On my 2000 vette I have been getting a random misfire code; no specific cylinder is given. It manifests itself with a rough idle and hessitation when starting off, but seems to be running OK at higher RPM's. two other symptoms are it seems to idle ok in open loop, and techron seems to improve the issue although it does not eliminate it. I have pulled the injectors and ran carb cleaner through them and the spray patterns look OK. The plugs and wires were changed a couple of thousand miles ago, so I am not sure where to take this next so I am looking for any available wisdom on the issue. Full disclosure, the car has 351K miles on the original engine.

 

If you had this issue 2 years ago before I moved down to Florida I would have diagnosed this pretty quickly for you when I was living in northern Monmouth County BUT you'd need some expensive diagnostic equipment which the average owner doesn't have to quickly and accurately find out what's wrong...now how do you know it's running in open loop ??...if you have a scan tool you can usually tell if it's ignition related or injector related by looking at your long term fuel trims...with 315K on the clock it may also be a mechanical issue (low compression or valvetrain issue)...hold the gas pedal to the floor and keep it there while trying to start the engine (clear flood crank)...it won't start but only crank over...listen to the cadence of the engine as it turns over...it should be even and consistent.

 

need to isolate which cylinders are misfiring, but you need a scan tool that monitors misfire for each cylinder, random means more than one often.....once you narrow it to certain cylinders you can go from there.

 

Thanks for the input. I do have an OBDlink MX+ which seems to be a pretty decent scan tool, but I am not an expert at interpretation. I monitor the short term and long term trim along with the O2 sensors for each bank and to my untrained eye they are behaving the way they are supposed to, but my expectations may be off. I thought that if it is one particular cylinder the code would give that cylinder number as the last digit in the P0300, for instance P0301 for cylinder 1,but perhaps this reader is not that sophisticated. It just yesterday threw a code P0332 for the knock sensor, but that is new and I suspect unrelated since this issue has been going on for some time.
I will be pulling the manifold to change the knock sensors so while I am working on it I will pull the plugs and do a compression check to try to get some more information.

 

I forgot to mention I will try the clear cold crank procedure you mentioned.

 

you need a scan tool that has misfire monitor for each cylinder 1-8 when there are multiple misfires on numerous cylinders it often won't record a specific cylinder which is why it sets a random p0300

 

OK thanks

 

Even cheap scanners will give you the mode $06 data that shows misfire count per cylinder. That will at least give you a hint at what cylinder or cylinders to look at.

 

OK thanks, I found mode $06; I will check it.

 

It seems that on thee 2000 the only thing it shows is TID $0C mfg defined. and the number is indeed quite high. On my 2007 Z06 it does show the individual cylinders with any misfire counts. so no new information. I am getting ready to pull the intake manifold to replace the knock sensors so I will look for any broken or leaking hoses in the process.
I would like to run a compression test and check the spark packs. they are all original. I will check for any threads related to the best way to test them, but any info there would be appreciated.
Thanks again for all the help.

 

If you have an IR thermometer you can take the temperatures of the exhaust manifold from each cylinder as it exits the head. Look for one that is colder than the others. That's most likely your misfiring cylinder. P0300 doesn't just mean that there are multiple cylinders, hence the "random" part of the code definition. For whatever reason, the PCM can't identify which cylinder is missing, so it sets P0300.

Now, the thermometer method might indeed reveal multiple cylinders misfiring, so don't stop diagnosis when you find the first cold pipe, be sure to check all 8 of them.

 

I did some more checking before I pulled the intake manifold.
I one by one removed the wire from the spark pack and each resulted in an obvious miss, and each spark pack was arching to whatever metal it could.
I pulled the plugs, and they all looked like they were burning clean. I will replace them anyhow since I have them out.
I ran a compression check and the range was 174 to 184 with most around 180. It has been quite a while since I checked the compression, so I am not sure what the ideal numbers are, but the motor does have 351K on it so I don't think those numbers are particulary bad.
I did check the temperature on the headers. numbers 7, 2 and 8 were lower then the rest. It also seemed that the spark pack on number 7 was not as strong as the rest so I will keep that in mind after I get it back together.
I did the clear cold crank procedure but I am not sure exactly what I am listening for. The cranking had a consistant cadence, but I would not say that it was completely uniform. Can I run the same test on the 2007 Z06 or our 2013 GMC sierra to get a baseline on what to listen for or will they start on wide open throttle? With this milage I am wondering about the timing chain.
Thanks again for the responses. they have been helpful.

 

A compression test will not tell you all things mechanically that may be wrong with the engine...it only tells you the ability of the engine to seal...you still may have other valve train issues...you can do the same "clear flood crank" on your GMC. Personally, I'd say take the car to a DIAGNOSTIC shop...not to a general repair shop or to a dealership...99% of both know nothing of auto diagnostics...you can try Monmouth Auto Diagnostics...they are in Ocean, NJ...I'd ask them if they diagnose using a scope (they should) and also if do "in cylinder pressure transducer" testing...with this advanced procedure they engine does not have to be disassembled...they can tell you if you have a worn timing chain or other valve train issues with this method without having to take half of the engine apart by using a cranking, running, and then a snap acceleration test !!

 

Supprised that you can't get that misfire data. I know you can get it on an 01 and I get it on my 04. Some code readers do not decode all of the data that is available.

 

Thanks again. Yes I may be at the limit of my diagnostic ability. I put everything back together yesterday with new knock sensors, sensor cable and new gaskets in the manifold and throttle body. I took it out for a test drive with the code reader on and it seemed to be running well until I tried a rapid acceleration. at that point it sounded like I lost half of the cylinders and no acceleration. To say it was running rough would be an understatement. I then tried a more controlled acceleration and it ran fine all the way up to the rev limiter. I checked the codes when I returned and the knock sensor code was again set P0332. The way the engine was behaving I guess it does not surprise me that a knock was detected. I will clear the codes and drive it normaly for a few cycles and see what codes I see, but it does look like I need an expert to help me at this point.

 

I am surprised that I cant get that info also, but the same code reader does give me that info on the 2007 so I am inclined to believe the PCM is not being forthcomming with it.

 

HPTuners will show individual cylinder misfire in the data stream. But only shows P0300 when a misfire is detected.

My guess is a hyper fancy scan too like Tech 2, Snap-On etc would allow you to see the same.

 

I am ready to through my hands up. I can rarely get the OBDLink MX+ to pair at this point and when it does I am not sure I trust it, the wifi is so unreliable. It used to work quite reliably, but I upgraded their firmware and it has not worked since. THey sent me a new one but it has the same behavior so I am ready to ditch both of them.
Anyhow I took it out for a reasonable drive, it seemed to work fine but I checked the DIC and had P0300 H, and P0332 H C and the mil is lit, so it is time to find help on this.

 

I bought a Launch CRP 123X OBD scanner and it will detail which cylinder is misfiring.

LAUNCH OBD2 Scanner CRP123X Code...
I've been happy with it, and it works on a bunch of other cars.

 

I think you are right.I found a document that shows the obd codes available for each year of the C5s. MY 2000 didnt have codes 301 thru 308 listed while the 2001 thru 2004 did. Those extra codes identify misfiring cylinders. So learned something new.