I have a 1999 Chevy Blazer 6 Cylinder 4WD with 113,000 miles. I just got a P0303 check engine light (stored code), cylinder 3 misfire. Also a P0121, Throttle Position Sensor Circuit Range, pending code. Are these codes related or two different problems? Difference between stored code and pending code? Is this code an ignition system problem or air or fuel system?
The light flashes for a few minutes when you first start driving it, then remains lit. I think it has rough idle. When I open the hood, it looks like the engine? (middle part area of car) is vibrating/shaking a bit. Is this normal when car is running? or is it rough idle? I don’t think I hear any strange sounds, only the sound of the belts running.
I can start the car fine so no starting problem. I just tested the car by by leaving it in park and idling for 30 minutes and didn’t notice anything other than maybe rough idle. When I drive the car, I don’t think I notice any problem other than the front of the car shaking/vibrating somewhat. I don’t notice any problem with accelerating and braking.
I changed #3 spark plug and wire last month with new AcDelco parts so it is probably not spark plug and wire. Ignition coil was changed last year. I replaced the distributor last month using Autozone’s warranty so the distributor cap and rotor are not AcDelco but Duralast brand. Is it really necessary to use AcDelco cap and rotor. A new Duralast cap and rotor shouldn’t fail this fast, right?
Can a vacuum leak cause this code? I just noticed today that on the passenger side of the car, near the air injection pump check valve, there is a short elbow hose/tube that is dry rotted cracked. Small pieces are falling out on the sides so you can see holes. It is held on by two clamps. I think this connects the check valve to the air pump down there? Can something like this cause a misfire? I don’t know much about air injection pump. Is it important or it doesn’t affect engine performance? But I am not getting any air system codes such as system too lean or oxygen sensor.
P0303 is the only code I am getting. I also have a P0121 but it is pending code so I am not sure if it causing the misfire or a different minor problem. I am not sure why I’m getting a throttle position code. I cleaned the throttle body and replaced the throttle body position sensor and air idle control valve last year. I am not good at diagnosing problems like this where it could be anything. I just want to know what part is bad so I can change it. But how do I fix this problem other than changing one part at a time to see if it goes away?
Have you ever seen someone advertise shop with disbelieve, be fearful of our parts they suck??? lol… I will not use any AZ electrical part, or most any other AZ parts for that matter… that a side…
1st thing I would do is a compression test…
2nd thing I would do is swap #3 plug with another plug to see if the misfire moves with the plug since you already have the plugs removed while preforming the compression test… unless you find a bad/dead cylinder while doing the compression test…
3rd if still nothing, swap two ignition wires (at both ends) to see if the misfire code follows…
If all that checks out you may has a spider injector clogged or something…
Check fuel pressure…
Check to make sure you don’t have any carbon tracking under the dist cap…
It doesn’t hurt to check for vacuum leaks either…
There is a short tube/hose that connects the air injection pump check valve to something down there, I assume the air pump. It is cracked and I can see holes on it. Is this a vacuum leak? Can problems with the air injection pump cause a misfire?
A worn out/split hose may or may not result in a vacuum leak. Depends what the hose does. Common sense says $$-wise, that before starting on a difficult to diagnose problem like a misfire, fix whatever is obviously broken. B/c it has to be fixed in any event, and there is at least some possibility that will also fix the difficult problem. Still a misfire on 3 is unlikely to be related to air pump, b/c air pump injects air into exhaust stream, not into cylinders, and exhaust stream is common to all 6 cylinders. If air pump problem was causing a misfire, most likely it would cause a misfire on all 6.
As mentioned above, the throttle position sensor problem is unlikely related to the misfire on 3. Separate problems.
Stored vs pending code. The car’s computer is constantly making tests. The tests take a certain amount of time and miles of driving to finalize. Done so glitches are not constantly turning on the check engine light. If the computer thinks it has found a problem, it will first flag it as “pending”, then if it finds the problem again, maybe a day or week later, it will flag it as “stored” or “current”, and usually turn on check engine light.
As mentioned above, if check engine light is flashing with a misfire code, best option for your wallet to immediately turn the engine off and tow the car home or to the shop. Expensive to repair damage otherwise.
Misfires are usually caused by either an ignition system problem, or a fuel problem. First step is to figure out which of those two is the cause. Best method is pro mechanic using a pro-level scan tool. Diy’ers often swap parts to see if misfire follows the swap: coils, spark plugs, injectors are the most common swaps. If I had to make a wild guess, your problem is caused by the ignition system, number 3 coil is defective.
So I have been trying different things to fix this P0303 misfire without any success. I changed the #3 spark plug and wire, ignition coil, and cap and rotor, and cleaned throttle body and MAF sensor. The car starts and idles ok, maybe some rough idle but not really noticeable. I just noticed that when I am test driving it, the car runs ok if I keep pressing the gas pedal. When I press the brake to slow all the way down like coming to a stop/red light, that is when the car suddenly loses power and shut off. No warning at all, just shuts off on its own. If I brake while driving, it is ok.
After it shuts off, I have to put it back in park and turn the key again to start the car. It will restart ok but the same thing will happen again. If I brake all the way down like coming to a stop light, the car suddenly loses power and shut off. It is unpredictable so you don’t know when it will happen. But so far test driving it, the car can’t even make it to 3 minutes without shutting off. The strange thing is if I keep pressing the gas pedal, I can somewhat keep the car running. Why is that? Is this a fuel problem? Is the misfire caused by the fuel system?
This is the only code I am getting. The car starts ok and restarts ok and I think I already tried most of the ignition system so probably not an ignition problem? I think the only thing that is left is fuel injector. After it shut offs, I restart it but put it in park and let it idle for 25 minutes without any problem. Can anyone explain to me why this happens? I am trying to understand how a faulty fuel injector or pressure regulator can make my car to run like this.
Sorry, I meant when you brake when coming to a stop/red light. Like almost stopping but not down to a complete stop. You are still pressing the brake pedal and the car is only moving a little bit but it isn’t fully stopped. If you are driving and brake to slow down a little, it is ok. If I put it in park and just let it idle, it is ok.
Could it be that the additional load on the engine from the power steering involved in the turn, combined with the loss of power from the #3 miss and the closed throttle position causes the engine to stall.
Reasonable ideas to try. Seems those weren’t the cause of the no. 3 misfire though. Is there anything you know needs repairing, but haven’t fixed it yet? It seems like you’ve pretty much eliminated the ignition system as the cause. Next up as potential culprits, the fuel system and engine compression. hmm … were I in this situation I’d probably do a compression test, as that’s pretty easy to do. Not just on no 3, but on all cylinders. You can post the results here if you like some more ideas.
A pro mechanic’s next step would probably be to check the fuel system using a pro-level scan tool, injector balance test, as mentioned above. Other fuel system tests, fuel pressure problem could conceivably show up on only one cylinder, if slightly out of spec. I’d be inclined verify the no 3 injector is actually being pulsed, using a noid test light, or an o-scope…
Which tests to start with, common sense says that definitely must influenced by how much time they’d take to do. The best strategy is usually to start with the easier tests , and that depends on the particular vehicle’s configuration. I’ve no experience w/OP Blazer’s configuration, but wires have two ends, one at the injector, and one at the computer. Maybe the pulse test could be done at the computer end of the wire. Imperfect test, wouldn’t spot a broken wire in between. Starting with an injector balance test makes sense, provided OP has that testing capability. If not, the optimum strategy might be to pay a shop for an injector balance test. This decision depends on whether OP has more money than time, something we don’t know.
