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92 SC 400 rough idle

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Old 10-22-14, 12:53 PM   #1
ifr2lax
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Default 92 SC 400 rough idle

Hi all,

Working on my "new to me" SC400. Got the car from a client in a trade. Very equitable!
Looks good, straight body. Interior is good and has new tires. 230,000 miles. Previous owner said the " valve adjust" at 160,000 miles. Think he said it was "resealed" what ever that means.

He also thought the rear fuel control computer was bad...but there is NO code for that.

When it first came home ran great at speed 30-50 and idled OK. Still STARTS instantly hot or cold. At idle No hunting around idle speed, or surging at speed. Had a code for a 02 sensor (pass. side REAR 02 "Sub"). I replaced with new Bosch. That cleared the code and currently it shows NO CODES.

But now a regular miss at IDLE which I am guessing is a bad plug wire...moisture could be an issue too. Wires may be original...are all numbered, black rubber seem old and stiff. Look what I imagine a OEM wire would look like rather than fancy aftermarket...

1. shouldn't the computer show a CODE for a misfire at idle? or is that only with the next gen OBD systems?.

2. pulled the plug wires one by one on drivers side and the idle seemed to get worse. One plug shows moderate wear, did not look at each plug yet.

3. Passenger side plugs...realized I had to pull the throttle body...stopped as I wasn't ready to have the car unmovable for a few days as I got into it.

4. Rather than pull the TB yet, just pull the plug wires at the COIL?

5. If I pull the throttle body, may as well clean it and the EGR valve? Isn't there a RUBBER WATER hose that comes off the throttle body? (need a new on hand, unless regular heater hose would work.)

Realized need to test the coils, rotors and caps, if a computer is involved am aware of the leaking capacitor problems...have recapped several old Marantz stereos.

Open to suggestions or threads I may have missed.

Thanks, Pete
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Old 10-22-14, 02:01 PM   #2
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1) I wouldn't expect you'll get any codes for that on OBDI.

3) You only need to remove the air intake, not the entire throttle body. That is, assuming the '92 isn't entirely different in that respect from my '94.

5) There is a coolant and air hose connected to the bottom of the TB that are a major PITA to remove (and even worse to reinstall), being wedged in so tight against each other that you can barely get to the clamps, let alone move them around enough to get the TB off its bolts and pull it off the hoses. I put off fully cleaning my TB for awhile because of that, but finally broke down last week and rerouted that coolant line to bypass the TB entirely. Regular coolant/fuel line will fit the barbs. With that bypassed, it's not too terribly difficult to hook the TB up to the one air hose while sliding it onto the bolts. For me, it was at most 5 minutes vs an hour or two previously.

I would assume/hope your plug wires are not original, but maybe they never got changed with a tune-up... My aftermarket ones are numbered, fwiw.
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Old 10-22-14, 03:06 PM   #3
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t2d2

Thanks for the tip on possible differences 94" vs 92". WIll lookt more closely...My reference here are photos of a LS sedan at web site "Lexus . com" tutorials. Probably similar to the "SC" in most respects BUT you mention a TB WATER and AIR line...the L:S photo tutorial says there are TWO water lines...so they could be using different set ups. This is a CALIFORNIA car and smog systems could be different...I do not know..

Good to know the early OBD may not monitor plugs...

At "Lexus . com" They DO mention the same PAIN getting the TB off...almost sounds like it easier to CUT the hoses off and replace...unless the OTHER end is worse to attach...learned that the hard way.

One thing at a time...first guess is...a wet/black plug and a bad wire/plug. The miss is so regular would think only one, but an old wiring harness breaks down and things go to total chaos.

As none of my symptoms seem related to TB I will pass on that for now. Thanks for the tip to just use regular hose.

Not sure how to diagnose a bad fuel injector...will look that up next. From here if a bad plug points to new wires and THAT won't fix it then on to the coils, caps and rotors. Anyone with more experience weigh in on what may cause a miss at idle only (or so it seems).

Will have more questions on vacuum line routing and lots of other stuff once I fix this miss.

Thanks, Pete
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Old 10-22-14, 09:31 PM   #4
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Sorry, I didn't word that carefully enough. When I said a coolant line, I meant it's routed through the TB, as in there's an inlet and an outlet. That's two lines, the way you're thinking of it.

The coolant lines on my TB were reusable -- although I ended up replacing them with a straight through bypass the second time around -- but the air hose was completely shot. I think it's crank case ventilation or something, and like the PCV on the other side of the engine, it'll feel like brittle plastic after being cooked like that for a couple decades.
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Old 10-22-14, 10:43 PM   #5
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T2D2

Went out and looked again and indeed the passenger side plugs are quite accessible with out removing the TB...

Do have to remove the air inlet elbow and a rectangular plastic extension underneath it...that extends to the rear of the engine...hard to see in bright daylight due to glare but seems to have one bolt at the very rear. Looks to be reachable without a huge problem. With the intake elbow off a lot will be easier to see/reach.

