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Old 08-13-14, 12:07 AM   #1
TOSU76
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Default SC400 Trouble

Hey Guys,
I'm new here I know but I wanted to ask a question because I'm coming up stumped on this one. I bought a 1994 SC400 that wouldn't pass smog here in California and was on its way to the junk yard. I really like cars so i thought I would give it a shot. The car has all of the problems that I was reading can be caused by the bad ECU such as idle is all over the place, car runs rough when started and almost dies, brake light is on in the dash etc. Needless after spending a good amount of time reading I ordered one off of eBay and plugged it in this evening with the same problem. At this point I'm confused as the car has new spark plugs and wires as well, I figured the ECU was a sure thing. I checked the MAF and the throttle body and they both seem clean. Also a computer can't read codes because it can't connect to the ECU is apparently what it says. Now would this mean the ECU I bought is bad as well? Or is there something else that is wrong that I am not aware of? Any help would be greatly appreciated, thanks guys.
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Old 08-13-14, 06:26 AM   #2
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Open the ECU and check for any bad signs it's faulty...usually the capacitors leaking or a burnt spot. You should have sent your old ECU to DriftMotion to repair it, this way you know you have a good ECU and can rule that out.

I don't know about an ECU reading codes, but to find out why Check Engine Lights are on you need to check the OBD1 sensor. You have a 1994 so you just need a paper clip. You need to put one end in the TE1 slot and another in E1 and then count the lights that blink on your dash....it's a process and to do it correct you need to check here.

http://www.troublecodes.net/Lexus/


Since you swapped ECU's you now have one you can send to Driftmotion while still being able to drive the car when needed to. Just make sure your ECU is the exact same year ECU and the same car model as I believe they slightly changed over the years.

Also if you didn't take the negative wire off the car for about an hour to reset everything it's likely using the old damaged ecu's settings. Computer needs to relearn and adjust. Disconnect the battery for a while, reconnect and let it run for a few minutes. This may help.
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Old 08-13-14, 09:16 AM   #3
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Originally Posted by Biddles View Post
Also if you didn't take the negative wire off the car for about an hour to reset everything it's likely using the old damaged ecu's settings. Computer needs to relearn and adjust. Disconnect the battery for a while, reconnect and let it run for a few minutes. This may help.
How would that work? Isn't the ECU itself the entire source of memory? Putting a new one in would wipe away that memory, wouldn't it? I suppose the replacement one could have its own memory, but it would have been disconnected for some time in shipping.

I disconnected my battery to remove and reinstall the ECU, just to be safe.

To the OP, definitely do a visual inspection of the ECU boards, but note that the damage may not be visible until you remove capacitors. That was the case with mine.

However, I'd guess the ECU isn't your [primary] issue. The odds of having the identical symptoms with two different ECUs is pretty slim. Various bad wiring seems more likely to me.
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Old 08-13-14, 09:57 AM   #4
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Thank you both for the reply it is very much appreciated. I opened my ECU and could not see any burns on the board by any of the capacitors. By what I have read and what you said t2d2 it may not be visible so I figured it was worth a shot to change it out. I did not disconnect the battery when I put it in; I suppose that would have been a good idea. I am normally fairly good with cars, but normally I play with American cars. I must say I am impressed how amazing of a vehicle this car was in its time. I mean for 1994 it was way ahead of its time just the way it was put together.
Thank you very much for the troubleshooting link Biddles, That’s awesome you can use a paper clip. I will try that this evening. Also I will probably send my old ECU to get it repaired by DriftMotion just to be safe as you say.
T2d2, This car was unfortunately not treated very well what so ever. That’s why I felt bad for it and wanted to try to get it up in running again. The original owners have broken lots of things and taped them back on or not at all. It’s very sad; once I fix the problem with the way it runs I plan to find a car to get parts off of to correct the other issues. If you believe it is a wiring problem where in the world would you suggest one starts then? Also are there any other sensors or anything these cars are known to have fail that could cause these issues other than the ECU? Thanks again guys.
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Old 08-13-14, 11:31 AM   #5
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No idea where to start looking if it's wiring. Any bad signal to and from the ECU would wreak just as much havoc as ECU problems. I've found several broken connectors and bare wires in my engine bay, so it could be just about anything.
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Old 08-13-14, 11:45 AM   #6
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Welcome to CL, your thread has been moved to the performance and maintenance section.
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Old 08-13-14, 01:45 PM   #7
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sounds like a vacuum leak to me either that or the IACV start with looking for a vacuum lkeak
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Old 08-13-14, 02:29 PM   #8
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sounds like a vacuum leak to me either that or the IACV start with looking for a vacuum lkeak
Good call. I thought he had checked the IAC valve already, but looking back, I see it was just MAF and TB.
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Old 08-13-14, 03:03 PM   #9
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I would try the coilpack test first since you sort of ruled out the ecu. also I would put the original ecu back in if the new one did not fix it there are differences over the years on the sc400. 1994 is the last year the 1uz had batch fuel injection and in 1995 went to sequential so if you use a 95+ ecu the car will sort of flood out and inject double.

