Idle Drops Then Returns While Driving Only When At Operating Temperature - ClassicOldsmobile.com


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Old August 8th, 2018, 08:14 PM   #1  
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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!
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Old August 12th, 2018, 04:52 PM   #2  
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I hollowed out the catalytic converter and the problem seems to have gone away. Will go about buying a Milzy Motorsports ECU with the O2 sensor code removed.
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Old August 14th, 2018, 02:56 PM   #3  
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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:
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Old August 15th, 2018, 02:41 PM   #4  
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what makes you think you can overlook a code for crank position sensor? if the computer doesn't get or gets an erratic signal from this it wont fire the ignition or injectors.
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Old August 15th, 2018, 03:04 PM   #5  
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Because it's been faulty ever since I've owned the car (over two years now) without these issues I've been facing recently.
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Old August 15th, 2018, 03:42 PM   #6  
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it's entirely possible there was a problem with the crank sensor that's gotten worse.those obd 2 computers are pretty reliable at self diagnosis.you can't just overlook it and say what could the problem be.
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Old August 15th, 2018, 04:01 PM   #7  
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Not a good video to comment on. Needs to be longer and focused on the tach, not steering wheel. Verbal explanations are ok for some things but not everything. This could be IAC or TPS or O2 sensor(s) related.
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Old August 15th, 2018, 05:06 PM   #8  
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As Allan mentioned, The 5 sec video is not very good at all.
I would check the Temperature control sensor. The engine will act just as you described. Usually the engine will not reach proper engine temps before sputtering and stalling out.
Just my thoughts..

Eric
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Old August 15th, 2018, 07:20 PM   #9  
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I've had defective TCS on my 95 Regency Elite. It caused a slower engine speed (around 500 rpm or less sometimes instead of the normal 750). It could be the sensor OR it could be a broken wire to the TCS shorting out.
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