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93 sc400 running rough on idle

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Old 03-27-12, 09:24 PM   #1
dontmachme
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Default 93 sc400 running rough on idle

ok so i bought a 93 sc400 about a month ago i thought it needed a head gasket. this last saturday i finally got around to diagnosing what was wrong turns out the guy had put ign wires on it and got two backwards i switched them around and it ran real nice at higher rpms but at idle it runs a little rough i put new plugs cap and rotor on it. i put a different iac valve on from an ls 400 because i tested my old one and it was bad. i did a diagnostic check on my engine light and it was code 28. out of ideas and energy i pulled the spark plugs out and bank one was bburning normally but bank 2 was burning rich and the plugs were covered in carbon. so my question is has anyone ever had this happen on their lexus and could an o2 sensor through off the fuel ratio that much to make it run ****ty on idle?
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Old 03-28-12, 08:01 AM   #2
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I got my SC for $800 for the same reason but I had two pairs, or half the engine, with the plug wires in the wrong order. It's a miracle it cranked at all!

I think you need to replace all the vac hoses under the hood. They're old, stiff, and dryrotted. You'll need 6-7 feet of 5/32 or 4mm hose to do them all. Things like the fuel pressure regulator and the EGR valve rely on vacuum to operate and either of them could cause a bad idle. It's a pretty simple job -- far easier than on any turbo car I ever owned!

I cleaned my ISC -- don't even think of buying a new one - you'll fall over when you see the price, replaced all the ignition components, and did all the vac hoses. I now have a solid idle, and thanks to the 12v fuel pump mod it cranks instantly too.

At some point I'm going to put together a list of everything that needs to be done to a stock 400 upon purchase. These questions pop up every few days.
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Old 05-08-12, 08:53 PM   #3
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Sorry to jump...
I just got a 1992 SC400.

Seats in front are not very good, (1 is vinyl)
Chugs at low speeds and when slowing down, surges, like the third member is going out.
It has 148,xxx.
About the surging and chugs, I cleaned the IACV, TPS, and throttle body.
I have not cleaned the MAF yet, and will check the coil in the mere future.
Cleaned fuel injectors extremely, and changed spark plugs and wires.
What would be the most likely to check out?? im thinking the maf is dirty...
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Old 05-09-12, 07:49 AM   #4
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Is the TPS calibrated properly? The how-to is easy to find with a quick search. Basically you verify the idle switch on one set of contacts and and the variable resistance throughout the range of operation on the other.

They say the MAF is too sensitive to clean without breaking it. I don't get why -- I've cleaned similar sensors in other cars many times without issue but I haven't touched the one in this car yet. Try unplugging your MAF and see if the car runs better. That's a good place to start.

Don't forget the vac hoses either. It's quick and easy and eliminates one more potential issue.
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Old 05-14-12, 12:21 PM   #5
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Thanks

I'll try.

I heard what happened when some people cleaned their MAF sensor.
It broke, and 1000 bucks slipped away...
Could it be the EGR?
It sounds mechanical to me.
Only when it warms up, it has very slight tachometer movement when its rough.
No loss of power or anything, (accelerates quickly)
also, how do I check the coil, cuz it might be the coil... 1 out of 2 may be broke
(FYI I fiddled W/ the TPS. when moved top the right idle went higher...
thanks for your help
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Old 05-26-14, 04:00 PM   #6
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Quote:
Originally Posted by MooJohn View Post
I think you need to replace all the vac hoses under the hood. They're old, stiff, and dryrotted. You'll need 6-7 feet of 5/32 or 4mm hose to do them all. Things like the fuel pressure regulator and the EGR valve rely on vacuum to operate and either of them could cause a bad idle.
Thanks for this suggestion. I've been trying to solve a start-up hesitation issue and had more or less narrowed it down to the fuel pressure regulator. I also was experiencing some rough idle yesterday that seemed to be contributing to tripping the knock sensor (may or may not be a related issue, we'll see) and found this thread searching for solutions.

I changed out the vacuum hoses and early indications are that the FPR is okay and it was just a bad vacuum issue. Several of the hoses were in pretty poor shape, as you described. With the new hoses in place, all four start-ups in a row were within a couple tenths of a second, as opposed to the usual 2-5 seconds of cranking. Fingers crossed that that holds true in the coming days.

Oh, your estimate on the total length was a bit low, though. 7' left me with about 4" left over, but that's without doing the two long hoses that run under the air intake. To do everything is probably 10-11'. I was going to get 8' at the store but they had a 7' section already cut so I figured that would be perfect.
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Performance: Alternative BFI (BeeFI?), A/T Solenoid Bypass, M2/Manzo axleback exhaust (awaiting mid-pipe resonator delete), 2-Tone Driver's Seat ("performance" as measured by the butt test).
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Old 06-08-14, 08:05 PM   #7
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Oh, your estimate on the total length was a bit low, though. 7' left me with about 4" left over, but that's without doing the two long hoses that run under the air intake. To do everything is probably 10-11'. I was going to get 8' at the store but they had a 7' section already cut so I figured that would be perfect.
I finished up the rest of my vacuum lines today. In addition to the two long runs I didn't have enough hose for last time, I noticed another long one that runs to the back of the engine block, plus a couple short ones it meets up with. I bought an additional 6' and had right around 1' left over, so at least for my '94 SC400, 12' is the magic number.

While working on that and throttle body cleaning, I noticed one of the bigger vacuum hoses that connects to the air intake had a split where it fits over the barb. There was enough slack to just cut that part off, since I didn't have that size hose on hand and didn't have enough light to see where it went down below. I'd like to think cold start and idle were improved after finishing up all the vacuum stuff, but I've thought that before and eventually decided it only partially addressed things...
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Performance: Alternative BFI (BeeFI?), A/T Solenoid Bypass, M2/Manzo axleback exhaust (awaiting mid-pipe resonator delete), 2-Tone Driver's Seat ("performance" as measured by the butt test).
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Old 06-08-14, 08:05 PM
 
 
 
 
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