'97 still having P0300 or P0303 only under load or on steep roadway

asilverblazer · December 4, 2024

On 12/3/2024 at 10:29 AM, bcrooms said:

All that you have mentioned has been checked by 3 different mechanics within the past year and none have resolved the issue.

The P0303 only appears when the truck is under heavier load (steep road, more throttle, high MPH) and is only replicated in those scenarios.

In my daily commute driving, it doesn't seem to have any issue.

Example: Intermittent miss cylinder #7, this only occurred on 'wetter' days. Visual inspection seemed fine, a new cap fixed it.

Also, found that the cheaper caps terminals would corrode on the inside causing intermittent miss fires. Pop the cap and clean the corrosion and miss went away.

I also have had a brand new set of plug wires with one bad wire straight out of the box.

Or, you can start looking at internal engine components that are specific to cylinder 3 that are more expensive to inspect and replace and less likely to be intermittent or load based.

Cam shaft lobes

Push rods

Lifters

Valves

Rockers

Intake gasket

Head gasket

An intermittent miss fire with no other symptoms such as smoking, noises, other codes, etc. doesn't indicate an internal failure to me.

Fuel delivery up to injector 3 is not the problem...

Ignition up to distributor cap terminal 3 is not the problem...

Air intake up to intake valve 3 is not the problem...

Sensors that monitor the entire engine or cylinder bank are not the problem...

... otherwise the problem would be more widespread.

bcrooms · December 9, 2024

When the misfire occurs, or is replicated…it seems to happen during the last shift as the transmission is changing gears.

it will misfire for a few seconds, lose power / rpm’s then it feels like it shifts down a gear and rpm’s jump a bit and then it’s fine

asilverblazer · December 10, 2024

22 hours ago, bcrooms said:

When the misfire occurs, or is replicated…it seems to happen during the last shift as the transmission is changing gears.

it will misfire for a few seconds, lose power / rpm’s then it feels like it shifts down a gear and rpm’s jump a bit and then it’s fine

I'd say unrelated except the increased engine load is inducing the miss fire.

Without the ability to actually diagnose in person, my guess is a bad injector, can't keep up with fuel delivery under load.

How did the mechanic(s) confirm all the above suggestions were not the problem?

All other possibilities are more and more unlikely.

bcrooms · December 10, 2024

42 minutes ago, asilverblazer said:

I'd say unrelated except the increased engine load is inducing the miss fire.

Without the ability to actually diagnose in person, my guess is a bad injector, can't keep up with fuel delivery under load.

How did the mechanic(s) confirm all the above suggestions were not the problem?

All other possibilities are more and more unlikely.

The MPFI injectors were replaced for the 3rd time in Jan ‘24 by mechanic #1

Others diagnosed distributor, cap, rotor, wires, plugs and those appear fine

Compression was checked by all 3 guys and it was good.

each mechanic kept the truck for nearly a month at a time and couldn’t determine the root cause

to me feels like something gets stuck open or closed at the time of misfire / higher load and then frees up but then it has the rough, stumble idle (sounds like a muscle car at times)

Edited December 10, 2024 by bcrooms

bcrooms · December 31, 2024

On 12/10/2024 at 12:01 PM, asilverblazer said:

I'd say unrelated except the increased engine load is inducing the miss fire.

Without the ability to actually diagnose in person, my guess is a bad injector, can't keep up with fuel delivery under load.

How did the mechanic(s) confirm all the above suggestions were not the problem?

All other possibilities are more and more unlikely.

I have replicated the misfire on a mostly flat highway if I punch the gas and get it above 65 mph+ and when the truck shifts to the last gear, it immediately starts having the symptoms previously mentioned. I either keep the throttle open until it quits or slow down until it stops.

asilverblazer · December 31, 2024

14 hours ago, bcrooms said:

I have replicated the misfire on a mostly flat highway if I punch the gas and get it above 65 mph+ and when the truck shifts to the last gear, it immediately starts having the symptoms previously mentioned. I either keep the throttle open until it quits or slow down until it stops.

Do you have a data logging device? Some way to monitor how much fuel and air are being consumed and burned - or not burned...

bcrooms · December 31, 2024

4 hours ago, asilverblazer said:

Do you have a data logging device? Some way to monitor how much fuel and air are being consumed and burned - or not burned...

At the moment I have a basic Autel AL319. I am planning on getting a better DIY scanner like the Autel AL539B or AL629

bcrooms · January 14, 2025

I was doing an oil change recently and pulled the spark plug from #3 to inspect. This is how it looked at time of removal.

I also checked plug #1, and it looked similar to #3.

I am planning to replace the plugs soon.

Edited January 14, 2025 by bcrooms

asilverblazer · January 14, 2025

On 12/9/2024 at 11:55 AM, bcrooms said:

When the misfire occurs, or is replicated…it seems to happen during the last shift as the transmission is changing gears.

it will misfire for a few seconds, lose power / rpm’s then it feels like it shifts down a gear and rpm’s jump a bit and then it’s fine

Are you feeling torque converter slip or poor TCC action? This can sometimes feel like a shudder or miss fire. Also a torque converter that doesn't lock up will cause lower fuel economy. You could still have a miss fire on cylinder 3 (not necessarily at the times you describe) in addition to failing TCC solenoid that isn't bad enough to trigger a code yet. (Transmission component slip detected or something like that...)

Plug looks like it has a miss.

Post a picture of the underside of the cap and rotor tips.

Fuel trim for that bank is still way negative? (Showing a bunch of unburnt fuel is making it through to the O2 sensor.)

