Emissions/Check Engine Light Help
 

I just bought a 1993 SC 400. I test drove the car and it ran perfect. The person I bought it from had to get the car smogged. I picked up the car today and after I drove the car home the check engine light came on. Also, the car would hesitate at idle. I looked under the hood and found a couple vacuum hoses disconnected. I connected the hoses and I runs fine now but the check engine light is on. The light will go off once I hit about 20MPH. When I stop accelerating and the RPMs start to come down, the check engine light comes back on. Again, the engine seems to be running fine. I checked the codes and it is throwing code 31.
I reconnected the hoses where I thought they would go, but I would like someone to look at the pics and confirm I connected the hoses properly. Or if someone can shoot me a pic from their engine so I can see where the hoses go that would be great.
I included a couple of pics to show what I am talking about and how I connected the hoses. Thank you on advance for your help.
This is the first time I am posting so I apologize if I screwed the post up or have the post in the wrong place.
https://cimg4.ibsrv.net/gimg/www.clu...2c735f12c.jpeg
https://cimg1.ibsrv.net/gimg/www.clu...427e86e1d.jpeg

I can confirm, your hoses are connected the correct way.

Those hoses are for the SC’s heater control valve pressure actuator and Vacuum Switching Valve (VSV). They are totally independent of the emission system and the little VSV only gets signals directly from the HVAC/Climate Control unit in the dash.

CEL Code #31 refers to some issue in the mass air flow sensor or circuit. This could be an issue with the sensor itself or a bad ground somewhere.

HOWEVER— DO NOT CLEAN YOUR MAF SENSOR!!! YOU WILL RUIN IT IF YOU DO!!

The early SC300 and SC400 use a Karman-Vortex style MAF unit. They are NOT to be cleaned under any circumstances! That won’t help anything. If the actual sensor unit inside the metal housing is truly at fault then it would need to be replaced with another good working one.

But that’s after its values are tested with a multimeter in accordance with the TSRM factory shop manual procedure.

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This segues into another important question:

Since your car throws this CEL #31 only at low engine speeds but it goes away at high engine speeds and higher load AND your engine still runs smoothly regardless. That is a bit strange.

Has your main engine ECU ever been sent out for capacitor replacement service? This is getting to be a highly recommended and common long term maintenance need for many 1992-1997 SC300/400’s and other similar vintage Toyota and Lexus models due to capacitor bulging and potential leakage of electrolytic fluid.

Usually getting brand new OEM brand and spec capacitors (has to be the exact replacements and yes they are readily available from Digi-Key and Mouser Electronics) solves all ECU malfunction issues.

A malfunctioning SC ECU that has never been cap serviced is not entirely uncommon to come across these days simply due to age and weird problems can result from that condition.

If your ECU has never had this service done to it we generally recommend Tanin Auto Electronix for the work as they have handled correcting this issue for SC owners for many years.

This is a good baseline long term car health service to have done anyway.

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Because it is odd that you’d have a CEL 31 (MAF related) being thrown but have no drivability issue associated.

Now as to how the previous owner managed to get your car smogged successfully while not really solving this issue that you’re experiencing I’m not sure. OBD1 cars don’t retain CEL codes if you reset the ECU by disconnecting the battery for 2-5 min. While the freshly re-energized ECU begins re-learning again any CEL code issues may then take as many as 100-120 miles before the dash light will come on to alert you again. Some many less miles. Maybe the car was smogged successfully within that window? It’s hard to say.

That is great information! The previous owner said he sent out the ECU two years ago, the last smog. He gave me a bag of little capacitors from when he sent it in. Sounds like I need to get more information from him on what was replaced and where he sent it.

This problem is a bit confusing. When the hoses I have in my pic was disconnected, the car wanted to stall when I stopped and idled. I connected the hoses and it that went away. Does that make sense for vacuum hose that control the heater system?
Additionally, I noticed my heater didn’t heat up this morning. This is confusing.

Would the heater control valve, vacuum switch valve cause my check engine light to come on?

It's good that the previous owner got the ECU serviced with new capacitors not long ago. Okay! That rules out one big potential issue that would prevent accurate diagnosis.

