Okay, Here's what I have done so far after reading the blogs...
I started with a tune-up on the 175K 1996 Blazer I bought recently because it has the infamous surge at idle that bounces the tac about 150-200 rpm's. It almost died on some occasions when it was really cold out.
I changed all the original plugs to NGK and purchased the AC Delco plug wires, cap and rotor based on forum feedback. Nothing changed.
I will also say that I had some o2 codes and the egr code as well as evap codes going at this point so I changed all the evap parts. front and rear purge solenoids as well as the vapor cannister and the code still exists. errr
I changed the egr. no change. I upgraded to the new style Delphi spider injection system. (That helped for 1 day and then it went back to surging!) errrr errrr
I have checked and can not find any vacuum leaks at this point.
I changed distributor because the forum also mentioned the gears were junk and indeed the gear on the original distributor was shot!
Here's also something interesting though. I cam upon the cam retard threads and my scanner shows cam retard anywhere from 18-26 degrees at idle to 1000 rpm. Driving down the road, the ccam retard is around negative 31-33 degrees. I did elongate the distributor mount today and clocked the distributor to the left a tad bit but it do much and had little effect if any.
The oil gauge is all over the place so I have a AC Delco sending unit inbound and the fuel pressure is key in my opinion at this point, given that the coil, the ignition module and also the IAC have all been replaced. I have a tps switch also inbound.
Fuel pressure kicks up to 60-61 pounds on initial hit but then drops to around 51 after about 2 seconds. when the engine is running it reads around 52 but then surges occasionally? At shut down, the fuel pressure bleeds down to around 35 lbs. after roughly 10-15 minutes.
Do you think my main issue here is the need for the fuel pump? I saw decent pressure when it was running but I question that now with the eratic pressure I noticed when the engine was running.
I have also replaced the O2 sensors and the MAF but the darn thing keeps flashing= egr, evap, and now the cat is flashing as well on this thing.
For safe measure I also changed the altenator and battery thinking maybe there was a volt issue at idle but the ground up front is tight as well.
I'm at a loss folks and can't afford much more with this truck. I'm almost wondering if it is a timing chain at this point the way the cam retard bounces all over the place?
Could the oil pressure sending unit be causing this as well? Oh, and I did try the trick of pulling the front )2 sensor to see if the surge would disappear but it didn't.
I have read all the forum blogs but am still trying to slay the dragon. Please help me.
Chaplain Randy

 

Cam Retard is not the same as Ignition Timing, Timing will vary based on what computer thinks is needed.

Cam Retard setting is 0 deg plus or minus 2 deg. and will vary a degree or two. Set by twisting the distributor while running with the holddown bolt loose. There is "slop" in the top hole of the Intake Manifold to allow a small amount of adjustment. If you are a long way from 0 then remove dist and move shaft a tooth and retry. Get this set and move on.

Fuel pressure is the problem, you will need to further test pressure and leak down while gauge is connected directly to the pump output.

 

Just out of curiosity, is -64 degrees at idle normal? That is what my Innova scanner is claiming...would I then turn the shaft clockwise 1 tooth to advance it? MPG totally sucks on this truck and I am wondering if the distibutor gear is possibly retarded by one tooth?

I'm ready to take this to a GM dealer and have them flash the ecm because the evap code won't go away either. I don't have a "smoker" so it's hard for me to say. I replaced the cannister and both front and rear purge solenoids and still get the evap code on my scanner. It's not throwing a code but gives the yellow light thing on my scanner. estimates for time and money involved by dealer to zero out cam retard?

I'm almost wondering if it is the ecm based on the fact that when I turn off my heater blower, the truck sometimes idles normally? Weird stuff! Sometimes it idles fine and then other times it is a bucking hog! could this possibly be a bad timing chain? It just seems so erratic...sometimes it idles fine and then other times, it just surges.

New fuel pump now too so pressure is good!

I also noticed that it idles fine when cold until it warms up to operating temp, then it starts to surge when in drive or reverse. My O2's seem to cycle but maybe they could be lazy and not set the codes?

I'm stumped...

 

when I turn on the headlights, it drops the idle about 300-400 rpm and then compensates itself . Is this normal? I have also replaced battery and altenator...

 

From your first post:
These engines run best with AC Delco 41-993 plugs with a gap of .060".
O2 sensors can cause surging if they are lazy or dead. The surge usually becomes noticeable when the PCM switches to closed loop operation, (the PCM ignores O2 sensors in open loop).
Depending on the EVAP DTC you got, a very common problem is rust holes in the fuel filler neck. All it takes is one pin hole .030" (or smaller) to set a DTC.
Your camshaft retard is well out of spec and DTC P1345 should be in memory because of it. More than likely this is what's causing the surge. Camshaft position sensor data, along with crankshaft position sensor data, is used to detect and identify cylinder misfire. When camshaft retard is adjusted properly, the rotor segment is aligned with the terminals inside the cap when the ignition coil fires. If camshaft retard is out of spec, crossfire will occur inside the cap. The farther off it is, the more misfire there will be. Before camshaft retard is checked, engine RPM must be snapped over 2KRPM and back to idle. There are 13 teeth on the distributor gear, each tooth accounts for ~27 degrees of rotation. If camshaft retard is more than ~27 degrees, (positive or negative) it means the distributor is off at least one tooth. Your camshaft retard reading is 31-33 degrees, which means your distributor gear is off by one tooth, plus a little bit Installing the distributor properly, (P1345 not present) is the "rough" adjustment. The "fine" adjustment is done by rotating the distributor. If you can't fine tune it to zero degrees by rotating the distributor, you need to remove the original hold down bracket and install one of these: Mr. Gasket Distributor Hold-Down Clamps 1009 - SummitRacing.com