Have you tried a spark tester on it? A visual confirmation of spark...

bcrooms · January 15, 2025

So I ran some engine cleaner through it earlier today, and then took it for a good 25-30 minute drive. The drive ended with a test on the steep incline road near my house with more throttle.

Got the P0300 again.

Edited January 15, 2025 by bcrooms

asilverblazer · January 15, 2025

From the freeze frame data looks like bank 1 is running richer than bank 2 based on short term fuel trims. Not much else to glean off the data though.

Still could be problem unique to cylinder 3 - potentially a more global problem. I would have to go back to re-read what has been done so far, but, there is a possibility that the bank 1 upstream O2 sensor is failing. That could result in an inaccurate/incorrect fuel trim for that bank. It would cause problems more wide spread than just cylinder 3 though. Hard to tell based on information so far. We don't know what the bank 1 sensor reading is, but if we infer that because of the fuel trim for that bank being leaned out the sensor must be experiencing a rich condition. For that to happen there has to be too little O2 in the exhaust. If the sensor has some carbon build up, it could not read all the O2 that is actually present in the exhaust stream.

Quote

Rich Air/Fuel Mixture: The signal is high (0.7 to 0.9 volts) when the air/fuel mixture is rich (little O2 in the exhaust).

Lean Air/Fuel Mixture: The signal is low (0.1 to 0.3 volts) when the air/fuel mixture is lean (Too much 02 in the exhaust).

If you're the gambling sort, and running out of things to bet on you could throw one on it. It might come down to spending money on better diagnostic tools, such as one that can record a log of the whole trip, or spending money somewhat guessing at parts.

The alternative to that is that the sensor is reading correctly and is seeing the rich condition because a cylinder is miss firing and letting a bunch of unburnt fuel into the exhaust stream. We know a miss fire is occurring because at least we have a code telling us so and you can feel them. Still inferring from the short term fuel trim that the miss fires are on bank 1, it doesn't look like a global problem. It could still be just cylinder 3 or maybe another cylinder on that bank too.

Someone would either have to go back through all the items we've already talked about to confirm they are good, or start a more intensive diagnosis; in no particular order of magnitude, maybe smoke the intake to confirm no intake leaks, pop valve covers and inspect valve train, inspect cam/lifters/pushrods, borescope to look in the combustion chamber for anomalies.

asilverblazer · January 15, 2025

A few other comments, I'm suspect of the ignition system, but didn't see that fuel delivery hasn't been ruled out yet.

Does the miss fire ever occur at low speeds, like taking off from a stop?

Does weather (temperature, humidity) cause the miss fire?

If it only is occurring at higher speeds, it could be that the fuel pump can't keep up. Check pressure and volume.

Check this link for reading on ignition conditions. The "higher" the discharge voltage the more stress on the ignition system. If the ignition system is weak in the truck then it will be noticed in the conditions described in the link.

https://www.denso.com/global/en/products-and-services/automotive-service-parts-and-accessories/plug/basic/spark/condition.html

bcrooms · January 15, 2025

10 hours ago, asilverblazer said:

A few other comments, I'm suspect of the ignition system, but didn't see that fuel delivery hasn't been ruled out yet.

Does the miss fire ever occur at low speeds, like taking off from a stop?

Does weather (temperature, humidity) cause the miss fire?

If it only is occurring at higher speeds, it could be that the fuel pump can't keep up. Check pressure and volume.

Check this link for reading on ignition conditions. The "higher" the discharge voltage the more stress on the ignition system. If the ignition system is weak in the truck then it will be noticed in the conditions described in the link.

https://www.denso.com/global/en/products-and-services/automotive-service-parts-and-accessories/plug/basic/spark/condition.html

The misfire seems to only be replicated while accelerating on an incline with higher RPM greater than 2k and letting the truck reach a speed greater than 55mph while on the inclined road. It appears to misfire as soon as the truck reaches the last gear change during this scenario. To me it feels like something gets stuck during the misfire (10-30 seconds most of the time) and then it frees up / stops misfiring. Under typicall daily driving conditions to from home to work, it runs fine from what I can tell, I don't feel any misses. Every so often it may have a rough idle but nothing that I can detect with my current scanner....

This truck has had the fuel injector kit replaced 3 times within 6 years. I just recently replaced the Fuel filter about two months ago. PCV valve elbow was also worn out, so I replaced it thinking that may have been causing a small vacuum leak...but didn't really make a difference.

As far as I know, the truck has the original fuel pump, I'm sure it could be replaced, but it has been pressure and volume tested each time the fuel injector kit was replaced.

I have considered changing the two upstream O2 sensors to see if it changes anything.

Keep in mind it's been through diagnostics and tests by mechanics. This issue leaves all of them scratching their heads.

asilverblazer · January 15, 2025

6 hours ago, bcrooms said:

It appears to misfire as soon as the truck reaches the last gear change during this scenario. To me it feels like something gets stuck during the misfire (10-30 seconds most of the time) and then it frees up / stops misfiring.

It seems you can readily replicate this scenario, the next time it does this, while it is miss firing, let your left foot rest on the brake pedal enough to illuminate the brake lights but not enough to really apply the brakes. Change nothing else as far as the throttle pedal.

Does the miss fire go away?

bcrooms · January 15, 2025

38 minutes ago, asilverblazer said:

It seems you can readily replicate this scenario, the next time it does this, while it is miss firing, let your left foot rest on the brake pedal enough to illuminate the brake lights but not enough to really apply the brakes. Change nothing else as far as the throttle pedal.

Does the miss fire go away?

I'll try that and see what happens