The more I stare and poke around on this thing the less intimidating it is...compared to old pre-smog carbureted engines a JUNGLE of stuff...even my 2001 Jeep GC 4.7 is less cluttered.

As soon as I can count on not having to move the car for a couple of days I'll get into the plugs...hope there is a obviously bad wire.

If that goes well after the miss is taken care a few vacuum lines need to replaced...the ones on the front off the Fuel Injection plenum housing run down the front-right side of the engine. Without jacking the car I have NO idea where they go...

Is there a thread on all the vacuum lines as to lengths and size of the hoses? Prefer to do one line at a time, would be great to buy most all the hoses at once.

Thanks again, Pete
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Old 10-22-14, 11:00 PM   #6
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It's easiest to remove the air inlet elbow if you loosen or remove the air box, as well. There's not much foreward-back range of movement to get the elbow out of there otherwise. There are two 10mm bolts up in front of the airbox, one next to it, one behind the MAF (be sure to unplug it), and the one you mentioned at the back of the elbow. The clamp that holds the elbow onto the TB and the two clamps holding the accordion hose onto the intake pipes are also 10mm. Loosen a few things and it'll all pull apart with a few tugs, then you'll have your access to the spark plugs and assorted plastic covers.

For the vacuum lines, get about 13' of the small stuff, which I believe is 5/32". There are a couple longer ones you might also end up wanting to replace, so you could just get 20' and not have to make additional trips to the store. Don't worry about pre-measuring lengths. Just take one off at a time and cut a new one to length. Vacuum line is super easy to cut with scissors.

The ones that disappear down the right side of the engine go to a VSV (vacuum switching valve) and to the power steering pump. The latter is impossible to see from up top, but you can feel around with the air intake removed and get to them blind. Be sure to only remove one at a time or you'll have a tough time figuring out what goes where even with the help of the schematic on the hood.

My last round of replacing hoses, the guy at AutoZone gave me a sample platter of a 1" snip of all their sizes so I could match everything up at home and come back to pick several up at once, since there were a few that I didn't dare remove simultaneously.
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Old 10-30-14, 01:36 PM   #7
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Default thread on pulling the SC400 distributor caps removing the minimum stuff?

Got the plugs out and changed.

Only 2 looked to be sooty, the rest more or less ok...the pointy electrodes makes it hard to judge for me to judge for wear.

Only one plug seemed to have been torqued correctly and made a small " click" as it came loose the rest felt like they had been glued in place and maybe never seated...someone may have used gasket cement on the threads..? Lots of residue in the area where the plug seats on the head. Odd.

We shall see if new plugs makes any difference...spent a lot of time cleaning the goo off the threads in the head plug holes. New ones went in cleanly with aluminum paste on threads.

Will try to run it with the old plug wires but am picking up new set, caps and rotors today just to see,,,,if it runs good I do not want to change them out...

If need be have you removed the distributors lower distributor cap covers? Drivers side has stuff that may or may not need to be removed and the pass side I dunno...do you have to move the header tank?

Trying not to get things screwed up so moving carefully.
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Old 10-30-14, 01:41 PM   #8
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I was able to do the caps and rotors without removing the lower distributor covers, but it was a tight fit and I had to be super careful to not drop things down there.
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Old 10-30-14, 05:35 PM   #9
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T2D2,

If you did the dist caps with the lower shroud ON you are a braver man than I....and more careful as I always drop something it seems...but guess I'd have to try it that way first.

At the moment things seem OK...the new plugs seem to have cured the miss...yea!!

Started it up, let it run for a while. Put in gear and loaded it up a little...seems smooth so far...idling in the driveway.

Sunday may have time to drive some hills and see if the miss is really gone...heavy load is the real test sometimes.

Thanks for the tips.

Pete
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Old 11-01-14, 01:47 PM   #10
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T2D2

Success! The new plugs alone did it...

I think in part the old ones were never property torgued to actually SEAT...hard to think there was blowby, but they all were covered with sticky goo on the threads, so who knows. Of the eight, there were two dark, carbonish looking plugs. Looked like they may not be consistently firing, All the rest had a "tan" coloration I associate with good ignition.

Also these Lexus type spark plugs with the pointed electrode are new to me and hard to judge the wear compared to a standard squared off electrode I have been looking at for years..

Drove the car ~25 minutes, including some steep 1/2 miie hills and ran fine...pulled smooth and when it downshifted, felt good and strong...wowza.

Very glad I didn't have to get into plug wires, caps and rotors.

From here out will be getting the interior door panels reinstalled, paint clayed and buffed and some other detail stuff.

TsD2 thanks again for the time and tips!
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Old 11-01-14, 02:25 PM   #11
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Glad you got it sorted out! Sure feels good to be able to sit back and enjoy the car the way it was meant to perform, doesn't it?
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