onto your problem though, check the coils. disconnect one at a time and see if there is any difference. do not disconnect the high voltage part while its running, disconnect the 2 wire clip that is on the side of the coil. If you find disconnecting one to not change how it runs, you may have found the bad coilpack. this is the most common problem on sc400's other than a bad fuel pump ecu where the car wouldn't start so its not that, and so the coils are worth a check. also the rotors inside the distributors (there are 2) often never get changed before they are so worn they cannot spark reliably. if someone changed the plugs and wires and forgot this it basically has no benifit. if the car has 200k on it it needs new rotors about 80k ago or more. start with these.

The 1uz is hard to get to misfire, its probalby ignition. now if it was just idling low or high I would say tps or IACV, but missing thats a ignition or fuel issue most likely. I would rule out timing as its locked on the 1uz and non adjustable but the timing belt in very very rare cases could skip but reguarly speaking timing is non adjustable on the 1uz, well that version anyways.
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Old 08-13-14, 06:41 PM   #10
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I'm going to give all of this a try and hopefully find what is giving the problem. SexCoupe is the rubber line that comes off the intake tube between the MAF and the throttle body a vacuum line because I discovered that it was broken off and a piece of electrical tape was placed over it and the hose was just dangling there with the plastic fitting from the tube still inside. Lovely!
Ali SC3, thank you for the coil pack check idea, I am going to look into that as well. When you speak of the rotors inside the distributors where are those located exactly? I am sorry under the hood of that car is quite a mess.
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Old 08-13-14, 07:11 PM   #11
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That sounds like the vacuum hose to your IAC (idle air control) valve. That's definitely worth replacing before trying to solve other stuff. Or is it the smaller of the two hoses?
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Old 08-13-14, 07:56 PM   #12
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Quote:
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That sounds like the vacuum hose to your IAC (idle air control) valve. That's definitely worth replacing before trying to solve other stuff. Or is it the smaller of the two hoses?
It looks as though it is the same size as the hose that goes into the front of the plenum.
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Old 08-13-14, 08:07 PM   #13
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that is indeed a vacuum leak... thread a screw in the hole it may or may not fix your problem but it needs to be addressed and it doesnt go to the IACV it goes to your power steering valve.you dont need it but it needs to be capped off on both ends. the PS valve is next to the Banjo fitting on the pump there are 2 vacuum hoses one goes to the front of the plenum to the valve the other from your taped up air tube to the valve so cap off both.you you will need 3 rubber caps and one screw
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Old 08-13-14, 08:21 PM   #14
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that is indeed a vacuum leak... thread a screw in the hole it may or may not fix your problem but it needs to be addressed and it doesnt go to the IACV it goes to your power steering valve.you dont need it but it needs to be capped off on both ends. the PS valve is next to the Banjo fitting on the pump there are 2 vacuum hoses one goes to the front of the plenum to the valve the other from your taped up air tube to the valve so cap off both.you you will need 3 rubber caps and one screw
SexCoupe, Just so I am understanding you. You are suggesting I remove the hoses that are still connected and cap the off that are going into the plenum and cap off the ends? Do you feel as though that this is causing all of the problems? Also if this doesn't do anything at all? Why is it on the car? haha
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Old 08-13-14, 09:51 PM   #15
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its suppose to stiffen your steering at high speeds but they leak way to often and it causes you to loose PS fluid and makes the car smoke i suggested removing them because one is already broke its a two in one valve.it wont effect your high speed steering much at all maybe a 15% difference
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