Misfire AGAIN P0300 and P0430

[nimrod.sixty9]

Well the Yukon is acting up again; original thread here.

This time it is not pointing out a specific cyl.
Only have;

• P0300 - Random/Multiple Cylinder Misfire Detected; and,

• P0430 - Catalyst System Efficeiency Below Threshold (Bank 2)

Seems to be every startup now, and then smooths out. This is hot or cold; could be running fine, turn off for a few minutes and it starts all over again; SES flashing and all.

 

[GTPprix]

My vote is for clogged cat

 

[nimrod.sixty9]

My vote is for clogged cat

It does seem to resemble that, but it doesn't do it all the time. I've never had a misfire from a a cat that was inconsistent.

 

[GTPprix]

I've seen it when the brick is breaking and shifting causing huge swings in back pressure but it's not super common.

 

[nimrod.sixty9]

Random frame capture. Not sure if I even trust this crappy app anymore tho (I was getting misfires with the truck OFF).

 

[SunlitComet]

backpressure test the exhuast system on both banks. Also if you can open your throttle and see if you can smell any gas in there. I say this cause you have 3 cylinders with current misfires as you are running your test and there are close to the fuel body. Two nearest the FPR. Is it hot where you live when you did this test? Engine seems awfully warm.

 

[nimrod.sixty9]

backpressure test the exhuast system on both banks. Also if you can open your throttle and see if you can smell any gas in there. I say this cause you have 3 cylinders with current misfires as you are running your test and there are close to the fuel body. Two nearest the FPR. Is it hot where you live when you did this test? Engine seems awfully warm.

Unless Im reading it wrong, doesn't it say Im misfiring on all but cyl 1?

For the fuel smell check; running or no?

It has been fairly hot today as well as not having the cover on the air box. I imagine that would explain the temp.

I will have to rent the tool to do the back pressure test. So is there anything else I should get while Im there?

---------- Post added at 04:24 PM ---------- Previous post was at 04:10 PM ----------

Just went to smell the TB... Open I could smell fuel, but only a little and with my nose buried into the TB. Could be old cleaner or oil for all I know.

---------- Post added at 05:01 PM ---------- Previous post was at 04:24 PM ----------

Also need to state, it does not lack power. Not like a clogged CAT, I get into the pedal and it rolls.

 

[SunlitComet]

Yes but I do not know how far back that history goes. Could be since the pcm last lost batt power. Current is live as in the past minute or so. I was talking engine temp not intake temp. For the fuel smell prime the system a few time with out starting and see if the fuel odor get stronger. If so can you borrow a boroscope to look inside the manifold?

---------- Post added at 02:04 PM ---------- Previous post was at 02:03 PM ----------

then you can probably skip the backpressure test then.

 

[nimrod.sixty9]

Yes but I do not know how far back that history goes. Could be since the pcm last lost batt power. Current is live as in the past minute or so. I was talking engine temp not intake temp. For the fuel smell prime the system a few time with out starting and see if the fuel odor get stronger. If so can you borrow a boroscope to look inside the manifold?

---------- Post added at 02:04 PM ---------- Previous post was at 02:03 PM ----------

then you can probably skip the backpressure test then.

How do I see current?

Had roomy prime the pump while I was nose deep in the TB, no change. If anything, the longer I held it open the less I could smell it.

Now what?

 

[SunlitComet]

Current as in current misfire count for each cylinder. I think a no cost thing to do is read the plugs and see if anything stands out. Did you see any spark arcing inside the dizzy cap before?

 

[nimrod.sixty9]

I thought that's what I did. If not, how do I do it?

No arching.

---------- Post added at 07:54 PM ---------- Previous post was at 07:08 PM ----------

Okay, well I officially no longer trust this app. Shows current misfires all over the place. Often they match each other in numbers, yeah right. Shows RPM going from 9000 to 600 to 3500 and I'm just cruising. Total BS.

 

[SunlitComet]

does the vehicle dash tachometer and vehicle speed remain constant when the pcm has really wild numbers?

 

[nimrod.sixty9]

does the vehicle dash tachometer and vehicle speed remain constant when the pcm has really wild numbers?

Maybe I should explain more how it feels; most of the time it feels fine with just a slight miss here and there. But sometimes, it's crazy missing and hesitation with massive loss of power. If I floor it, it can get up to speed pretty well. Slower seems worse as well as going up bridges (my hills).

RPM does not match the PCM numbers at all. Every frame refresh shoes it's all over the place.

 

[SunlitComet]

The rpm that the pcm puts out is based on the crankshaft sensor. That sensor is required for starting and running. Ignition and fuel timing is based off it as well as misfire monitoring. So again say you are cruising down the road in the consistent 60's and say your tachometer(see's it signal from actual ignition wiring so very separate sources.) says 1800 rpm while in lockup overdrive. Your pcm should be saying saying about the 1800 rpm too. If it at all says 9000 (Should never see that kind of number) rpm or some numbers that does not match the tach something is very wrong.

Possibilities include:
1. a screwy diag program, head to a local store and use there scanner and verify say with the throttle open part way.

2. Defective crankshaft sensor.

3. Defective sensor wiring.

4. Damaged reluctor inside the timing chain cover(highly doubtful tho.).

5. A defective pcm.

If you have to change the pcm or crankshaft sensor you will need to perform the relearn procedure. It is used to learn the variances in the sensor so it gives proper misfire data detection to the pcm. Will need a high end scanner for the procedure that can initiate the function. Put it in park and set the parking brake. Start the function on the scanner. Once the scanner says accelerate, scream for your life real loud and floor it WOT your engine will head for the redlining. On it way to blowing up in a ball of fury it will sudden start dropping a few cylinders. Immediately release the pedal to avoid redlining it and consider the test done unless the scanner says you failed in a crash. If all goes well you will be done 2-3 seconds after you mash the throttle open.

 

[nimrod.sixty9]

Just checked it with Tourqe Pro on my cell and the RPM reading is spot on. Like I said, the app. So now what?

 

[SunlitComet]

If you are sure the program is wrong then tell the maker of it so they can solve the issue that way you can get some accurate data to help diagnosis. Read those plugs yet. remove them, label and take multiple pics to post.

 

[nimrod.sixty9]

If you are sure the program is wrong then tell the maker of it so they can solve the issue that way you can get some accurate data to help diagnosis. Read those plugs yet. remove them, label and take multiple pics to post.

Thanks Sun. Ill get the plugs changed out and posted as soon as I can.

Im going to reinstall the app and see if I can get some trustworthy readings.

Hopefully soon I can find my fuel pressure gauge. Im thinking maybe weakening fuel pump. We will see.

 

[SunlitComet]

Don't really need to change them out just inspect them for now. Platinums can cost about $5 each you know.

 

[nimrod.sixty9]

Just as I was about to go out and pull the plugs, it occured to me that I do not trust what ScanXL is showing for all the misfires. Why would I trust it for the codes?

Sure enough, codes did not match.

It was one again cyl 3! Changed just that plug and once again runs like a dream...

Im suspecting it was fouled up from last time it had a misfire from what I assume to be the cap (what I replaced 4ish months ago) and steadily got worse from there. What say you?

 

[SunlitComet]

I say if it happens again let me know. Still does not solve the bad data issue in the diag software.