P0302 - Misfire DTC

[Trail X]

I've had a P0302 that I've been chasing for a while. This means misfire cylinder 2. The code will only come on when the truck has been idling for several minutes. The misfire is most felt when at idle, or when slowly creeping to a stop.

At first, I figured this was most likely due to old spark plugs, so I instinctively changed them out. As I recall, it seemed to improve the idle for a while.

The code however kept coming back.

I changed out the coil on #2, cleaned the throttle body. Boom, ran like silk. I figured it was the coil replacement that fixed it. However, a few days later I started noticing the misfire at idle, and eventually the P0302 came back.

This leads to yesterday, when I remembered I had used injector cleaner when I changed out the coil. So I went out and bought another bottle of fuel system cleaner. After a few minutes of running, the engine went smooth again. Now, I'll have to give it a week or so to make sure the P0302 doesn't come back, but based on feel, the misfire seems to have gone away due to the additive.

Now, does this logic make sense: since the fuel additive seems to be the only thing that helps the misfire - does that mean it's an injector that is dirty? I'm contemplating replacing it myself - although it seems like it will be a decently large job. I've also contemplated getting a professional cleaning (where they use high pressure to help clean the injectors). It seems that the professional cleaning and a new injector might cost about the same.

Any thoughts or opinions? Thanks guys.

[Blask3]

I would say change out the injector. I have never ran into this problem on the trailblazer. on past vehicles it was easier to change it then to clean it. I'm not even sure how the injectors are set up on the TB.

[bgwolfpack]

James have you done a compression check yet? If not, get that one out of the way first. I know a PO302 is random, you said this happens during warm up? A scorched valve may seat after heat up. Just good to check. You do have a compression tester right?
Once you know it's ok than either fuel or ignition is the fault. Sounds like you've changed the ignition components associated with cylinder #2 only. Next change the fuel side of equation. I do not suggest changing only one injector. Change all six as a set. Each injector has millions of open a closes on them and are warn. One new injector could make things worse (lean). Once you've changed them all and the issue continues or if different codes arise then...
Many miles on these trucks now. Parts will fail and will need repair or replacement. Just make sure you have proper books like Helm Inc.

[Trail X]

I have not done a compression check because the issue has gone away intermittently. I was expecting that poor compression would show up no matter the fuel additive. However, maybe your point is valid. Maybe the additive is allowing combustion under lower compression pressures.

I have also recently read that the misfire sensor can sometimes be wrong about the location. Maybe I should go back and check the adjacent plugs for spark.

Another quick question regarding compression checks - they are supposed to be done on a warm engine - but you aren't supposed to remove spark plugs when it's warm. So... how does that work?

[bigezzyk]

did you check the connection that plugs in to the coil pack i have had one go bad due to getting mud in it

[bgwolfpack]

Another quick question regarding compression checks - they are supposed to be done on a warm engine - but you aren't supposed to remove spark plugs when it's warm. So... how does that work?

You didn't throw your old set of plugs away did you?

I'm perplexed at your comment. Why wouldn't you pull a warm or even hot plug?

[Trail X]

It says not to on the left engine cam cover - right? Or maybe that's do not install when engine is warm. I forget.

And yes, I threw them away once I thought the misfire went away.

[bgwolfpack]

It says not to on the left engine cam cover - right?

I don't know. Maybe it does say that. I would imagine it refers to letting the head be cool down before installation of plugs for proper torque specs.

Hmmm

Now I've got to go look.

Never the less, your PO302 should go away with new injectors or some other related part.

Oh and, my new injectors will be here Friday or Saturday along with new Radiator hoses, Thermostat, CPS, Belt, TB gasket, FIP Reg and Coils. Never seen a code off this truck but I know this intermittent mis you speak of. Already changed Plugs, OS's, Air filter, Fuel filter and used gas additives with the same effect.

[fishsticks]

In my experience, dirty injectors generally like to stick in the open position or just have a terrible spray pattern. I'm trying to wrap my head around how some injector cleaner could fix the problem for a short amount of time.

If you can afford to have the truck down for a day or two, check in your area for places that do ultrasonic cleaning. You pull your injectors and drop them off with them. They use a combination of heavy duty detergents and ultrasound to knock everything out of them. The place I used also flow benched them before and afterwards to show how dirty they were and to see if any had problems after the cleaning. Mine were like new when they were done.

