Rough Idle Issue

[suprajim]

Hey guys, I need some help with a rough idle issue. I've searched, but couldn't really find any other posts with the same symptoms. My car (92 Turbo 7MGTE A/T) starts up fine and idles at about 1200 RPM for about 30 seconds then it drops to about 700 and sounds like it wants to die, but never actually does. It drops RPM's, but then recovers and goes back up to about 1200 before dropping again. The only code I have is 21 for the O2 sensor. I had recently replaced the vacuum hoses with a new silicone kit, so ruled a vaccum leak out. I thought maybe I had a misfire from oil on top of the plugs, but had replaced the #3 head gasket and replaced the plugs (NGK platinum gapped to spec). The wires are NGK and I tested them with an ohmmeter when off the car and resistance was well within specs. I also checked the ignition coils and they seem fine as well. I drove the car around the block afterward to burn off the oil that dropped in the cylinders after pulling the spark plugs and seemed to have plenty of power and it doesn't sound like it is misfiring when I get on the gas, but the idle issue remained. I adjusted the timing, which was at about 25*, back down to the factory spec of 10* and noticed no change at all. I don't see any smoke and the previous owner replaced the head gasket with a metal one about 10k miles ago. I'm going to check out the ISC valve next, but didn't know if anyone had any other suggestions.

[cre]

Have you made sure the TPS is properly calibrated?

Well, the O2 sensor is easy enough to check... unplug it. It's just a trim sensor, but the ECU is allowed to add quite a bit based on the O2 sensor's feedback.

Recheck the vacuum hoses. As silicone is non porous and rather slick you should use clamps or sip tied at every connection to be sure none leak or blow off.. I had two which just refused to stay on.

When adjusting the timing I trust you had the diagnostic jumper in place.

Does the idle issue occur both in park and in neutral? I don't know a whole lot about automatic transmissions, so I'll leave this to someone else.

Have you tried to clean the AFM at any point? The AFM is VERY easy to screw up by 'cleaning' it as it is an optic device... most cleaning agents will fog the lenses and sensor. you can unplug the AFM and see how the car runs as well, but I don't think the AFM is likely.

[suprajim]

I have not checked the TPS, but was going to pull it off and make sure it is correctly calibrated using the instructions found on a UK forum. I had previously replaced the O2 sensor, but the light stayed on even after trying to clear the code. So I'm not sure if it's a wiring issue or possibly related to this problem. When I changed the vacuum lines, I did zip tie each one of the ends so I think that rules that out. I did jumper the connection when I adjusted the timing. I haven't actually tried to see if I experience the same symptoms when in anything other than park, but will try that when I get back to working on the car. I will also take a look at the AFM. I have not done anything to it at all since I purchased the car. Previously the car seemed to be running fine, although a little rich. One day I noticed a vacuum hose that was capped with a bolt (leading to the factory BPV) and once I tried to fix that, it all started snowballing into these other issues. I was initially going to take it to a mechanic to look at for the O2 sensor, but found all of these other issues and now want to try to solve the issue myself to learn and save some cash. As always cre, thanks for the advice.

[cre]

Start with the TPS. http://www.cygnusx1.net/Supra/Librar...spx?S=FI&P=101 It has a switch in it which is used by the ECU to determine if the ISC is to be employed when other certain criteria are met.

[suprajim]

Ok, so it is too dark to really mess with the car right now, but I had unplugged the ISC valve electrical connector while the car idled and there was no change in behavior....still idling erratically. Could this alone point to a bad ISC valve or could the TPS still be a potential cause?

[cre]

Any diagnostic codes stored in the ECU?
http://www.cygnusx1.net/Supra/TechTi...ror_codes.aspx

TPS Calibration:
http://www.cygnusx1.net/Supra/Librar...spx?S=FI&P=101

ISC (Idle Speed Control) testing:
http://www.cygnusx1.net/Supra/Librar...ction=FI&P=106


More testing, less guessing.

[suprajim]

Ok, sorry if these are stupid questions, but I want to be sure I do this right. Do I need to have the engine running when I test the TPS? Secondly, how do I apply "battery voltage" to the ISC valve? Do I just run a short wire from the battery?

[MA70-3.0GT]

If you take a bit of time to properly read through the tsrm links cre provided you'll find it answers your questions... More haste = less speed as they say, stop trying to rush through without researching properly as I suspect you're doing & you'll find it's a half hour job!

If I'm honest the isc test is one you can usually skip provided you can hear it resetting (clickclickclickclickclick) after engine switchoff, just make sure it's clean inside & the one way valve isn't held open by any bits of rubbish...

EDIT-: and I assume you have no codes stored? That's obviously the first thing to check.

[cre]

Hell, the car may still not idle properly once the original problem is fixed due to the manual adjustments made. In this case the idle is usually just very high but reasonably steady.

[suprajim]

Ok, so I had just checked the resistance of the TPS and I kept getting about 7K. I followed the instructions to the T and inserted the .7mm feeler blade and loosened the screws, turned clockwise and the needle of the ohmmeter never deflected and constantly stayed at about 7K. So, am I doing something wrong or is my TPS bad? I'm sorry if I seem like an idiot, but I am trying to learn. I've done many other mechanical jobs on this car, but these sensors and electronics are new to me. Thanks again for the help. As a side note, I checked again and still only have code 21 which the car has had for the 2 months I've owned it, even after replacing the O2 sensor and pulling EFI fuse and battery to try to reset.