00 Denali - Random Misfires under load

[TurboGSR96]

[/COLOR]just put #1 on a piece of paper nearby so you don't have to take time editing the photo with labels.

No need to label them anyway, they are not arranged anymore... they were so similar I didnt bother keeping them matched.

 

[SunlitComet]

Daaaaammmnn! acdelco 41-993's LIKE NOW! One of them looks to have physical damage is that something you did? Also if you have the stock ignition system the spark will be a little weak unless platinum is on both electrode.

---------- Post added at 11:25 PM ---------- Previous post was at 11:20 PM ----------

Almost looks like they were the wrong heat range as well.

 

[TurboGSR96]

I put the correct plugs and wires in today, no difference.

---------- Post added at 02:43 AM ---------- Previous post was at 02:42 AM ----------

One of them has anti sieze on it

 

[SunlitComet]

Don't suppose you have a boroscope to stick down your cylinders. If a compression test turns up negative you have got to have an injector problem.

 

[TurboGSR96]

How can I have an Injector Problem? I had this exact same issue with the original 180,000mi Injectors and no better with the MFI Upgrade....

If you feel its a fuel problem, then how far out there can my clogged strainer/pre-filter theory be? I just find it odd a bunch of powerful additives designed to looken up gunk made the truck run good for a little while then when the additives stopped the issue returned.

I know you have been suggesting a comp test, I still feel the issue does not lie there... I wouldnt have a good running truck until precisely when the trucks goes into 3rd gear if I had internal issues with the motor... it doesnt smoke or eat oil and its a strong running engine until 3rd gear.

 

[gpracer1]

Here is the sad part about fuel delivery. Lets say the fuel pump, filter, injectors and regulator are all good. You have to find out if it really is running lean like you think. Either a wide band, or log the O2 sensor voltages while driving normal and WOT. At WOT they should both be up at .800 - .900. Normal driving, they should be bouncing high and low like .2xx - .7xx up and down constantly.

It could be a fuel calculation problem. In other words one or more sensors is not working correctly (or even the PCM) and you are not getting enough fuel because the A/F ratio is is being told to be lean.

So ASSUMING you have all good fuel components and plugs, there could be a bad coil, dist cap, wires, compression, MAF, MAP, TPS, PCM, O2 sensors, IAT, ECT, etc.

If you can log these sensors while driving down the road at 50 mph, I can do the same and compare to see if one of your sensors is way off or something. That is all I can think of at this point in trying to troubleshoot over the internet (which can be very difficult).

Have you tried unplugging the MAF and then start and drive it?

 

[TurboGSR96]

I plugged the truck in yesterday, didnt look at 02 voltages while driving but at idle 3 of them were sweeping pretty good and one wasnt sweeping as much. After I revved the engine a little the slower moving one moved a little more but not like the others. They were sweeping from like 10-70 area while idling.

I can confirm, IAT and the ECT sensors are working properly and the 02 while old, are still sweeping enough not to cause this issue. Plugs and Wires are OEM Delco and not the issue, fuel filter is a few weeks old not that either. The coil was replaced when I did the cap and rotor a year ago, issue never changed. I am about to re-do the cap and rotor again for the hell of it, but that brings us back to why would the ignition system decide to go screwy in 3rd/4th/OD and not 1st and 2nd? Also after the truck goes through one of its fits the issue with clear up if I maintain 45-50mph with very light load like the truck has adequate fuel flow, once the demand increases in a high gear the truck will start breaking up again until I get back out of the throttle again... 15sec passes with no throttle at all and it clears up again. (almost like the pump is straining to get the pressure back up where it should be)

What would unplugging the MAF do besides making air/fuel control go crazy? I was unaware the truck had a MAP too how do the two work together? Thought you only needed one or the other?

What effect would a PCV have on this issue?

Thanks for all your help fella's! This truck is a borderline bastard SOB money pit...

 

[gpracer1]

If you unplug the MAF it will run in open loop controlled by the map in the PCM. It just takes out the airflow value and changes it to fixed calculation based on rpms, temp, MAP, TPS, etc just like on the TBI motors that dont have a MAF. (speed density)

Yes it has a MAP, manifold absolute pressure sensor. It helps in calculating engine load.

I would love to see a log of your sensors at WOT say 4K and at cruise at 2K rpms to compare each sensor vs mine.

Does the problem happen right from the start, or does it take a few minutes?

I would laugh if your gas tank wasnt venting causing it to starve.....wishfull thinking.

 

[TurboGSR96]

Understood on unplugging the MAF, I will run out and do that in a minute.

I am very familiar with MAP's I run that setup on my turbo car...

I will try and borrow my friends good scan tool, this has to be something simple at this point...

The problem occurs warm or cold, normal acceleration in 1st and 2nd gear... shift into 3rd any more than like 20% throttle the truck feels like it just drops 2 cylinders and starts misfiring and shaking real bad. If you quickly get out of the gas for 10-15sec the shaking and misfiring stops, I can feel the truck pick back up and it will be fine until I load it too much again.

Dude how hard is it to check the vent? This truck has seen very dusty dirty conditions most of its life. The fuel pump is louder than the twin externals I have on my racecar... makes me think the pump is pulling a vacuum on the tank and once I get off the gas the vacuum drops causing the pump to return to normal capacity after some time to recover.

---------- Post added at 11:57 AM ---------- Previous post was at 11:31 AM ----------

Unplugged the MAF and took a quick drive... truck has no throttle response and shifts like the converter was constantly locked up or something. Never really got it into 3rd gear pulled over and plugged it back in. It definitely is reading the lb/min of flow increases as I give the truck throttle.

---------- Post added at 12:20 PM ---------- Previous post was at 11:57 AM ----------

The interesting thing is the truck was always suggesting cylinder 6 misfire, but by looking at the plugs you can clearly see its an "all cylinders" misfiring issue. Its weird because it doesnt feel like a lean misfire to me, I run my race car on E85 and sometimes I have to tweak the fuel columns to adapt for changes in the Ethanol Blend, I have gone from pump gas to E85 without any Air/Fuel Tuning and the car was obviously running stupid lean and it didnt feel like this. Also if the truck was running lean why is my fuel economy so poor? lol

 

[gpracer1]

Try taking off the gas cap or leaving it real loose.