2001 Blazer ZR2, 79,900 miles. There's a lot of info here so I tried to make it as short as possible.

This car has very low miles and I have babied it and taken really good care of it for the last 10 years of owning it. But....

I have a P0300 (random misfire) code. No other codes present. It has been very difficult for me to diagnose since it is not located on just one cylinder. I have watched the behavior of these random misfires on my scanner and will tell you how they behave and what I've done to diagnose it so far. I really need to fix this as I can't get my car smog checked with the SES on. Otherwise I wouldn't worry about it so much. Maybe someone will read this and have an idea what it could be given the info below.

It normally affects only the front four cylinders which are numbers 1, 2, 3 and 4. Cylinders 5 and 6 (the two across the back) rarely show a misfire. With that said, of the front four affected cylinders, number 1 and 3 are significantly more active than 2 and 4. These are the drivers side front two cylinders. (so the two closest to the A/C compressor.)

BEHAVIOR:

The car runs like its brand new. I detect no vibrations in the engine or drive-ability issues - idle and acceleration seem great. I would have never known there was an issue if the SES didn't come on. Monitoring the scanner: No misfires occur when coolant is cold. Car sat overnight so coolant is cold measuring 60 degrees on scanner. First misfire was at 123.8 degrees as engine started to warm up to 195. Once warm, AT IDLE, cyl #1 and #3 seem to offend the most while #2 and #4 not as much. Almost no misfires on #5 and #6. When revved up and left at 1,500 RPM, the scanner shows that 99% of the misfires go away. This would explain why sometimes the SES turns off because the misfires go away at constant highway speeds.

Here is a list of new parts I've recently put on the car:

Fuel pump and related assembly canister
Fuel filter
Fuel pressure regulator
Fuel injection spider (the upgraded version)
Spark plugs and wires
Cap and rotor
Lower intake manifold gaskets
Gaskets related to above job (for plenum, etc)
Thermostat

TESTING THATS BEEN PERFORMED:

FUEL:
Fuel pump and related pressure was heavily tested as noted in my prior thread. All pressure and leak down tests passed. Pump, filter, and fuel pressure regulator are new so I didn't expect a failure. Bank 2 LTFT looks a little off. I thought they were supposed to be in the +-2% range? This is very strange because Bank 2 is the side that barely has any misfires. Bank 1 is the major offender! Long Term Fuel Trim Measurements:

LTFT B1: -0.4 at idle . . . . -1.2 at 1,500 RPM
LTFT B2: -4.3 at idle . . . . -5.9 at 1,500 RPM

IGNITION:
I checked the CMP Retard value and made sure its correct. I was actually able to get it to zero degrees! Since cyl 1 is one of the offenders, I swapped its plug with a non-offending cyl (#6), No change in misfires. I swapped the spark plug wire from #1 and a non-offending cyl (#5), No change in misfires. So ignition appears to not be the culprit. I didn't think it would be with new wires, plugs, cap and rotor. I attached a timing light to the plug wires to see if I could detect inconsistent flashes or other bad behavior. Checked cap and rotor and all looks new. All looked good.

AIR:
EGR tested flawless. Did a comprehensive pin-out test. Also did a visual inspection and pintle moves freely and easily with zero built up carbon in EGR or both ports in engine block.

The scanner read that the Manifold pressure was 10.1 inches of mercury idle. I thought that value was supposed to be 18-22 inches of mercury?!? I checked for vacuum leaks with two different methods, the propane gas method and spraying water test. I did both of these TWICE so that was four total vacuum leak tests and I found nothing. I checked all things in the engine bay that I knew to check. I don't know if that Evap canister thing back by the fuel tank could cause a vacuum leak because I'm not sure how it connects to the engine. I know it's supposed to use vacuum though. Not sure what other things to check for vacuum.

As you can see, I've done lots of testing before I came to ask for help. I understand this motor pretty well by now after 10 years of ownership. But I don't know what else to check.

I figure to an experienced person, all this info should point to something. Any ideas?

 

Cap & rotor: You can't tell if they're good by looking at them. There are some brands that cause no-start problems right out of the box! Really needs to be AC Delco cap & rotor, with a dab of silicone dielectric on each terminal, (inside and out).


EGR valve: "pintle moves freely and easily" This is an all electronic EGR valve using nylon gears to operate the pintle. Plain and simple, the gears will break if you attempt to move the pintle by hand. If the pintle moves freely, the damage is done and the valve needs to be replaced. That could easily explain why cyls 1 thru 4 misfire, and 5 & 6 do not. 5 & 6 are the farthest away from the EGR discharge tubes in the plenum.

