1995 Toyota Previa sc/awd low idle issues

[x2cy]

I figured i'd start another thread just to keep it separate. My van has fairly low miles (238 km). It runs fine and has no engine lights on but it seems to idle fairly low, especially when stopped at lights and especially when the a/c is on. I have cleaned out the throttle body, double walled cooling line (with the T intersection) and the IACV and it didnt seem to help much. Just had the plugs, wires, cap and rotors done as well not too long ago. Any sort of help or ideas would be great. I have ordered gaskets to complete the egr system cleaning but almost hate to do it as there are no engine lights on.


Thank You

[cvtroger]

My classmate's previa has some low idling issues as well.

It is after starting the car, it kinda bogs down and dies, but after a few tries, it becomes "fine".

Even after cleaning out her MAF and doing the whole throttle body and IACV, it seems kinda intermittent. It doesn't happen all the time.

I also wonder if there is a load issue. If her A/C is one, it will die shortly after. Kinda like your issue.

Yesterday when we were leaving class, her an was idling good, but as soon as he put in reverse, died. This happened twice.

[x2cy]

Thank you for the reply. hmmmm, one of my older previas had the exact same symptoms as your classmates but the van was so rotted I just set it aside. Mine current van isnt stalling as of yet, just has low idle when in gear (forward or reverse) when stopped and very low when the A/C is on. Im going to steal the TB and IACV from one of my other vans and see if anything changes. I will keep posted

Thanks

[x2cy]

ok. So I took the TB, IACV, MAF, VSV, intake hose and air filter lid from another one of my previa's. Higher idle and noticeable increase in idle when the a/c is on so i believe I have corrected the issue. only other issue is, because I switch all the parts....i dont know what helped and which part is faulty...

[TheMAN]

more than likely IAC
if it's the ISCV, it'll cause low idle when A/C is on

[nobrush]

What is the difference between the IAC (Idle Air Control?) and the ISCV (Idle Speed Control Valve?)? I'm having issues with a really low idle to where it died within about 10 seconds after starting. After taking off the Idle Speed Air Control and testing it (appears to work correctly), taking the Throttle Body off and cleaning it thoroughly and putting it back on with a new gasket, its still idling low. I just overrode this by raising the idle and now it just takes longer to warm up, doesn't idle as low at stop signs, but I figure it can't be hurting anything. If anything, I think it's better for the engine since it doesn't rev so high when cold like it's supposed to and then come down after warming up. Am I wrong here?

I'm thinking next step should just be to do a standard tune up and then go from there. I was thinking also to artificially activate the ISAC with 12V just after start up to see if it's not getting the correct signal from the computer, unless I can just test for that at the plug maybe.

On a side note, I accidentally broke the little BVSV (bi-metal switching valve) putting everything back together but just bypassed that with the vacuum lines to just always have suction until I can get one at a junk yard. I see they're not made anymore and dealers/parts stores can't order them.

[eastman51]

The IAC valve is what the ECM uses to control the engine's idle at any given time. The ISC valve would be activated with the AC switch to raise idle when the AC is activated.
The higher rpm cold idle is wholly intentional, higher rpm + richer fuel mixture forces the engine to warm up faster. Realistically it will not harm the engine, especially if you waited for it to idle down after a few minutes; driving right after a cold start, especially when reaching rpm over 3-4k would be significantly more harmful but realistically not taking that many miles off the engine's life.

I've not had experience with a low idle like this on a fuel injected car, but have had similar issues on a carbureted engine. Of course, on a carb, idle speed is all analog; on fuel injection, idle speed should be adjusted by the ECM at the IAC valve. If the IAC is cleaned and such, you should check that the wires on the harness side are still good; such as reference voltage, 12v, ground, etc. You'll probably want a wiring diagram and if possible the Toyota FSM since it has troubleshooting steps.
On the same car, when the AC kicked on, idle would drop drastically low and made it hard to drive without stalling constantly. The fix ended up being adjusting the plunger for the ISC diaphragm (carb tech) so it would actually catch and pull on the throttle to raise the speed. On an EFI car, this is all done electronically, so more troubleshooting would likely by required.


If I'm not mistaken, the BVSV is basically a temperature actuated vacuum valve that prevents the evap system from venting into the intake before the engine coolant has reached a desired temperature. It should not matter if it's bypassed. My 95 Corolla has been running with the BVSV bypassed for years and still gets excellent mpg and has no issues running.

[nobrush]

Thanks for the reply and the detail on what those do. One thing I forgot to mention is that I've had a somewhat similar issue before where it would die at stop signs (manual tranny so it'd be out of gear). I cleaned the throttle body/butterfly AND replaced the fuel filter at the same time and it fixed it on 2 different occasions so I think I'll try that next before digging into electrical.

[nobrush]

So I put 12 volts directly to the IAC and it fixed the low idle. Apparently it's not getting a signal from the computer. I have the Haynes manual but it doesn't show the IAC on the starter circuit or engine control pages. Does anyone know if there's a good schematic on this site or where to find one? I went by color (I'm colorblind or really color deficient) and didn't seem to be getting any continuity to any of the 3 that looked similar going into the harness that plugs into the computer. They look to be white, red/black, and black wires. I could just jump them or test the computer with the switch turned on since there's supposed to be 12V there but not sure which pin on the computer it should be.

