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Hi, I'm new here and I appreciate anyone that takes the time to respond (in advance). Last week, I purchased a '95 SC300 with only 128k miles on it, from it's original owner. The day before it was smogged and passed with no problems. The day I bought the car and test drove, the 'check engine' light came on but, the car was running smooth, no problems. I took it to Pep Boys to get it scanned. They did not have a OBD-1 scanner but, told me they could rig a scanner they have and they came up with codes 31 and 71. When I got the car home, I replaced the air filter and took out the mass air flow sensor and sprayed it with mass air flow sensor cleaner. After it dryed, I put it back in and the car would barely start and when it does it idles rough and the exhaust is a thick and awful! Can a MAF go bad if you clean it?? Any/all input is greatly appreciated here....I could really use some help. Thanks, PDM
Thanks very much for the quick response and for the knowledge! I now see that fine print on the can. Ironically, this did not affect cleaning the MAF of my SC400 when I had that model. Wisely, I have not been driving the car until I get another sensor in hand and replaced. I'm going to get one today. Do you have any experience with codes 31 and 71? I've read on here that only the front (upstream) O2 sensors would need to be replaced and not to bother replacing the back one located next to exhaust manifold. I'm a little concerned about code 71.
CEL Code 31 refers to a malfunction in the Mass Air Flow Circuit (actually a Karman-Vortex system in the SC300 but it's referred to commonly as a MAF). Since you sprayed something in there that is a sure way to kill it unfortunately. If you need a replacement sensor I have one and also an entire MAF unit for a 92-97 SC300.
CEL Code 71 refers to: open short or malfunction in the EGR gas temperature sensor (located on the EGR valve itself), a short or failure of the EGR VSV (located on the front part of the intake manifold), a cracked or punctured or disconnected vacuum hose in the EGR system, or insufficient flow in the EGR system due to a carbon clog. It can also refer to an issue with the main engine ECU but unless you have had some other telltale red flags of ECU malfunction it's definitely not that.
First, you need a new MAF sensor. You don't need the entire MAF assembly, just the sensor. Sorry to say but spraying in the cleaner did it in. You would be surprised at how many mechanics do not realize that they cannot spray cleaner onto that delicate electronic component unless they unscrew it from the aluminum MAF assembly first and refrain from getting any liquids onto that sensor.
Second, you will need to start diagnosing your EGR system. You will have to test the EGR VSV's +12V on/off function to confirm it's working, then test your EGR temp sensor for continuity while the engine is running (should measure 69-89k at 122F, 12-15k at 212F, 1-4k at 302F), then you need to actually remove the EGR valve, EGR pipe and the remove the upper intake "Y" manifold in order to clean all three of them. Also you need to make sure your EGR vacuum modulator's internal diaphragm is working properly. Rarely do the EGR valves go bad but anywhere that EGR gas flows you can have carbon clogging.
I went through an entire cleaning process with my stock GE engine's EGR system a couple of years back to get rid of the CEL 71 in order to pass smog. I bought a little flexible cleaning/reaming carbon busting bit that goes onto a *variable* speed drill to get the carbon out of those passages, I used a LOT of carb & choke cleaner (with a ventilator mask and googles on my face and 7mil nitrile gloves from Harbor Freight all for my own safety) and a very thin but dull screwdriver to get at the carbon buildup in the EGR valve, the, EGR pipe, and the intake "Y" manifold which turned out to be the final major area of clogging.
I have some cleaning procedures and correct removal procedures posted in another thread for all of those parts. I actually just took some pictures for a future DIY on how to clean out the "Y" manifold of carbon buildup in its EGR passages but haven't posted it yet. Toyota lists no procedure for its cleaning in the TSRM so I just had to figure it out. Actually it's not that hard but I was concerned about whether or not the little freeze plugs on that manifold could be re-used once they were popped out for cleaning. It seems they can but I took measurements and looked up potential aftermarket replacement steel plugs just in case.
Basically that's what you'll need to get ready to do. The EGR passages most likely have to be cleaned of all that carbon buildup. And there is a specific way you can remove the EGR valve and its pipe to make it easier. I'm not going to say it isn't extremely tedious because it is but this is correctable with some effort and shouldn't revisit you for many years after you've got it all cleaned out again.
Also, being in CA it is a good idea to replace your two upstream O2 sensors (go with Denso OEM replacement sensors) if they're old and you are coming up on a new smog test. Get your MAF sensor sorted out first. And change out your spark plugs for Denso Iridiums and your plug wires, OEM/Denso distributor cap, and get a new OEM/Denso rotor installed. Then do your O2 sensors.
Once you solve your EGR system clog and/or malfunction the only major parts left to trouble you with smog are your cats, both the front and rear units. On an original CA Emission SC300 the 3rd O2 sensor bolted into cat #2 is a four-wire heated "efficiency check" sensor which does not directly influence engine performance or calibration but which can throw a CEL code if your #2 cat isn't working or if that sensor has failed. Generally it is your two front O2 sensors that need replacing. For 1995 and up models I believe your Denso O2 sensors became three or four wire heated types so you will find that front O2 sensors for 1992-1994 SC300's are not compatible with your 1995.
