00 Denali - Random Misfires under load

[TurboGSR96]

Just tried that, left it all the way off. Started out dead stop went to 3k rpm in 1-3rd clean shifted to 4th and then instant multiple cylinder misfire and heavy shaking.

---------- Post added at 01:47 PM ---------- Previous post was at 01:45 PM ----------

I would do a WOT pull and then key and pull a plug but honestly im afraid to, just put these expensive ass fresh plugs in there dont really wanna pound them out already...

 

[SunlitComet]

I would like to see a log file to with the o2 readings as well. If your problem disappears with fuel additves and comes back when they are gone that indicates a possibility of bad fuel chemistry. You mention e85 in your other vehicle, what are you using in this vehicle?

 

[TurboGSR96]

I would like to see a log file to with the o2 readings as well. If your problem disappears with fuel additves and comes back when they are gone that indicates a possibility of bad fuel chemistry. You mention e85 in your other vehicle, what are you using in this vehicle?

I will try to get a log, this truck uses regular every day pump gas like every other car. I only did the additive thing once and that was at least a year ago, but the problem definitely went away enough to pull a trailer with a 2600lb car on the back of it. I used a high grade motorcycle carbon cleaner in a few gallons of gas ran that out and then i added a few more gallons of gas and a can of sea foam and let it sit in there overnight and next day it ran good.

 

[SunlitComet]

I can't recall did you do a fuel pressure test like the ones I have posted around in other threads? If not would you like it.

 

[TurboGSR96]

On key on I get a 60psi spike that sits at 55psi no drop
At idle I get 50psi and jumps to 60psi when floored in park.

Are issues this difficult to pinpoint normal with these motors?

---------- Post added at 05:08 PM ---------- Previous post was at 04:58 PM ----------

I just checked the old codes, sometime recently it blew a P1351 IC Circuit High Voltage... I dont think unplugging the MAF would of caused that?

Maybe the coil is bad?

 

[gpracer1]

I thought you said you already replaced the coil ?

You need to do the fuel pressure check while it is misfiring in 3rd or 4th gear under load......you might find it dropping to 20 psi or something. In park there is barely any load or fuel consumption.

 

[SunlitComet]

print before it changes. OBD2 for these specific year were still kinda vague on ignition faults because they are so hard to track thousands of spark cycle per minute to analyze every single one. That code would indicate that voltage spikes are possibly leaking from the Ignition control module to the pcm output that could be the ICM or coil but it would have to leak thru the ICM to reach the pcm monitor for that output.

Follow this chart as best as you can.


P1351

Circuit Description
The Enhanced Ignition system uses the Crankshaft Position sensor in order to provide a timing input to the Control Module (PCM/VCM). Ignition Control (IC) spark timing for each cylinder based on this input. The Control Module provides the Ignition Timing Signal to the Ignition Control Module (ICM) to control the Ignition Coil. Each timing pulse detected by the ICM allows it to energize the Ignition coil. A large Secondary ignition voltage is induced in the secondary coil by the primary coil. This high voltage is switched to the correct spark plug by the distributor.

This DTC will set if the VCM detects an unusually high voltage on the ignition timing signal circuit.

Conditions for Running the DTC

• The ignition control is enabled
• The engine speed is less than 250 RPM

Conditions for Setting the DTC
The Ignition Control voltage is more than 4.9 volts

Action Taken When the DTC Sets

• The Control Module illuminates the Malfunction Indicator Lamp (MIL) the first time the diagnostic runs and fails.
• The Control Module will set the DTC and records the operating conditions at the time the diagnostic fails. The Control Module stores the failure information in the scan tools Freeze Frame and/or the Failure Records.

Conditions for Clearing the MIL/DTC

• The Control Module turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
• A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the Engine Coolant Temperature is more than 70°C (158°F) during the same ignition cycle).
• Use the scan tool Clear Information function.

Diagnostic Aids
Intermittent test - If the connections and the harness check OK, monitor a digital voltmeter connected between VCM terminal C3 pin 9 and C4 pin 18 while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change. This may help to isolate the location of the malfunction.

An intermittent may be caused by any of the following conditions:

• A poor connection
• Rubbed through wire insulation
• A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. See: Symptom Related Diagnostic Procedures

Test Description
The numbers below refer to the step numbers in the diagnostic table.

1. This determines if the DTC is an intermittent.
2. This step checks if the IC signal from the VCM is available at the Ignition Control Module.

 

[TurboGSR96]

Sunlit, I read all that... seems overwhelming its hot as hell outside this bastards about to get dropped off at the stealership or parted out.

I thought you said you already replaced the coil ?

You need to do the fuel pressure check while it is misfiring in 3rd or 4th gear under load......you might find it dropping to 20 psi or something. In park there is barely any load or fuel consumption.

I did replace the coil a year ago and the issue never changed, not sure what coil went on it... got it at Autozone they sell AC and another brand. Wish I still had the old one.

No way to check pressure while moving or I would of done that already... the lead on the borrowed gauge was way to short.

 

[SunlitComet]

If that is the case then icm is $125 at oreily for a acdelco unit if you want to take another shot at it before you may get rid of it but also inspect its wiring to for damage. Otherwise let the dealer scope the ignition power patterns for you.

 

[gpracer1]

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.