 

At this point I have actually seen each plug fire and appear to fire correctly by putting the timing light on each plug wire. There weren't any skips or unusual behavior. I realize there are a billion things that can cause a misfire like this and I am going to have to sensibly start ruling them out one at a time. I think I can rule this one out safely. I think its also safe to assume that if the cap screws up at 625 RPM (idle), then it would continue to screw up at simply doubling that RPM which isn't *that* much more (around 1,500)

As for the EGR, I agree it does seem a bit suspect. And the behavior of this random misfire does seem, in my opinion, to be vacuum related. But I extensively tested the EGR by following the test procedures in *both* of the following:

Part 1 -How To Test the GM EGR Valve -Buick, Chevy, Olds, Pontiac

http://www.tomco-inc.com/Tech_Tips/ttt30.pdf

The EGR passed all the tests. The other thing is that the EGR has lots of codes that'll spit out if it messes up. Some are:

1. P0401 EGR System Flow Insufficient.
2. P0403 EGR Solenoid Circuit.
3. P0404 EGR System Performance.
4. P0405 EGR Pintle Position Circuit Low Voltage.
5. P1404 EGR Valve Stuck Open.
6. P1406 EGR Valve Pintle Position.

Well, I have never, ever received one of these codes in that vehicle. I was actually able to apply 12v to it while it was unbolted from the car and was able to see the pintle go in and out as it is supposed to. So its not internally damaged and if instructed to by the PCM will move accordingly. I did notice one thing that was strange though. In the first link above, you will find "TEST #2" on page two. It simply tests whether the unit is getting 12v on Pin "E" of the 5 pin connector. Well, I got different readings with the plug plugged in *and* unplugged?!? Check this out: When the connector is plugged into the EGR, the volt meter did not show 12v. But if you UNPLUG the connector from the EGR, the volt meter reads 12v! at Pin "E". If you plug it back into the EGR, it will again not read 12v. How the hell can THAT happen? So, I bolted it back into the car, started it up, it ran fine, then I applied 12v to Pin "E" while its idling and the car immediately died. So I know it actuated it like it should. If the EGR were not operating as it should, the car would run like crap and I'd get an EGR code. If it weren't seating correctly due to carbon build up at the port, it would allow air to get in (a vacuum leak) but its completely clean and seats properly.

I am open to further testing as I want to nail this problem, but since I've never gotten an EGR code, it seems to test okay and work like it should, I don't know what else to do with it? Some people remove the EGR and plug up the holes. But I dont see how plugging the holes would help me test it any further. The only thing I know to do is to bolt on a new one and see if the misfires change at idle (which is the only time it misfires). But where am I gonna get a new one without having to buy it? They're around $150 so that doesn't seem like much of an option.

 

I have an update on what seemed like odd EGR pinout test behavior. In the test, the author of the article in the first link above does not differentiate between plugged and unplugged for showing 12v at pin E. This is an error on his part and he should correct his test procedures.

Here's why: If you UNPLUG the EGR, you also remove the sensing wires from the circuit back to the PCM. (These are the three wires in the middle of the plug, pins B, C, and D.) This means the PCM has no idea in what position the EGR is in as long as its unplugged and therefor it feeds it 12v to move it! When its plugged in, the PCM knows the position of the sensor (showing a closed pintle) and therefor turns OFF the 12v PWM signal because its not SUPPOSED to be open in the first place! So again, the EGR in the car is fine.

 

Your big clue was no EGR DTC's. If there was a problem, a DTC would have been set. If you unplug the EGR connector when the ignition is in the RUN position, that alone will set a DTC.


Have you performed the crankshaft position sensor relearn and made sure camshaft retard is within spec? Those two things enable the PCM to detect, and accurately identify cylinder misfire.


If the timing cover or crank sensor are moved, removed, replaced, or disturbed in any way, the crankshaft position sensor relearn MUST be performed. If it's not done, the engine will still start and run, but ignition timing and injector timing will be incorrect. Also, if the distributor is moved, removed, replaced, or disturbed, camshaft retard must be checked and adjusted if necessary. Both of these things can cause P0300 to be set.


EDIT: Using a timing light at idle, (no load on the engine) will not detect a problem while the engine is under load, (the most likely time for a misfire).

 

Yes, thats why I ruled out the EGR because I did not get any EGR codes and I had tested it. Yes, my earlier post mentions CMP Retard was literally at zero degrees. I'm not sure what a crankshaft position sensor relearn is. But I did disconnect the battery for about an hour so that the PCM would do all it's relearn stuff. Would that do the relearn that you are referring to?

I had to use the timing light at idle because thats the only time I get misfires. The scanner reports no misfires at higher RPM, free way speeds, or engine under load conditions. They only occur at warm/operating temperature idle.

 

Disconnecting the battery is basically a waste of time. It does nothing more than lose the preset stations in the radio, reset the clock, and turn off the SES light. If there are no hard faults in the OBDII system, the SES light will remain off. If the light was on due to a failed monitor, the next time the monitor runs, the light will come back on. If there are any DTC's in memory, they will remain there, as will any freeze frame data. Crank relearn is initiated with a capable scan tool.

 

That doesn't really answer the last sentence in my prior post...lol If the answer is No, then what do I need to do? Do you have a link that shows procedures?

 

Sure it does NOTHING is relearned by disconnecting the battery.

 

The relearn must be done with a capable scan tool. The $500 El cheapos at the auto parts stores are not capable. After the scan tool is programmed to the vehicle, the relearn menu is accessed. It will give the safety instructions first, (apply the brake, engine at operating temp, trans in park, etc). Engine RPM is slowly brought up to 4K. At that time, the PCM takes over throttle, performs the relearn, and returns throttle back to you. Done in less than 5 seconds.