The infamous P0306!!!!
 

So, now that I have replaced just about every sensor, pump and injector on this vehicle, I'm now getting this P0306 error code. I have a new coil, cap, rotor, plugs and wires (all AC Delco except the wires, they are Autolite Professionals recommended by the Captain).

The error comes and goes along with a slightly rough idle while it's in drive.

Oh, and I just recently replaced the air charge temperature sensor because I was getting a P0113 error code as well. Well, I'm still getting a CEL for that one too.

The only thing I haven't checked is for a vacuum leak or cylinder compression.

Could these two some how be related?

The last time I checked my mileage I recorded 18.9 mpg (this was after installing new injectors and a fuel pump).

check #6 spark plug wire. got hot on exhaust and is grounding. intake gasket causing a vacuum leak. could be quite a few things...
:icon_twocents:

Changed out spark plug and wire looks good. Re-torqued the intake manifold bolts just to make sure there was no vacuum leak. I'm at a lost.

Anything else?

Retorqing the bolt probably wouldn't fix a leaking intake gasket. Compression test?

The reason I re-torqued the bolts because I just recently replaced the fuel injector assembly and I wanted to be sure I didn't over look one of the bolts.

I did find a small vacuum leak at the PCV elbow. After replacing it, the idle smoothed out a bit. Now, I'll reset the CEL to see what happens next. If this isn't the problem, I guess a compression test will be my next course of action.

oh, ok. were you into it far enough that you had the whole intake manifold off? the lower intake manifold gasket is right at the head and can be problematic is why i ask.

When did the misfire and P0306 start showing up? Was it just after doing work to it? What was replaced before it started missing?

Hey Captain!

Well, I replaced the injectors and fuel pump about two weeks ago. Everything was running great until the truck over heated lasted weekend.

By the time my wife was able to pull it off the road, the thermostat read 260! The upper radiator hose gave out (looked original) so I replaced it and the radiator (found a small leak). Since then, it's been idling rough, again and I think I know why...

I just came in from checking the compression for each cylinder and I just about threw a wrench at the garage when I came to number 6. All of the cylinders read somewhere between 148 and 151 with the exception of cylinder 6. It said 100 - twice.

So, I'm at a point now where I think I'm going to count my blessings and get rid of the vehicle.

But, if by chance a buddy is willing to help me work on it, can cylinder 6 be re rung from the bottom side? Or, do you have to pull the motor to do something like this?

Did the hose & radiator blow and then overheat? Or did it overheat and THEN blow the hose & radiator? I know.... it's hard to tell. Did you notice any coolant leaks before it overheated?

If a head gasket sprouts a leak into a coolant passage, you end up with compression pressure entering the cooling system... hoses and radiators are not designed to handle that kind of pressure and POOF, they blow out. If you fill the cooling system and put the radiator cap back on tightly, does the upper radiator hose pressurize immediately when the engine is running? Like to the point of being rock hard? Any white smoke from the tail pipe?

She noticed smoke first, then I saw the temperature gauge sky rocket.

The old hose was really worn around the end where it connected to the radiator (the clamp couldn't hold it on very tight) and the radiator itself had a small piece of plastic broken off (that's why I bought a new one).

No, I never really noticed a coolant leak before the vehicle overheated.

Okay, I just cranked over the engine to check the upper hose and it's still soft. And no, there is no white smoke coming from the tailpipe.

I also just ran the compression test on cylinder 6 again, about ten minutes ago, but I added about a table spoon of oil into the cylinder. The readings came up 120 is stead of 90 like the first time.

Come to think of it, the truck does run smoothly in the mornings or at least until the engine is completely warmed up. Does this have something to do with the oils viscosity?

If the radiator hose remains soft, and no smoke from the tail pipe, I'd say the head gasket is OK. Really rare for them to be a problem on the 4.3L engine. Most of the time they're replaced due to a misdiagnosed leaky intake gasket, which gets replaced when doing the heads anyway, presenting the question: was it head gaskets or just an intake gasket? ;)

I was just going to suggest a "wet" compression test, (using oil). The oil helps seal the rings during the test... good job ;) Compared to the other cylinders, 120 is still a little low. Could be any number of things causing that, but if it runs smooth cold, I'd look for something other than the compression issue causing the rough running when warm. During cold start ups the PCM operates in closed loop. It ignores oxygen sensor data and applies a fuel mixture based on data from MAP, MAF, ECT, & TPS sensors. When the PCM switches to closed loop operation, fuel mixture leans out. If there's a vacuum leak affecting cylinder #6, it will run even leaner and could easily have a lean misfire, and NOT show up as a lean DTC. It's a stretch, but worth looking in to. Could be a leaky intake gasket around #6 intake port. Could be internal or external. Using an analog tachometer and propane is probably the easiest way to detect it. For an external leak, direct the propane along the intake gasket and watch for an RPM increase on the tach. To detect an internal leak, remove the PCV valve from the rocker arm cover and insert the propane nozzle into where you pulled the PCV valve out. It will take a few seconds for the propane to arrive at the leak, if there is one, and the RPM will increase. SO, for the sake of safety, put a 4 or 5 second shot of propane in and then plug the hole with your finger. Don't fill it up with propane!



As for oil viscosity making a difference: Oil does thin out as it warms up, and the oil pressure drops slightly as a result, but it shouldn't make a difference in how the engine runs.

