thanks everyone for the responses! I am still struggling with jumpy timing and rough idle! So what does the pcm use to determine what the advance is? What can cause it to just constantly vary the timing?

 

If you can wait for a couple days we can go over things in detail. On trip and don't like doing this on a cell phone. Let me know if you still want the info.

 

Yes, If possible ide appreciate the help. It seems like warmed up at idle it has a noticeable miss, I have taken care of the common things, plugs, wires, cap and rotor. The Fuel spider has also been replaced, i had two leaking valves so i had the heads gone through by the machine shop. Going down the highway it runs awsome. At idle it feels like it has a miss, but no ses light. While on vacation sometimes after a drive if i let it sit and start it, the idle would sorta go up and down.

I am beginning to think fuel pressure, it does not leak off, but with the key on it reaches 58 psi and then 51psi running.


I do have a scanner that does some live data but nothing to in depth like cam retard, it does show the timing, fuel trim, o2 sensors, ets

I know the engine could idle right, it idles good when first started, then it gets worse

 

Just a shot in the dark but I had the same issues and after 2 weeks I found a sparkplug that was tight enough at startup but as the truck warmed up it loosen enough to cause a skip

 

Timing depends on a lot of inputs into the PCM. Coolant temp, MAT sensor, MAF sensor, TPS, manifold vacuum, engine speed, etc. IMHO, timing varying would be a result, not a cause.

Contrary to what you read in the fuel pump sticky, with the AC Delco MPFI upgrade spider I have found 58psi to be normal regulated pressure with engine off but fuel pump activated - so I personally don't believe the 60+ psi spec applies to the upgrade spider. Also, if I remember correctly, mine ran 51-52 psi at idle as well. I originally replaced my spider because of the "low 58psi" and the replacement was exactly the same and it fixed nothing. As long as you have the 52psi at the time of the miss, then fuel pressure is not your problem IMHO.

I would think a small vacuum leak would be more likely to cause your observations.

You can use your existing meter to look at short term fuel trims while idling hot and that should tell you if that is the case. Look for values swinging outside of the -10 to +10 range on either bank of injectors - or if one bank is different from the other. If you don't see something there I'm going to recommend that you get what you need to read the Enhanced GM Powertrain PIDs for further diagnosis.

You have a couple of inexpensive choices for Android. Car Gauge Pro at $8.95 works nicely on a 1999 (not for 1996-1997 Blazer). Dash Command at $10 + $10 more for enhanced PIDS per car, works nicely on all 1996 up Blazers. Either one uses the BAFX Bluetooth OBD2 adapter found on Amazon for $23. These will both do the misfire counters and the CMP retard on your Blazer.

Let us know what you find with the fuel trims and we can go from there.

 

my distributor had so much slop in it that the tip of the rotor was hitting. Replaced the complete distributor. 2002 Blazer extreme.