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2005 chevy trailblazer_lt_ext
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Hello all,
I have a problem with my 05 TB 4.2 EXT LT (160,000 miles). Here is the short story:
I did an engine swap after the original engine needed a valve job and original block was damaged beyond repair trying to remove broken head bolts.

Got an engine from salvage yard.

Removed head and installed new valves, timing chain kit, water pump, Camshaft sensor, coils, spark plugs, exhaust manifold, cat, 02 sensors, new head bolts and all the gaskets.

Once engine was replaced it started right up but with a P0300 that I cannot figure out how to fix. Flashing check engine. Rough idle and drive. gas smell from tail pipe.

Here is what I have done:


[*]Checked compression - all are 170 - 175 psi
[*]Checked fuel pressure - 60 psi
[*]Replaced ALL coils and plugs - all have spark.
[*]Replaced intake gaskets and made sure that all the bolts were tighten to specification.
I am at a loss here and was looking for direction in fixing the P0300.
Also, P0300 is the only code that is coming up.

Thank you in advance for your help.
 

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I know this is an old thread but did you ever figure this out? I'm in almost the exact same situation and I cannot figure it out. Any help would be appreciated, thanks!
 

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Just spitballing this here ----

I had this kinda problem from a vehicle where a guy did a junkyard install (why-o-why? Junkyards are places to buy cores, not search for running or useful parts) --- and when he took the old engine out, he let coolant get into the exhaust system - killing the O2 sensors.

.... just spittin' at ya ..... like I said.......
 

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Thanks for the spitballing. What mine turned out to be was kind of crazy but it may help someone else, hopefully.

Mine started after I replaced the timing chain. I fired it up and it purred like a kitten. I let it idle for about 5 minutes and the check engine light came on. It turned out to be the coolant fan which I forgot to plug in. Plugged back in, fired it up and it ran like ****, PO300 on the scan tool. I started out with the usual looking for vacuum leaks, I put the old plugs back in in case I cracked a few of the new ones, and then I started unplugging coil packs one by one. I found that when I unplugged #1, #2, #3, and #5 it ran worse. When I unplugged #4 and #6 the idle didn't change. So now I'm thinking coil packs. I swapped them around but nothing changed, the idle didn't change when I unplugged #4 and #6.

So I call a mechanic buddy of mine and he said because of all the work I did on it and the symptoms I was describing that the timing chanin had jumped a tooth. About the only way to check that is to pull the oil pan again. I ended up buying a small camera, 4.5mm I believe, pulling the valve cover, and painting the timing marks pink so they showed up on the camera. It is a pain in the ass but you can thread a small camera down the timing cover and see the timing marks. Everything lined up....****, now what?

My next thought is bad or clogged injectors so I pull the fuel rail to swap injectors around and that's when I start looking at the valves. Something seemed off about the #6 intake valves. On further inspection the farthest back roller rocker had fallen off of the valve train. How you might ask? Well when I reinstalled the fuel rail from when I had pulled it earlier to clean the intake manifold mating surface on the head the #6 injector plastic collar hadn't come off with the injector. No big deal I had figured, I'll just lube up the end of the injector and tap the rail in gently with a rubber mallet. Well what happened was I forced that plastic collar into the intake port and it must have bounced around in there for a while until it became lodged between the valve seat and the valve, permanently holding the valve open. The roller rocker then fell off because it is only held on by pressure.

I removed the plastic collar carefully so it didn't fall into the cylinder and reinstalled it on the injector as it had no damage if you can believe that. Put everything back together and BAM, runs like a champ again.

Now I doubt this will happen to anyone else but if you are getting a PO300 and you can't narrow it down to plugs or coils or injectors or loss of compression, etc. Check your valve train for broken springs or bad valves.

Oh, I almost forgot to mention, I had done a compression test during all this and even with the valve being held open it was still 170, low, but within spec.

I hope this helps someone, good wrenchin everybody!
 

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A random misfire code usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty fuel injectors, low fuel pressure, a weak ignition coil, bad plug wires, or compression problems.
 
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