C5 Pete

I've done some searching, but the threads mostly seem related to long tube header installs and/or rough running engines.

2004 convertible.

My car is bone stock. Drives and idles fine.

Every time I take it out, once it warms up, the CEL comes on. I pull the code and it shows P0300 H (I've not ever seen it show C)

If I clear the code, it goes away and does not return until the car has been sitting for a while, cooled down, and then driven again.

So it seems, in my case, the random misfire has something to do with engine temp.

Plugs and wires are new and good.

Prior to this code, I was getting a code for the front right O2 sensor, so I replaced it. I'm no longer getting an O2 code.

What other systems will trigger this code? Could the other O2 sensor be bad (or going bad) but not throw a code - but trigger the random misfire?

I have a handheld scanner, but it's not showing a specific cylinder.

I have a spare set of injectors I could try.

Any suggestions?

Thanks.

dougbfresh

If you get a CEL, does it flash? If so you need to hook the scanner up and try to catch it in the act when it's happening. ALL engines have misfires ALL the time, with better scanners you can actually see the count-it's when this count gets too high in a time period is when the computer sets the code. Some cars were getting this with broken valve springs (Z06's for the most part).

The car runs in 2 modes (Open and Closed loop) OPen is when the car is COLD, lookup tables are used by the computer to retermine fuel/timing/... the O2 info is NOT used since it takes a while for the )2's to heat up to the 600 or so degrees where they function correctly.

Once WARM, the system goes into Closed loop mode where O2 sensor output is used to calculate fuel and spark on the fly.

You could have a LAZY O2 sensor (one that does not switch from lean->rich and back at the correct rate. Since when you read it is an H code and not a C code, you need to catch it in the act. A misfire code is almost always preceeded by a flashing CEL, this is the time to try to catch it with the scanner.

I have seen GM's get random misfires with clogged injectors-I'd try to catch it in the act 1st before I went down the injector route even though changing injectors is about a 30 minute job.

Mikedoc

I am having a similar issue. I get the quick flashing cel at 75 or so. I can feel the car buck a little when it happens. What should I look for, it also doesn't happen all the time

DOUG @ ECS

With the milage on that car Pete I would swap out the injectors since you have a set, but if you would like me to check it out for you let me know.

C5 Pete

The CEL doesn't flash, it comes on steady.


Thanks Doug. I'll likely give you a call this week.

Trios

Were the plugs replaced before the O2 sensor? It's possible the failing O2 caused the car to run too rich and fouled the plugs, despite that they're 'new.'

C5 Pete

I just pulled them all back out and re-installed. They all looked good. I put in new GM performance wires at the same time. Code still shows up.

Every problem I've ever had with either C5, has been rather straight forward and easy to fix... I guess I'm due for this...

It all starts with the proper tools for the job. I now have the repair manuals. I need to get a better code reader/scanner.

Or, I could just bring it to Doug...