I scanned through this thread as I was curious about your issue in case it ever happened to me.

A couple things mentioned reminded me if issues that I had once with some much older cars I had.

First was that you were having misses at high rpm. Was this while accelerating and under load? Second was that you saw some electrical arcing under the hood.

In my 68 Mustang, I once had the symptom of the engine missing when at highway speeds while accelerating, e.g., to pass someone. Turned out it was a bad coil. It was partially shorted so that the voltage generated was reduced. When cruising, the pressure in the cylinders was lower and the voltage was sufficient to make a spark at the plugs. But when accelerating and the pressures were higher, the voltage was not high enough and I lost spark. A new coil fixed it.

The second incident was with my 71 Dart. (Boy I miss that car!) It had the 318 and so the spark plug wires got close to the exhaust manifold. Some of these wires got damaged from the heat so that spark woukd arc to the exhaust manifold causing a misfire.

Applied to your case, my first thought is that there never should be arcing from any of the ignition components. With our solid-state computer-controlled ignition symptoms, damage to the circuitry could be possible. I would suggest this needs to be fixed no matter what.

But, I can imagine this arcing could be related to your symptoms. High voltage arcing is hard to predict since it is so dependent on the placement and shape of conductors and the properties of the air through which it might pass. With this in mind, I can see arcing happening preferentially that short-circuits some cylinder spark plugs and not others depending on where the wires run and the driving conditions.

Good luck and be sure to let us know what works.

 

Does your clacking sound like this?

 

I did some more tests related to fuel trims, and they are around 0-2% short-term, but they are consistently at positive 20% on long-term trims, which I assume is proof of a vacuum leak somewhere. I have checked all the spots around the upper intake for leaks with brake cleaner on the working engine, the only small one that I could find was on the MAP sensor, which I now replaced.

Unfortunately the issue is still there, I ll look into coil next, but it sure does look like a vacuum leak to me, per fuel trims and intake pressure that goes down when you throttle, instead of going up (on IDLE), surprisingly it goes upcorrectly when I drive in the city (most likely cause throttle body is not fully closed as on IDLE and vacuul leak more forgiving)

 

Hey Christine! I really appreciate the stories you shared, that's a good points to look at, but I think my issue is slightly different at this point, cause I am getting misfires only after high RPM's, when it goes down from lets say from 3000 to ~500 instead of working 600, and that exactly when I get misfires, but when I keep holding the throttle on high rpms, there are no misfires at all

So some additional info that I gathered so far: misfires are erratic, and they happen all across all cylinders on IDLE, and when I throttle to ~3000RPM, the most amount of misfires go from cylinder 6, but the rest are affected as well

Judging by: long-term fuel trims, misfires across all cylinders, and the pressure in the intake goes down from 35kPA to ~20kPA or less when I throttle on IDLE, it sure does look like a vacuum leak to me, and that's why it's more sensitive on IDLE, because the throttle plate is fully closed

So, since I have tested all possible vacuum spots around the upper intake with brake cleaner and replaced the MAP sensor, I am coming to the conclusion that it's most likely a cracked intake manifold gasket (which is quite a problematic spot on s10's, plus keeping in mind my truck has a bit over 200k miles on the odometer, it must be it)

Or as an alternative, if we include the clanking noise that can be heard when misfires happen (especially when the engine is hot), it also could be worn-out hydraulic lifters, which is also a weak spot of Vortec engines

 

On the video you shared, it sounds like hydraulic lifters, isn't it?
The clanking noise that I have occurs most prominently when the engine misfires or when the rpm's go below working 600
Here is the video:

 

Arishynb, yes after reading what you wrote, I see why you are focusing on the vacuum leak theory. If you are interested in keeping your Blazer, it would be worth doing the lower intake manifold replacement anyway. But it sure would be nice to know what the cause is first, right?

Have you looked at the condition of the spark plugs? As you very likely know, on at older cars, their condition can tell you if you are having an overly lean or rich condition. I don't know if those old rules apply to engines with ECMs though. This was the clue that helped me figure out that I had a massive vacuum leak on my 82 Prelude a few years ago. It might be worth checking if it is easy for you to do.

BTW, the only reason I'm offering advice is because I'm now invested in your story and hope you can figure it out and definitely not because I think I have all the answers! LOL

 

I am most certainly interested in keeping my Blazer, it's in "medium sage green metallic", quite rare or I just couldn't find many of these in this color, beautiful dark warm green, looks astonishing in the sun, love it!

But yes, good point! I do believe the OEM intake manifold gasket is made out of plastic, and they are quite fragile; that's why they fall apart, especially on such high mileage. It is usually replaced by a higher-quality metal-based gasket

I v changed my spark plugs about a month ago, old ones had burn marks all over it, up to ceramic part, tops were not ugly but all black, also ceramic parts of a couple of spark plugs cranched into small pieces while removing, which either means there weren't replaced for a long time (before I bough the truck), or they were significantly burned

Assume spark plugs overheat due to a lean air-fuel mixture caused by a vacuum leak. Lean mixtures burn hotter, increasing combustion temperature and stressing the plugs, but I might be wrong

Thanks a lot for your contribution, I will check my new plugs too, see if the condition is getting about the same or not

 

In order to use fuel trims as an indicator for a vacuum leak you need to do two things: record fuel trims at idle and again at 2500 and always add st and lt trims together for a given bank.


George

 

Hey George, thanks a lot for the additional info
I did try to measure it, it seems that on high rpm's they are around -2% on bank 1 and +3% on bank 2 (short-term), but consistent +20% long term
Does it says anything?

Thank you

 

Supply the info in post #18 and I’ll see if I can help

Your also going to need to graph the o2 sensors in live data

George