Chris Taylor Posted November 30, 2015 Share Posted November 30, 2015 new to the forum, hoping somebody can shed some light on my issue. 1998 Legacy Outback had it for around 1 year after buying it needing head gaskets, drive shafts, and a couple of other minor issues. Son has been driving this with no problems since I did the rebuild. Last week the battery went bad, got a new one from the local parts store and fitted it. On restarting the engine the idle was rough for a couple of seconds before shooting up to 7000 RPM and staying there. Nothing else was done other than replace the battery. I have cleaned the IACV after checking the forums yesterday. have seen many discussions on rough idle and the need for the ECM to re-learn, but none have mentioned this issue. Vehicle is the 2.5L with 115,000 miles. No other issues in the last 12 months. Help needed....thanks in advance. Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 30, 2015 Share Posted November 30, 2015 Nobody accidentally hooked the battery up backwards did they? Does it do this repeatedly? Or just the one time after you replaced the battery? Couple things that would cause that. Major vacuum leak. Dead TPS. Dead computer. Link to comment Share on other sites More sharing options...
Chris Taylor Posted November 30, 2015 Author Share Posted November 30, 2015 cant vouch for the fitting of the battery as I wasn't there when they put it on. The high RPM happens every time the car is started. I tried putting it in gear to see if this dropped the RPM's but that didn't work either. I have checked all the vacuum lines and cannot find any bad areas. Would a dead computer cause any other issues that would point me in that direction? Link to comment Share on other sites More sharing options...
crazyhorse001 Posted November 30, 2015 Share Posted November 30, 2015 (edited) Nobody accidentally hooked the battery up backwards did they? Does it do this repeatedly? Or just the one time after you replaced the battery? Couple things that would cause that. Major vacuum leak. Dead TPS. Dead computer. A major vacuum leak would lean the car out causing a high, rough idle A dead tps would still idle normal, but stumble on acceleration I would think a dead computer wouldnt allow the car to start. It wouldnt get fuel or spark at all. Id be looking at the throttle cable, and linkage. Going to 7000rpm says its hitting the rev limiter. This screams hung throttle to me. The engine cant rev like that with the throttle closed. It cant get the air any other way. Edited November 30, 2015 by crazyhorse001 Link to comment Share on other sites More sharing options...
Chris Taylor Posted November 30, 2015 Author Share Posted November 30, 2015 Throttle linkage was my first thought, but it appears to be OK, moving freely etc. Also when I had the airbox off today the flap in the throttle body was closed. That is why I went for the IACV as I think this allows air to bypass the throttle body on a cold start. If I am correct. Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 30, 2015 Share Posted November 30, 2015 I've had bad TPS report WOT even with the throttle plate closed and redline the engine immediately. Heartless had a major vacuum leak a few years ago that ran the engine into redline immediately upon starting. Fried computer can cause any number of problems. Just depends on the failure mode. Could be one transistor that burns out and causes it to think throttle is wide open. Throttle plate stuck open is possible as well. Should be pretty obvious by looking at the throttle cable and lever. Link to comment Share on other sites More sharing options...
Chris Taylor Posted November 30, 2015 Author Share Posted November 30, 2015 I will take another look at all the vacuum systems, are there any obvious or common culprits. If not I guess it is worth buying a TPS and trying it. Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 30, 2015 Share Posted November 30, 2015 Check voltage on the TPS center pin. Should only be 0.5v with throttle closed. 4.0-4.5v with throttle wide open. Brake booster hose would be the only obvious one large enough to cause that. I've seen it caused by bad intake manifold gaskets/loose manifold bolts. Heartless' case was caused by missing fuel injector o-ring on the base of the fuel rail. Link to comment Share on other sites More sharing options...
crazyhorse001 Posted November 30, 2015 Share Posted November 30, 2015 Not doubting your word fairtax (awesome screen name btw ) How did the engine get enough air to run away like that? The iac cant supply that much air, can it? I can somewhat see the major vacuum leak, but the ecu shouldnt fuel if it sees idle conditions. In either case it should lean out well before 7k rpm. There should only be enough fuel to support 4.5-5k rpm before it leans out so much it cant burn anymore. [baby Sinclair] I dont get it [/baby Sinclair] lol Pardon my trying to learn in your thread Chris. Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 30, 2015 Share Posted November 30, 2015 Engine really doesn't need much air to support 6000 rpm with no load on it. If the ECU thinks the throttle is open it'll give enough fuel and timing advance to run right up to redline on its own, and it will get enough air between the gaps around the throttle plate and the IAC, which is gonna be open pretty far for a cold start anyway, to do it. Especially on a 4cyl engine like these. Before I said Wide open throttle, but that really isn't right since generally if you open throttle 100% the ECU will cut fuel entirely to clear a Flood condition. But if a TPS fails so the ECU thinks throttle is 50% open that's just as good as wide open for an engine with no load on it. If you've ever reved the engine to 3,000 rpm or so and held it there it only takes about 8-10% throttle to get it there. Big enough Vacuum leak, it'll run right up as high as it can. Cold start, the ECU is running the injectors wide open trying to get idle mixture rich. Plenty of fuel there to run away. If you were to allow the engine to warm up like that, it would eventually slow down because the fuel mixture would lean out more as coolant temp rises. Funny thing about cold idle fuel mixture, it's actually burning lean because cold fuel/cold air doesn't allow the fuel to vaporize completely. The fuel condenses into droplets too large to burn completely during the combustion process. So that cold high idle when you think its running rich, you've actually got a lean burn, yet lots of excess unburned fuel out the tailpipe because it just can't burn those giant fuel drops. Link to comment Share on other sites More sharing options...
