I have a 2001 Blazer 2WD. I now have my second new Jasper Engine. The first one last 1 1/2 years. This is one seems so much better. Problem is, I picked it up Thursday and it is running great. Got in it Friday and the Service Engine Soon light came on. I ran it to Autozone and the the P0300 code for multiple random misfire and P0135 for O2 Sensor were the results. We changed the O2 sensor, but took it back to the shop that put in the new engine for the P0300 code. They can not find the problem! Any ideas! It has all new plugs, wires, various pigtails, radiator, injectors, EGR valve, battery, and now a new O2 sensor.

 

New engine requires the crank sensor relearn procedure be performed. Was this done?

 

I am not sure. I will check. Thank you!

 

You are welcome. Takes a scanner that has the capability to trigger the relearn procedure built into the car's computer (not cheap). If it was a professional shop replacing engines, they should definitely have the capability. Is a common thing that needs to be done every time the crank sensor is disturbed for any reason. If they can't do it, then they owe it to you to arrange for and pay someone who can. It's part of installing the engine.

I'm assuming that the PO135 is now fixed and the PO300 remains? If not you might want to take a second look at the PO135 before expecting the PO300 to go away as the random misfire could be real and caused by the PO135. I was always taught to repair the lowest numbered codes first.

Another thing to consider is if the codes were left in the vehicle (not cleared after repairs). How do you know which one is turning on the SES light? What does your shop expect you to do? Live with it? They are giving up after looking at it for a max of one day? This entire thing sounds a little strange for a pro shop.

 

You may also want them to check the cam retard since they will have the scanner out for the crank relearn.

 

Of course they said the computer was re-learned. Also, they did not clear the codes since the O2 sensor was changed. The tech says it doesn't really show a misfire while running with the scanner on. There might be a slight misfire when idleing and first starting up (how can he not be sure about that?). He is thinking that maybe there is a problem with the harmonic balancer, or maybe the torgue converter/tranmission. At this point, who knows. I am suspicious of the wires. The manager that billed me for the work that was done that was not under warranty for the engine said he used the plug wires from the new engine (that failed)that was installed 18 months ago, and now he says that he used new ones. Don't trust them at all now. I might mention, they changed the oil sending unit and the oil pressure guage was not working when I picked it up. Really? Who changes and oil sending unit and does not make sure that the guage works?

 

Also, could the faulty O2 sensor cause the P0300 code/misfire?

 

Ok, now they are saying that it may be the harmonic balancer, bad torque converter and/or transmission putting the engine out of balance causing the code. Can this be the cause?

 

Yes, see post #4.

 

Yes, those are some of the things that can cause a false misfire code to be set. Far below is the list from the factory shop manual. The $10,000 question is if it has a false PO300 or if it is real. I would say a recurring PO135 needs to be fixed first, as that could potentially cause a real misfire.

If there are no codes being set other than the PO300, I personally first recommend evaluating the ability of the cars system to detect a real miss induced into the engine by disconnecting (and grounding) a plug wire while monitoring all 6 cylinders for misfire counts in real time. The monitoring can be done with a variety of software/scanners. You should see a steady rise in counts for the cylinder that has the disconnected plug wire. If that is what you get, then the misfire detection system appears to be working properly and I would trust that the PO300 misfires are real. If you get anything else, then misfire detection is definitely not working properly and any misfire codes coming from it are suspect (may or may not be real).

If it is still setting a PO135, then start there.

If no codes other than the PO300 and you cannot feel the engine miss, then evaluate the misfire detection system for operation as described above. If it cannot detect the induced misfire, the misfire detection system needs fixed. Start with performing another crankshaft sensor relearn and make sure it is successful (fuel cutoff engages). If that does no good, then and only then can you start chasing the possible causes for false misfires IMHO. If it does detect the induced misfire I would trust that your PO300 is real and you will need to chase that problem to wherever it takes you (but it's highy doubtful to be the commonly maligned and replaced crank sensor).

I have some experience chasing false misfire codes in my own 2001 Blazer 4x4. In my case it turned out to be worn engine bearings (especially worn bearings on #1, #4 main and #1 rod) causing false misfire codes. Mine could not detect an induced miss. I can attest that it does not have to be a thrust bearing bad in order to whack out the entire misfire detection system. I worry a little about your statement about picking the vehicle up with oil pressure gauge problems.

What was the reason the last two engines were replaced? Was the radiator (oil cooler) replaced with either engine? As I understand, it can harbor a lot of metal particles. Indeed my Goodwrench engine instructions specifically stated that engine oil cooler must be replaced (this means the radiator)(I assume for warranty).

Good luck. I'm sorry but it sounds like your shop is trying to use the S.W.A.G. system of diagnosis. (see far below for meaning of acronym)



S.W.A.G. = Scientific Wild-Assed Guess