I've got a 2003 4.3 4x4 Blazer with 76k that will do something utterly psychotic.

I just got back from changing the plugs, wires, cap and rotor with quality delco parts, all plugs looked fine and cannot be told apart, cap, rotor wires were all factory when replaced at 76k to try and resolve this issue.

Issue is that under HEAVY load (such as 0-80mph foot to the floor) Cyl 3 will rack up 1000+ misfire count, but the engine sounds fine, runs fine, PULLS /great/ with what I think is great power, and is very smooth.

Cyl 3 is the only misfire, and its repeatable every time, plug looked fine when pulled, can't tell it apart from the others.

I have received 2 "bank 1 sensor 1" errors for the O2 heating element, and I plan on replacing that sensor, however I am having difficulty blaming the sensor because no other cyl in the bank it services throws the same error.

The code does NOT set under normal driving, it was found to happen after almost a year of ownership while attempting to head up a STEEP incline on the freeway with a 30-40mph headwind.

Only seems to happen above 65mph, requires that the load be present on the engine for more than 10 seconds or so (hence the 0-80mph, only sets after about 70mph), the CEL flashes fast, but does NOT stay on, it goes away and stores a code P0303 with 1000+ misfires in the Cyl 3 misfire history.

It idles OK, it drives fine, it returns average gas mileage (16-18 mostly city) like it always has, there are absolutely zero drive-ability issues and it catches with the flick of the ignition key, first time, every time.

Emissions tests all pass after codes are cleared, except for the evap, which is forever pending...

.... What... in the hell!?

Alldata says injector/cat damage, but, it doesn't run hot, all the plugs looked fine, and it can cruise at 75-80mph without setting a code, it only does it if you REALLY put your foot into it and ask it to give it all its got under a load.

Sometimes I think it sounds like it has a bit of a 'tick', but its nothing at all like something others have recorded on video that I've seen as legitimate issues. It does not backfire, or smoke, or lack power, when commanded it will snarl, get up and run like its got more than 200 horses to move its big bulk.

I can get in and drive the truck without issues.... but it has... an issue?

Yes, I've changed the fuel filter, and the air filter. In my ownership its had a trans flush, coolant flush, diff fluid changed front/rear, transfer case fluid changed, and after the code, its had cap/rotor/plugs/wires all changed.

I'm lost, help...

 

GM has a TSB addressing this exact issue. Your description of the symptoms fits it to a "T". It is caused by insufficient valve guide clearance allowing the valve stem to bind in the guide, only under a load. The guides need to be reamed out a hair. Can't recall the TSB #, but a google search should locate it. I have the TSB on my PC, but will not be able to access it until later this afternoon. I'll check the thread later, and if necessary, post the number for you. A call to your local GM dealer might get you an answer, but don't hold your breath

 

Considering the local dealer was adiment that my engine did not have a distributor until I brought it in, I wouldn't hold my breath either.

This is just a TSB right, not a recall?

That sounds..... Expensive to say the least!

If you can nab it, let me know, I want this problem to go awaaaaay :c

 

Reaming the valve guide will require removing the cylinder head which can get expensive from a labor standpoint.

Here is the TSB:
SOURCE

If you know someone with a small head fiber-optic camera, you can diagnose the problem described in the TSB through the plug hole. Barring that, pulling the head is the only real way to see what's going on in there.

 

Way to go Kyle! Thank you!

 

If you really think it is not missing, you can check the vehicles ability to detect misses by inducing one and watching the misfire counter for that cylinder. if the OBD2 system can't detect an induced miss, then the P0303 is probably a false code and that's an entirely different can of worms. Even if it does recognize induced misses, doesn't mean the PO303 is real. False misfire codes can be caused by a variety of problems including but not limited to converter/front balancer, front accessories balance problems and thrust bearing problems. Mine was a mystery P0304 in a 2001 with 65K miles, however mine ticked heavily and oil pressure was low when warmed up. Mine was eventually proven to be loose bearings (very loose thrust) whacking out the crank sensor. Always a constant PO304 but couldn't recognize an induced miss on other cylinders. Had been worked on a lot, apparently attempting to fix this and I bet a crank relearn was done with the bad crank bearings further complicating things. I bought my creampuff with the SES light wire disconnected from the instrument cluster and taped.

Yours does sound a bit like the TSB for the P0304, but I assume a stuck valve would actually make it miss and I don't understand why a guide would suddently get tight after 74K miles unless it was carbon. Maybe a can of good old fashioned GM EOS (or it's modern equivalent) in the oil and down the intake would help clean out carbon on a valve stem? Back in the early 80's in the dealership I used to take low-mile Caddys with rough idles out back and would dump EOS down the intake to kill the hot engine while running about 2500 RPM. Let it soak for 30 min. Start up and blow it out (don't get too carried away with RPMs) until running smoke free. Then repeat another soaking. One more EOS in the gas tank. Screw in a new set of spark plugs, adjust carb as best I could, and tell them to come back in 1 week for final carb adjustment. Usually the idle speed would increase a couple hundred RPM and needed to be set back, along with the idle mixture needing readjustment. Never had a complaint. Guess this dates me pretty well! I can say that the early 80's was a very dark time to work for a GM dealership, with all the engineering abortions being put out. They finally got their act together a bit and then OBD2 was mandated.

 

I've had the truck since about 69k, but since I've had it, I've driven it pretty gingerly, and although I hauled a good 3k-lbs or so on a HOT summer day once with it, I kept it at or under 55 in 3rd gear... mostly because I don't like the idea of getting a new transmission, the first time I've really and I mean REALLY put my foot to the floor trying to keep up with traffic doing 65-70mph on what must be a 30% grade with a massive headwind (you can always smell unburnt gas from all the cars going up that hill).

It passed smog when I bought it in May of last year, doesn't seem to run any worse, its just something I drive sorta 'slow' and 'gentle' out of fear of the dreaded 4L60E explosions I always hear about, but I still want the /ability/ of putting the engine to use when needed, I bought it to haul stuff, tow things, and make use of the 4wd so not being able to make it **** *** when required negates its intended uses.

I kinda like this boroscope idea, those things are fairly cheap in comparison to pulling the head! What /exactly/ would I be looking for if I was to, you know, go buy one, pull the #3 plug, and take a look...

I'm not a big pushrod engine guy, I understand general diag and can grasp many mechanical concepts when presented with visuals or an explenation :C

The irony (get it, IRON block and head ;D) is that I bought the Blazer over anything else for the want of the bulletproof 4.3L that I'd not have to worry about opening up or have trouble doing diag on because it was 'simple' ;P

 

The link provided is to a ZR2USA forum post shows the attached picture. In that case, it was cylinder #5 (right most chamber on the head as pictured) that was having the difficulties.

 

Thank you good sir! Now to buy a borescope and weep softly at my tax return most likely disappearing in the near future!

If I have more to report soon, I'll be back and provide an update here, thank you!

 

I have never used them, but there are a number of small tip USB bore scopes on Amazon.com for pretty cheap. Anything under 12mm tip diameter should be able to get through the plug hole. Only downside to them is that you need a computer relatively close to the truck.