I am the Original Poster and wanted to give an update on the corroded CMP in the distributor under the rotor at the 2'o'clock position. I decided to remove it and clean it and see if I could replace it without doing a CMP relearn like you have to do when you replace the distributor. I pulled the distributor cap and then removed the rotor and found the CMP is held on by two small machine screws just like the rotor. The problem is you can't just pull it out because of the metal cup under the rotor. There is no room. But GM provided a small 1/2" slot in the cup to allow you to wriggle it out. You have to rotate the metal cup around until that slot lines up with the CMP sensor. To do that, you have to carefully 'bump' the starter with the key to move the cup around until it lines up perfectly. This takes patience and several tries to get it lined up perfectly. This procedure is easier with as a two man team---one to bump the starter and one to watch the slot line up with the CMP. You will go around a couple of times before you get the line up correctly. Another way is to put a ratchet with socket and cheater bar on the harmonic balancer bolt and turn it CW until the slot lines up with the center of the CMP. This is also a two man job.
I took my CMP off to clean the white corrosion off and see if there were any cracks or other damage to it. I used a soft bristle brush and WD-40 to do this. After cleaning, It looked good and I replaced it carefully back under the metal cover. I tightened the two screws down carefully. I didn't want to strip the treads and have to replace the distributor. The good news is after plugging in the 3 wire connector, replacing the rotor and the dist. cap. It cranked right up. That answered my question: to replace the CMP sensor and leave the distributor as it was,do you have to go through the 'relearn' procedure. The answer is ---no, you don't. I have driven it several times since then, and it still starts and runs great. However, I have decided to go ahead and replace the old CMP with a new AC Delco CMP from Amazon. Cost ---$38.xx plus tax. The AC D. part from local part stores is $46.99 plus tax. My thoughts were if the outside were so corroded, the insides of the CMP may be just as corroded. So I will be going through this procedure again after the part comes in from Amazon. BTW to get a OEM AC Delco part from the local parts stores requires you order and wait for deliver. AC Delco part no. 213-920.

 

Just FYI, the engine will run fine if the CMP retard is off by a few degrees or more, but that doesn't make it "right". It would still be a good idea to check/adjust it if you mess with the sensor. There are a couple of phone apps that can do it if you have the Bluetooth adapter. You don't need an expensive scanner.

 

Tom A., You are correct. I may have just been lucky. I am planning on getting the Phone app and Bluetooth set up and have everything on hand but the Phone App. What is the latest recommended app for this project ? It has changed a few times as the app's and hardware changed and improved. I am in the Android world with a samsung S7 and am excited about using it for this project as well as all the digital gauges that are offered. I like gauges. You can''t have too many.
I borrowed the following from a good write up in the 'Troubleshootmyvehicle.com/GM 4.3L,5.0L, and 5.7L' ; on- line article. This on line resource had a lot of info not only on the CMP but other systems on the GM Vortec engines. They give a lot of trouble shooting tips using just a simple Digital VOM.
The CMP is a Camshaft Position Hall Effect Sensor. It talks to the PCM=fuel injection computer.
The symptoms of a bad CMP are 1./ Rough idle, 2./ Hard starting/excessive crank time, and 3./ Poor gas mileage.
It is located in the distributor and is under the rotor and under the metal vane that the rotor is attached to. The rotor and vane are spinning together and trip the Hall Effect sensor each time it passes by and generates a single 12 VDC pulse. Here is a trick to lining up the CMP sensor to the notch in the metal vane so the CMP can be maneuvered out, Use a 1/2" ratchet handle and a socket that fits the harmonic bolt and turn it CW until the slot in the metal vane lines up with the CMP. you may need a cheater bar extension for the ratchet handle since you are spinning the engine. Removing a few easy- to- get- to- spark plugs would make this job easier. Also you need a spotter on top to let you know when you are getting close to the mark. I know I mentioned this trick in my post no. 11 above. I just added a few details to it.

