The P0300 means more than one cylinder is randomly, (not continuously) misfiring, and not necessarily the same cylinders. The only way to tell which cylinders are misfiring is with a scan tool capable of accessing misfire data, (entry level scan tools will not access it).

The MAF sensor affects all cylinders equally. With that in mind, it's unlikely that's the problem. As for the secondary air pump, it pumps fresh air into the exhaust to help the catalytic converters do their job which can't cause a misfire.

P0300 is almost always caused by a fault in secondary ignition, (plugs, wires, cap, rotor, ignition coil, worn distributor bushings and gear, and incorrect camshaft retard. Low fuel pressure can also cause it, but there are usually other symptoms present, which you haven't mentioned. There have been instances with members experiencing P0300 when using other than AC Delco plugs. However, if I understand correctly, the problem was present before you replaced the plugs, so that may or may not be the problem. If camshaft retard is out of spec, there will be random misfires, guaranteed. At this point, I would suggest checking fuel pressure and leakdown, and having a shop check camshaft retard, and adjust it if necessary, (entry level scan tools will not access it).

 

Ok, i will take it from the top again and re-check everythin.. but just so we are all clear on the issue... the vehicle doesnt misfire or have any problems at all until i am on the highway and doing 65 mph or more, thats when the miss firing starts and its only for a fe seconds that it miss firies and the tach drops or flutters.. if i hit the gas, it accelerates and doesnt appear to misfire.. its only when i am at a steady 65-70 mph stat if flutters.... So what would make it misfire at 70 mph and not at any other speed

 

Is what your seeing on the tach possibly from the torque convertor locking and unlocking? If its in the shop have them do the crankshaft sensor relearn. It takes 30 seconds and can solve some miss fire problems. Going out on a long limb here. It definitely looks like you had something going on with those numbers.

 

Camshaft retard is adjusted by slightly rotating the distributor. The adjustment does not affect ignition timing. It aligns the camshaft position sensor with the camshaft, and at the same time, it positions the distributor cap so when the ignition coil fires, the gap between the rotor segment and the cap terminal is at a minimum. This prevents crossfire inside the cap. As engine RPM increases, so does ignition timing, along with the gap that the spark has to jump in the cap. At lower speeds the gap is minimal, but at 65-70MPH the gap is substantial. The larger the gap, the more likely it is for crossfire to occur. If the gap is too large, the spark can jump to the wrong terminal, and the engine misfires. The spec for camshaft retard is zero degrees, plus or minus 2 degrees. If it's out of spec, crossfire will occur. It may or may not set the P0300, and you may or may not feel it, but it's happening. The farther out of spec camshaft retard is, the more crossfire will occur.

As mentioned earlier, worn distributor bushings and or drive gear, will also cause crossfire. Those can only be accurately tested using an oscilloscope or high end scan tool.

 

If your cam retard is within spec as mentioned by Captain hook, then you may have a sticking valve.


Here is the TSB:

Subject: SES Light and P0300 When Towing , Cruising Uphill or on Hard Acceleration - 4.3 5.0 5.7 accelerate cruise cylinder head RPO L30 L31 L35 LF6 LU3. DTC P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 PIP3081 - (07/23/2004)

Models: (96-02 Chevrolet Astro) and (96-02 GMC Safari) and (96-00 Old Body Style Chevrolet Suburban) and (96-00 Old Body Style Chevrolet Tahoe) and (96-02 Chevrolet Express) and (96-00 GMC Yukon) and (96-00 Chevrolet Old Body Style C/K Truck) and (96-00 GMC Old Body Style C/K Truck) and (96-02 GMC Savana) and (96-99 P32 Cab and Chassis) and (96-02 Chevrolet S-10) and (99-02 Chevrolet Silverado with 4.3L V-6 Engines (96-02 GMC Sonoma) and (96-02 GMC Sierra with 4.3L V-6 Engines).

The following diagnosis might be helpful if the vehicle exhibits the symptom described.

Condition/Concern:
The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3,4,5 or 6 will be the ones to experience this.

Recommendation/Instructions:
If the P0300 diagnostics did not isolate a concern, the following may help:
Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:
Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.
Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.
Reassemble the cylinder heads using new valve seals.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

 

So it looks like I am looking at a new distributor or at least a camshaft retard adjustment... both which are probably beyond my limited mechanic skills.

I cleaned the MAF, even thgough it is new and everything was bright and shinny, I sprayed it with MAF cleaner and lightly brushed it and disconnected it, no real imporvement. The last few days, it has still been misfiring at speeds around 65-75 moh, the engine light flashes, but does not stay completely on. It doesnt seem to happen, at least not noticable when driving up hill or at accelleration, only seems to be when I am at a steady cruising speed of between 65 and 75 mph.