The only reason the disconnected HCV vacuum hoses would cause any abnormal engine behavior is due to the vacuum leak that a disconnected hose causes. You reconnecting the little vacuum hoses corrected that vacuum leak and thus returned your idle to normal.

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Now what you will need to do in order to check whether the actual valve, vacuum-pressure actuator or the VSV of the Heater Control Valve Assembly are all working correctly or not is to have a friend or family member sit in the car and operate the climate control system from the inside while you observe whether or not the little actuator rod is moving the little pivot arm on the heater valve inside that assembly (it's all in plain sight to observe working with a flashlight aimed at it).

If switching from low heat to mid-heat to full heat settings and then back again don't make the valve arm physically move then you may have the hoses connected wrong (though it looks like you have them set up correctly now) or there may be a tear in the hose or a worn out/loose end. Replace any worn out hose in that size with a length of readily available Gates #27042 5/32nds inner-diameter vacuum hose that you can get anywhere online. Gates is a good brand for this application.

Also you should check to see if the two wires on the connector that plugs onto the little VSV are in good shape and not ripped, exposed or shorted out in any way. If they are or if the plastic connector is badly cracked and not seating and locking properly then you'll need to replace the connector and/or terminal repair wires with new OEM parts. We can get you the part numbers for those if that's the case.

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If on the other hand there is a broken HCV actuator, bad VSV, or some break or fault in the heater control valve itself that is bolted to the metal HCV bracket assembly then you may need to replace the whole assembly with a new one. The Lexus P/N for that assembly is 87240-24040 and you can order it at a good price from MyLParts (which is a California Lexus dealer's online ordering site). Amayama is also good but they ship from Japan after purchasing from Toyota/Lexus dealers there so work out the shipping cost vs any savings from buying it in the U.S. if using them.

But... so far we have no confirmation that your HCV assembly has any issue with its components so check the other things I mentioned above first before going to those lengths.

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Also, this should not be the case as a hypothetical "what if" scenario but hopefully the previous owner didn't have a failed heater control valve which they decided to "fix" by disconnecting the heater control valve hoses and connecting them together so as to bypass the HCV unit entirely.

You'll have to do some investigating with the above tips first to see further what is going on with the heater issue.

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The heater issue still unfortunately is not going to be related to the issue of your Check Engine Light (CEL) Code #31. For that I think you may need to take an electrical multimeter and test the resistance values on the pins of the Karman-Vortex MAF sensor in order to determine if they fall within the normal operating range that Toyota/Lexus specifies in the factory TSRM service manual.

The full SC400 early model factory Lexus TSRM should be posted on this forum.

KahnBB6, thank you for the great info. I will look into all that you mentioned.
This is a great forum and I really appreciate the input.

If the MAF is bad and I need to replace it, can you recommend where I could get that part?
Thank you.

You're welcome! And welcome to the Clublexus forum! :)

You need to test the MAF sensor unit first to determine that, since as you said there are no drivability issues, no idle issues, no bogging or misfiring. Other than the HCV issue (which is unrelated) you just see the CEL light come on when the RPMs are low and go off when the RPMs go up to a certain level.

This also might be something to consult the Troubleshooting Matrix in the TSRM for, since more often than not a *truly* bad MAF sensor will cause rich and sputtering idle or poor metering of the actual air temperature in the intake tract... which would force the ECU to default to assuming the ambient air coming in is 68F and which would also cause a lot of difficulty with any kind of engine speed acceleration.

And you don't seem to have those issues... just the CEL light. It is worth further investigation.

However to find a replacement I recommend going onto ebay and getting a used OEM Lexus 1992-1995 SC300/400 Karman Vortex MAF sensor (just the sensor itself is all you need, not the entire metal housing it bolts into). You cannot use an SC300/400 MAF sensor from a 1996-2000 SC300 or SC400.

RockAuto might also have some remanufactured MAF sensors for the 1992-1995 model year SC300/400's. I still recommend a good used OEM part that has been confirmed to be working normally.

Yet I think you not having any symptoms despite the CEL Code 31 is a bit odd. If I were in your shoes I would probably pick up a good used sensor at the very least just to have a good spare in my parts bin... but I would want to check the sensor values of your old factory sensor.

