I'm new here but I've been lurking for a while. I bought a 1995 SC400 about a month ago and I've been having a small issue with it. First off, a little info about the car. It's got 110k original miles, 2.5" x-pipe into dual straight pipes (stock headers/cats), and a MAF adapter with an open element filter.
Recent maintenance done by me and the previous owner-
NGK BKR6EP-11 plugs and wires 5k miles ago
t-belt done at 89k
oil/filter change 100 miles ago
trans filter and drain/fill with TIV
Dizzy caps and rotor buttons
regapped plugs to .032 and then .044 (no change)
Cleaned IACV as best as I could (my IACV looks different than the cleaning writeups on here, the spring isn't exposed)
checked for vacuum/intake leaks
Jumpered fp and b+ to see if it was the fuel pump ecu, same issue
93 octane fuel
NO CEL's or trans codes
seafoamed through vacuum line and gas tank
Now, on to the problem. The car runs like a champ 99% of the time. Unless I'm under extremely light acceleration, like 1/8th throttle, (car temp, PWR/NORM mode, outside temp, speed, and gear do not affect it) the car will sort of studder/buck/stumble slightly enough for my passenger to tell and will continue doing so until I push the accelerator down a little more. I'm no beginner to old Toyotas (mainly Turbo MR2s) and it feels ignition related but all of my ignition components are basically brand new.
Do you guys have any ideas or suggestions as to what it may be?
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Just did this resistance test on my MAF with my miltimeter set at 20k ohms and the reading stayed at 1.97 ohms at around ~76*F (from what my home thermostat says) so I believe the MAF is still good from this test?
Reset ECU, plugged it back in, went for a drive, and it still did it. As soon as I felt it doing it again I unplugged the MAF and it stopped once more and felt like the car had much more power with it unplugged.
generally unplugging the maf just covers the real problem because the ecu defaults to running super rich and less timing across the board. sounds meaner but you are probably using a bunch of gas that way.
I would double check the IACV. alot of people clean it and it doesn't really restore its full operation unless the plunger can move very freely, it needs to be able to operate in the full range of motion. the ecu uses this and timing control to hold an idle, and if the iacv isn't up to par its swinging the timing trying to compensate and weird things happen. I am only mentioning this first because you said the car shut off on you that one time. generally thats a sign of a going bad iacv or a bad coilpack, and you already replaced the coils you said. you can try unplugging the idle valve and see if it makes a difference in how it starts and idles, if there is not much change at all I would suspect it at that point.
also if when you start your car for the first time in the morning (cold start) if your rpm's don't shoot up very high and the engine is idling at 900 or below you pretty much have iacv problem, it should be above 1200 and when cold the IACV is pretty much responsible for all the extra air the engine needs. when running warm the iacv is in more of a rest position (the position it usually gets stuck in), so alot of times when it sticks you will have start issues especially in the winter, but then the car will run alright when warm but not perfect since the ecu cannot fine adjust.
also you can check the tps and also check the PCV hose for cracks, as well as the other line going from the crank case to the intake pipe, a crack in either of these can cause hesitation issues that are related to the maf.
also in your test above you test tha, which is the air temp sensor on board the maf, you may want to actually test the maf signal part, which is the middle pin "KS"
My cold start idle is fine and it idles at around 650 RPM in drive after warm and about 800 in park when warm. I checked the PCV hoses and both were in tact. I couldn't figure out how to loosen the bottom screw of the TPS unless I got a 90* phillips head or remove the FPR so i haven't messed with it yet.
My IACV looks different from all of the cleaning guides here. When I took it out to clean it all you can see is the bulb, not the entire spring like in most of the pictures on here. I tried pushing it down with a pen using a great amount of force and it wouldn't put which leads me to think it was all the way open when removed. Should the plunger be fully up against the base when I remove it? Here is a picture of my IACV.
Also, do you have any directions on how to test the actual MAF part?
are you sure your cold start idle is fine? you basically just removed the carbon buildup off the top of the plunger, but you have to actually remove the plastic part of the iacv from the metal base to expose the bottom part of the plunger with the spring. the plunger will be in the last position it was in on key off, which is normally where it was last or sometimes it will ready to an engine start position for the next start if the key is moved to key on, that is of course if its working.
often times things inside get grimed up and the plunger wont go all the way in or out. you should be able to turn the plunger and as it turns it will go in our out, as there are magnets in the base which the ecu controls to spin the iacv, and this moves the plunger in and out, but over time the magnets wont be strong enough to turn it due to buildup etc. what I usually do is shoot that part with a bunch of wd40 and work it free till it spins right, then grease it a little bit and reinstall. or just replace.
pushing down on it wont work and if it really wont spin at all its probably stuck around that position or has limited travel. when you go to reinstall you may have to give it gas to get it started the first time if you reassembled it in a more closed position, until the ecu gets a few minutes to sort it out.
if your cold idle is high enough then its probably not the iacv, but from the way you described cleaning it that is defintely something that can be an issue thats why i asked about the idle and the stalling because sometimes it will fail in a favorable position (the position its in the most) so it can go unnoticed for a while.
on the tps yes, a right angle screwdriver goes a long way. If you have never messed with the tps before and it was running properly before, then maybe don't mess with it. if the car has had the problem the whole time, then maybe someone could have played with it and you may want to test/adjust it. I drove for like 2 months with a bad tps and no check engine light, you will be surprised how much the ecu can compensate for things.
O2 sensors to my knowledge haven't been changed. I'll try adjusting the TPS when I get that screw driver but I'm pretty sure the IACV is okay. I don't really want to throw parts at the car. I'd rather narrow it down and work from there.
The tps adjustment is just a guess and that is why I asked if you have played with it before or if the problem was there when you bought it because then there is a chance the previous owner played with it. If it was running fine before and no one has touched it I would recommend you not adjust it, if you have to do anything just remove the connector and do checks in place.
well if the o2 sensors haven't been changed it is likely having an affect and could explain your issue. If the o2 mixture is off or wrong, you could have issues on part throttle. they are supossed to be changed every 60k, after that they are usually slow and over 100-150k they pretty much aren't doing their job anymore, but the older ecus rarely throw the code for it they just think its still warming up. Plus you only need to replace the front 2 o2 sensors, and the non heated ones are under $20 a piece. the savings alone in gas over time is worth it.
Just drove with the TPS unplugged and although I didn't have any hesitation (the issue is random anyways) the transmission wouldn't shift until 3k RPM and it idled pretty high. I'm getting a right angle screw driver tomorrow and seeing if it's in spec/needs adjusting.
TPS is in spec. Another side note, it seems to do it a bit worse when cold and it almost feels like the timing is being severely retarded and making the bogging noise/feeling. This is becoming very frustrating.