On deceleration from highway speed, (not touching the throttle or brake pedals) the PCM will go into open loop after approximately 10 seconds. Just barely touching the throttle will cause the PCM to go back into closed loop. During the "decel" several things change: injector pulse width, ignition timing, IAC position, etc etc. As you mentioned, from a constant highway speed to decel, you can actually feel it change from closed loop to open loop, and back to closed loop when you touch the throttle. If you're paying close attention, you can watch the tach drop about 75 RPM also.

For all intents and purposes, camshaft retard on this engine is the same as "base timing" on an engine that has an adjustable distributor. With that in mind, your timing advance at idle of +16 to +20 degrees is "ballpark" however, that is what the PCM is commanding and does not include "base timing". The PCM has no way of "knowing" if camshaft retard is correct so it has to "assume" that it is.

Everything has to be "in sync" when the engine is at idle: throttle plate angle, TPS adjustment, cam retard, timing gear calibration etc, etc, (all stuff that the tech needs to make sure is correct) before the PCM can control the engine properly.

If you have a scan tool that can access cam retard, check it. If not, find a shop that can, and have them adjust it if necessary. It's not a big deal, takes 5 minutes to check it, and less than an hour to modify it so it can be adjusted, and make the adjustment.

 

Thank You Captain Hook! I checked the CMP Retard this morning and, at idle with the system in closed loop, it was -5. Now, I have gone thru the procedure listed in the forum and my question is: Does a person, such a Fivspeed or myself, have to remove the distributor in order to get to 0 degrees (+/-2)? Or, can one merely loosen the hold-down bolt and turn it slightly in order to advance it the 5 degrees needed? Swartlkk appears to say that there is about a 15 degree amount of 'play' using this method; correct? For what it's worth, the Knock sensor is showing 0 degrees and the idle is smooth.

 

There might be 15 degrees of total adjustment. Depends where it's at now, whether you'll have enough to get back to zero. All you can do is try it. If you can't get back to zero, you'll need to pull the distributor, remove the original hold down bracket from the distributor, and install a SBC, (small block Chevy) hold down bracket. Most auto parts stores stock them, made by Mr. Gasket, ~$5.

To make things easy when it comes time to reinstall the distributor, before you remove it, take the cap off and manually turn the crankshaft until the rotor lines up with the "6" in the distributor housing. When you reinstall the distributor, the rotor must point to the "6" when it's fully seated.

 

Not sure how relevant this is but I thought I might share. I have emissions testing coming up next month so I wanted to make sure everything was squared away before going. I have a 2001 blazer with the AIR injector/pump system so I usually take that off until about a week before I get my emissions tested (got tired of replacing corroded pumps). This time around after I re-installed the AIR pump, I hooked up my OBD2 reader to clear out the usual P0410 and noticed that I had both of the OPs DTCs (P0171 & P0174). I thought it had something to do with either the amount of air the pump was sending through the exhaust or possibly a exhaust leak through the hoses back-feeding the pump.

After a few days of messing around with everything I thought may be the problem, I took a look at this forum and noticed a lot of emphasis on air intake leaks past the MAF. That reminded me that I took my MAF off a couple of months ago for cleaning. Sure enough when I had taken the MAF out I had accidentally disconnected the hose from the back of the shroud that covers the throttle body. Hooked it back up and the codes went away.

Like I said this may not be relevant to the OP (it looks like he may have checked all of this) but for anyone else with these issues, make sure to a quick idiot/sanity test before you dig in too deep!

 

That can definitely cause a lean code, but leaving that tube off most often results in some noticeable driveability concerns (bucking/etc).

 

I thought the same but oddly enough I didn't experience any of them. I streamed live data on the OBD2 while driving (when the hose was disconnected) and definitely saw lean condition numbers for the O2 sensor readings, but other than that I had no symptoms.

 

i had the same codes 2 days ago and parts guy said it maybe the oxygen sensor so i pulled one out of a wrecked blazer before i bought a new one and now it runs even worse.the one i pulled out of my jimmy appeared to be burnt is that a bad sign? should i replace with new one?and do i need to replace both sensors? oh also when i started it today it was hard to start and when it did it ran reeally rough and had a lot of gas smell. any suggestions?

 

Really after all this there is no outcome. I just broke a ypipe and drove it for a few days. I now have the same problem P0171 & P0174. I guess I will have a look at that air pump and EGR.

 

Is it possible that you damaged your Mass Air Flow sensor? The drive cycle does need to complete. Those are the last two systems checked and it can take up to 5 hot cycles to complete the catalytic converter. I got the drive cycle at www.obdii.com/drivecycle.html

 

I had the codes po171 & po174 on my 2001 blazer. It turned out to be the MAF sensor. Even though the sensor should show its own code, it never came up on the scan tool. Ater testing the fuel pump and replacing the fuel filter the fuel pressure increased a bit but the codes kep coming back. I did a lot of research and came up on a YouTube video on how to check the MAF sensor with a voltmeter. I did the test and I was surprised. The voltage shoul increase when you throttle up but it did not move much. I cleaned it and still no help. I finally bit the bullet and replaced the MAF with a rebuilt AC delco unit and the codes have not come back.

I read somewhere during my research, that sometimes if the MAF sensor does not provide any information to the computer, the computer uses default values in order to keep the car running. In my case, I think the computer only thought that the engine was getting to much air and showing a lean condition.

It's been several days now and the codes have not come back. The only issue I had doing the MAF test is that I could not get the probe on the volt meter to make contact with the signal wire. The connector to the sensor is well sealed. I finally stuck a sewing needle through the connector.
I hope this helps because I had originally taken the truck to a shop where they suggested I have the fuel injection spider changed. That is costly!