Yes -- Another P0300, Random Misfire CEL Problem

Discussion in 'Chevy Blazer Forum (GMC Jimmy)' started by KMN, Oct 10, 2010.

  1. KMN

    KMN New Member

    There are so many threads regarding CEL Code P0300, it is overwhelming. So, I thought I'd start another thread particular to my issue (well, my buddy's issue).

    A buddy of mine has a 1997 Jimmy which has been giving him problems for almost a year now and his mechanics are at their wits end on this one. Sure, they'll eventually find it, but not without a significant drain on my buddy's wallet.

    He describes the problem/experience as this:

    "...When I am trying to accelerate rapidly when getting onto an interstate, or when I'm trying to go up a hill at a high rate of speed (65 MPH+) it will begin to lose power, the RPM's will kick up -- first to around 2500 RPMs, but then as I continue to climb the hill they may jump all the way to 4000 RPMs. Sometimes I'll gain power as I go up the hill, and sometimes not. The Service Engine Soon light will begin to flash due to the engine misfiring...".

    I hooked up my AutoEnginuity scanner and the only code I got was P0300, Random Misfire. The results are below

    1997 GMC Jimmy (P0300 CEL).jpg

    I asked my buddy what has been replaced and/or checked out with regards to the problem and the list is below:

    • Fuel Injector Assembly
    • Spark Plugs
    • Mass Airflow sensor but the original was put back in since the new didn't fix the problem
    • 2 Oxygen Sensors
    • Fuel Filter and Fuel Pump

    My buddy is getting frustrated and is at the point that he won't drive the Jimmy on long distances. He (and I) would like to find the problem, but doesn't want to spend a significant more $$ for something that may not fix the problem.

    There are so many things that can cause a random misfire, I was hoping that given the list of parts already changed out, the results of the scanner , that somebody can provide a few suggestions.

    Anyway, the collective wisdom and knowledge of this forum is requested and I/we would certainly appreciate any help.

    03 Suburban 1500 Z71
  2. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    What did he put in for plugs? These engines like AC Delco plugs best. Plain old platinum plugs gapped at .040. Bigger gaps put a tremendous load on the ignition system, which can result in random misfires especially under load.

    You didn't mention plug wires. Hard to beat Belden Blue Max wires from Napa. ACD wires are also good, but generally more expensive. Duralast and Bosch wires from AutoZone are both Lifetime Warranty, so they should be fine.

    Duralast Gold Cap & Rotor kit are very high quality, solid brass terminals. And of course ACD are top shelf but big bucks.
  3. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Also something you will want to look at is the fact that the long term fuel trim on Bank 2 is way way out of sync.. What brand of fuel injector did they install, and also something to look at is the Idle air control valve, this can cause a lean condition and allow the motor to misfire. im not saying change it im suggesting that you remove the throttle body and clean the entire unit. Also something to look at is the Fuel pressure regulator since those are a common failure point with these trucks. Simply take it for a drive, and let it cool down for about and hour and then open the throttle blade and smell for raw fuel if the FPR is bad it will smell like raw fuel. Then you can also take a look into the upper intake when you have the TB removed and see if there is a washed area, and or a wet area from the FPR leaking. Also if the mechanics didnt clean out the injector runners in the lower intake then they may have left a big pile of carbon in the tube, and as a result the fuel is spraying poorly under heavy engine loads. I have 2 of these trucks and have had the same problem as this. Keep us informed please and we will all try to chip in as well...
    Oh another weak point is the distributor, and while you are in changing the cap and rotor (which corrode very easly, and i end up changing mine yearly reguardless of milage) check for endplay on the distributor since the bushings go out pretty easly.
  4. KMN

    KMN New Member

    You are giving me/us some good we just need to absorb it and figure out a game plan...

    Regarding the spark plugs...I'll check to see what type/brand...but given that the problem existed before the change out, I wouldn't think it would be the spark plugs...but I'll check just the same...

    I too was wondering what was going on with the long term fuel trim on Bank 2 since it seemed to be so far out of bed with the others...however, I have no clue what it all means....I can always hook up the scanner with the car running and take a look at it as see what it does under "normal" conditions...will that help?

    What about the catalytic converter...? Is there a way that I/we can test it?

    We'll keep you posted...
  5. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    There really isnt any way to test the cat that i know of, but more than likely if it is the stock cat, it is burnt out (both of mine have done this) as i said id look at the other things first since a P0300 code is usually linked to fuel or spark. As to the fuel trim yes i would give it a whirl under "normal" conditions but something is telling me that you wont find a change.. Just keep us informed as i said that is just the first line of attack. Good luck
  6. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    The cat won't impact how the engine runs, unless it's clogged. You can measure exhaust backpressure in the upstream O2 sensor port. A manometer is the best way to measure, I believe less than 10 inches of water is acceptable.
  7. KMN

    KMN New Member

    I believe we are ready to start tackling this issue. I think we are going to try to replace the distributor, then check out the fuel regulator and remove the throttle body.

    We were thinking about replacing the distributor first since I don't believe it has ever been changed. I've got the GM service manual, so I have something to go buy, although it reads to be more to it than I thought (e.g., "...The engine front cover has 2 alignment tabs and the crankshaft balancer has 2 alignment marks (spaced 90 degrees apart) which are used for positioning number 1 piston at top dead center (TDC). With the piston on the compression stroke and at top dead center, the crankshaft balancer alignment mark (1) must align with the engine front cover tab (2) and the crankshaft balancer alignment mark (4) must align with the engine front cover tab (3)...."), but we'll give it shot.

    Is it really difficult as the SM makes it out to be? Any words of wisdom? I've replaced the timing belt, water pump, tensioner pulley, etc on my Chrysler 300M, so I don't believe I'm completely out of my element...but it never hurts to ask....

    Oh by the way, it passed it emissions and then soon after it acted up again and the CEL came back on...

    '03 Suburban 1500 Z71
  8. KMN

    KMN New Member

    Update - We changed out the distributer cap and rotor. We'll see how it goes. We also checked the Throttle Position Sensor and it checked out fine....

    I would like to change out the Fuel Pressure Regulator, but I do not know where it is located. Does somebody have a picture of where it is located or a description?

    '03 Suburban 1500 Z71
  9. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    There really isnt a good way to show you (i just did one on my 2000 Blazer and forgot to take pics since i was in a hurry) it is under the plastic upper intake, mounted on the backside of the spider injector assembly. You have to remove the throttle body, and a few 10mm bolts, and more some of the wiring harness out of the way, as well as disconnect the fuel lines, and unplug the sider injector wiring harness (should have a 10mm that is holding both sides together) and then just carefully slide the upper intake off. then on the back side you will be able to find the regulator. I reccomend that you remove the spider and change it out of the truck since it is a PITA to do with it still mounted (ive had to do it both ways)

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