2002 Chevy S10 4.3L 4x4

Good evening,

So I am back in the automotive field after being out of it since 2012. I was working on a 2002 Chevy S10 with the big V6 4.3L. It had a P0300 (Random Misfire), P0341 (CMP Sensor Circuit Out of Range), and P1345 (CKP/CMP Correlation Problem). After scanning the vehicle I checked the retard and it was 0 degrees (I remember the gear on the distributor would always go bad and the retard would be roughly 20 degrees). I went to the fuel trims and they were in range. I pulled up the misfire monitors and saw cylinder number 3 was missing and all the other cylinders were not. I pulled out my Verus and checked the secondary ignition waveform and was noticing a lot of turbulence in the firing line. At first, I was thinking fuel so I checked fuel pressure and it was good. I did the fuel injector balance test and all my pressures were the same so that ruled out an injector (I remember these injectors always going bad). My firing line was good so I ruled out a secondary ignition problem. The only two things I can think of are a possible worn cam lobe or a leaking valve. The misfire happens at random times while the engine is hot (DTC freeze frame data showed 194 degrees), but I have not checked it cold yet. One last thing, I through a vacuum gauge on it and the needle was steady at 20 in.hg. if that is any help (not an expert with a vacuum gauge), but I thought maybe it would help me if there is a valve seating issue or worn cam lobe. Any help anyone can provide me is greatly appreciated as I have lost a lot of my diagnostic abilities these last few years. The pictures attached are from this truck while snapping the throttle.

God Bless,

Chad

I should also add for the P0341, I have good ground power, and signal. The sensor also displays a good square wave. Also, there was no CMP/CKP correlations data on the scanner and I did not feel like getting the Launch or Autel to see if it had that data (I was being lazy) so I did back probe the CKP signal wire and the CMP signal wire and compared it to a known good waveform which looked good while the engine was not misfiring.

Hi,

The CMP sensor produces a magnetic field whenever the ignition is ON. The CMP sensor is mounted near a reluctor wheel that is attached to the distributor shaft. When the distributor shaft rotates, or when the engine is cranking or running, the reluctor wheel changes the magnetic field. The CMP sensor converts each change in the magnetic field into a PULSE. The number of teeth on the reluctor wheel determines how many pulses the CMP sensor detects per camshaft rotation. The control module uses the CMP sensor signal in order to calculate the correct timing for sequential fuel injection. If the powertrain control module (PCM) does not detect the CMP signal while the engine is running, this diagnostic trouble code (DTC) will set.

I see from your report that you have a Verus, good.
What I would do is back prob the cam crk and record them. Look at the scope till you see the cam and crk act up. I think you will find one of them drop out, I would bet its the crk…


I would also check the ref sig / power/ ground at each if all good then .

I don’t think that engine needs both cam crk to run but it does to start.

One other thing you can do is do a leak down test to see if Cyl 3 and one or two others are close this will help in knowing if you have a valve ring or other problem.

Hope this works.


Doc

Thanks Doc! I did check my power, ground, and signal wires at the CKP and CMP sensor. Also, I did scope the signals on both and as the engine was misfiring, there were no drop outs in the signal. I was just curious why there was so much turbulence in the secondary ignition wave forms I captured. I am going to do a running compression test, but the customer will not be back until after Spring Break so I still have time.

Hi.
I understand the compression test for misfire but i guess you have two different issues. P0341 and P1345 are telling you something else.

I don't know that engine, then i'm going to ask in general speaking.

Does that engine have vvt valves?
Does it have timing chain?

I wonder if u have a valve hanging up intermittently. vacuum gauge should show defect if it is steady, also cranking engine in no start condition will show a weak cylinder amps will be normal on the rest, but the one not holding pressure will show a drop, so on your scope a different wave form than the rest of the cylinders. pull fuel pump fuse, or coil wire, and put amp clamp on posi battery terminal and crank afew cycles. this will tell you if it is mechanical or not.

Good evening wacoeagle,

So that vacuum gauge was steady at 20 in. hg., even while misfiring. I did not do a relative compression check, but that is probably the first thing I should have done. I guess I didn't think about it because it was an intermittent misfire and only did it while the engine was hot. However, I did do a running compression check on it and the snap throttle and cylinder number 3 was significantly lower at idle and the snap throttle. So, I pulled the valve cover off and noticed the valve spring was breaking. Thanks for everyone's help! I am still curious about the secondary ignition waveform though. I guess I will have to research that to see if that was the cause of all the turbulence on the firing line.

Respectfully,

Chad

maybe triggered a knock, and sent the timing retarding ? you have to fix your known issue and recheck. just thinking outloud.
the conundrum with motors for me is the deciphering if it is an electrical or mechanical problem. its always fascinated me how inextricably intertwined they are, and can show similar symptoms.
70 percent of troubleshooting to me is figuring out if its mechanical or electrical.
I am a shadetree mechanic and dont know nearly as much as these guys in here but it always goes back to basics, ALWAYS, ! give me fuel , give me fire , give me that which I desire!!!! AND compression and timing, ha