Hello everyone I'm new to posting on the forum i have been reading info from this website for years finally have a question I cannon figure out. I have a 99 4.3 Vortec Chevy Blazer LS 4x4 it has 270,000 miles on it this truck has run really really strong all these years with routine maintenance and.

So I decided to do a tuneup on the truck the lower intake manifold gasket was leaking externally on the passenger side. No coolant in oil yet. So I disassembled everything and labeled all the connections vacuum hoses Etc.... disassembled the intake plenum and removed the distributor. Now i know better than to do this and even though I know better I pulled the distributor without marking it and went on to finish the gasket replacemnt wial i was in there i noticed the cpfi spider unit lookong a little fryed. So i figured wial im in here i will install the updated mpfi new fuel injector spider assembly new water pump thermostat and s belt so i put it all back together the truck starts and runs but has a multiple cylinder misfire P0300 code now I've never seen this on this vehical. So i checked all connections and hoses and everything is making contact and vacume is strong . No vaccuse leaks around plenum checked with unlit propane torch and can of carb cleaner I use my scan tool and took a look at the live data and noticed that the spark plug advance was at 30* . Now before I did the tune up the truck ran great never had a misfire at all.
So I am thinking that I am 1 tooth off on my distributor gear. Like I said the truck starts and runs but is missing and and has no power if you try and drive it. I've removed and installed the distributor over 20 times double checking that the marks on the harmonic balancer are correct. "2 marks on the balancer line up the second mark with the notch on top of the timing chain cover on compression stroke of number one cylinderand that the rotor on the distributor cap lines up with the 6 mark on body of distributor."
so I'm on compression stroke number 1 distributor lines up with 6 mark it bolts down flush can I really be out 30* when it wasn't before?

Please help I've taken it to Chevy dealer and they where blaming it on cylinder 1 being 5psi under the rest on a running compression test cylinder 1 measured at 85 the rest measured at 90 psi running compression 185 and 180 for static leak down compression test.

 

Coolant at operating temperature, engine at idle speed, spark advance should be ~17 degrees, (it is not adjustable). The + or -2 is camshaft retard, (CMP retard). It tells if the camshaft position sensor is aligned correctly with the camshaft. If/when the distributor is removed, CMP must be checked, and adjusted if necessary. The adjustment is made by slightly rotating the distributor. If it's not within spec, you can expect misfires due to crossfire inside the distributor cap.

 

Thank you Capitan.

so I have my truck 99 4.3 s10 up to op temp 200 set screw on throttle body adjusted so throttle is pinned at 1500 rpm and I have a spark advance of 30* on my live data. Before I removed the distributor I had zero misfires no codes . Now I have p0300. I checked distributor gear for signs of wearing but it looks in really good shape and there is only a tiny tiny tiny bit of rotational play when it is installed. Plugs wires cap and rotor are new also.
Had to remove distributor to do intake manifold.

 

"...set screw on throttle body adjusted so throttle is pinned at 1500 rpm and I have a spark advance of 30* on my live data."

This is why your spark advance is showing 30 degrees, (The engine is not at idle).

The set screw on the throttle body should not be tampered with. It is set at the factory, and GM does not publish any instructions regarding adjustments to it. The best you can do, is get it "ballpark". It will require a bi-directional scanner that is capable of controlling engine RPM.

 

EDIT:
As for the P0300: CMP retard needs to be checked and adjusted if necessary. When you removed the distributor, the CMP adjustment was lost and needs to be readjusted. Back in the "old days", when the distributor was removed and reinstalled, ignition timing needed to be adjusted. It was part of the job. The distributor on yours does not affect ignition timing when you rotate it. But it does change the positioning of the terminals inside the cap in relation to the rotor. When CMP retard is adjusted correctly, the ignition coil fires when the rotor is pointing directly at each terminal inside the cap. This reduces/eliminates crossfire/misfire inside the cap, (which needs to be AC Delco). Whenever the distributor hold down bolt is loosened, CMP retard must be checked/adjusted, it's part of the job.

 

Hi Captain!!! I find that a small flat bladed screwdriver fits nicely behind the throttle body screw to hold rpms anywhere that you want. Want more rpms? - just use a thicker part of the blade.

 

Couldn't one display IAC counts and adjust screw to obtain a typical value for idle while letting the PCM control? Of course correct adjustment would be dependent on no vacuum leaks.

 

So i have not messed with the set screw yet this is something that i read here on the forums somewhere and was asking this sounded correct correct. I have actually been using the arm on the side of the throttle body where the throttle cable connects. I was just holding it steady at 1000 and also 1500 when the motor is up to temp and everything. I can see the rpm on my live data so i could see the spark advance seemed to stay at like 27-30 no matter the rpm range.

 

There are several "cells" that are programmed into the PCM. Each cell is for a different driving situation, (idle, starting, cruising at a steady speed, WOT, deceleration, etc). Each cell has a specific set of parameters. When the engine is at idle, coolant at operating temp, closed loop operation, transmission in neutral or park, etc, the PCM controls the engine in cell #10. There is a list of probably 100 different readings in each cell. Ignition advance should be ~+17 degrees, idle speed should be ~550RPM, and CMP retard should be zero, +/- 2 degrees, (after a brief throttle snap to 2K rpm and back to idle). If the PCM is not in cell 10, there's a problem somewhere. What brand and model of scanner are you using?

 

Hi Les, I've never tried it that way, I suppose it's worth a try though. I always command the IAC as low as it will go, and then adjust idle rpm to ~350, (TPS should end up at .5 volts or less). I have always had good results doing it that way. Rule of thumb is, when you command the IAC all the way down, the engine should not stall, but close (usually around 300rpm it will stall). The idea is to keep the throttle blade open just enough so that it doesn't stick in the bore at idle. And that's assuming the throttle blade and bore are clean... on the underside too