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Mass Air Flow Sensor--maybe?

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Hello!

I have a 1995 SC400. Just recently, I have a "check-engine" light and the engine is running a little rough idling. Also, the engine does not run smoothly when driving using very little gas. I'll explain:

At about 45mph, on a level road, it takes very little gas to maintain this speed. The gas pedal is hardly pressed at all. But, since it needs just a little gas to maintain this speed, the engine starts to "balk" for lack of a better term. I can take my foot off the gas pedal and it smooths out. I can accelerate with no problem. But, if I try and maintain about 45mph, the car is very "jerky." Now for my own trouble shooting:

I guessed that there might be a problem with the Mass Air Flow sensor. So I unplugged it. The car runs fine. No "jerkiness" or idling problem. So, I replaced the MAF, with a used one, and plugged it in. Same problem I had before. So, I guessed that maybe the ECU is not working properly. Well, a new ECU can be as much as $1500, but I found a place on Ebay that will rebuild my old one (with a lifetime guarantee) for only $249. So, I had it rebuilt and installed it today. Same problem I had before. So, I unplugged my MAF and it runs fine (except I still have the check engine light).

Also, as an extra note, when I turn on the A/C or turn it off, the idle speed responds appropriately maintaining a steady 600rpm. Smooth, with the MAF unplugged, or "rough" with the MAF plugged in.

So....... whatdy'a think? Should I just drive it with the MAF unplugged? I'm hoping someone out there has a better idea.

Thanks in advance for anyone who can help.

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90%+ of the time the oxygen sensor is faulty when a check engine light comes on.I have never had a car run that poorly due to the sensor but it's worth checking it out.

Download the service manual (search this forum for the link) and go from there.

An aftermarket sensor will work just as well.

Only if your SC was one year newer with OBDII.....it would have been all plug and play with a code scanner.

Oh, and check the throttle position sensor as well while you are at it.

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Thanks for your help.

I'll check out getting a replacement oxygen sensor on Ebay. It has to much less expensive than an ECU. I'll search for that service manual.

You know, when I was a teenager, we would put in new points, a new condenser, check the vacuum advance, and we were good to go. Too many sensors on these new fangled cars!! ;-)

Thanks again.

Phil

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Thanks for your help.

I'll check out getting a replacement oxygen sensor on Ebay. It has to much less expensive than an ECU. I'll search for that service manual.

You know, when I was a teenager, we would put in new points, a new condenser, check the vacuum advance, and we were good to go. Too many sensors on these new fangled cars!! ;-)

Thanks again.

Phil

TRUE....I'm 30 so I got my license well into the era of fuel injection, sensors, ECUs, etc....

But I grew up with my dad having a VW bug so we got another one (1973) for nostalgia. Only the radio in the car is electronic....everything else is mechanical.

Sooooo cheap to work on. I rebuilt the entire brake system for only $210 using all German parts.

$210 won't buy even a remanufactured master cylinder on an SC.

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hello, well hopefully you are not too far deep into the spending because this is the exact same thing that happened to me...when i unplugged my MAF, the car ran fine..! however maybe you've noticed your gas mileage get absolutely cut in half when its unplugged...so i bought one from ebay and thru it on there and nope, didnt fix the problem...the problem was that the one from ebay was also defective, its tough cause it really doesnt make you want to buy another new one, but i did, i bought a second one and presto, no more check engine lights, no more "balking" no more engine coughing.

the second ond from ebay was also used, however it was functional. good luck to you i remember how awful that was havin to deal with that

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Another thing that causes the same symptoms is the IACV Idle Air Control Valve, located top front of engine, those get carboned up and will make your car have bad idle, and even at highway speeds will act up.

All you have to do is take it out and clean it good so that the insides move freely again. Good luck

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Thank you all for the advice on my problem. I was hoping I wasn't the only one with this problem.

So, far, I've had the ECU rebuilt by a nice company on Ebay that offers a lifetime guarantee. http://www.autoecu.com For only $249, I sent them my ECU and they rebuilt it, returning it to me in only a couple of days (from the time I sent it to them in Tennessee, until I got it back was about a week). Unfortunately, that did not the fix the problem. They said they'd be happy to check it out again, but I don't think the ECU is the problem. I bought a used MAF on Ebay, but I still have the same problem. Same thing that you mentioned. I might try that again, but I think I'll check out the IACV Idle Air Control Valve that was discussed. At least that is a low cost solution.

