Ok. I've done my research and searched through a ton of threads about this issue and have generally come up with a steady answer to my problem. Basically, what is going on is that when my 92' SC400 is in park it idles up and down from about 1250-2000rpm. From my research I've found the general culprits to be the MAF, IACV (Idle Air Control Valve, for those who call it some other name), or a vacuum leak.

So far I have checked for vacuum leaks and found none, cleaned the MAF off with CRC, and removed the IACV and cleaned the plunger (which was all gunked up). Throttle cables aren't sticking either, b/c I did see that in one thread. Prob something I'd have overlooked.

Unhooked the battery to clean the MAF and tried starting it up. Fixed the problem for like 20 sec (wasn't looping, but was idling at 1100) and then when back to the idle looping. Same story with the IACV. Figuring that this 20 sec of "good" idling was just b/c the computer was getting reset by unhooking the battery.

Tentative plan is to take the whole throttle body of and clean it up. I have also heard that the TPS could be the issue. Not really sure what I should be looking for or adjusting here.

Any help on theories or suggestions are welcomed. I'll take pics of all the work I'm doing, for a write up on this issue later. Don't have a working multimeter though to check pins on MAF or IACV or anything though (but I believe there is already a write up on that)

Thanks,
Sean

 

If it's bouncing up and down at idle then it's probably your tps.

Has it ever stalled?

 

Do you remember if there is a thread about fixing/tuning the TPS in the tutorials sticky? I thought that I remembered seeing one but I'm not finding it right away. I'll check it after I get off work.

If I remember correctly though I should loosen the screws and turn it clockwise? Also, I may just go ahead and take the whole throttle body off, and clean it out this weekend, so are the throttle body gaskets reusable? Also, any thoughts on it being an EGR issue?

 

tps requires a multimeter and a set of feeler gauges. just loosening and twisting will cause more problems.
There was a post recently about using a molex cable from a computer as an adapter for the tps harness. so far, I haven't seen a true "write-up" (then again, I haven't looked, either).

 

You know what? I might have even gone through that thread. I'll see if I can dig something up. My multimeter isn't working, might have to pick up a new one and put this off till the weekend. Unless anyone is feeling really generous. lol.

 

It could actually be your IACV. The IACV is just a simply stepper motor that is controlled by your ECU. I had a similar (but not exactly the same) issue with my 93. I replaced my IACV ... did not fix. Replaced the ECU ... fixed.

Good luck ...

 

Definitely going to run some more diagnostics on it and do a more thorough disassemble of the throttle body and IACV to make sure. Also picking up a multimeter to check harnesses. I'll try and do the diagnostics and everything step by step for a write up on this issue. Seems to happen quite a bit.

 

IACV would be my bet. I had the exact same issue and replacing the IACV with a working used one fixed it.

 

What's the word on where you got one and for how much? They are looking nasty expensive. I found a guy near me on Craigslist selling a used one for $150 with 130k on it. That's about what my car is at and I think there is a tutorial on taking these apart and servicing em. i'll look more into it.

Against better judgment and warning, I adjusted my TPS, turned it counter clock-wise. Seemed to get rid of the idle running in a loop, but now it's sitting around 1750-2000 rpm.

What are your guys' thoughts on it being the EGR?

 

Luckily, I got a used one for $80 from my local shop. Looking around, I found two online which were both around $120 - the one you mentioned seems a bit pricey.

There is a tutorial on servicing these; here's a good one from Peter Scott:

http://planetsoarer.com/IAC/iac.htm

 

Yep! thats the tutorial that I was looking at earlier. luckily I checked this first b/c I was about to go search for it.

As far as the IACV that I found on craigslist, I'll prob hold off on it till I run through this valve cleanup.

 

Does anyone know if the EGR can be the cause of this problem? I unplugged the EGR while the car was running and it didn't have any affect on how it was running.

 

As I stated prreviously The problem may very well be your IACV ... but the valve is just that, a valve. it is controlled by the ECU. If you clean your IACV and it still loops then consider a used ECU.

IACVs = about $600 at the dealer but you can get them on eBay used for about $50 (or more).

ECUs = about $2,500 at the dealer but can be had for as little as $100 or so used on eBay. If you buy used make sure you get the exact same model number as they changed even within model year.

 

Any tutorials on checking whether the ecu is working properly? I will see if I can get my hands on a manual in the mean time

 

Might be just your tps.