97 Legacy GT - P1507, P0500, P1540

[osalmon]

Hello, new to the forum but I'd like to start off with a problem that has been plaguing my '97 Legacy 2.5GT for the last 8,000 miles.

The vehicle is on the original engine and 4-speed auto with just over 128,500 miles and no modifications.

Straight to business, there are three codes that my car has been putting out:
- P1507 = idle control system malfunction (previously found in another forum)
- P0500 = vehicle speed sensor 
- P1540 = Vehicle speed sensor
- Trans temp light - flashes about 4 times after restarting vehicle (mentioned below)
- No ABS lights (in reference to the speed sensor codes)

The main problem: when driving, the vehicle will suddenly have a significant drop in idle rpm and will run incredibly rough to the point where it is no longer drivable, and then die. This is indicative of the P1507 fault and as of right now, I have not made any progress with repair or diagnosis on this front. There is almost no predictability on when this happens, sometimes it is on a hill, and sometimes it is on level ground. it has happened in the same location a few times, but not every time. My short-term solution to this issue has been putting the car in neutral while still moving, turning the ignition off and then back on. The CEL will stay, transmission temp light will flash for a moment, but everything will return to normal afterwards. The part that is weirdest to me is that the CEL will come and go. The car will be fine for 70% of the time and be fully operational, and even with the check light on it will operate just fine. Also, it is worth it to note that there is no problem with the idle UNTIL it does its little slow death thing. 


The smaller connected problems: my speedometer functions erratically and intermittently, however still works for the majority of the time. I bought a replacement speed sensor which I will install when it arrives (in which case there may be a cheap and cheerful solution to my entire problem...if only). I have had the vehicle on a lift many times and have checked the current speed sensor for damage or metal stuck to it (as it is magnetic) and there was nothing to report. Upon replacing the VSS I will use a multimeter to compare for "scientific comparison." In classic Subaru fashion, there is a hard 1st to 2nd shift. Another symptom that I have found, in situations of hard acceleration I have found that the car will literally fall on its face and not rev above 3500 rpm. This problem only happens when the CEL is present and will not happen when the CEL is cleared or absent. I will take a look at the IACV later today to see if there is a problem stemming from there.

My initial thoughts and attempted fixes: This problem first occurred when my GT clicked over to 120k miles (currently sitting around 128,500) and the first thing that I did was change the transmission fluid in hopes of seeing metal flakes or some tell-tale sign that my auto was toast. At this point I was delusional that I could just swap in a manual but now I see that there's no point changing something that isn't broken. Point is, the outgoing fluid was totally fine, I put new in and went on my way. Later I found an ATF leak from my oil cooler (built into stock radiator), so the radiator was replaced, and the leak went away. My thoughts initially were that the problem centered around the ATF overheating - due to the trans temp light that came on after it died - however that seemed to have little to do with anything...new parts though! 

The next move and my current hypothesis: First and cheapest step; clean the idle air control valve as well as the MAF sensor. I am waiting for the VSS so that I can compare the resistance of both sensors to see if the one in the car is truly bad. Concurrent to the VSS, I will also perform some diagnostics on the idle control system, primarily the EGR valve (smoke test or something), as that was the leading cause of a similar problem on my '02 Tacoma. My hesitancy with this is that there were no codes relating to emissions; no P0301 or P0171 like I had on the Tacoma. HOWEVER, I believe that this could be a problem as the exhaust on my car has smelled particularly fragrant lately. Replacing the neutral safety switch is the absolute last thing I want to do - they are so unreasonably expensive and I'm not sure if I trust a junkyard part but honestly I'd rather spend $30 at the junkyard than $300 for the AutoZone special - and especially given that the function of the NSS has nothing to do with the problem that I am having, it seems to be an unlikely culprit. 

This being my first blog post, I have no idea what I'm doing, how I am doing, or how I got there, but it seems that I may have answered some of my own questions in the process of doing it. I sincerely hope that someone out there has an answer and that others are able to find some use from this. Advice is appreciated, suggestions would help, comments and criticism of not dealing with the problem sooner is also warranted.

Each problem a car has is unique to every other problem that it has.
-O 

[doublechaz]

I believe there is a separate VSS for the speedo and for the computer.  If the computer is getting bad speed info that will cause some problems.  The IACV cleaning is where I would start.  Did you find info on how to?  Correctly cleaning a MAF never hurt anyone.  Do it wrong and you'll be in a world of hurt, so be sure of that as well.  They make a special cleaner just for that task for a reason.