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

[bcrooms]

It's been a while since I've posted here about my truck's intermittent engine issues. 

 

It goes through a cycle of having misfires, they "go away" and reappear.

 

I can replicate the issue only by driving up a steep roadway with approximately 50%+ throttle between 55-65mph.  

 

I recently replaced the fuel filter, PCV valve, PCV elbows and grommet, and cleaned the MAF sensor to see if anything had changed.

 

Here is the freeze frame reading at the time of the most recent misfire (P0303):

Load PCT = 21.6%
ECT = 203F
ShortTermFT1 = -27.3

LongTermFT1 = 15.6

ShortTermFT2 = 1.6

LongTermFT2= 13.3
MAP (inHg)= 28.64

RPM = 1715
VSS = 53.44 mph

MAF (lb/s)= 0.10

TP% = 56.1

 

I've been through 3 MPFI sets on this truck due to failed injectors.  I hope it's not that again. 

 

I'm trying everything I can myself at this point because I have dumped a lot of cash in this truck for mechanics to diagnose the problem which never seems to be 100% fixed.

 

Any ideas from fellow members?  Has anyone else had a very similar issue?

 

It has had at least 3 compression tests done within the past year to verify that part.  Also had valve job done.

My next shot in the dark is the Oxygen Sensors or new Cat Converters....

[tbarn]

Sounds like it's time for plugs and wires. pull the plugs and look for something like this. If you find it, DO NOT replace plugs only.

 

[bcrooms]

7 hours ago, tbarn said:

Sounds like it's time for plugs and wires. pull the plugs and look for something like this. If you find it, DO NOT replace plugs only.

 

That has been done, I should’ve mentioned that in the OP

[asilverblazer]

My experience with that generation always had issues at the distributor cap. I've also had brand new plug wires that one was bad. Presume that cylinder 3 (P0303) is the main problem. See if swapping plug wires from that cylinder to another moves the miss to the swapped cylinder. 

 

Take a look at this thread:

 

[bcrooms]

2 hours ago, asilverblazer said:

My experience with that generation always had issues at the distributor cap. I've also had brand new plug wires that one was bad. Presume that cylinder 3 (P0303) is the main problem. See if swapping plug wires from that cylinder to another moves the miss to the swapped cylinder. 

 

Take a look at this thread:

 

I’ll check it out. My truck also has a relatively new distributor, previous one was worn. cap, rotor, wires, plugs were also replaced at the same time. It’s been a while so I can’t rule that our just yet, but with the issue being present before and after all those parts were replaced…leads me a different direction

 

the truck was recently “diagnosed” by three different mechanics that say it’s fine other than when it decides to misfire.

first mechanic replaced the MPFI again, the injector nozzles were cracked

Another guy mentioned the slim chance the PCM is not working correctly for Cyl 3 or a sensor is flaking under load

 

3rd guy said it was 100% fine, which is obviously not the case.

 

 

 

 

[asilverblazer]

On 11/20/2024 at 1:11 PM, bcrooms said:

Another guy mentioned the slim chance the PCM is not working correctly

Still have NEVER seen a failed PCM

 

On 11/20/2024 at 1:11 PM, bcrooms said:

a sensor is flaking under load

Maybe

 

On 11/20/2024 at 1:11 PM, bcrooms said:

I’ll check it out. My truck also has a relatively new distributor, previous one was worn. cap, rotor, wires, plugs were also replaced at the same time. It’s been a while so I can’t rule that our just yet, but with the issue being present before and after all those parts were replaced…leads me a different direction

Its either ignition or fuel... 

[bcrooms]

21 minutes ago, asilverblazer said:

Still have NEVER seen a failed PCM

 

Maybe

 

Its either ignition or fuel... 

I dont believe its the PCM. 

I've read a few other posts about P0300 / P030# and some were resolved by replacing O2 sensors.

I did replace the ICM early on, but that's been over 5 years,  could be the ignition coil...

