Hi everyone! I know there are a plethora of posts about this, and have researched a lot so far, but I figure I would tell my story so that I can get a better focus on what I should do next.

My 2001 Jimmy had an issue where it was losing coolant from the system, and not leaking anywhere that I was able to tell. Started developing a slight tapping knock, and decided to get it checked out, thinking it could be a lower intake manifold gasket issue that is common with these engines. Sure enough, the gasket had to be replaced. Once replaced, I was no longer losing coolant, and the knock went away.

About a couple days after that, I started to get the P0375, and the trucks RPM's would jump up and the truck would stutter like crazy with any load put on the engine after 2000 RPM's. Because of this, I had the crankshaft position sensor replaced on it, and had to add the shims so that the RPM's wouldn't jump anymore. (without the shims, the new sensor still caused the RPM's to jump, but not has horrible as before.) I read here that after you get the sensor replaced, you should have the relearn done on the car. My brother in law said that it didn't need it, even though I insisted it did. Anyway, for 2 weeks it was running great without the relearn.

Then, when I was on the highway, the SES light would start flashing whenever I was driving around 65-75MPH. It would then go solid when above or below the 65-75 range, giving me a P0300 code. Because of this, I wanted to bring the truck to someone that can use the tool to relearn the crankshaft sensor. I brought it to a local Chevrolet dealer that had the sensor, and asked to have them run the relearn process on it.

When I get back to pick it up, I start driving it, and now anytime I put any load on the truck above 30MPH or 3000 RPM's it gives me a blinking SES light, now with only the P0304 code, and I do not have the P0300 anymore. I brought it back to them, thinking that they did not do the relearn correctly, and had them do it again. Once more, it still does the same thing.

Whenever I drive the truck now, I feel no performance issues at all. It feels like it runs great, but I do not want to drive it until I get it checked out.

Now, what I would like to know is could this be an issue with the crankshaft sensor doing this, or should I have the cap/rotor/wires/plugs checked out first on cylinder 4? I know this can range from anything, from fuel injectors to delivery, to a vacuum leak. I had cap/rotor/plugs/wires replaced a few years ago, but I know sometimes it can be faulty. Also, I don't have the proper tools to run some of the tests, but maybe it's time I should start haha.

Again, just trying to focus myself after having all of this work done to it. Any help would be greatly appreciated!

 

Short story:

Using shims on the crankshaft position sensor adjusts ignition timing and injector timing. The problem is, you have no idea how much, or in which direction. The crankshaft position sensor relearn assures they are both correct.

Long story:

The P0375 is related to the crankshaft position sensor. The problem could be in the circuit or the sensor itself. The crankshaft position sensor creates a magnetic field that is focused in a certain spot. As the crankshaft rotates, the tone ring passes through the magnetic field and sends a pulse to the PCM. Each sensor has a unique spot where it is focused. The relearn "tells" the PCM exactly where the sensor is focused. The PCM uses the data to determine when #1 piston is at top dead center, (TDC). Using simple mathematics, the PCM can determine when each piston is at TDC. The data is also used for ignition timing and fuel injector timing. The relearn data is burned to a file in the PCM, and it remains there until the next relearn is performed, (disconnecting the battery has no effect).

If the sensor is mounted too low in the bracket on the timing cover, (not enough shims) the tone ring might not pass through the magnetic field, or it might contact the sensor and destroy it. If it's mounted too high, (too many shims) the tone ring might not pass through the magnetic field either, it's got to be "in the ball park".

If the crankshaft position sensor, or timing cover, are moved, removed, replaced, or disturbed in any way, the focus point of the sensor changes and the relearn data that the PCM uses is incorrect. If the engine or PCM is replaced, the relearn is necessary as well. Incorrect relearn data has a direct affect on ignition timing and injector timing and will cause a multitude of driveability issues, guaranteed.... with NO check engine light!

With all that in mind.... after having the relearn performed, if you're not seeing any DTC's for the crankshaft position sensor, the sensor is working properly. P0300 through P0306 DTC's are almost always caused by a fault in the secondary ignition system, (spark plugs, plug wires, distributor cap, rotor, and ignition coil). They can also be set by low fuel pressure, clogged or leaking fuel injectors, worn distributor gear or bushings, or a mechanical problem with the engine, (valves, valve springs, etc etc). These engines are very fussy when it comes to ignition parts. Plugs need to be AC Delco 41-993, cap & rotor need to be AC Delco. Might want to start with those first, then check fuel pressure and leakdown, these are the most common issues with this engine.

 

So I brought the Jimmy to a mechanic to have it checked out, and so far he replaced the plugs, wires, cap and rotor. He said he also added a second shim to the crankshaft sensor, and said he checked the camshaft sensor as well, and ran through the relearn 3 times.

He said he doesn't get the P0304 code anymore, but now gets the p0300 code, and still flashes when driving. He said that it drives smoothly without any roughness or stuttering, but I am going to find out what kind of results he sees, if and how many misfires are occurring.

He noticed that I installed a CAI and wanted to try the original intake setup, so I gave that to him to try out, just so he can rule that out as a possible issue. If that doesn't work, then hey may check the injectors and spider assembly. I believe he has not changed out the ignition coil, but I will mention that to him. He also said it could be the PCM, but hard to believe since he has never had to replace one for these models yet.

 

These engines are very fussy when it comes to secondary ignition parts, (spark plugs, wires, cap & rotor) and do not perform well with aftermarket stuff, even when it's brand new. These are basic parts that must be functioning properly, especially when trying to diagnose a driveability problem. They all need to be AC Delco.

The camshaft position sensor can not cause a misfire, however, if it is not adjusted correctly, misfires will occur due to crossfire inside the distributor cap. DTC's P0300 through P0306 may or may not be set in memory. Depending on the scan tool he is using, the data is listed as "Camshaft Retard" or "CMP retard". Spec is zero degrees, plus or minus 2 degrees. It is adjusted by slightly rotating the distributor.

The CAI will not cause a misfire. It does very little, if anything, for performance. The injectors may or may not be the problem. Checking long and short term fuel trims, and fuel pressure and leakdown would help in the diagnosis. The PCM's in these things are very substantial and rarely fail. Far more are replaced as a result of misdiagnosis than are actually faulty. I have to agree with him, that it's extremely doubtful the PCM is the problem, (I've never replaced one either).

EDIT: A flashing SES light is indicating a misfire that could potentially destroy the catalytic converter. Incorrect camshaft retard could be the cause of the misfire.