: CTS V P0305 Engine Misfire Code

03-26-11, 04:09 PM
I really need advice on a problem with my CTS V. I have been getting a p0305 code for a couple months, it runs rough at idle, and it smells rich at startup. However, it feels fine under throttle.

So far, I've:

Swapped injectors between cylinders 3 and 5.
Swapped coils between cylinders 3 and 5.
Swapped plugs between cylinders 3 and 5.
Replaced the wires.
Swapped the coil assemblies from the right bank to the left bank.
Put a noid light on the Fuel Injectors to check for signal.
Checked for Spark on cylinder 5.
Taken off the valve covers to look for broken springs and make sure the valves open/close correctly
did a compression test on 5.
Tried to check for a vacuum leak on the intake with propane as well as starter fluid
Looked at the O2 sensors with a Scan Tool... both banks look the same and seem to fluctuate like they should
Looked at the long and short term fuel trim valves. They seem high at startup, but they look good in closed loop
Did my best to look for bad spots in the wiring harness

Everything seems to function as it should. I did find a spot in the harness on the front left side of the harness that looked like it was abraided on a bolt slightly, but the wire looked intact. It is a Brown wire with a white stripe, but it didn't look damaged.

I may have tried more diagnostic measures, but I can't think of them right now. I am at a total loss of what to try next.

Any ideas? Or anyone now a good LS guy in the Houston area?

Thanks for the help.

03-26-11, 05:42 PM
You may need to pull the intake manifold and check gaskets.

Sounds like you have looked at almost everything else.

03-27-11, 12:21 PM
Yeah, I think that might be my next step. I've been trying to avoid that. How hard is it to get the manifold off on these?

03-27-11, 12:38 PM
Yeah, I think that might be my next step. I've been trying to avoid that. How hard is it to get the manifold off on these?

Easy cheesy. If you are nervous about the process (doesn't sound like it from your diagnoses procedure to date) you can look in the Maggie install manual for step-by-steps.


03-31-11, 05:33 PM
I finally got a chance to look more closely at pulling the intake. I thought it was like the old small blocks and the seals were a nuisance. I guess I'll try to get to that this weekend. Anything to look for there other than a bad seal going to cylinder 5?

I did do a couple more things, though. Checked the fuel pressure and it was a solid 60 psi. Also pulled the computer and ohmed out all of the wires that were unique to cylinder 5 and the cam position sensor, and they came out OK.

04-09-11, 03:30 PM
Pulled the intake, replaced the seals, same problem.

I also did a leak down test and pulled out the pushrods to make sure they weren't bent.

I am thinking it must be a small crack on the intake. I sure can't see it on the leak down,though. Still at a loss as to what to try next.

04-09-11, 04:26 PM
I sympathize with ya.

Took me over a year to finally nail mine.

Mine indicated "random miss fire" rather then the specific cylinder.

It also ran fine at any temp.

I would tell ya: at first, when it does it, turn it off and remove the spark plug wire and attach one with a plug and start the motor again.

If you can see a nice fat spark, your problem is not ignition related.

You could have a lifter going funky, but that is just guessing.

What is your oil pressure reading?

How old and how many miles?

Engine mods?

04-09-11, 09:24 PM
I sympathize with ya.

Took me over a year to finally nail mine.

Mine indicated "random miss fire" rather then the specific cylinder.


I decided to make P0300 my bitch. I get the code if I let the cold engine idle within 2 minutes of start. If I keep it at 1k rpm for 30 seconds after start I don't get it. It must be something with my tune that it thinks with the cam loping that its a misfire. Oh well.

Good luck with your issues. You are in good hands here.

04-09-11, 11:04 PM
The usual fix I have read about is that the tuner turns the tables "off" that reference a miss fire.

These are acceleration tables that look at the acceleration between each firing.

Lopy cams just exacerbate the "problem".

I run adjustable rockers (Comp Cams #1500) so it became a matter of "dialing" it in, which became very critical when I replaced the RHOADS lifters with MOREL's.