Of course you can also swap sensors (with the car off) and see if there is any change. If the CEL #31 still comes on despite swapping sensors then we will know that the problem is somewhere else in the system.

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To note... the thing about OBD1 CEL flashing codes is that it is a very rudimentary issue and diagnosis system. With OBD1 usually a fault code often refers to an entire individual electrical circuit rather than just a specific part within that circuit. This is especially true of the CEL code for an EGR issue on SC300/400's. It's a good system and some 30 years into people owning these cars we have usually learned the common things that need to be fixed when a particular code comes up but sometimes old fashioned investigation of an electrical circuit and its critical sensor(s) has to be done to figure out what the true issue is.

It's just that *usually* the fix for a particular CEL code isn't too hard to figure out.

With any 1996+ OBD2 vehicle and especially any made after 2007 or so the fault codes and system diagnosis got a LOT more sophisticated and a lot more specific as to what failed and where.

You have to remember that these SC's were still made in the early days when Federally mandated Check Engine Light fault code diagnosis systems had only just become implemented a few years prior in the mid-to-late 1980's. These cars were designed between 1987-1990(early) and first went on sale in Japan in 1990 (for model year 1991) and in the U.S. and Canada in 1991 (for model year 1992).

Interesting thing happen when I drove it yesterday. It started to run crappy. When I came to a stop (idle) it wanted to stall. It actually did stall once.
Once I got home and let the car sit for an hour, got in to drive and then it ran fine.
My point is this, it seems it is having intermittent driveability issues. I took the car to a local shop. I’m waiting to see what they find out.
Again, this info has been extremely informative and helpful. I will keep you posted on the outcome.

That does seem to be more consistent with getting the CEL Code #31 for the Mass Air Flow Sensor. Again, cleaning it will only make it even worse and destroy its function entirely.

At this point I would pick up a good used working sensor off ebay. Read the part number on the top of your sensor on the MAF assembly and get that same one made for SC300/400's from 1992-1995.

I have noticed you have mentioned a couple times to pick up a good used working OEM MAF sensor off eBay or somewhere. Getting an aftermarket is not a good idea. Is the aftermarket ones no good?
I have to say again, thanks very much for all the help!

Ok, I found a OEM sensor on eBay. Fingers crossed.

It's not that it's a bad idea to get a "new" aftermarket one but rather that a used good working original one should (currently) not be too expensive off ebay at around or from a wrecking yard between $30-$40 and that I think the aftermarket ones are rebuilt original Toyota parts. But the quality of the rebuild/remanufacturing work may vary when compared to a working stock original. I just tend to find that OEM electronics parts in most cases are the better bet.

Ideally I'd say get one good used working one and then get a "new" aftermarket one from RockAuto or a re-manufacturer as well.

But for now stick with the purchase that you have made. As long as it hasn't been cleaned it should be fine.

However at some point in the future we may be looking at aftermarket remanufactured KV MAF sensors and specialist companies that rebuild their electrical components as being the only options. At current time that is currently not the case.

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I'm wishing you luck with this. Hopefully your CEL #31 will go away once you have swapped in the replacement sensor. I myself put an eBay purchased used OEM sensor of the exact same type in my SC300 many years back when my mechanic screwed me by cleaning it and the replacement sensor put the car's operation right back to normal.

I couldn't stop him in time, just like one of those slow-motion commercials where something staining is beginning to spill all over onto a rug and someone is very slowly yelling "Nooooooooooooo!" while scrambling in vein to stop it from happening. Needless to say I never went to that mechanic again.

Fingers crossed!

Ok, installed the OEM MAF sensor and it seems to have fixed the issue. CE light is out. It seems to running smoother. No more rough idle or stalling at idle. But, I am now able to notice a very, I mean very minor hesitation when I am accelerating hard. Right around 2500 to 2800 rpm before it shifts into 3rd gear I get a stutter. It only hesitates for a brief second and I can’t make the hesitation last. If I’m cruising with normal acceleration, I don’t get the hesitation. Any ideas?

Thanks by the way for the solid advice on the MAF. The input saved me a lot of headache!