The computer can indeed get confused by the TPS if the TPS isn’t correctly reporting the throttle position. You still haven’t determined what exactly is causing the big increase in idle speed with the IAC disconnected. It sounds tho like the increase in idle speed might not be from extra air getting into the engine, but that the timing gets advanced. Advancing the timing will usually cause the idle speed to increase too. So you may be on to something there. The computer may be increasing the timing incorrectly, b/c it is getting a false report of the TPS moving. A bad crank or cam timing sensor could cause this too.
The TPS is a pretty simple device usually. It is either just a switch that indicates when the throttle is at idle or wide open. Or, more likely in your case, it is a variable resistor that smoothly changes its resistance as the throttle moves from idle to wide open. There should be a procedure described in the factory service manual to test it. Find it and test the TPS is probably the thing to do at this point.
Don’t have the FSM? There’s several options besides the local public library. First Google it. Maybe somebody has already posted the procedure on the internet. Second, visit a local Sube dealership. At a time when they are not busy. The service guys there may be willing to print the procedure out for you. Third, ask if Sube offers a service where you pay a small fee to view the FSM via the internet. Many manufacturers offer this service, you pay $25 to view the FSM for a week via your computer. I think AllData also offers a service where you can view their condensed version of the FSM via the internet for a small fee.
GeorgeSanJose - The rpm did drop with the IAC disconnected. Warm idle went from 1050 rpm to about 200 when I disconnected it, then recovered and went to a lumpy 850 rpm idle and stayed there. Advance was at 10 deg at this point ( maybe if the PCM doesnt find an IAC connected anymore, it goes to an open loop, ‘limp home idle mode’).
I did find a FSM online; its for a 2002 Forester, not a 2003 Baja. However, it tells you to connect a Subaru Select diagnostic tool to troubleshoot idle problems, and I suspect a 2003 Baja FSM will tell you to do the same.
GeorgeSanJose - I forgot to mention that there is a TPS test in the Clymer manual. Test is done on vehcle, plugged in, to see if it has about 0.5 volt at white wire with throttle closed, and full open it should be about 5 volts, all with Ign ON engine OFF. It passed this test, and it also seemed to have smooth voltage transition across whole range of throttle travel
Also I removed the TPS from the throttle body and did bench test of resistance vs position. Turns out there wasnt any result- it was open circuit at all terminals at all positions. This is not as strange as it sounds - there must be a solid state device of some sort inside the TPS that is very hi resistance across all terminals.
The MAP on my Corolla is the same thing, you can’t test it by measuring resistance, although it functions sort of like a variable resistor, you have to test it by measuring the voltage output. Some kind of solid state gadget inside I guess.
It seems like you may be approaching the end of the line of what you can do w/out the Subaru-specific scan tool. Sometimes the ECM circuit boards get solder cracks from the thermal stress over the years, and the only way to fix the problem is to replace or repair the ECM. I haven’t heard of this problem on Subies much, but it could happen. Do you live in a really hot climate? The Subie-scan tool should be able to test the ECM for good/bad. Might be worth it to pay someone with the Subie scan tool for a diagnosis at least. Best of luck.
GeorgeSanJose - Thanks, I figure on finding a guy with the subie scan tool to see what he charges, if its $$$ I will try Ebay for junkyard replacement ECM or throttle body (they’re surprisingly cheap).
Before springing for a replacement ecm or throttle body, a less expensive guess, maybe try another IAC? You mentioned you used an aftermarket IAC, right? Maybe you can find an OEM version from a recycler. Sorry you are having this idle speed difficulty. I have the sense you are very close to solving it. But you’ll probably have to enlist some expert-Subie help to get you over the top.
GeorgeSanJose - I could not find an OEM Subaru IAC at any of the local junkyards (all have been stripped away and they only have a few junked Subes in correct years), but I did get a good price on a throttle body assembly on Ebay, should have it tomorrow.
Cougar- how would I test the fuel pressure regulator?
To check the fuel pressure regulator by checking the fuel pressure getting to the injectors. If you haven’t already replaced the fuel filter that would be a good thing to do first.