 

I believe Car Gauge Pro is the app of choice for checking cam retard. It costs a few bucks, IIRC, but it wasn't bad.

Thanks for the tips on maneuvering the sensor out of the distributor but, to be honest, I think it's easier to pull the distributor and do it on the workbench. It really only takes a few minutes.

Pro-tip: As long as you don't turn the engine while the distributor is out, you don't need to worry about setting it to TDC, lining up the rotor to the 6, etc. Just mark the rotor position on the distributor body with a Sharpie before you pull it out, then put it back in the same spot.

 

Thanks, Tom for the quick reply. I will look for that app. Jim

 

I just received the AC Delco CMP and it is made in Mexico. Welcome to 2020 world. The GM OEM part used to mean 'Made in the USA'. The AC Delco part no. is 213-920, the GM part no. is 19417445. I received it at 445 pm and the sun sets at 530 pm. Yes, I am a true shade tree mechanic and long for a heated and A/Ced garage. So far, Santa hasn't delivered. I am tempted to try it, but it gets cold went the sun goes down. so mainly on that reason alone, I will postpone the install til Tuesday. I want to be sure to do this job right. That reminds me of another tip on the removal and install of the CMP. The tiny six screws for the CMP, the rotor, and the distributor cap---two each--- all have the inclination to disappear behind the bell housing if dropped. Never to be seen again. Tip: put some old sheets or other cloths behind the distributor so when you drop the screws , they don't fall behind the engine and bell housing. One positive note is the good folks at AC Delco provide 6 screws of different lengths to fit different applications. This also means you probably have a spare if you were to drop one.

 

I just paid $40.xx for an AC Delco CMP from Amazon. When Skip White was selling our 4.3 L V6 2nd Gen distributors for only $41.99, I thought that is a good deal but now Skip White no longer carries them.. But now you can buy a complete new distributor from E Bay for only $31.xx. plus shipping and tax. That is incredible. It comes with a dist. cap, rotor, and a CMP, but they are not the recommended OEM AC Delco. If anyone has tried one of these distributors, please write a review. In the past with the Skip White 'Made in China' distributors, many were recommending swapping out the Chinese cap and rotor with an AC Delco and most were satisfied with this set up. It would be nice to hear how these new Made in China dist. are performing and holding up day to day.

 

Wednesday, Jan. 8 was a good day. I got the CMP installed in the distributor. I was able to turn the harmonic balancer bolt CW with a 5/8 inch socket with a 1/2 " ratchet in increments. Each time, I would have to crawl out from under and check to see where the gap was. This is a two person job ! I went around once and on the second go round, I hit it dead on. I was able to unscrew and remove the old corroded CMP and install the shiny new AC Delco 'Hencho en Mexico' part and screw it down. I had towels wrapped around the distributor so I wouldn't lose any screws. Got the CMP.; the rotor; and the cap on tight. You can only put the rotor on one way due to the plastic pins on the under side of the rotor. I had cleaned the distributor out to get rid of the white corrosion. I have a small Porter Cable air compressor. I snapped on the 2;4; and 6 spark plug wires and the coil wire on the left side of the distributor. BTW one kind poster said there was a TSB regarding the distributor corrosion being caused by moisture build up inside the distributor. The TSB recommended punching out the screen over the hole in the bottom of the distributor. I guess this allows the distributor to breath and prevent moisture build up. I found the small screen covered hole in question. It didn't look corroded or clogged up with dirt. So I sprayed it with WD-40 and then with compressed air and buttoned up the distributor cap. Snap on the 3 wire connector to the CMP and you are ready to hear it fire up. It did, but as Tom A said in his post no. 12 above, the fact that the engine runs doesn't mean it is right on Zero degrees plus or minus 2 degrees. I am trying to get a 'Car Gauge Pro' app /still only $8.99/ and a Bluetooth OBD II adaptor and take a look at it. Thanks to Les Myer and his extensive work on that project. It is good to be back on the road again.