I recall paying around $250 for 8 injectors to be done (which was half of what new ones would have cost me)... it's been a few years though.

[bgwolfpack]

Rock Auto GB REMANUFACTURING Part # 83211205 $36.89 ea plus $9.00 core.

[Trail X]

Hmm, I normally try to replace with OEM whenever it has to do with engine internals - but reman equipment could be just as good. The OEM AC delcos were running more like 90 each if I recall.

Donny, I was trying to wrap my head around it too. It seemed to me that it pointed to the fuel system being the culprit. However Randy made a good point about checking compression in the cylinders. It's possible that the additive is allowing combustion to occur at a lower pressure.

[bgwolfpack]

One other thing to ponder. In the past 'Sir Bill' has recommended changing the Camshaft Position Sensor (CPS). I tried this about six month's ago with one off a wreck. It seemed to work for a short time but then the mis was back and slowly getting worse. Maybe a new fresh one would work properly as Roadie said.
One thing I'm not sure of, and I don't have my Helm available, does the CPS work with the Fuel or Ignition side or both? In any case if it's worn or damaged it may have a great impact.

[fishsticks]

One other thing to ponder. In the past 'Sir Bill' has recommended changing the Camshaft Position Sensor (CPS). I tried this about six month's ago with one off a wreck. It seemed to work for a short time but then the mis was back and slowly getting worse. Maybe a new fresh one would work properly as Roadie said.
One thing I'm not sure of, and I don't have my Helm available, does the CPS work with the Fuel or Ignition side or both? In any case if it's worn or damaged it may have a great impact.

CPS simply takes a measurement for the PCM. I could see an overall rough running condition caused by a bad CPS but not one cylinder.



I'm hoping a compression test turns up nothing for you James. Head work can really ruin your day.

[The Roadie]

I'm a big fan of changing the CPAS (cam position actuator solenoid), especially now Dorman has one for $40. Not remembering discussing the CPS except I changed mine to no effect when I had some unexplained idle RPM fluctiation (not the motor mount vibration issue, but at the same time). In any event, all that system does is affect exhaust cam timing. Not sure how it would cause a one-cylinder misfire or vibration, since the CPAS is relatively slow-acting compared to the crankshaft spinning.

[bdp1978]

JD- is your misfire the type where the truck at idle will drop RPM's to the point of almost stalling and then rev up well above idle rpm's and then drop to idle again?

I've had that issue for awhile, it smoothed for a couple months after I did the brake booster sea foam treatment but has returned in the past couple days. Hasnt thrown any codes, but has dropped enough to stall at stoplights a couple times.

[Trail X]

Mine has stalled only once since this issue occurred. It seemed to stall directly from the 650 rpm idle - it didn't sputter at all, just seemed like I turned the ignition off.

The idle has been rough, but RPMs were steady - never dropped below 650 rpm or so.

[bgwolfpack]

James if the truck stalled from idle and the only code you have is a PO302 (Random Misfire Cylinder #2) then this is a fuel issue only. If this was Ignition related on #2 only. Stalling would be highly improbable. The engine starved and it died, then it's Fuel.

(CPS from memory. CPAS from his fingers. Around 100,000 miles. Less than $60.00 for both...)

[Karo]

Hi James,

wondering if you have fixed this problem, mine has Cylinder 4 misfire (P0304) ctually, sometimes it disappers, next day it´s there, New Ignition coil is waiting for installation.
So was it the injector?
Karsten

[Trail X]

Still haven't fixed it. I recently purchased a compression tester, but have yet to have time to use it. I've been elbow deep in other maintenance issues (front suspension mainly). Anyway - the bottom line is, not enough time.

The funny thing, is that the engine idle only seems to be temporarily fixed when I use Gumout brand injector cleaner... no others seem to help, including Techron. It also seems to be worse on cold days - whatever that means.

[Trail X]

On a side note, if you're not running premium gas I would recommend it. After looking at some tables, my car was pulling up to 5* of timing on 87 octane. I found that a little surprising.

Very interesting. The 5* of timing was to reduce ping I assume? I've never heard mine ping, but I still have a phantom cylinder misfire at idle.