 

Hate to throw money at it without knowing for sure what the problem is. May or may not be the distributor, won't know until you have a competent technician look at it on a high end scan tool. Not all that expensive to have someone check & adjust camshaft retard, probably less than an hour labor. Just have to make sure they know what they're doing. Talk to them and see if they know what camshaft retard is, what it does, and how to adjust it. What were the fuel pressure and leakdown numbers? Did you check fuel volume? Those fuel trim numbers, were they negative or positive? Seemed to be some question about it in earlier posts.

 

I havent done the fuel pressure yet, will do that this weekend

But here is a history of the numbers as I worked and replaced various items. (yes, initally the numbers were all positive)

This is what the initial numbers where before I did anything.
Initially I was getting the codes P0440, P0420 and P0300
My engine code reader gave the following in the freeze frame
ABSLT - 34.1 %
engine rpm 2251
calc load 27.4
MAF 47.1
MAP 80 kpa
coolant 194deg
st ftrm1 7.8%
lt ftrm1 21.0%
st ftrm2 14.0
lt ftrm2 23.4
fuel sys 1 & 2 - clsd
speed - 68 mph

a check of the I/M monitors showed all applicable montiors were ready except the EVAP SYS was not ready
Codes after replacing plugs, wires, rotor, and dist. Cap.
P0300
ADSLT TPS 63.1
Eng Spd 2397
Case Load 38.4
MAF 66.38
MAP 98
coolant 197
ST ftrm1 5.4
Lt Ftrm1 19.5
St Ftrm2 29.6
Lt Ftrm2 13.2
Speed 48 mph
fule sys 1 & 2 clsd

I/M monitors were all ready, except the secondary air system was "not ready"

Codes after replacing cat, o2 sensor after cat. And the vapor canister valve selonoid.
Code P0300
absltt tps 22.3
speed 2367 rpm
calcload 23.1
maf 40.05
map 63
coolant 194
st ftrm1 -.8 ( thats negative.8)
lt ftrm1 17.1
st ftrm2 -.8 (negative .8)
lt ftrm2 18.7
veh spped 73 mph
fuel systm 1 & 2 closed

I/m Monitors , all applicable are ready except the evap and secondary air sys are not ready.

These re the codes after I replaced the Cat. as the honey combs had come lose and where clogging the exchaust. I replaced the gas filter, all plugs, wires, distributor capt and rotor, I replaced the O2 sensor after the Cat, MAF sensor, I replaced the vapor canister vent selonoid that sits above the gas tank, and fuel filter and reset the coeds.
after about a week of driving, the misfiring is still happening, although not as bad. it seems to be more of a fluttering after travelling at 50+ for 5-10 mins or more and its random..
The P0440 and P0420 codes have not come back at all only the P0300

Code P0300
ABSLT TPS 45.4
Eng Spd 2997
Calc Load 59.6
MAF 99.19
MAP 81
Coolant 194
ST FTRM1 -7.1
LT FTRM1 -4
ST FTRM2 10.1
LT FTRM2 -3.2
Speed 54
Fuel sys 1&2 Clsd


I did however, noticed that the one elbow for the PCV valve was spongy and sucked in with a small crack in it. So, I just replaced that along with the PCV valve and reset the codes
These are the codes that came up after replacing the secondary air injection pump
abset tps 52.1
2602 rpm
calc load 51.7
maf 90.63
map 95
temp 199 deg
st ftrm1 -1.6
lt ftrm1 -6.3
st ftrm2 3.9
lt ftrm2 1.5
spped 79 mph
fuel sys 1/2 clsd
The vehile still misfires, the engine lite flashes, but doesn’t stay on, so no codes are being registered yet.

 

You are not going to get a code for the misfire when the CEL is flashing.
Mine did the same thing when I got it up to 70+ mph or floored it up a hill (flashing check engine light).
I hooked up my scanner and watched the live data as I floored it up a hill. When I got up to 60 mph, cylinder #5 started to missfire constantly then stopped as soon as I let off the gas.


You may have the same problem. Not uncommon on these engines.

 

The fuel trims are still all over the place. If the numbers are positive, it means there's a lean misfire, (too much air or not enough fuel) and the PCM is adding fuel to achieve 14.7:1 fuel ratio. Usually a vacuum leak, or a restricted fuel injector nozzle. If they're negative, it's a rich misfire, (too much fuel or not enough air) and the PCM is subtracting fuel to lean out the mixture. This usually indicates a secondary ignition fault, (fuel not being burned) but it could also be an injector not sealing shut 100%. In the last freeze frame snapshot, the only one that sends up a red flag is LTFT B1, (cyls 1, 3, & 5). -6.3 is indicating that at least one of those 3 cylinders, and maybe more, is/are running slightly rich. Ideally, in a perfect world, all fuel trims should hang close to zero. Inexpensive things to try would be switch to AC Delco 41-993 spark plugs and check fuel pressure and leakdown. The leakdown test might spot a leaky injector. The other option for checking injectors is an injector balance test, but that requires a high end scan tool, and a very accurate fuel pressure tester. Without one of those big money scan tools, you're really limited when it comes to pinpointing problems like this. Just need to analyze the data you have, and make an educated guess