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Old 01-15-2007, 01:40 PM   #1
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Tow Vehicle OBD II debugging

We are still working through debugging a couple of OBD II codes on our 2003 GMC Sierra 2500HD. I'm posting our trials and tribulations here to see if anyone can offer pointers or next steps to try out.

P0455
The first code is a "P0455" which is an evaporative emission control system leak detected (i.e., "gross leak"). This one has been appearing off and on since October of 2006. It is quite elusive as it goes away on its own only to reappear a month later. We took it to the dealership and they smoke tested the system and found no leaks. The dealership indicated it was probably related to the gas cap. Since then, we have reset the code (after checking the gas cap) only to have it come back on 100 miles down the road. Should we try replacing the gas cap next?

P0300
The second code is a "P0300" which is "Random/Multiple Cylinder Misfire Detected." While we were checking things out under the hood, we discovered that there were two broken bolts on the exhaust manifold cover. We ruled this out as the cause of the code after we replaced the bolts and the gasket and the code reappeared after 100 miles of driving. Any thoughts on what the next best thing is to check for this code?
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Old 01-15-2007, 02:07 PM   #2
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P0455
The first code is a "P0455" which is an evaporative emission control system leak detected (i.e., "gross leak"). This one has been appearing off and on since October of 2006. It is quite elusive as it goes away on its own only to reappear a month later. We took it to the dealership and they smoke tested the system and found no leaks. The dealership indicated it was probably related to the gas cap. Since then, we have reset the code (after checking the gas cap) only to have it come back on 100 miles down the road. Should we try replacing the gas cap next?

This is most likely the Gas Cap $gets you a new one. But I think you can get a seal for the cap alone.

P0300
The second code is a "P0300" which is "Random/Multiple Cylinder Misfire Detected." While we were checking things out under the hood, we discovered that there were two broken bolts on the exhaust manifold cover. We ruled this out as the cause of the code after we replaced the bolts and the gasket and the code reappeared after 100 miles of driving. Any thoughts on what the next best thing is to check for this code?[/quote]

You may have a bad spark plug or spark plug wires. You may have a bad ingtion module but I would check spark plugs and wires first.
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Old 01-15-2007, 02:48 PM   #3
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The emisssions people should be able to test the gas cap for you. They aren't that expensive to buy. I had a bad ignition module on my F150 that gave similar results. In the Ford trucks, it tells you which cylinder is having the problem, I simply swapped that module with a known good one to see if the problem followed. Maybe that helps a little.

