Idle Drops Then Returns While Driving Only When At Operating Temperature

I have a 1999 Olds Alero with the 3400SFI V6 about 173k on Engine (up until rebuild) and 65k on transmission.

So a while back I fitted a Pacesetter Cat-Back exhaust to my car, about a month later the RPMs would drop to idle then come back up (while driving at operating temperature) then a week later it progressed to the RPMs sticking at 3000 once that RPM was reached and refusing to accelerate further. Then about a week later, the engine gave up the ghost, the rear center piston looked as if it exploded, the head was ruined too (valve was pushed in and holes in surface of head). I went and got two new heads, and all new pistons and wrist pins, and rebuilt the motor. I retained the old belt drive components, the old timing components, oil pump, rods and bearings, crank and bearings, cam and bearings, injectors, intake components, and exhaust piping. New parts were pistons and wrist pins, used heads, valve stem seals, head gaskets, intake manifold gaskets (upper and lower), fuel injector seals, exhaust manifold seals, spark plugs, 7x crankshaft sensor, thermostat and gasket, throttle body gasket, and oil pan gasket (maybe forgetting some things).

About two weeks ago now, the car began dropping RPMs to idle and then back. But only when in motion and at operating temperature. I haven't driven it since it began again, for fear of blowing up the motor again. My suspicion is a bad lifter, I know these have hydraulic lifter and when the engine gets warm and the oil thinner, I think maybe a valve is closing or staying open because the lifter is collapsing or sticking open. Just a suspicion though. Either that or the timing chain as the chain and the lifters were the only two things not really changed. The motor sounds completely normal no ticking or scraping. Before it blew I heard piston slap and lifter tick, the slap is gone now, but the tick is only present when at operating temperature. Only code stored is for the 24x Crankshaft Position Sensor and that has been there since I've owned the car. Any help would be greatly appreciated as I've spent so much money on this thing as of now and to just have it sitting here is quite depressing.

~Thanks!
 

melsg5

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I'm a little puzzled you had the engine completely apart at 173k miles but you kept the old oil pump, old timing chain, old lifters, all the old bearings, old cam. When the piston let go it generated lots of small pieces of metal which would have gone all through the engine.
You can try a compression test but you should not have left all those old parts in the engine.
 
I'm a little puzzled you had the engine completely apart at 173k miles but you kept the old oil pump, old timing chain, old lifters, all the old bearings, old cam. When the piston let go it generated lots of small pieces of metal which would have gone all through the engine.
You can try a compression test but you should not have left all those old parts in the engine.
I'll try to do a compression test, I was going to replace all that stuff, but due to unfortunate events, the budget got cut drastically, and the things that didn't look bad and still was within spec, stayed. Only reason all the pistons got replaced was to prevent another old one from letting go further down that line.
 

melsg5

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you disassembled and cleaned every lifter and the oil pump to make sure there were no metal particles in them?
you measured the bearing clearances and determined you didnt need new bearings?
I understand the financial aspect but it might end up costing you more due to short cuts you felt you had to take.
let us know how the compression test goes.
you are sure you have no other codes?
you are sure the engine timing isnt thrown off by the crank sensor causing detonation which destroyed the piston?
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you disassembled and cleaned every lifter and the oil pump to make sure there were no metal particles in them?
you measured the bearing clearances and determined you didnt need new bearings?
I understand the financial aspect but it might end up costing you more due to short cuts you felt you had to take.
let us know how the compression test goes.
you are sure you have no other codes?
you are sure the engine timing isnt thrown off by the crank sensor causing detonation which destroyed the piston?
All lifters and oil pump were removed and cleaned (although no metal was found).
The clearances were measured, and everything was in spec.
Only code stored or pending is the 24x Crank Sensor code (it's there because the ring inside the harmonic balancer is gone) the code has been there since i owned it and its never caused a problem.

A member on another forum also reccomended that I removed the O2 sensor and see if the problem goes away. They said if it does then the Catalytic Converter is plugged (which is probable considering the entire contents of the cooling system went the cat)

Will try a compression test and try removing the O2 sensor and let you know how it goes.
 

melsg5

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I guess no emissions tests in your area, easy fix.
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Well, this is my new problem. Went out to run an errand in what I thought was my now working, nice sounding (but not smelling) Alero. The car got all the way up to what I recognized as normal operating temperature. And every single time the engine tried to come to an idle, the RPMs bounced and then the car would die. I had to be quick to throw the trans in neutral, start it up then back in drive before it died again. I took a video but its very fast because I was at a stop sign with people waiting behind me (how embarrassing). I mean every single time that I came to a stop, the RPMs bounced then the car died. I even tried putting it in neutral and coasting to a stop but it still done the exact same thing. This is without the AC on so as to have little load on the engine. This morning I pulled the throttle body and cleaned both it and the IAC valve. They were both pretty dirty, but they cleaned up pretty well. Is it normal for the IAC to only move a very little amount? But anyways cleaning the IAC seemed to make very little difference.

Here's a link to the video: https://youtu.be/nBxImZ5RR88
 
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