2004 MsM error codes
 

I keep getting engine misfires, mostly #4 ... but if I let it warm up and reset, no codes and runs great. Turn it off, let it sit overnight, error codes return, etc. Help. Thanks.34,000 miles

What do the plugs look like? Could swap injectors and see if the problem moves with the injector.

Misfiring when driving it cold or just at cold start idling before driving it? Heavy throttle misfires or steady miss? Can you feel it missing or do you just have a code but no tangible symptoms?

Kinda need more info to really help more accurately.

I'll pull the plugs and look. Swap injectors how or just put some cleaner in the tank? Starts just fine and will idle but cannot be driven until warm without stalling. if it runs rough, it is hardly noticeable after warmed up. Thanks and keep asking and I will keep looking.

Whoa wait hold up, tell me more about this inability to drive it until it’s fully warm or it stalls…? That’s very abnormal. Is this a stock car?

Inspect the plugs for damage/discoloration.

Do a simple compression check.

Run a bottle of Chevron Techron fuel system cleaner through it if the other two tests check out. You can also then put Seafoam in the fuel for cleaning injectors and combustion chambers.

Don't be afraid to clean and test the IAC valve, too, but that's after the above.

start the car, idles fine, give it a little gas and then let off the gas peddle, rpm's drop and engine stalls. redo, same results until engine fully warm, then no issues at all. Only non-stock item is the air filter. Let me add one more thing, after resetting the error codes and starting over the next day, when it is idling to warm up, this is when the error codes kick in. There is a slight roughness during warm up.

what is an IAC and where is it? Thanks

Your inability to troubleshoot or do any kind of research / critical thinking isn't encouraging...you can literally google "IAC car" and figure this out...

already done this, albeit after I asked. My apologies. Some of us, even at 73, are still learning.

Do what sixshooter said, and report back if the problem persists.

Just want to add some additional thoughts. I disconnected/reconnected the spark plug wires last night. Reset the codes. Restarted this morning and did NOT touch the gas until fully warmed up. Seemed a little rough during start up/warm up but did settle down. Let it cool for several hours, temp gauge bottomed out cold, restarted, no immediate error codes, pumped the gas a few times, bingo, error codes, PO300, PO304 but no stalling or drop in RPM. Maybe this helps. I'll start pulling plugs tomorrow. Appreciate the help.

If you don't have one and / or don't want to buy one, your local Autozone / Advance Auto will have a loaner compression test kit for free (I'm assuming you're US-based)

If it were my car I'd be doing that at this point in addition to chasing other potential issues. It can't hurt.

And Harbor Freight sells cheap compression testers.

I'll do the compression test but do not understand what this has to to with PO300, PO304 codes. Help me learn, please and thanks.

Two symptoms of poor compression are misfires and shitty idle. The fact that all of your misfires seem to be on Cyl4, it stands to reason that you MAY have poor compression in that cylinder.

just one more question for tomorrow. compression test cold or warm? Thanks, again.

Hot is better, cold is fine. Test all cylinders. They should be reasonably close to each other.

https://cimg6.ibsrv.net/gimg/www.mia...38e2185805.jpg
here plug #1, front of engine. Look OK or ???

Looks ok, though I can't tell if it's just glare or if the threads are wet. Pull Cyl4, the P0304 code is for that cylinder.

https://cimg3.ibsrv.net/gimg/www.mia...c88a42cd2f.jpg
plugs one (top) thru 4. All look the same.
https://cimg0.ibsrv.net/gimg/www.mia...ce93077092.jpg
Plug 4 (rear). None wet. Gaps look big to me but cannot find my old gap gauge .. new plugs, gap gauge and compression test tomorrow. Thanks for all the help.