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Rough idle/CEL after cap & rotor change

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Old 10-21-12, 03:31 PM   #1
kyddo112
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Default Rough idle/CEL after cap & rotor change

Just changed out my cap and rotor, pcv valve, and ECT on my 95 ls400 because I didn't get a chance to from my tune up a month and a half ago. Car was running fine last night before I did any work. After I bolted everything up it fired right up but once it hit operating temp it started to idle rough.

Idle would fluctuate up and down from 200-1000 but not a consistent fluctuation like a vac leak. Then the CEL came on for code p0336 (crank position sensor). It even stalled once. I can hear a rattle noise (sounds like an exhaust leak but not sure if it is or not) when I punch the throttle but is barely noticeable when in neutral. I unplugged the clip of the crank sensor and the car would not start. Plugged it back in and it started right up.

Can anyone help me? Maybe I broke one of the wires going to the sensor? I didn't completely unbolt the cam gear over on the driver side and just kinda pushed it aside. Maybe that stretched the wires but I can't see it cus it's wrapped up.


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Last edited by kyddo112; 10-21-12 at 03:43 PM..
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Old 10-21-12, 03:48 PM   #2
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sounds like you either mucked up the wiring or unplugged a cam or crank position sensor.
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Old 10-21-12, 04:26 PM   #3
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Yeah, you should really double check what you disconnected!
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Old 10-21-12, 04:54 PM   #4
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If I disconnected anything else (other sensors) would that have thrown a CEL? Can anyone tell me what the sensors are by the driver side ignition coil and can gear cover? Those sensors all attach to the same harness as the crank sensor plug.
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Old 10-21-12, 09:13 PM   #5
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you engine does not sound well - are you *very sure* you have the wires going to the correct ports on each distributor cap and to the corresponding correct cylinder?

I think I would re-do this job from scratch, and go slow & meticulous

I triple checked mine when I did my caps, rotors, plugs & wires, people tell me I am a slow mechanic, but this is the reason why I am extra slow and A.D.D. about things like this, not worth the resulting headache to go fast!

Quote:
Just changed out my cap and rotor
and you changed "cap and rotor" - you know you have two caps and two rotors, right?

you can check the electrical integrity of the wiring from your ECU to the cam and crank sensors with a multimeter if you have the Lexus wiring diagram book

best $39 you wil ever spend!

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Old 10-22-12, 06:21 AM   #6
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LOL yeah i meant to say Caps and Rotors. Did both sides. I actually retraced everything and installed my old caps and rotors in to rule out bad parts. I still get the same effect. When I was removing the spark plug wires off of the old caps, I did them one by one and installed them onto the new caps right away. I've checked the wiring and can't see any chaffing or cuts or anything. The plug seems to covered in gunk and trans fluid from the PS though so that might have caused it.

I took the old crankshaft position sensor (sensor on the bottom of the engine right behind the crank pulley right?) and tested the resistance on it. I'm getting 1150 ohms. I checked the resistance on the wire plug and got 650 ohms.

According to this it should be within spec.
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Old 10-22-12, 08:46 AM   #7
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shortly after replacing my plugs, wires, rotors, and caps - my ECU capacitors failed

since this is a known design defect on the LS400, I would strongly suggest you do this, whether it needs it or not, because they will fail soon anyway.
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Old 10-22-12, 01:12 PM   #8
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Quote:
Originally Posted by LScowboyLS View Post
shortly after replacing my plugs, wires, rotors, and caps - my ECU capacitors failed

since this is a known design defect on the LS400, I would strongly suggest you do this, whether it needs it or not, because they will fail soon anyway.
while it's possible, the chances of spontaneous ecu failure following unrelated work is highly unlikely.

OP needs to check the harness on the front of the motor for shorts/breaks.
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Old 10-22-12, 02:21 PM   #9
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Anyone have a wiring diagram of the engine bay? The harness that the crank and cam sensors attach to disappear behind the driver side cam gear.
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Old 10-23-12, 05:16 PM   #10
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Changed out the crank sensor to a new one. Went to start and got a big puff of white smoke out of the throttle body. Tried to do a compression check but my tool kept showing up as 0 on all 8 cylinders. Finally got it to start and it ran ok until it warmed up then the crap idle came back. Went to check timing and saw that the belt might be off by a tooth. This looks off right?

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Old 10-23-12, 05:25 PM   #11
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Looks off to me. How many miles on the belt?
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Old 10-23-12, 08:28 PM   #12
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belt def has some miles on it. car has 230k and is due for a belt change. i was hoping to do it in the next couple weeks but i'm guessing I'm going to have to do it sooner. I'm hoping i can get the belt lined up and that the valves aren't bent or anything. when i got it running i don't hear any tapping or anything at idle. just wish i could get a good compression test.

Here's a vid of a weird sound (backfire inside the intake?) the car does randomly when it starts.

http://s12.photobucket.com/albums/a2...A4E530012D.mp4
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Old 10-23-12, 11:23 PM   #13
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you should be praying to all you know that it didnt damage the valvetrain. 95+ are interference.
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Old 10-24-12, 12:00 AM   #14
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That sounds bad! hopefully it's not damaged
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Old 10-28-12, 06:41 PM   #15
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Ended up being the timing. Was putting off doing the timing belt but ended up replacing it. Wasn't as hard as I expected (done various timing belts before) but it def runs like a champ again. Also, I didn't do the 50* atdc. Just lined it up and put the belt on.
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Old 10-28-12, 06:41 PM
 
 
 
 
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1997, 400, 95, cap, car, high, idle, lexus, low, ls400, plug, replace, replacing, rotor, rotors, sc, wire

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