Hi All I am new to this Forum, I am working on my daughters 2003 ZR2 with around 150k miles.I am dealing with the Dreaded P0300 code. I have the flashing service engine light, car runs OK. I have changed out the following items. Plugs, wires, cap, rotor, crankshaft position sensor, spider fuel injector. I have checked for Vacuum leaks, checked the fuel pressure 55lbs no leak down. after all that I reset the codes and still had the Flashing Service Engine light and the P0300 code at around 2500 RPM.

I broke down and took it into a nearby shop that we trust, they put it on there mega dollar scanner and could only also see the P0300 code, no individual cylinder codes. After doing some looking they connected the car to an older SUN scope and noticed some possible problems with the spark pattern. They came to the conclusion that the Distributor needed to be changed. I told them that it had been done a few years back but if they wanted to try it go ahead and put a new one in. That was done and the car ran better, the flashing Service Engine light is out, But after a wile running, the computer still is throwing and holding a (Pending P0300) and now the Mega Dollar scanner is seeing misfires on cylinder 1. I had them check the compression on that cylinder, it came in at 125psi. Not great but OK. The shop thinks it may be in the Heads,I think not But ????

So I have the Car back from the Shop, no flashing light but with a pending P0300 code.

Any Ideas on were to look next.

Thanks MJ

 

Did the truck ever run lousy?

What did they see on the ignition wave form? Is that resolved?

Have you done compression on all cylinders?

When you changed out the ignition components what brand did you use?

Any work been done on the ignition module or ignition coil?

George

 

A certain amount of misfires are acceptable, and that is why the code is only pending. I wouldn't typically get my panties in a big bunch over pending codes, but you do have a history of setting a P0300 and you know that the new distributor changed things.

Start by checking the cam sensor retard value with the mega dollar scanner (or if you want to do yourself there are a few phone app/hardware combos that will work for under $40). Should be 0° ± 2°. The cam sensor retard (cam sensor is in the distributor) also doubles as an accurate indication of distributor rotor position. If this is off, you can get the spark jumping to the wrong place inside the distributor. Cure if cam sensor retard is off (due to apparent timing chain stretch/wear), is to unlock the factory distributor hold down so you can turn the distributor (which is normally locked in place with the factory hold down clamp). The other possibility for cam sensor retard being off is a worn distributor gear, but yours is new so that possibility is now out (and is probably why the new distributor improved things). Remember ignition timing is determined solely by the crankshaft sensor and the PCM, but on our Blazers the spark still has to travel through the distributor on it's way to the spark plug. IMHO cam sensor retard being off due to timing chain stretch/wear is the likely source of your remaining pending P0300.

FYI I have seen an occasional pending P0300 on my own Blazer (with both new engine and distributor as well as cam sensor retard set to exactly zero). Also I get a few counts of misfires on various cylinders during normal operation, but SES light never comes on or sets a misfire code, and I never feel a misfire. This is why I say a certain amount of misfires on the misfire counters are acceptable.

If you just can't stand a pesky pending P0300 in an engine with no apparent felt misfires, if all else fails you might need to have a Crank Senor Relearn performed. This restores the crank sensor rotational timing the computer uses to identify misfires. I imagine this timing can change a bit over time due to mechanical wear in the engine, etc. But if you have a pending P0300 all the time, the PCM may refuse to run the procedure unless you successfully play games with it by clearing codes and then immediately going into the procedure in same key cycle. Definitely don't jump to this first and don't jack around with your crank sensor or you will definitely need to do a crank sensor relearn (at least according to the factory manual).

Sounds like your shop did a good diagnosis with the distributor (gear), so kudos to them. Man, I sure wish I had an old Sun scope. They are great and very useful with anything that has one coil and an actual distributor.

Good luck and best wishes for a successful repair. Let us know.

 

Hi George. Sorry I have been out of reach of the internet for 4 days. To answer some of your questions here we go.

Yes the Blazer has run with a miss at idle and sometimes the RPM will fluctuate.

The waveform had what they called trash were it was not to be.

No I have note done all cylinders yet.

The new distributor was a delco.

No I have not changed out the coil yet, it is on my list.

The new problem when I got home this week is a noise from underneath the middle of the truck. I am going to look tonight to see if it is the muff or cat or ???

Thanks Matt

 

Thanks Les, I read your post all good items some that we have done. The timing chanin is new put in by the same shop. The Daughter now tells me that there is a noise coming from under the middle of the Truck, Tonight I will look into that, Maybe the Muff or the Cat ????

Thanks for all the ideas I will look into all and let you know what I find.

Thanks Matt J