My 2002 LS 4 door 4x4 has an interesting issue, after many hours of lurking and searching with no luck I have finally decided to make a topic about it.

When the vehicle has warmed up it will randomly idle high (1000-1500 RPM) then low/normal (500 - 750 RPM) then high again, then low again and so on it doesn't do this every time the vehicle is running only on seemingly random occasions. I've also noticed that it will do it while I'm driving (why I'm actually concerned about it.) If the vehicle is in drive when it has its "issue" it will slowly and consistently accelerate up to 45MPH (even changes gears) without my foot touching the gas pedal. Luckily though my brakes can over power the acceleration and slow me down, but still... safety issue to say the least...

I have one code: P0300 Random misfire (I'll address this later)

I have ruled out a sticky throttle cable by turning the blazer off and having a helper press and release the gas pedal slowly and quickly multiple times so that I could watch the air intake open and close without issues.

The following is a list of parts that have been replaced recently:
Fuel injection spider & Fuel pressure regulator (converted it to MFI) (delphi)
Spark plugs & Wires (ACdelco)
TPS (ACdelco)
Distributor (Autozone)
Cap and rotor (Acdelco)
Fuel pump (ACdelco)
Water pump
Computer
Fuel filter
Engine (Completely rebuilt by a well trusted local professional)

I have checked for vacuum leaks using carb cleaner and have not found any YET (although I did change some lines that were very cracked and ragged.) To be honest I'm starting to run out of ideas... I do have a semi powerful scan tool program on my laptop that allows me to record my drives, however it never has the issue while I have the laptop plugged into it. Could this issue be caused by the idle air control valve?

 

did you change the idle air control valve its right next to the TPS

 

I have not, I wanted to know if anyone else has or has had this issue before, I would also like to know if it is possible for the idle air control valve to be the issue, all the research I've done has insisted that it will cause issues related to stalling.

 

my first thought would have been idle air control. but you are going to have to tackle the misfire as well.

 

I guess I would suggest starting with cleaning the IAC and the passages in the throttle body...which I would just remove to make cleaning it easier.

Also agree with Twip that the ramdon misfire will need addressed. Does your scan tool provide the "cam retard" reading? If not, need to find one that does. Should be 0±2° when in Park and engine RPM just above 1000.

 

Before you go any further, the crankshaft position sensor relearn must be performed. If the PCM is a "rebuilt" one, it should have a default value programmed into it, which is close, but not close enough. If the PCM is a used one, it will have data from the last relearn that was done to it, which is not correct with your CKP sensor. CKP relearn data is used to base all calculations that the PCM makes. Without performing the relearn, you're wasting time and money. After the relearn we can help you with diagnosis.

 

Sorry for the delayed response, I plan on taking the problem child to the mechanic who did the relearn procedure last time I had to replace the crankshaft position sensor. I'll give you another update after that.

The cam retard reading is ~ -1.5

 

Desired camshaft retard is zero degrees with a tolerance of +or- 2 degrees, so your's is within specs. Keep in mind that the throttle must be snapped above 2Krpm and back to idle before camshaft retard reading is accurate. Next step is to have the crankshaft position sensor relearn performed. Takes about 15 minutes, if they know what they're doing

 

Once again, sorry for the delayed response. They have completed the crankshaft position sensor relearn it seems to run a bit smoother now. I still have the P0300. Today I was messing around under the hood and found out that the PCV valve was almost completely pulled out of its "socket." (I pushed it back in and it doesn't seem to want to wiggle itself out.) could that have been the cause of the high, low (seemingly random) issue?

Edit: It appears that the PCV issue I described above was not the cause of my on-going problems. I would also like to add that my blazer is one of the 2002's that does not have an EGR valve.

 

P0300 is almost always caused by a fault in the secondary ignition system: plugs, wires, cap, rotor & ignition coil. It can also be caused by camshaft retard adjustment being out of spec, worn distributor shaft bushings and low fuel pressure and or excessive fuel pressure leakdown.