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

[bcrooms]

I tried the brake tap while misfiring but I didn’t notice anything except the brake pedal nearly went to the floor without much pressure, and I was barely touching it.

 

Engine continued to misfire.

 

i’ll try the test a few more times to see if I notice any differences

Edited January 16, 2025 by bcrooms

[bcrooms]

I attempted your suggested brake pedal tap while driving to work this morning.  

 

The highways and streets on my route are pretty much flat, and speed varies between 35-55mph.

 

I did the test at least 4 times while at 45-50mph and noticed the engine would jump in RPM from 1.5k to 2k RPMs.

 

Keep in mind, the misfire doesn't appear to be occurring while I am commuting.

Edited January 16, 2025 by bcrooms

[asilverblazer]

5 hours ago, bcrooms said:

and noticed the engine would jump in RPM from 1.5k to 2k RPMs.

That sounds like the torque converter unlocking, like it should.

 

18 hours ago, bcrooms said:

Engine continued to misfire.

So long as both of these conditions are occurring at the same time it is not the TCC shuddering.

 

18 hours ago, bcrooms said:

I tried the brake tap while misfiring but I didn’t notice anything except the brake pedal nearly went to the floor without much pressure, and I was barely touching it.

So, this isn't right... I'm inclined to suggest plugging the vacuum line to the booster and go for a test drive to see if the miss fire goes away - weird vacuum leak in the booster. Plugging the line with a vacuum gauge might be even more interesting. 

[bcrooms]

So I decided to replace the two upstream O2 sensors to see if anything changed. 

 

Here are pictures of the (I believe original) O2 sensors that were replaced.

 

Sensor #1

 

IMG_1110.HEIC IMG_1111.HEIC IMG_1112.HEIC

Edited February 19, 2025 by bcrooms

[bcrooms]

Sensor #2

IMG_1114.HEIC IMG_1115.HEIC IMG_1116.HEIC

[bcrooms]

I can tell that cold & warm idle is smoother. I have driven the truck as I normally would and tested it on the same inclined highway where it usually wanted to misfire. I haven't been able to reproduce the misfire since the O2 swap.  Will keep posting updates as I have time.

[asilverblazer]

Had to go back and refresh myself with the thread.

 

Looks like the last freeze frame data was showing a dig differential between the banks on fuel trim. That is controlled by O2 sensors - doesn't look like you were able to determine whether that was a cylinder specific problem though. 

 

You may have gambled correctly the O2 sensor could have been miss-reading and altering the fuel trim incorrectly to the point of an induced miss fire.

[bcrooms]

Another update that could be relevant: I changed the spark plugs on my truck.  The 4 plugs closest to the rear on both sides have oil on the ends.  Not a ton, but its enough to immediately notice and be concerned.  My truck tends to "use" oil between changes, and I change the oil every 3500 to 4000 miles.  My truck does have the oil filter adapter kit installed.  

 

I replaced all 8 plugs.  I didn't do the wires or anything else as I was short on time over the weekend.  I have noticed some improvement in the smoothness of the engine.  

 

I still believe the O2 sensor change helped the most so far for my truck.  

 

I found a reputable shop in the area I am in now and I am planning to have them check it.  They have confirmed they have more advanced tools and skilled techs that can do a deeper inspection and diagnosis.  

 

 

 

Edited April 2, 2025 by bcrooms

[bcrooms]

Well after driving my truck awhile without being able to replicate the misfires, it returned yesterday. Not as rough of a misfire as before but same scenario and symptoms

[bcrooms]

Providing an update.  

 

Since my last post, I replaced the cap, rotor, and plug wires.  No CHANGE. Misfire is still present under the higher load/throttle and incline scenario, and it is more noticeable (truck shaking, loss of power & speed, etc).  

 

I've done extensive research on other forums and YouTube videos of similar issues, as well as on different vehicles with a similar problem and have seen several mentions of faulty sensors such as the Knock Sensor, TPS, etc. 

 

With as much $$$ I've previously put into MPFI repairs (3 MPFI kits so far in 8 years) this issue creeps back until it becomes so bad that I can't drive the truck.

 

I did take the truck to a local shop in TN (recently moved here) and from the minor diagnostics they have done so far (planning to do more) they believe its the MPFI nozzles again OR the cam shaft is worn enough to cause misfiring on Cyl 3, 4, 5 & 7 where I see the counters go crazy when it does it.

 

The shop is going to give me an estimate to diagnose deeper into the engine.  I know its gonna be $$$$.

 

What are the chances, that its something super simply like the Knock Sensor throwing everything off?

 

I'm tired of the problem being "fixed" by a shop for about a year or so and then returning with a vengeance.