I'm loading up the parts cannon with things I can DIY.  I don't have the tools for deep engine inspection/repair.

[bcrooms]

Gas mileage has also suddenly gotten worse after I changed the fuel filter, replaced PCV valve and rubber elbows, cleaned the MAF sensor....

 

I was getting 300+ miles on a full tank, now I can only get around 200 miles. 

[asilverblazer]

MAF sensor is delicate, hope you didn't mess it up cleaning it.

 

A poor running engine won't get good gas mileage, sounds like it isn't fixed yet. Parts cannon approach can cause more problems than it solves. You may now have two problems, the original miss fire plus a potentially damaged MAF sensor. (Not trying to criticize.)

 

Proceed with caution, first, try data logging if you can to see if some cylinders are miss firing more than others.

 

[bcrooms]

1 hour ago, asilverblazer said:

MAF sensor is delicate, hope you didn't mess it up cleaning it.

 

A poor running engine won't get good gas mileage, sounds like it isn't fixed yet. Parts cannon approach can cause more problems than it solves. You may now have two problems, the original miss fire plus a potentially damaged MAF sensor. (Not trying to criticize.)

 

Proceed with caution, first, try data logging if you can to see if some cylinders are miss firing more than others.

 

I've cleaned MAF sensors before, and I inspected it before and after cleaning.  Doesn't appear to be messed up unless it has something wrong that isn't visible....

Right now it seems to be stuck with a P0303.  It has had P0300 in the past at times, but not currently.

Edited December 2, 2024 by bcrooms

[richard wysong]

303 is cylinder 3 misfire, 300 just means a misfire was detected. You can not visually tell if the MAF was damaged, you need a scan tool with live data

[bcrooms]

5 minutes ago, richard wysong said:

303 is cylinder 3 misfire, 300 just means a misfire was detected. You can not visually tell if the MAF was damaged, you need a scan tool with live data

I have a basic DIY scanner that shows some live data but not a complete live capture.

[asilverblazer]

20 hours ago, bcrooms said:

Right now it seems to be stuck with a P0303.

You need to figure out what is wrong with cylinder 3 and only cylinder 3. 

 

Things that are unique to that cylinder:

Plug

Wire

Distributor Cap Terminal

Injector

Valves

 

New parts does not equal good parts. Try swapping the plug wire then plug to known good cylinders to see if the miss follows one of those parts.

 

Thoroughly and carefully inspect the cap, I noted earlier that my experience with this generation of cap is not good. Especially with intermittent miss fires like this. make sure it is a good quality part (AC/Delco and not a counterfeit one at that).

 

Inspect wiring to the injector.

 

MAF sensor, O2 sensors, MAP sensor, fuel filter, fuel pump and the like will not cause a miss on only one cylinder.

 

Considering the head has been off and the valves have been worked on, confirm the compression is good - If this were a problem I would expect a dead miss, not intermittent though.

[bcrooms]

6 minutes ago, asilverblazer said:

You need to figure out what is wrong with cylinder 3 and only cylinder 3. 

 

Things that are unique to that cylinder:

Plug

Wire

Distributor Cap Terminal

Injector

Valves

 

New parts does not equal good parts. Try swapping the plug wire then plug to known good cylinders to see if the miss follows one of those parts.

 

Thoroughly and carefully inspect the cap, I noted earlier that my experience with this generation of cap is not good. Especially with intermittent miss fires like this. make sure it is a good quality part (AC/Delco and not a counterfeit one at that).

 

Inspect wiring to the injector.

 

MAF sensor, O2 sensors, MAP sensor, fuel filter, fuel pump and the like will not cause a miss on only one cylinder.

 

Considering the head has been off and the valves have been worked on, confirm the compression is good - If this were a problem I would expect a dead miss, not intermittent though.

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.

Edited December 3, 2024 by bcrooms

[richard wysong]

a worn camshaft may cause that,Pull the valve cover and see if the lift on #3 is the same as the other 3 cylinders on that side