00 Denali - Random Misfires under load

[TurboGSR96]

I have a Matco Pro Scan Plus MD1000 in my possession, how would I post a log to my computer for you to view? I do not have a software disc for it nor do I see a way to connect it to my computer, it does have a phone jack though...

---------- Post added at 12:52 PM ---------- Previous post was at 12:49 PM ----------

I am starting to suspect the fuel delivery after weighing all the info.

Runs fine in 1-2-3rd and has problems in OD, then you let off for a few and baby it and it runs ok at light load. Sounds like fuel starvation. Ill bet the O2's are reading totally low and not even bouncing.

You did say your coil was from Autozone.....lifetime warranty?

---------- Post added at 03:36 PM ---------- Previous post was at 03:29 PM ----------

I just remembered something that happened about 17 years ago when I was a tech.....same type problem and the rubber fuel line section had come apart inside and a flap of rubber would block the flow.

I would take a flashlight and check all the lines for maybe being smacked shut by a rock, or bent from a wreck, etc.

Really need to check the fuel pressure while it is misbehaving.

9 times out of 10 it doesnt pull through 3rd very cleanly either, normally just 1-2nd gear. I will check the lines when it cools off later, I have been thinking fuel too after looking at the plugs.

I got a scan tool, no idea how to upload to a computer though. EDIT I found a phone jack connector for it, gonna go drive and long some miles in a minute... hopefully we can get this figured out!

---------- Post added at 01:41 PM ---------- Previous post was at 12:52 PM ----------

Anyone have a clue where I can download the software at? No software disc in the scanner case...

Took a drive and did some logging, there is a misfire history on every cylinder but cylinder 1 which has 0. Cyl 6 almost always starts first followed by cyl 4 and so on. Oddly enough after driving a while I couldnt get the truck to do it anymore idk.

At one point before it got better I thought the truck was going to die it was misfiring so bad, got a good whiff of something out the exhaust too when that happened.

 

[SunlitComet]

Can you identify the whiff at all? I have bee up 28 hours now will come back later.

 

[TurboGSR96]

Couldnt identify the smell, didnt resemble rotten eggs....

Went through the list of sensors and tested them all individually, couldnt find anything out of range even slightly at idle except:

Inj PWM 1&2 avg. 3.0-3.3 (range 3.6-3.8) not off by much
MAF: 6.1g @ idle
MAP 9.4 @ idle
Spark 22-24deg at idle

everything else seemed in range. No luck on finding software to be able to upload a log, need a Serial to USB adapter as well...

 

[SunlitComet]

I think if the map was reading 9.4 INCHES of VACUUM, with misfiring, a screwy O2 plus rotten eggs you may have burned out a catalytic converter. Have an exuast shop do back pressure test to confirm or deny it. If you need a new assembly I reccomend the high flow assembly for magnaflow. straight bolt on from manifold to muffler and 50-state legal for $550 would also recommend reevaluate the O@ sensors again afterwards if you don't replace all of them all ready.

 

[TurboGSR96]

I think if the map was reading 9.4 INCHES of VACUUM, with misfiring, a screwy O2 plus rotten eggs you may have burned out a catalytic converter. Have an exuast shop do back pressure test to confirm or deny it. If you need a new assembly I reccomend the high flow assembly for magnaflow. straight bolt on from manifold to muffler and 50-state legal for $550 would also recommend reevaluate the O@ sensors again afterwards if you don't replace all of them all ready.

9.4 was at idle as I stated, it reads 160ish at WOT in 2nd around 4400rpm. I will look into a backpressure test, I never said it smelled like rotten eggs.

 

[SunlitComet]

sorry about the rotten egg thing. What exact unit of measurement are you using for the map sensor reading that gets you around 160. I am confused. I was just guessing the unit of measure. Perhaps it was something else.

 

[gpracer1]

Mine is read in kPa. It is 35 at idle and 95 at WOT.

 

[SunlitComet]

In that case skip the back pressure test then. Thought you might be showing sign of clogged cats.

 

[TurboGSR96]

Pretty sure the scan tool was measuring grams for the MAF, cant remember what the MAP was.

I have not had time to mess with it or get a backpressure test done, took it for a drive yesterday before I returned the scan tool. I noticed that before i did the new plug/wire the misfire always started in cyl 6 before moving to other cylinders now it starts at cyl 4 and moves to cylinder 3 after the plug/wire install.

The misfire history had cyl 6 at 20000 misfires while the rest were only in the 500-1000 range, cylinder one never had one misfire logged ever...

The truck also has alot more useable throttle position and I can stay into it longer before it starts misfiring, just now the first cylinder to go is cyl 4 every time and after a few seconds cyl 3 starts acting up too.

 

[SunlitComet]

Damn forgot about you turbo, did you ever look over the cylinder compression? I don't recall.