 

One significant question keeps coming up. What are the symptoms of a defective CMP/ Camshaft Position sensor ? They are 1./ Hard starting, 2./ Rough idle, and 3./ Poor gas mileage. Sound familiar ? I am sure it does to many Blazer and Jimmy owners. As they age and go through many heat cycles, they will crack or corrode. When they do, they are prone to give the common symptoms above. My Blazer's CMP was corroded and it showed up as extended crank time, especially when there were wet, humid conditions. I just replaced my 2003 Blazer's CMP and soon after we had two heavy rain storms come through. My drive way is still saturated with puddles of water. But now my 4.3L starts quickly and runs smoothly.
Here's a site that has a lot of good info on the 4.3L Vortec engine. The CMP testing info is on pages no. 4 and 6 of this site.
Troubleshootmyvehicle.com/GM/4.3L/5.0L/5.7L

 

If CMP sensor is defective or malfunctioning, the PCM will turn on the SES light. Engine will run fine with the CMP sensor unplugged. Try it! I also think that CMP sensors (like crankshaft sensors) are not parts that are prone to failure but corrosion is never good.

Just a FYI. There is no such thing as a CMP Relearn or CMP Relearn Procedure (post #11 - you were apparently confusing this with a crank sensor relearn). However there is a CMP retard setting that indicates the position of the cam sensor relative to the crank sensor. This should be 0° ± 2.0° and the distributor has a lock built into the hold down clamp so it cannot be turned and altered. This cam sensor retard setting also indicates that the relative position of the rotor to the distributor cap terminal is correct when the spark comes through the distributor so on the it is important to get it right (on distributors with ignition triggered by pickup coil in distributor, or on distributorless ignition this mismatch doesn't happen). Since you didn't remove your distributor and you are most likely using the factory hold down, you have not changed the cam sensor retard setting when changing the sensor and that is good if you don't have a way to read CMP Retard. (Note how much CKP Relearn sounds like CMP Retard - and that's the source of the confusion). If you had pulled the distributor out, the factory hold down lock would keep it pretty much adjusted correctly as long as you get the distributor in on the correct tooth (as evidenced by no SES light).

So why check CMP Retard value to make sure 0° ± 2.0°? After time, wear and stretch in the timing chain/gears, as well as wear in the distributor gear can allow the distributor (and distributor rotor) to fall further and further behind in rotation (additional negative CMP retard). This can cause misfires from misalignment of the rotor to cap terminal when the spark comes through, especially when timing is most advanced. The Cam sensor also give the PCM information for it to sequentially fire the injectors properly (this is typical use for a cam position sensor in modern engine). However, the cam sensor info isn't a critical thing to the PCM. You can unplug the cam sensor connector completely and the PCM will simply revert to pulsing 2 banks of injectors and run fine, but with the SES light on and probably at reduced power and increased emissions. So for actual operation of the engine, the adjustment of the cam sensor is more important than the cam sensor signal itself because the 4.3 still used a distributor. Go figure.

Of the Android Bluetooth softwares, Car Gauge Pro definitely does the most - but for GM this is only on 1998-2005 vehicles. It was really made for Ford/Nissan and is a bit clunky to use until you get used to it - also has a lot of unfiltered features that will not work with your vehicle, or will not work correctly. For example, in Car Gauge you will find two different cam sensor retard PIDs to choose from (* and **) they both appear to work but give different values (use the *). Dash Command is a better interface and works on 1996/1997 but you have to pay $10 per vehicle for Enhanced GM PIDs (CMP retard is an Enhanced GM PID).

To adjust the cam sensor retard, you just turn the distributor a touch while watching the cam sensor retard, goose the throttle once and let things settle down to idle, and read the new setting. This may or may not require you to modify the distributor hold down. You usually get a couple of degrees slop in the hold down itself from the factory. Elongating the bolt hole with a Dremel is a typical fix.

Good luck.