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Discussion Starter · #1 ·
I have a 01 Blazer 2dr, 2wd,5spd, 4.3l, and the original motor blew up so I put a 4.3 from an Astro Van in it.
The van was running perfectly fine before someone rear-ended it, and i was able to snag the motor.
The van was automatic but everything looked the same, i plugged everything in, and it ran fine.
Then about a week later it started random misfiring when under load. Going up hills, even straightaways, i was chasing this random misfire for years.
Replaced full distributor assembly, coil, plugs, wires ,fuel pump and filter, spider injector assembly, crank sensor, and I'm still getting that intermittent misfire. I did take it to a shop to have the timing re-learned after replacing crank sensor and distributor. Checked timing and compression, everything is good.
Then i was decided to go over the wiring harness thoroughly and i found this plug just hanging back by the distributor, i looked all over and i couldn't find anything that it would plug into.
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The numbers on the plug are PED41.
I think there was something on the original engine from a Blazer that isn't on the Astro Van for some reason.
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The last picture is where the plug comes from on the wiring harness.
The green mark is on the main harness, the blue is on the distributor plug, and the red is on the unknown plug.
Hoping someone knows where that plug goes.
Thanks!
 

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That's the knock sensor connector for the later model 4.3 S-series engines which use a 2 wire non resonant KS.
It's located below and behind the distributor. Your other engine may not have had that style KS. All you need to do is drill and tap the back end of the block with the correct drill and tap for the bolt that holds it and make a flat surface for it to rest on. Darned difficult to do in the truck. IDK if that could cause your missing but w/o it connected your timing could be off since the PCM uses it to figure out how much advance it can get away with before knocking occurs.
Since you have the good 0411 PCM it may be possible to have it programmed for the earlier type sensor. You'd have to ask a tuning Guru.
Your original 2 wire KS looked like this:
It is held on by a metric bolt.
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The earlier single wire resonant KS looked like this:
It has an SAE pipe thread.

350974
 

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Discussion Starter · #3 ·
@odeerslayer that was it!
I felt around on the backside of the block and found the sensor with my fingers, and plugged it in! Thank you so much for the help!
The misfiring used to shake the truck insanely and make the MIL flash, and now its actually drive-able. But the misfire is still there. During normal around town driving im getting random misfires on all cylinders except 2 and 5. And when im in 5th gear at around 60mph, I'm getting like 300+ misfires on cyl. 3. Looks like im gonna need to do more digging to figure this out. Might make another post lol
But again, thank you SO much odeerslayer for helping me with the knock sensor!
 

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With a miss at every where except the far corners of the intake, I'd put the fuel pressure regulator high on my list of suspects. If it's leaking extra fuel into the middle cylinder that would explain your misses. You can check it by doing a fuel pressure leak down test. After reading your fuel pressure KOEO turn off the key and see it the pressure is holding. If it decreases relatively quickly your regulator is leaking. Not too bad to replace. Just need to take the top off the intake. Get an intake gasket set.
If you didn't replace the lower intake when changing engines you might want to consider a complete intake set.
Get only the very best ones, Mahle MIS16168 or Fel Pro MS98002T and just hang onto the 2 lower intake to head gaskets. You'll likely need them someday because the OEM ones usually fail sooner than later. It also appears that neither company is including the little seals and O-rings included in the complete set if you buy just an upper intake set. They used to include them with either set.
 

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Discussion Starter · #5 · (Edited)
Did the leakdown test. Started at 50psi and went down to around 44psi after about 4min. Im guessing thats pretty good.
I talked to some mechanic acquaintances of mine and they say it might be a sticking valve, because the misfire is so intermittent.
I also have a P0453 EVAP pressure sensor/switch high input code, but they say that wouldn't cause my misfire
Another moment of shame for me, when i was installing this engine, i somehow cracked the threads on the EGR port on the lower intake. So instead of replacing the intake i just screwed the pipe in as much as it would go and threw some JB weld on it. Not sure if that would cause misfires but it is something that came to mind.
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If 50 psi was all the fuel pressure was with the key on, that's your problem. Minimum is 60. Dropping to 50 when you turn the key off isn't good, but not unheard of. 4 minutes to 44 is OK. I'm surprised it starts with only 50 psi. That's usually the point where they just aren't getting enough fuel to start. If 50 is the peak you need a new fuel pump. I just put one in my ZR2 and it runs a lot better. It was to the point where it wouldn't start when cold.
The EVAP sensor is just a test sensor in the gas tank and won't effect the performance. Your fix of the intake shouldn't be the cause either, because that little front section of the intake manifold is separated from the rest and just has exhaust in it. If the crack extends beyond the exhaust section it could be causing a vacuum leak which would effect the performance. Spray a little starting fluid or carb cleaner in that area when it's idling. If the idle changes then you have a leak. Be careful to not overdo it. Both those fluids are highly flammable. I'd do it on a cold, just started engine so that none of the exhaust is real hot yet.
 

