Why does it have to be a bad sensor? I already commented in post #4.

 

The cap and rotor are new, and the cam position sensor is a sensor lol. I'm not sure how to reset the retard on it and am currently not able to check it, so I'll have to look into that more.

Ignition control module I haven't looked into either. The thing is that the outside air temperature has an effect on when it misfires. If its below 30 degrees out it will misfire under load like crazy. If its above the 30 it rarely does it.

 

This is a felt misfire, correct? Not just a SES light with a P0300?

CMP Retard value isn't going to indicate anything as simple as a bad cam position sensor that needs replacement. It's going to check for a worn distributor gear, or a stretched/worn timing chain/gears, or a bad distributor installation - all causing a misalignment of the rotor with the terminal in the distributor cap when the spark is supposed to come through. Misalignment means spark can jump to where it shouldn't inside the cap, but this will depend on the timing at the moment as well as the voltage required to jump the gaps (which also varies). The spark jumping where it shouldn't definitely will cause a misfire and needs to be ruled out, especially on these Vortec engines where the timing is independent of distributor rotor position. A lot of things come into play. For example, temp of the engine or MAT can affect timing so could possibly aggravate the problem when engine is cold (but not necessarily, it's just possible).

You need a capable scanner, not just a code reader. Look at the actual live data from your various sensors and see if anything sticks out as abnormal when the misfire is occurring. We can tell you normal values. Check the cam sensor retard and adjust the distributor position if necessary. Check the resistance of the plug wires with an ohm meter. Pull all the spark plugs and look at them for cracks in the porcelain. Check the fuel pressure per sticky at the top of the 2nd gen forum - yes, check it again. None of this requires any parts...... unless you actually find something definitive.

For a 2004 Blazer and Android Phone I recommend Car Gauge Pro (Play Store) / BAFX Bluetooth OBD2 adapter (Amazon). Car Gauge Pro is $8.95. About $30 total. Car Gauge Pro comes with enhanced PIDs for many different vehicles, but are not filtered. So you will see many, many things that will not work on a Blazer (but works on some other car). Keep this in mind. Just because it appears in this software does not mean it is functional on your vehicle.

For a 2004 and iPhone I recommend Dash Command with Enhanced PIDs for your particular 2004 GM vehicle and a WiFi OBD2 adapter (no recommendation of the WiFi adapter). Dash Command is about $20 for the base plus the enhanced PIDs. It does filter the PIDs, so what you see should work. Don't get the cheapest Wifi OBD2 adapter.

Dash Command is also available for Android but is more expensive than Car Gauge Pro and does less in the end - but software is slicker presentation.

Either of these programs will allow you to read the cam sensor retard and live data from many different sensors on a Blazer. They will not do crank sensor relearn, ABS brake bleeding, or injector balance testing - these things require bidirectional control. Car Gauge Pro appears to be able to read and clear codes for other systems on the data bus such as SIR, ABS, BCM, TCCM, etc .... but no PIDs are provided to read device positions and sensors (on the Blazers).

Most Auto parts stores have loaner fuel pressure gauges (with deposit). Everyone should have their own digital Volt/Ohm meter.

Good luck and ask if you have any questions.

 

 

Yes this is a felt misfire, no codes. On occasion the SES light will flash when the misfire is occurring. Like I said, it only happens under a load in 4th gear/OD, usually up a hill. I've had timing issues on these motors before but this one is definitely unique, if its a timing issue.

I've changed out the air temp sensor already so don't think its that.

I'll look into those scanners, though I think my buddy might have one that will work that does live feed.

This year blazer doesn't have an EGR valve

 

I'm sure it has a "pending" misfire code and your scanner or code reader just isn't picking up that fact. Misfire codes have to repeat (per design) in order to set a hard code. Flashing SES light is pretty much a dead giveaway of a detected misfire.

Ignition timing is determined by the PCM based on the crank sensor input, along with various sensors (not including the cam sensor inside the distributor) and the PCM calibration table. Ignition timing is completely independent of the distributor and the spark will occur even with the distributor taken out of the vehicle and is sitting on the workbench. Distributor on this vehicle only functions to distribute the spark to the different plug wires and to tell the PCM what cylinder is firing for the purpose of sequential fuel injection and misfire detection (this is what the cam sensor inside the distributor does). But if the rotor isn't pointing exactly to the right spot inside the cap when the spark comes through, the spark can jump to the wrong place and misfire will occur. This would be aggravated by ignition timing where the PCM times the spark with rotor furthest away from the correct terminal - and this might occur under your particular set of conditions when you are noticing the misfire. Outside air temp could conceivably cause the PCM to give different ignition timing. Also, outside air temp could change the voltage required to jump the spark plug gap and it could start jumping elsewhere inside the cap. These arguments can be circular, confusing, and very frustrating. But know that this problem is well-documented on both 4.3 and 5.7 Vortec engines.

So how do you know if the rotor is pointing to the correct spot? The cam sensor retard value. It is a measure of how centered the cam sensor is, on the crank sensor signal for each cylinder. The cam sensor could have very well been placed elsewhere (as it is in distributorless ignition systems), but by putting it inside the distributor it serves double duty by allowing the distributor rotor position to be clocked exactly as designed.

If you want to check a temp sensor, display it's value on a capable scanner while the misfire is taking place.

VERY few inexpensive scanners will display cam sensor retard on these Blazers but you never know until you try. It is NOT timing, so don't confuse it. It is sometimes referred to as CMP retard or simply CMP.