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.

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

Does anyone know the correct size of the factory rear speaker bolts? I have replaced my factory speakers with aftermarket speakers that required an adapter for proper fit, and the adapter obviously has added material between the bolt washer and the body panel. The factory bolts are too short to use with the aftermarket speaker + adapter. I tried using the screws and such included with the new speakers but those screws aren't the right size for the factory holes.... I've checked a few hardware stores in my area but I can't find anything that is close to the factory bolts. I just need the same type but slightly longer.

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.

Not yet, but I can give that a try

I can see and feel the rear of the truck bouncing up and down at times when I apply the brakes.

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.

Sensor #2 IMG_1114.HEIC IMG_1115.HEIC IMG_1116.HEIC

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

I would say the rear is the cause for the hop. The fronts only have a grab/release pattern.

It hasn't been smooth for quite a while. I thought the last guy would be able to resolve it...but no different.

It's always had this intermittent brake bounce.... the last guy that changed the front rotors/pads and rear drums said it was as good as its gonna get for my truck...which I dont believe is true. I should have smooth braking if all the brake components are operating as they should, right?

Could this be an issue with worn out wheel cylinders on the rear?

I can also feel the heat in the cab get much warmer if I have it on while this is happening. I can also feel it cool down.

Yesterday I noticed my truck's water temp gauge has started to give an inconsistent reading. While I'm driving the temp gauge shows below 210, closer to the mark between 100 and 210. When I come to a stop for a few minutes (red light, etc) the temp seems to increase back to 210 and at times goes above 210, but not by much. When I resume driving the temp drops again. I know it's been colder than normal lately in our area but the last few days it has been in the mid 40s to low 50s. Could this be relative to the water pump not operating as it should or temp sensor / sending unit?

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.

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

I'll try that and see what happens

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.

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.

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.

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

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.

Both were checked. Drums were out of round enough to need replacement according to the guy that inspected. Shoes looked almost new... It could be the ABS system on the fritz. The truck will hop like a rabbit at times due to the brakes. Has a grab, release, grab, release, feel even when apply the brake steadily.

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)