DTC P0302 - Misfire Advice

Akula

Akula

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Hello all,

My 97 3800 Firebird has set a P0302 code, misfire on cylinder 2 according to my scan tool. Seems legit, fairly strong smell of fuel on switch off and a few pops if I shift down from 4th to 3rd. Muffler has been deleted by previous owner, just to note.

The 3800 is a new engine to me, so can anyone tell me which one cylinder 2 is? Planning to look for a vacuum leak first, then check the plug, and then try swapping the coil packs around if the plug is good.

I've heard the plugs can be a bit of a pain to get to, so I'm kinda praying it isn't toward the firewall end!

Thanks in advance.
 
First one on the front, passenger side.

Check your injector wire and plug also, had one that had a bad clip before on a S10.

Doug in P.R. 😎
 
Thanks Doug, I'm just hoping it will be something obvious!
 
Okay - in total, two codes have thrown so far - P0302 and P0420. I think the previous owner had been driving it around with this misfire on cylinder 2 and just ignored it (or used a scan tool to reset it). There is definitely a smell of raw gas from the tailpipe.

So far, I've checked the spark plug which looked ok to my eyes, the gap was correct and there was no obvious damage. The HT lead I metered (off the car) and is well within spec, no damage or splits, no corrosion inside the boots. It's fitted correctly too (coil end on the coil etc).

I've fitted some garden hose onto a vacuum nozzle and blown some smoke into the engine, I couldn't find any leaks, but I didn't manage to test all vacuum hoses, as a lot are out of sight and tricky to reach.

I've swapped the cylinder 2/5 coil pack with the 3/6 coil pack and the car seems to display the same symptoms, but the P0302 code has not come back (my expectation was that cylinder 6 would show the misfire if the coil pack was the problem, as that terminal was previously feeding cylinder 2).

I've tested fuel pressure, the quick and dirty way mind, by switching ignition on for 2 seconds, then 60 seconds later tapping the schrader valve on the fuel rail - it was a VERY strong high pressure spray, not a dribble, so surely that rules out a stuck open injector?

I've held a long screwdriver to the injector nozzle with the engine idling - it's very hard to make out as it's a loud engine bay, but I'm almost certain I could hear a constant ticking sound (like a super quiet stopwatch sound) so I think the injector is ok. The injector clip is in place.

I'm still quite new to this engine, so I'm learning as I go.

At idle the rev needle is moving up and down, but only very slightly - it's not bouncing around all over the place. I guess that, ruling out internal damage, the last option is the LIM gasket, but I'm pretty sure the previous owner told me he had replaced that.

Does anyone have any other ideas I can try? For reference the car has covered 70,000 or so miles.
______________________________
 
So you only have the P0420.
Sounds like the coil swap worked, perhaps a bad connection.
You need to rent a pressure tester and the wait time is minutes not seconds.

www.obd-codes.com

P0420 Catalyst System Efficiency Below Threshold

How to diagnose, repair, and info on symptoms and causes of a P0420 engine code which means the catalytic converter is no longer performing at a normal level.
www.obd-codes.com
 
At the moment, the only code showing is the Cat code P0420 yeah. I'm continuing to drive it every other day to see whether the misfire code will come back, as I'm still not entirely convinced the misfire is gone. The coil packs are quite new also, replaced by previous owner, so perhaps he didn't tighten them down properly. I'm hopeful! I'll see about getting hold of a pressure tester - thanks for the advice.
 
Kinda resurrecting this thread a little - the issue is still apparent. I've had the car off the road since it failed it's roadworthiness (MOT) test recently. Emissions were one of the failure points, being a slightly too high fast idle lambda reading. I don't know if this relates to the problem.

That said, the car is now showing what I believe are symptoms of an injector fault. Before, the rev needle didn't move much but now, on cold start, the car is idling very poorly, rev ranging between 1000-1300, and the car was shuddering a bit. Even when it reached operating temperature, it continued to idle roughly. So, it's going in for some servicing this weekend, and I will ask them to pay close attention to the injectors - I don't have access to a pressure tester here in the UK, nowhere around me seems to rent them, so I'll see if that can be done at the shop too.

I've tried a bottle of Techron injector cleaner which I added to a couple of cans of 99 octane fuel, as it was highly recommended, so this may assist over time. I have run it on Premium 99 but I don't know if the previous owner did. Having said that, it's been in the UK for a decade, and we dont get anything lower than 95 octane here.

I need to scan it for codes again, so I will report back with what I find. If anyone has any further advice, I'd be very grateful.
Hopefully it won't be anything insurmountable - Rock Auto are still shipping here, thankfully!
 
Are you telling the shop to find the cause of the P0420?
That code does not list injectors as the cause
www.obd-codes.com

P0420 Catalyst System Efficiency Below Threshold

How to diagnose, repair, and info on symptoms and causes of a P0420 engine code which means the catalytic converter is no longer performing at a normal level.
www.obd-codes.com
______________________________
 
No, I know that's related to the catalyst, it's the original P0302 code that came up before all this I'm more concerned about, as it leads me to believe the injector is the cause of the poor idle.

That's why I'm going to rescan the car with my tool, see if that misfire code has come back.
 
Thought I'd drop back in here to update - the issue is now resolved: all three coilpacks were breaking down under load. Turns out the previous owner had indeed replaced them, but with the cheapest crap possible. So I've fitted all new AC Delco OEM units. Problem solved immediately.

The O2 sensors were cleaned, codes reset, no problems since. Injector code mustve been thrown because of bad ignition from the coils.

Hopefully this can help someone else in the future. Thank you for all your suggestions, I appreciate your assistance.
 
Thanks for the update
 
Another update - a month on, three more DTC's set. These are:

P0122 (TPS/Switch A Circuit Low)

P0300 (everyone's favourite random/multiple cylinder misfire code)

P0420 again (Cat below efficiency)

Now, a TPS is already on its way from a certain rocky supplier, so I will update once fitted. I know I shouldn't throw parts at the car, but they're only £25 so I figure why not in this case.

Speaking of which, on a 3800 Series II, does the TPS require twist-fitting to the same position as the previous one (ie: positioning first at an angle, and then rotating to line up before screwing down)? Or is it a simple pull off old, push in new?

Any advice very much appreciated. Engine is 75,000 miles now, I've heard heat can kill the TPS from somewhere, I'm sure.

Thanks again all. Hope you're all well.
______________________________
 
There is no adjustment if that is your question.
 
melsg5 said:
There is no adjustment if that is your question.
Click to expand...
Thanks for that, I was worried there would be some kind of manual adjustment involved, good to know.

Issue again seems to be resolved, though I won't blow the trumpet just yet.

I replaced the TPS and cleared the codes. So far (117 miles of driving) no codes have returned and the car runs much sweeter. Idle is also much smoother.

Probably a combination is issues, but hopefully with this all in the same thread, it can help others too. Thank you for your help, guys.

To summarise:

- Coilpacks were breaking down under load - replaced with new (AC Delco)

- Throttle Position Sensor may have been faulty - replaced with new (Delphi)
 
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