I'm thinking about taking it to the our local Lexus dealer for another diagnostic. There will be a charge, but at least they can tell me what the codes are saying that might be causing the check engine light. I had my non-Lexus mechanic check it out and he had a laundry list of codes including cam sensors. I'm hoping the Dealer will be better. I'm also wondering about the O2 sensors that were mentioned. Unfortunately, at my age and my bad back, I really can't crawl under the car and do that kind of work myself. So, O2 sensor replacement (all 4) could run over $1000.

So, tomorrow, I think I'll check out the IACV and see what I can find. I can work under the hood with no physical problems.

Thanks to everyone for their help.

I'll let you know what happens.

Take care,

Phil Lewis

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I know what you have been through as I went through the same with my first SC400, it would surge at idle, would even act up at highway speeds. I tried a different MAF, cleaned injectors, you name it...and no change. The Toyota shop told me originally it was the MAF and it was like $800 used....but got one off of ebay for $100. I then discovered the IACV and once I got it off found it to be the culprit....it was so dirty inside that it would not move freely, so I cleaned it up and put it back in and my Lexus purred again and was so smooth at idle....anyways this must be a secret because I don't read about it much. It is a treat to get off the engine because of the hoses that connect to it, not much give, but once you get it off of the engine and clean it and put it back in you will be happy with the results. These things are an expensive part from Lexus so it's great that it just needs cleaning. Good Luck and hope this takes care of your problems.

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Well, I opened up the IACV and it was very clean. I cleaned it anyway with high quality electrical cleaner (fast drying, no residue) and no help.

I'm about done trying to fix this thing myself. I'm planning on taking it to the dealer and having them check it out. I'm not taking it back to my regular mechanic (Premier Auto Service) because they basically gave up after they replaced the crank position sensor.

On a separate note, my catalytic converter was destroyed by what, THEY say, is a fuel problem. The converter was making noise like it was broken inside. They (Meineke muffler) is saying my warranty is void because of driving with the check engine light on.

Anyway, my next step in a dealer diagnostic. They I'll have to decide if I want to spend the several thousand dollars I know they will want to fix it!!!

Thank you all for all your help. If youall are interested, I'll let you know whether I keep it and fix it or sell it for parts!! :-(

Take care,

Phil Lewis

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Hey Phil, you took the IACV out that is at the very front of the engine? It is difficult to get out but I know mine was dirty and solved my problems after cleaning it. I can't believe yours was clean with all the problems you are having with it. Let me know if we are talking about the same part. It has small antifreeze line connected to it and bolts to top front of engine. Good Luck, Glenn

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Hello Everyone!

Well, I finally got my '95 SC400 fixed. I Googled for a good Lexus mechanic and found one about an hour from my house. It was a long drive, but it proved to be worth it.

When he did the check for my "check engine" light, he found several items. The first real problem is that my car (manufactured in September 1994) has the older computer check and only gives codes on general conditions. The codes showed an RPM problem, but that could be interpreted different ways by different mechanics. I was told it could be caused by several different things. Fortunately this mechanic diagnosed the problem correctly as one problem.

It seems that I had a bad timing belt tensioner. This allowed the timing belt to slip and this messed up the idle as well as cruising. It didn't slip much, just a couple of teeth. Anyway, the labor is much more than the part (but a lot less than what a Lexus dealer would have charged). I think the tensioner is only about $50, but you have a take a lot of things off to get to it. So, a few hundred dollars later and my SC400 is happy again.

My only advice to all is, if you have the timing belt replaced, replace the tensioner, as well. It's a very small additional charge.

Thank you all and happy driving!

Phil

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That's Great Phil, glad to hear you got it sorted out and back running again. Those codes are hard to interpret but as you said a good Lexus mechanic can save you some time and money in the long run.

Hope that's the end of your problems.

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Hey Phil, you took the IACV out that is at the very front of the engine? It is difficult to get out but I know mine was dirty and solved my problems after cleaning it. I can't believe yours was clean with all the problems you are having with it. Let me know if we are talking about the same part. It has small antifreeze line connected to it and bolts to top front of engine. Good Luck, Glenn

Hey Glenn i have a 92 SC400 with the same problem but i cant find the IACV could you put a picture up or explain it in great detail thanks

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