compression reading tells the crank and knock sensor somethings not right, or if it was a bad coil, wouldnt that also leave #4 misfiring also?

**Scottydoggs** · 03-24-2020

id get a junk yard icm, or buy a used one on line. most go for like 20 or so used, new they are over 100.

**BillBoost37** · 03-24-2020

If moving the #1 and #4 wires didn't change it, then it's not the coil or ICM. If the tower for one of the posts on the coil were cracked, the issue would move when you swap the wires. The ICM fires that coil for #1 and #4, the only reason it wouldn't fire on one of the two would be if you need to do a crank relearn. BrandonHall10's Regal recently had a similar issue and when he was done shotgunning parts on, he did a case aka crank relearn and magic occurred. It'll happen from time to time and I can't tell you the hours etc I spent trying so many things to learn that one.

If compression is low in #1, yes 50psi is plenty for the sensor to realize it's a lame duck cylinder. And.. it'll give you a misfire code.

I've worked on far too many of these engines and try to help others based on the experience. The way I would diagnose this is check that code is there for sure. Then swap plug wires, after that check the plug, next up is compression test and then crank learn. If you notice, my plan is in order of saving money and the usual order to find out what is going on.

Heck I once bought a Regal from four hours away. Body was great and engine wasn't running right. I couldn't quite place the sound it was making along with a misfire, knock retard etc. Got it on trailer and got it home. Scanner said cylinder 4 was the issue. I put a compression gauge on to find 0psi. The piston was completely missing from the cylinder with the rod/pin flopping around. Later I located the piston in the oil pan in about a million pieces lol.

**BigToyz** · 03-25-2020

Wow, I sure hope that isnt the problem here. LOL. The wires and plugs are new. And I ran it w #1 plug wire and a different plug grounded and it was lighting up like it should. I am going to do a compression test again tonight comparing #1 and a different cyl. Are you talking 50psi difference between cylinders would indicate a bad cyl or 50 psi in that cyl would indicate bad? T code comes up wight away after clearing and I can read how many misfires and it has to be everytime its at TDC.
I think Im at the crank relearn stage then. Can you explain that process?
Thanks guys!

**BillBoost37** · 03-25-2020

I'm saying that if #1 is 50psi lower than all the rest, it will cause a misfire code to be thrown.

The crank learn requires a scan tool that is capable, HP Tuners or an Aeroforce gauge. There are likely other tools that will do it as well. Dealer can perform it too.

**BigToyz** · 03-26-2020

Today I pulled #1 plug and one other. #1 compression was just as good as the other cyl so I think compression is no longer a question. Fuel rail press is 50psi. So I am stumped. Sooooo, new plugs. wires, Ive eliminated compression, fuel press, new injector, coil, ICM.
So, I am thinking it has to be a crank relearn like you suggest. My scan tool wont do that, so I think Im just going to have to have her take it into a dealer for that.
Atleast it has kept her daughter home during these lockdown day of the virus, so parent are happy.
Thank you all for your help and best wishes in these virus days.

**BillBoost37** · 03-26-2020

It is sure an odd one when nothing else fixes it as these are typically very easy and reliable motors. And then you come across that one oddball that needs the crank relearn.

**BigToyz** · 04-07-2020

Well guys, you want to hear a good one? And I appologize if I am WAY off base here but......
I did what I could (you all suggested a crank relearn) but sent it to the local dealership. Owners got it back, approx $400 bill. Parts replaced were approx $35, a new plug and a valve spring kit. They said #5 also showed a misfire (yes, but it was an extremely occasional one, I think caused by a leak in the plug thread adapter that someone put in) from an oil fowled plug. I had just replaced it w new, and that was the other plug I pulled when compression testing it so I KNOW it was like new.
THEN, the #1 constant dead cylinder ended up being a broken valve spring......yet it had 135-150 psi compression????? Ive been a HS automotive instructor for 38 yrs and to me, a broken valve spring, even a soft spring, wont open or close as it should and drops compression considerably! A broken spring wouldnt even pull the valve shut and would result in no compression. Am I wrong? Missing something?
My guess is they jumped the gun on testing, assumed a bad spring and replaced it, and later did a crank relearn and decided not to tell her it was part of the charge. Are the valve cover gaskets reused on this when they they are off? Because they didnt charge her for a new gasket. Something just doesnt even sound logical.
Just thought I would share what I learned after all your help.
Thanks guys, and best wishes.