Fuel pressure: First, GM does not publish any "engine running" fuel pressure specs for this engine. All tests are done with the engine off. Running pressure means nothing, there are too many variables involved. Your leakdown to 35psi after 10-15 minutes is excessive. You'll need to check fuel pump maximum output pressure and leakdown at the fuel filter outlet. All pressure and flow must end at the pressure tester. This is commonly referred to as "dead head" pressure. It must be 73psi to 108psi while the pump is running. It must remain above 55psi for at least 10 minutes. If it fails one or both of these tests, the problem is in the tank. If it passes both tests, the problem is in the plenum.

 

Well, I found something interesting and would like to ask your opinion on it, Captain. I went out this morning and went through the proper procedure of setting the distributor in place. I wanted to confirm the position in correlation to the cam as you suggested above. I rotated the engine until the mark on the damper lined up with the "V" after compression stroke, etc. The rotor was pointing about 1/4" before the "6" position in the distributor housing so I repositioned the distributor exactly one tooth counterclockwise in order to try and get the rotor "dead-on" with that mark on the housing. Upon initial driving the truck seemed to run better and the timing according to my Innova scanner was much lower at -10.5 degrees and it held constant there no matter what rpm's I ran it at. I shut the truck off after road test and restarted it and got your DTC P1345 code. I tried resetting it several times but it just came back. I want you to be aware also that when I advanced the distributor exactly one tooth counter clockwise that the marks did NOT line up "dead-on" and now the rotor was forward or in advance of the "6" mark on housing. I have since returned the position to original in order to get cam position sensor in range so that the DTC P1345 code would go away.
Honestly, at this point, just knowing that the distributor will not achieve the marks according to the procedure in Chiltons, leads me to believe that the timing chain has skipped a tooth. There was a difference of about 9 degrees timing on the scanner but when the truck runs in its current state, the timing will vary from 19-23 degrees on the negative side. -19 or it jumps to -23 etc. When I advanced it it was at -10.5 constant.
Feedback?
I have replaced the Delco cap with another Delco one for safe measure at this point and it it is still surging.
In my mind it stands to reason that if the cam is off its rotation, that no matter what I do at this point, it will continue based on the improper valve timing.

 

I saw some of your posts about timing chains etc, Cpatain. I ordered the distributor hold down and am going to get some new Os"s because I see 2 of them acting lazy.
I'll post back once work is complete to see if it fixed surge.

 

A friendly word of advice, don't rely on Chiltons for anything more than an oil change If you want complete and accurate information, specifications, wiring schematics and diagnostic procedures, get the GM "dealer issue" shop manual. Each set has 3 manuals that cover one model year in 3,000+ pages. You won't regret it Amazon & Ebay have them, both new and used.

When you view ignition timing on your scan tool, it doesn't really tell you a whole lot. It's showing you what the PCM is applying, and it's not adjustable. The PCM relies on correct data from the cam and crank sensors to make accurate calculations and adjustments to the various outputs. The PCM has full control of ignition timing and if the sensor data is incorrect from either or both sensors, ignition timing will be incorrect, (bad information in, bad information out). The camshaft position sensor is adjusted by rotating the distributor. The adjustment is called "camshaft retard" and it must be checked/adjusted if the distributor is moved, removed, replaced, or disturbed. The crankshaft position sensor is calibrated by performing the "crankshaft position sensor relearn" with a scan tool capable of initiating it. The relearn must be performed if the crank sensor or timing cover is moved, removed, replaced, or disturbed. As I mentioned in post #5, camshaft position sensor data, along with crankshaft position sensor data, is mainly used to detect and identify cylinder misfire however: The PCM also measures/monitors irregularities in the 2 sensors while the engine is running. These irregularties are perfectly normal but can cause changes in ignition timing if one or both sensors are not providing accurate data. Think of these sensors as a "benchmark", much like the throttle position sensor must indicate 0% when the ignition is on and the engine is off. These are all basic points from which the PCM makes its calculations and adjustments to ignition timing, injector timing, canister purge, EGR, etc etc.

As for the timing chain.... Unless the engine has been abused or neglected, the chain & gears are easily capable of going 300K miles without needing replacement. You can easily check timing chain slack without engine disassembly. With less than ~10 degrees of slack, it can't jump time. Visually inspecting slack requires replacing the timing cover and removing the oil pan.... serious work, especially if it's a 4X4.

 

Is that a Timing reading or a Temperature reading?
Please clarify what the exact parameter label is on your scanner.

Seems out of range for timing, but indicates a bad or disconnected Temperature Sensor if it is a Temperature reading. A bad or disconnected Temp sensor can make it run poorly. Some years have 2 Temp Sensors.

 

timing...I changed both temp sensors.