Chris Taylor Posted November 30, 2015 Author Share Posted November 30, 2015 Your comment about unburnt fuel is correct, you can smell fuel even after just a few seconds of running. I am going to try and get to the car today and check the TPS voltage as well as linkages etc. Link to comment Share on other sites More sharing options...
crazyhorse001 Posted November 30, 2015 Share Posted November 30, 2015 Thanks for the education fairtax I like being proven wrong, i learn stuff that way. Chris, good luck i'm gonna let the guys who know help from here #thumbsup! Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 30, 2015 Share Posted November 30, 2015 Not tryin to prove anyone wrong. When you think about it, it doesn't make sense that the engine could redline with the throttle closed, and I would tend to agree with you if I hadn't dealt ah this type of problem before. I've actually had that happen twice on older GM v6 cars. One that I owned and the engine revved on its own to redline while I was driving it... In 5 o'clock traffic. Luckily I knew enough to put the car in Nuetral and turn the key off. Same thing happened to a neighbors car several years later. But anyway, /threadjack] Link to comment Share on other sites More sharing options...
Chris Taylor Posted November 30, 2015 Author Share Posted November 30, 2015 checked the TPS at lunchtime. Minimal voltage throttle open or closed. I have a new TPS coming tomorrow but I am out of town and wont get chance to fit it until Thursday. Hopefully this is the fix. By the way I am always happy to be proven wrong, the only way to learn Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 30, 2015 Share Posted November 30, 2015 Did you happen to check to see if the TPS was getting the correct 5v supply? And you made sure you had a clean ground or grounded the meter to the negative battery post? When you install the new TPS you have to adjust it so you get 0.5v on the center pin at closed throttle, Otherwise the ECU doesn't know how to set idle speed. You will also want to probably disconnect the battery negative for 5 minutes or so to reset the ECU. Link to comment Share on other sites More sharing options...
Chris Taylor Posted December 1, 2015 Author Share Posted December 1, 2015 didn't check the 5V supply. Meter was grounded well. I will make sure we adjust the voltage to 0.5V as you suggest. May not be until the weekend now as Advance gave my daughter the wrong part and they may not have the right one for a couple of days. Link to comment Share on other sites More sharing options...
Fairtax4me Posted December 1, 2015 Share Posted December 1, 2015 Might want to just check and make sure its getting 5v. Also check the opposite pin (one end is 5v, the other is ground but I can never remember which) for voltage on the ground side. Should show less than about 0.2v on the ground pin. Link to comment Share on other sites More sharing options...
Chris Taylor Posted December 4, 2015 Author Share Posted December 4, 2015 update, daughter managed to get the right TPS from the store. Fitted it but problem still the same along with smoking alternator belt because of the max RPM's. Back to the drawing board. Hoping its not the ECU Link to comment Share on other sites More sharing options...
Chris Taylor Posted December 6, 2015 Author Share Posted December 6, 2015 update. All out of ideas, so today went to the local pic a part and got an ECU for $35 to try and eliminate the computer as the issue. Last night the battery was flat so charged it over night. Replaced the ECU, reset the CEL and started the car.....no luck same problem. So replaced the original ECU and TPS, checked the MAF and IAC with the meter as per the Haynes manual. Re-checked the TPS and got good voltages, so looks like I had a bad ground when I checked it the first time. Went to restart the car and the battery did not have enough charge to start the car, only 30 minutes had elapsed since I started it the first time. Is it possible a bad new battery could be causing the problem. My son now tells me that the radio unit is staying on after the ignition is off and the key is out and it wasn't doing this before the battery was replaced. I am now at a total loss, I am going to get this battery replaced and see what happens. Link to comment Share on other sites More sharing options...
Fairtax4me Posted December 6, 2015 Share Posted December 6, 2015 Bad battery won't make the radio stay on. It really shouldn't cause the high idle problem either. Maybe try unplugging the alternator? Link to comment Share on other sites More sharing options...
Chris Taylor Posted December 6, 2015 Author Share Posted December 6, 2015 I will try anything at this point!! Link to comment Share on other sites More sharing options...
Fairtax4me Posted December 6, 2015 Share Posted December 6, 2015 (edited) Do you have a code scanner? Have you checked it for codes? I've heard of alternators causing no start issues and some other odd things. Not a redline idle, but who knows?! The radio thing kinda smells like bad ignition switch. The only way it gets power to turn on is from the ignition switch. Maybe something goofy happened in there. Edited December 6, 2015 by Fairtax4me Link to comment Share on other sites More sharing options...
Chris Taylor Posted December 6, 2015 Author Share Posted December 6, 2015 I have a code scanner, not showing any codes. I am going to pull the alternator today and have it checked by Advance Link to comment Share on other sites More sharing options...
Chris Taylor Posted December 6, 2015 Author Share Posted December 6, 2015 well the alternator is good, no issues with it at all. So re-fitted it and started the car. Vehicle stuttered for a few seconds, idle was below 300 then it rose to 7000 again. Turned it off and tried it again, same again. It tried to stabilize but quickly went up to max RPM. Checked the codes and it is not showing P0122, which if I remember (don't have the book in front of me) TPS low signal input. Not sure if this is relevant or not. Link to comment Share on other sites More sharing options...
Fairtax4me Posted December 6, 2015 Share Posted December 6, 2015 Did you start the car with the alternator unplugged? Link to comment Share on other sites More sharing options...
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