My thought is that if I can't figure it out, I do have a spare computer from the last Previa I owned although it was an automatic and this one is manual. I'm hoping that doesn't matter but guessing that it will since the clutch needs to be pushed in for the van to start unless the switch is inbetween the battery and the clutch, and then goes to the computer from there.

Another thought is that I could run 12volts to a potentiometer/rheostat that I have on the dash or something that I could regulate the voltage on the IAC...not sure how I'd wire that up in my head ATM since the center post on the IAC is positive, and the 2 outer ones are both ground...

Hopefully I can find someone who's not colorblind who can help me avoid all that and we trace it down. The funny thing is the wire harness/plug going into the computer seems to be about the closes thing thru the floor thru hole to the IAC....

[John Kaufmann]

So I put 12 volts directly to the IAC and it fixed the low idle. Apparently it's not getting a signal from the computer. I have the Haynes manual but it doesn't show the IAC on the starter circuit or engine control pages. Does anyone know if there's a good schematic on this site or where to find one? ...

I have the EWD from my 91 Previa (recently departed). I have no idea how close it might be to the SC version, but can think of no reason for significant difference in that regard, so this may help. The only two schematics making reference to Idle Speed Control (ISC -- not IAC, which does not appear anywhere in the EWD) are pages 50-60 (section 14: Drivetrain-Engine_Control) and pages 118-123 (section 36: Drivetrain-ECT), both attached. FWIW.

[nobrush]

These seem to work. You're awesome! Thanks. I was able to cross reference these with my Haynes manual and I see the ISC inputs where it meets the computer, they just didn't label the box in the diagram with any identifier that the wires were going to.

Your schematic states "THE ECU EVALUATES THESIGNALS FROM EACH SENSOR (INPUT SIGNALS (1, 2, 4 TO 8, 11)), OUTPUTS CURRENT TO TERMINAL ISC1 AND ISC2, AND CONTROLS THE ISC VALVE." This is a tremendous help that I didn't see in the Haynes manual! My initial thought after reading this is that since the computer isn't putting out a signal when I turn the van (on w/o starting which it sounds like it's supposed to put out 12V) that it needs a signal from the water temp sensor since cold coolant is what make the ISC valve adjust when the engine is cold.

Now it looks like I have to go thru each one of those sensors to see which one is faulty/not sending a signal to the computer. I already did the paper clip jumper test in the diagnostic box under the seat and the check engine light flashes every .25 seconds, which is indicating the computer is working correctly with no errors. I would think that if a sensor is bad, that this is when it'd show up, but maybe not....

On a side note, I took out the computer and put in the old one I had from my previous Previa (was an automatic vs. current manual tranny) I had to see if that was the issue (maybe this was dumb!). I didn't screw the computer down and when I started it the engine made a funny loping noise. I immediately shut it off and put the old computer back in. It ended up working correctly at 1st but then it didn't later on and went back to the loping noise. I didn't remember to screw the 3rd mounting screw to the body of the car and guessed that it was a bad ground, and that ended up fixing the problem. My thought is to put the other old computer back in and make sure the ground is good this time, although it looks like there are several different PN's for Previa computers and wondered if I should chance it. It sounds like an engine really doesn't need an ISC to run so maybe I should just cut my losses, keep the idle raised, and let it be.

[nobrush]

So I put 12 volts directly to the IAC and it fixed the low idle. Apparently it's not getting a signal from the computer.

Correction here as I just retested the signal coming to the ISC connector (apparently I must've previously had the multi-meter leads connected for resistance still/not voltage) and I only get a reading of above 12V on the RSC pin (uppermost pin from ground) to the center B+ pin (battery positive). I'm wondering what could cause that?

I learned that the signal on this is "duty cycle" which apparently means that there's signal going to both ground pins and the computer is varying it according to the needs of the motor. This being said, I'm guessing the RSO (Lowest pin) should be having some larger value of voltage than the RSC pin when starting cold. The reason I say that is when I connected that RSO pin as ground with 12 volts positive to the B+ pin, it idles high like it should when the engine is cold. I already tested the resistance values when it's cold and they were the same (22.5~ Ohms) and this sounds right with other Toyota ISC's online, but if someone has that value that'd be great.

The RSO pin voltage shoots up to 9V and then drops immediately w/o me doing anything when the ignition is in the ON position w/o starting it. I double checked this by using a computer from an old Previa I had, and the same 9v initial voltage jump happened and went back to 0. Whoever is reading this, if you're idle is working correctly, taking this reading would only take a few minutes and would help me tremendously. Just make sure obviously to not let the 2 pins touch that are coming out of the plug- I just used small paperclips and alligator clips to my multi-meter.

Maybe when I fixed my MAF awhile back (apparently the solder joints are temporary - https://www.youtube.com/watch?v=CXCpYj-r_lI&t=758s) I soldered something that now is causing this problem...hmm....still thinking about that one...