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Old 01-15-2007, 03:00 PM   #4
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I would replace the gas cap first, they are pretty inexpensive. Also, if your truck is less than 8 years old, and has less than 80,000 miles on it, it is still covered under manufacturer's emissions warranty. This is a Federal law, and if your dealer gives you a song and dance, I will be happy to quote chapter and verse of the emissions law to them.
Second, on a GM vehicle, P0300 is not a random or multiple cylinder misfire. It can be, but is usually one cylinder. You may be able to find out which cylinder is misfiring using the generic functions part of your scan tool, and pulling up a freeze frame. If not, you can get a graphing multimeter and hook it up, and watch the voltage spikes for the coils. A misfire will show as a shortened spike.
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Old 01-15-2007, 03:09 PM   #5
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I can't offer repair advise other what was already suggested, but do know that GM is very good about their emissions warranty. Just last year I had an injector failure on my 97 Suburban, and GM bought the entire repair, without a question. Apparently this had been a problem on my model, so they prolonged the warranty.
The symptoms were poor idle/misfire and sluggish performance, together with a check engine light.
Be sure and go to a reputable dealer for your warranty repairs. The first Chevrolet daler I went to ( close by) would only do the rrepairs if I bought all kinds of associated parts to the injection system ( $ 1100.00 worth), while the second dealer I went to did the warranty work only, which proved to be all the truck needed.
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Old 01-15-2007, 06:27 PM   #6
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Quote:
Originally Posted by overlander63
Also, if your truck is less than 8 years old, and has less than 80,000 miles on it, it is still covered under manufacturer's emissions warranty. This is a Federal law, and if your dealer gives you a song and dance, I will be happy to quote chapter and verse of the emissions law to them...
I didn't know about that. Can you point me in the direction of where I can find more information about exactly what is covered under the emissions warranty.
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Old 01-15-2007, 09:46 PM   #7
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The PO 300 is a random misfire in that its not necessarilly happening on
one cylinder only.The PCM is not flagging any one cylinder in paticular. .if you have a PO300 ,it means you can have multiple misfires at the same time ,for instance ,the 2000 GM truck 3500 with a 5.7 vortec PO300 ,had minor multiple misfires on about 5 or 6 cylinders ,has bad heads on it ,the exaust valves are seating poorly at 230 000 miles ,there was no code for a specific cylinder stored ,but I could read (real time) misfires as they were happening.The 5.3 V-8 has been known for cold misfires ,say a couple of minutes ,and blackened spark plugs as well ,suspect a fuel pressure regulator leak into the intake vacuum hose .To check, engine off ,after running ,remove vacuum hose to see if fuel comes out of regulator vacuum connection .
If you have scan tool data stream ,you can see on the datastream function
readouts ,the history misfires and real time misfires ,you would see plenty
on multiple cylinders and not see a stored PO305 or other cyl, meaning a specific cylinder .The PCM will flag distinct cylinder misfires ,like a bad plug wire or injector .It will also flash quickly ,to warn of immenent catalyst failure
Ive heard of catalyst failures on these trucks restricting exhaust flow and
also causing misfires as well .That can be checked doing a pressure test
by removing the front O-2 sensor either side to test and utilizing the threaded O-2 hole to thread a test pressure gauge in there to read exhaust
pressure .Should not be nothing more than 1 to 3 psi,thats all ,if its higher
youve got a restricted converter .The EVAP code youve referenced can be
a fuel cap (very common) the more likely cause could be an EVAP soleniod
intermittant problem ,also they should check the fuel fill hose at the tank
fitting as well ,alot of late ford explorers and rangers have PO455 codes and
fuel fill hose splits at the tank connection (badly split).The smoke machine test is a good one if it works ,sometimes it won't .The VENT solenoid for
the EVAP canister is usually under the truck bed on GM pickups ,drivers side
outer frame rail ,they get coverd in dirt ,water debrise etc and do fail .
They don't hold a vacuum and do leak ,this may be the trouble.The EVAP
system can be fairly difficult to find the leaks sometimes ,go for the easiest
(if thats possible)things to check out ,in the end another good shop may
need to be tried ,and that Definately doesn't mean the dealer .So, then if you have a scan tool that has good datastream ,you can check the misfires
and see which ones are the problem ,otherwise it's a crap shoot to find out whats wrong .

Scott
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Old 01-15-2007, 09:52 PM   #8
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uwe ,your lucky on that repair ,because GM is backing way off on those now,
as they are suppose to have a 10 year 200 000 mile warranty .They now have the new updated injector spider assembly as we call it (looks like a spider) when installed ,they are a complete assembly ,not a single injector
so if they replaced yours ,they all were replaced .The emmisions warranty
follows the vehical ,not the owner and is federally mandated to cover emission hardware for 7 70 or 8 80 whichever the case may be .That said
the dealer will not jump at the chance to help you out ,but if you tell them
you know of the mandated emissions warranty and you want it covered
they must do it ,or a call to the BAR beaure of automotive repair is in order.
The injector warranty is only for 96 to 2000 5.0 5.7 vortec engines only
no 4.8 or 5.3
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Old 01-16-2007, 11:43 AM   #9
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Quote:
Originally Posted by yukionna
I didn't know about that. Can you point me in the direction of where I can find more information about exactly what is covered under the emissions warranty.
You can find more information on this on the epa government website.
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