S10 4.3 misfire after high rpm
 

Hi, I have been having an issue with my Blazer S10 2002 4.3 for a couple of months now
On IDLE, if you rev your engine to 2000 RPM or higher, when RPM goes back, it runs very rough, drops down below to around 500 or even lower. It recovers after less than a minute or so and runs relatively okay with rare shaking (under 600RPM)
I have replaced and done the following: MAF sensor, fuel injector, done throttle body clean, IDLE sensor, distributor cap, and then distributor as a whole, PCV valve+ PCV tube, engine oil with lubricant to clean the system, all filters, spark plugs, spark wires
It wasn't there before I started replacing parts, but now I also hear some metal clacking noise when it goes under 600 rpm, and it becomes slightly worse with time, and a bit louder on hot
No error codes, rarely P0300, and it also feels perfectly fine when I'm driving around the city, but it's noticeable on highways- it starts to flash check engine and goes away if I stop throttle
I'm losing hope at this point. If you have any ideas, please share them here. I would really appreciate any help or info

It started after which parts replacement?

Check your distr wire order

It would help to have misfire counts by cylinder along with your cam retard value

At idle in the dark, mist the top of the engine at idle and look for electrical arcing

There is more but lets start there.


George

Hey, thanks a lot for the response George
It seems that the issue had already been there when I bought the truck, I never dug into it cause I never used it for long trips until a couple of months ago
It showed P0300, P0304, P0306 time to time so that was where I started replacing parts(dist, sparkplugs, wires) errors were gone but main symptom was still there
One mechanic pointed and said 100% to the fuel injector so I replaced it, no results
When I asked the previous owner, he said he had the same issue but only when using LPG, which I never used, and removed it while installing the new spider injector- dont think it's related to LPG
Dist wires are correct, I made sure multiple times after installing- it won't start with wrong wires order
Heard about cam retard, but my scanner doesn't read that value, Wi-Fi OBD2, couldn't find GM scanners for iOS that can read it yet
I will try to look for electrical arcing, thank you!

Scanners for blazers:

ODBII Code Scanners for Blazers - Blazer Forum - Chevy Blazer Forums


George

Thank you George, I got DashCommand with GM add-on, works great!
So some additional info:
-CAM retard is fine, -1.4 on 1000RPM
-Electrical arcing in the dark, I have noticed few arcs, one on the dist wire 0 that goes from coil to dist, arc is where it connects to dist, and also one arc on the ignition coil side, both quite small and flashing dully but consist almost all the time - I doubt the issue is there cause misfire would be across all cylinders, wouldn't it?
-After reving, misfires are significant on cylinders 6 and 3 (mostly 6, 3 cylinder is most likely caused by PCV breather tube being is intentionally closed cause it was cut short by previous owner and was sucking dirt air, I couldn't figure out where it goes yet, not in the fender filter)
Thank you

Did you use AC Delco ignition parts? After market parts can cause problems.

So the PCV valve hose is just open? You may have a vacuum leak. What are your fuel trims at idle and at 2500 rpm?

George

Distributor and cap are "US Standard", wires and spark plugs are NGK should be top of the line, ignition coil seems to be replaced by the previous owner, can't tell if it's original or not

I have fixed vacuum lines on right side, PCV goes to PCV valve as it should, the breather goes to the air canister(fender) and HVAC, although the transfer case line is missing (nothing goes on the right from dist where it should go from transfercase), but no vacuum leaks from that side no more, 0% short term trims (it was 20% when breather was open), long terms are around 10-15% still

I have found an issue with the MAP sensor, it seems that the seal is no good, and when I press the throttle, pressure goes from 35 to ~20, when it should go up, gonna replace the seal with MAP, just in case, it definitely causes some of misfires but I doubt it's the root of the problem

Boris

If you still have a vacuum leak, the fuel trims will drop at 2500 rpm.

Unmetered air will cause problems (intake leak after the MAF). You can test the MAF reading. g/s at idle and again at the moment of a full throttle 1-2 shift.

An inexpensidve trial would be to replace the coil/distr wire with an AC Delco OEM and see if anything changes including the dark/misting experiment.

"Different brands of spark plug wires may have varying resistance per foot, and if the new wires are not within the acceptable range for your vehicle's ignition system, it can affect the spark quality and lead to misfires."

George

Interesting! So I don't see any short-term fuel trims on IDLE, although as I said, they are around 10-15% long-term, I will test them on 2000+ RPMs too

I have checked for any leaks around the MAF sensor, all seems to be good, data from MAF looks correct, but I found a vacuum leak around MAP sensor, seal seems to be no good and I also noticed when I rev to higher RPM's, pressure from MAP goes down instead of higher, which proves huge vacuum leak or incorrect data from MAP sensor- I have ordered a new one with seals, will let you know if that was the problem (It should fix random misfires on IDLE but I doubt it will fix initial issue with misfires on high RPM)

Wires might cause that, although I have ordered them through RockAuto, they were specified for Blazer S10, tested and liked by other people, plus that issue was there before I changed wires too, but that's a good point and I will keep that in mind- I heard a lot about dist and wires, that its better to keep them OEM at all time

Thanks George

Boris

You need both short term and long term fuel trims added together for a given bank for the reading to be useable.

George