Aside, an issue with your OEM fuel pressure regulator and/or injectors can also affect smog testing but at this time you shouldn't focus on those as there is no indication of any issue. I am only mentioning this just for your future knowledge because they're the last other factor (other than the ECU) that can affect your passing smog assuming all the other systems mentioned above are in order.
This is a set of links that highlight the majority of the EGR system troubleshooting I did in my 1993 SC300 Cali-Emission model three years ago. Your O2 sensors, injectors and main ECU are different for 1995 but the rest of the components are the same.
I covered a LOT of ground in that thread to get the car to pass smog so these links are not going into all that was entailed in that.
To finally pass smog after correcting everything else including the EGR issue I did require a new front and rear cat.
And the longstanding issue with odd rough startup for a few seconds which I mentioned a lot was totally unrelated to the EGR issue. That ultimately had to do with carbon buildup in the cylinders and head which I discovered much later... after I passed smog.
I digress. Focus on the EGR specific posts. They will help you get through the cleaning of it all.
I greatly appreciate your time! Just so you know, last week I unscrewed and took the MAF sensor out of it's casing and sprayed it...I did not take the casing off and spray inside that (at all). I was surprised that this MAF did not have any visible resisters; it's all plastic. I replaced the MAF sensor today and the car starts ok now but, will not idle like it used to. When I tried to drive it, it sputters and then eventually stalls. I will get a proper scan t'row. I'm lucky since the car already passed smog...how, I don't know. The check engine light did not come on when the previous owner took it to be smogged (or, it would not have passed as we both know). Like I mentioned earlier, I just bought the car last Saturday with 128,000 original miles from it's only owner that properly maintained the car. However, it was hardly driven in the last two years.
Taking off the EGR valve does look like a pain in the _ss, with several tubes involved. Right now, my thought process is to remove the EGR valve, replace all the tubes and replace the sensor. Testing the sensor the way you described is over my head. I'd rather just get a new one. Relay any other thoughts, if you don't mind. This is all amazing to me since the car was running fine (even with the 'check engine' light one)...before I pulled out that MAF sensor and 'cleaned' it. Thanks again!
No problem. I hope the notes can help. Yes, getting into this and cleaning everything can be a chore but it isn't as bad as you might think. After what I went through I feel like there is a pretty clear path to get and carbon clogs sorted out in reasonable time.
Even at low mileage like that over 100k minor carbon buildup in the EGR just depends on how the SC was driven. Puttering around town a lot for most of its life could have caused some carbon buildup for all we know. If you are aware of the car's history with the previous owner then I am sure they did take care of the car and maintain it well. Troubleshooting of this nature falls outside of what would be considered normal routine maintenance though.
The best way to remove the EGR valve properly is with the long socket wrench extension and angle arrangement I showed pictures of in that one post listed above. Then after it is removed you can take off the two nuts holding the EGR valve to the EGR gasket on the intake Y-manifold. The big EGR nut on that pipe is 32mm. It takes 47 ft-lbs of torque. The two little nuts holding the EGR valve flange to the Y-manifold and gasket take 13 ft-lbs of torque.
To torque it down properly later you'll need a 32mm "crows foot" adapter and you'll recalculate the correct torque setting for your torque wrench with this handy, easy calculator tool: http://www.cncexpo.com/TorqueAdapter.aspx
The EGR temp sensor may or may not be the issue. That's why I recommend testing it with a basic $20-$30 electrical multi-meter first. It is only a simple electrical resistance reading between the two wire pins on its pigtail that you would want to get. Toyota shows a procedure to test the sensor in a pan or pyrex with oil on a stove heating element but realistically it is a whole lot easier to just test it still installed in the car with the sensor connector disconnected and hooked up to a multimeter to grab the readings while the engine is warming up from full cold to full operating temperature.
If you don't want to though, get the OEM sensor from Toyota or Lexus (it should be the same part as for a 1995 Supra Base/Non-Turbo) or NTK and Standard Motor Products makes their own versions.
The EGR VSV is only made by Toyota. You can get one new or used. Those can go bad after a very long time but its still best to test the one you have on the car before jumping to replace it. It's very simple: get some small electronics electrical wires with alligator clips at each end, clip one wire each to the pins inside of your VSV's connector end, note which is positive and negative and put the other end of the negative wire to battery negative and then put the positive wire's other end briefly to battery positive. A good VSV will click on as you briefly connect the circuit and off as you disconnect the circuit.
Before proceeding though I recommend you get a "new" (ie: used good) MAF sensor unit and reinstall it exactly as you described. That is currently the reason your car will not idle or run quite right, why it will not stay running for long and why you have a CEL Code 31.
Again, I've got a good one if you'd like it to replace your old one. You shouldn't have trouble finding one used otherwise.
PDMichael, I'm not currently in LA. The MAF sensor would ship from Florida. I bought it along with another one in 2016 when my then-mechanic did the same exact thing to my original sensor. Tested both in good condition, used one and stored the other good one as a spare. Now that I've gone GTE I don't need either.
MAF sensor - '95 SC300