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Discussion Starter · #7 ·
So i positioned the fuel pressure gauge so i could see it when i was turning the key on, and it looks like it jumps up to around 58psi as soon as i hit the key, and then instantly goes down to like 55psi, then gradually drops to around 46psi over 5min.
I sprayed carb cleaner by the EGR pipe during idle and nothing happened so we're good there i think.
A mechanic i know said that my issue could be a sticking valve, because he said he heard loud tapping when it misfired, but i cant hear it when ive got the hood open...and i cant hear it when im driving because im inside the truck.
Maybe an issue with the ECU?
 

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ECU's rarely fail. Not saying it's impossible, but not likely. Especially for a single cylinder misfire. Are you getting any codes?
 

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Discussion Starter · #9 ·
I was getting a random misfire on all cylinders before i plugged the knock sensor in. After plugging it in and driving it around the block, it seemed to be driving ok, so i erased all codes, now im just getting the P0453 EVAP pressure sensor/switch high input code, no misfire codes.
I am seeing the misfires while im driving thru the live data option on my scanner
 

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Discussion Starter · #10 ·
mind you, i am getting misfires on cylinders #1, #3, #4, and #6. its just that #3 is the one that misfires the most during high speed driving
 

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Replaced full distributor assembly, coil, plugs, wires ,fuel pump and filter, spider injector assembly, crank sensor, and I'm still getting that intermittent misfire. I did take it to a shop to have the timing re-learned after replacing crank sensor and distributor. Checked timing and compression, everything is good.
You have done everything I can think of to eliminate the random misfiring. That type misfire can be hard to diagnose. As you are painfully aware. Any one of the new parts you listed could be substandard or defective.
Not much else. The theory of a sticking valve or bad lifters would have to be four of each to get misses all over the place.
The only thing I can think of if you're willing to try something else is another fuel pump. Reason I'm suggesting this is my 98 ZR2 with a stroked Vortec V8 in it wasn't running quite as well as I would have liked, but wasn't setting any codes. I replaced the pump with a Quantum 340 lph drop in assembly for $150. The fuel pressure immediately jumped to 62 psi and held that. The truck runs much better. I'm theorizing that the better flow and pressure made the difference. Not saying it is a cure all, but might be worth a shot since you've pretty well exhausted all other possibilities.
Other possible cures would be a new ECT sensor because if that is off so would your fueling, but usually they go bad totally or are OK. Have you considered replacing both O2 sensors? Any leaks in the exhaust ahead of the sensors? Then there is the possible clogged cat? If you only get the missing at speed or under a load that's worth looking into.

The P0453 is either the sensor or the wiring to it. Should have no effect on performance or driveability. Strictly a CA and tree hugger sensor.
 

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Discussion Starter · #12 ·
A clogged cat actually sounds like a possibility...is there a way to test if its clogged other than cutting it off and looking inside?
 

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Discussion Starter · #14 ·
well after 3 weeks of doing construction and farming i finally got a chance to test the cat, and its good. both voltages are doing what they're supposed to (as the video shows).
so now another moment of shame: Ive had a P1638 Generator F Terminal Circuit, and a C0265 Electronic Brake Control Module Motor Relay Circuit codes on this car for as long as i can remember.
I didnt mention it when you asked me if i had any codes, i only mentioned the P0435 code because i thought that would be the only performance-related code.
I did some research on the P1638, and read that the PCM can control engine speed to compensate for high electrical loads, and the red battery light just comes on and turns off randomly. The system is charging fine but the scan tool shows me a "Generator F Circuit - 0%" the whole time,even when the red battery light flashes off. Honestly have no clue if this relates to my issue in any way, but i really would like to have that red battery light off my dash.
Obviously solving this misfire is top priority so im just gonna take your suggestion and throw a Quantum 340 lph drop in assembly in the tank, pray to hay-zoos that fuel pressure is the answer
 
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