Since I run a stock cam, it was all in the adjustment and that was a bitch to figure out, but I finally did.

There was that much difference between lifters.

Science triumphed over superstition!

04-10-11, 01:10 PM
Its a 2004 with just under 100k. It is bone stock.

I am not sure of the oil pressure readings. I think I remember noticing it seemed a little low at one point, but not enough to ever set the Check Engine Light, though. Can that be related somehow?

04-10-11, 01:46 PM
A funky lifter could be the cause, but you really don't want to start pulling heads if you don't have to.

Since you say it is crappy at idle (all the time?)it could be.

If you do pull the head and you need a stock lifter, I have a few.

04-22-11, 12:54 PM
OK, finally gave in and sent it to the shop. Supposedly it is now fixed. What was the problem? Bad o-ring on an injector. I really don't understand how an o-ring can fail at such low pressures just sitting there, but apparently it happened.

Anyone seen this before? When I swapped the injectors between 3 and 5, both o-rings looked fine. It must have been one on either side of 5 on the firing order.

04-22-11, 03:56 PM
According to the literature - "If a fuel injector concern exists, it is possible to misfuel an entire bank of the engine, causing multiple cylinders on the same bank to misfire even though the root cause is a single fuel injector."

05-09-11, 11:29 AM
OK, it worked fine for a few days after he replaced the o-rings, but then started misfiring again.

Then, out of desperation I put a can of seafoam in with about 1/2 tank of gas. When I started it after work that day (after driving about 25 miles), it ran fine. After I got home that night, the Check Engine light turned itself off. I couldn't believe that the seafoam actually fixed it.

Two days later, started missing again. I am guessing it must be fuel related, but Don't know what to try. Any more suggestions at this point?

05-10-11, 09:20 AM
I know you said you monitored the O2's and they were swinging just fine but perhaps try swapping banks and see if the misfire follows? I know you think it is fuel at this point but that isn't that hard to do to rule it out.

From darkman's DTC .pdf.

"Swap the Position 1 O2 sensors side to side to see if the misfires move to the other bank of the engine. If
so, replace the O2 sensor."

Oh and also I looked at a stock file and it takes 2 fails to set the DTC just FYI.

05-11-11, 11:51 AM
I've always had a slight mis at idle but never a code and I suspected maybe a bad valve. That all changed recently when I installed a cam... Code P0300. Oddly enough when I was changing the valve springs I noticed that the first cylinder on the pass. side didn't pump up with air right away like all the others when I put 100psi to it (I actually used my leak-down tester to charge the cylinder). That cylinder took about 10 seconds or so and I could clearly hear the air coming from the throttle body especially when I opened the plate!
Other than that I haven't ever done any tests like leakdown or compression. My mis is very very slight which most don't even notice, I think with the cam it's just now more pronounced for the PCM setting the code off.

Although it's not a clear cut answer maybe it's a lead for you.

Btw, I've got 73k miles on the clock and I bought the car with 30k bone stock.


06-28-11, 09:30 PM
After much more diagnosis, I am still having this problem. Leak down tests, used engine block tester, cut open the wiring harness to find any problems, etc.

Brought it to the dealership and they said I needed a new computer. Got a new computer, still the same problem.

It was suggested that I swap o2 sensors on the front bank, but I can't see a good way to get to them. I have been searching on this forum, but can't find it mentioned. Anyone know a good way to get at the front o2 sensors?

07-10-11, 04:56 PM
I found the problem. The roller on one of the lifters on cylinder 5 was galled.

I just wanted to post in case someone else goes through these issues.

Now I have to figure out what kind of cam to put in. Any suggestions? I really don't want to have to put on headers and all that stuff, but might as well get some more power out of it.

07-10-11, 04:59 PM
Use Morel lifters.

You won't regret it.

07-10-11, 05:06 PM
I would call Geoff Skinner with Engine Power Systems. Geoff can set you up with an EPS-lobed cam designed to get the most power out of an otherwise stock engine.