I know the MAF sensor is okay, or at least it was before the incident. The ECT was changed out last Saturday as well as the MAP sensor. This leaves the TPS sensor. I'm pretty sure I have over looked this sensor.

As far as testing with propane, I'm letting the vehicle warm up now. Then, once it's at operating temperature, I CAREFULLY follow your recommendations.

Ahhh, just had another idea...... The MAP sensor plugs into the plenum right above the #6 intake runner.... if the seal/gromet is leaking, it could cause #6 to run lean. Again, just a thought ;)


Attachment 32099

Okay. I

So, I did the propane test. Nothing. I even had the mrs help me out but I think I may have found a temporary fix.

I was doing some research on cylinder compression and found a few websites which raved over the results they had using Restore. Well, after reading all the opinions I decided 10 bucks couldn't hurt.

After driving around town today with a 20 oz bottle of Engine Restore poured into the crankcase, the truck ran very well. In fact, there was almost no rough idle at all while I would sit at a traffic light waiting for my turn to go. I haven't checked the compression again but I plan on doing that tomorrow.

Why would a product like Engine Restore work? Does it just temporarily apply a thin sealant against the cylinder walls? Is there a product like it which is made for a more permanent solution?

Difficult to say what caused the improvement. There's always the possibility that a valve lifter, (#6 cyl) got pumped up and couldn't bleed down when the engine overheated. If that happened, it would explain the low compression, and the Restore may have freed it up. Typically I'm not a believer in additives, but if it works, cool. Put some miles on it and see what happens.

Well, after about 300 miles I'm getting the same code, again. The Restore must have done something for awhile but I guess it was just temporary. I haven't checked the compression on cylinder #6 yet but I'm also getting P0102 as well.

I checked the MAF sensor out yesterday per the guide below and ended up replacing it last night. Now, I'm still getting the P0102 code and P0306.

Part 1 -GM MAF Sensor Test (P0101, P0102, P0103). BAD MAF Symptoms.


Would a bad O2 sensor have anything to do with this problem? I replaced the one down stream of the catalytic converter a few months ago but the other two look like they could be original sensors.

What about a faulty ECM? If I replace this, do I need to have it programmed?

Does anyone have any connections with a St. Louis auto shop? I think it's time to take this truck in for a looksy. I've spent alot of money over the past several months replacing sensors and parts just to get stuck on these two codes.

:(

Not sure where you found that test procedure, but it obviously didn't find or fix the problem ;) This is straight out of the GM factory shop manual, follow the flow chart exactly as written, don't skip any steps, and it will take you to the problem. https://blazerforum.com/forum/diagno...plained-28157/

I followed the instructions you provided and this time the sensor and the harness checked out okay. I'm going to reset the DTC and see if I get another code later on tomorrow.

If I take off the MAP sensor, will I ruin the gasket? This was another sensor I recently replaced but I just wanted to check the sealed connection.

I'm starting to think it maybe be a small vacuum leak near the EVAP canister purge valve. That I know is original.

YES. If the MAP sensor is removed, the seal needs to be replaced. Same goes for the fuel tank pressure sensor, (same type of sensor and seal). Use it once and throw it away.

Okay, so I got a friend to help me tear into the truck this past weekend to see if we could find anything under the valve cover.

Well, the intake valve isn't completely closing during combustion. After looking through the spark plug hole we could see a large amount of carbon build-up on the valves seat. Thinking this was a possible cause we decided to clean it as best as possible without taking the head off. It did help some but there was still a small leak after doing another compression test.

Later this week, we may go ahead and check out the lifters. From what we where able to see and measure, the stems to the valves checked out okay as well as the push rods.

I guess my question is this - which is more likely to fail, a lifter or compression rings?

If a lifter is pumped up and can't or doesn't bleed down, it would not allow the valve to fully seat. Measuring valve stem height would confirm that possibility. If the valve face, or seat, is burned, valve stem height would be within specs, but carbon could still build up. It could be something as simple as a piece of carbon got stuck between the face and the seat. Carbon build up is common in the EGR valve, could be a piece made its way to the valve. Rings are not a common problem with 4.3's unless they've been neglected on the oil changes. Also, if it were a ring, the engine would most likely smoke out the tail pipe.

Okay, I see what you're saying about the rings. So, it's either a bad valve or lifter. I didn't think I could be the push rod simply because this is not a race engine nor do I neglect my oil changes ;)

Well, we'll measure the valve stem height before we take the head off. Is there a way to verify a bad lifter with the head still in place? Wouldn't simply pushing down on the rod confirm this idea?

If a lifter is pumped up, and the valve spring can't compress it, you won't be able to compress it either! If that's the case, you can disassemble the lifters, (one at a time) and clean them out, you don't necessarily need to replace them unless they're damaged. You don't need to remove the cylinder head to remove lifters.
Measuring assembled valve spring height will tell if the valve is fully closed. If the height is less than spec, it could be a pumped up lifter or carbon stuck between the valve face an seat holding the valve open.

Replacing an ECM
 

I think the ECM is bad in my 1995 chevy blazer...So when buying a new one will it be required to get flashed?

Thanks -pblaze-

To avoid confusion, it's best to start a new thread here:
https://blazerforum.com/forum/newthr...newthread&f=41

Change your fuel filter and the problem will go away. Service engine will go off too.