1997 GMC Suburban 1500 - P0302 code

Hi all,

New to this forum, it looks very functional. For my truck listed above, I had a misfire that was very noticeable and generated a check engine light. I hooked up my OBDII reader and it diagnosed a P0304 code. I then changed plugs, wires, cap, rotor, oil, oil filter and fuel filter. I started the truck and still have a noticeable misfire that now shows as a P0302 misfire. I checked for crossed wires as well as checked with a known good wire to see if that was it. No dice. I than removed the valve cover to check for a broken exhaust valve spring on any of the cylinders on that bank - they are all good. The truck has 198000 miles on it and is in very good condition. This happened basically overnight (ran good one day, bad the next). Any suggestions would be great.

Cheers
VeeThree

is the ING coil original? with that many miles it would be weak for sure. and at only 40-$50.00 to replace. also if u have a multi-meter check the voltage on high side !! I replaced mine after 100,000 miles had a low idle miss to it. hope this is helpful..also in the future instead of tearing into u could of did a compression test on that cylinder, this might be helpful to determine.

Thank you for the response. I will replace the coil tomorrow (seems inexpensive enough). I had already replaced the plugs, so it was easier to pull the valve cover. If I still have issues after replacing the coil, I'll do the compression check as advised - if nothing else, it will give me a good baseline to go from. I think I will check fuel pressure also, maybe it is a couple of components that are both at minimum specs. that are causing the issue

I'll let you know

cheers

helo

Also check as you said the fuel pressure, but also check the cylinder´s injectors that are producing the missfires, check if they aren´t clugged.
Has the engine mutiport injection system or TBI system ?

Howdy,

The engine has TBI, so I guess that rules out the single cylinder misfire (unfortunately), but a great idea none the less. Is there an advantage to having one of the quick oil change shops do the injector/TBI cleaning service they offer for about 80 bucks over adding chemicals to the gas tank (for quite a bit less than $80) and letting them run through the whole fuel system?

BTW, I have not yet gone to get the new coil - so I have not done anything else to the truck as yet ... maybe tomorrow .... (obviously not a daily driver).

Thanks
V3

Helo !

Well , actually is a good idea to clean and service the TBI, but is much better cleaning the TBI without gas tank chemichals, why don´t you try to service it in a garage with the special equipmnet so they can clean and tuned directly the injecor. But anyway these is not extrictly necessarelly in orther that you are having only a one missfire cylinder so then we can discard the fuel injector is the one is producing the missfire code.
I recommend you to recheck the ignition system. Recheck the spark and its wire from the # 2 cylinder.

keep posted

OP: Could you elaborate? Isn't that a Vortec engine? If so it isn't TBI, it's port. The Vortec engines had alot of problems with the CPI injection module under the intake man. They often set random/multiple misfires.

Darkrooms: Wow - thanks for the pointer, I thought this was TBI - there's my problem; I'm just an idiot (that is the first step towards recovery). Is CPI injection the same as Central Sequential Fuel Injection (I got that from the Chilton's manual that I am looking at)? If so, I'll take a look at the module and see if the injector(s) are bad/clogged and go from there.

Thanks again for for the steerage,

V3

Yes, that's the same thing. Actually, before you pull the intake you can do a injector test using a scan tool and activate each injector. When those CPIs go bad, they'll fire one second, and won't the next. It's like the flip of a coin. I know it sounds impossible, but that's the way they fail: random multiple misfires that move around the engine. If you've already pulled the upper intake, just replace the CPSI module, if you can afford it. Also, we found that BorgWarner sells a replacement module that takes the place of the spider module and relocates the injectors from the central module to the ends of the injection tubes. It's much better, but not much cheaper. I've posted about this spider injection module before, so if you searched for "spider" you'd probably hit it.

Hi all,

I replaced the CPSI module with a new one - it took awhile to get the right one from AutoZone. I looked for a Borg Warner module as recommended, but was not able to locate one. I used a Delphi module (I was told these are the OEM replacement for the original module) which has the modified injectors and control units on them.

Truck runs better than new - smoother idle, better pickup, waiting to see if fuel economy is better as well. Thank you to all that shed light on the problem I was experiencing.

Cheers,

VeeThree