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  #1  
Old 09-28-2004, 10:22 AM
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ECU Hiccup

My check engine light came on the other day. The car had been sitting all night and it didn't want to run when I tried to start it. It cranked and cranked for a while, kicking and coughing, but my battery didn't last very long because I had been running my stereo with the engine off for quite a while the night before.

So I hooked my gadjillion amp charger up and kept cranking, because after all, it was at least trying to run with all the kicking and sputtering it was doing. When it finally started up after about a cumulative minute of cranking, the engine light was on. Later in the day, around lunch, it started just fine, but the light remained. The next morning however... With a cold engine, it didn't want to start.

I pulled the codes and got 21, engine temperature sensor, and 34, EGR solenoid. Well, code 21 explained why the cooling fans were both running even with a stone cold engine, but I had just replaced the temperature sensor. Anyway, I replaced it again with the sensor from my other SVX, which has no codes or trouble lights.

The problem was still there. So, just on a whim, I reset the ECU because I was down there playing with the wires anyway. Ha! The trouble light goes away, and it starts on the first crank. WTF?! The 'problem' didn't even exist. The 'problem' wasn't causing any troubles, the ECU was causing my headache! How 'bout that?

My conclusion is that the low battery from running the stereo caused the ECU to go stupid. For some reason, it did more than just store imaginary codes, but also screwed with it's engine management protocol. It's been running fine for a while now.

I thought you guys would appreciate knowing that gremlins do exist.
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Old 09-28-2004, 01:48 PM
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Old 09-28-2004, 02:13 PM
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I believe the ECU can indeed take a temporary trip around the bend. In my case, I'd started the car just long enough to back it out of the garage to wash it, then again to pull it back inside. The next morning it was very hard to start, acting as if flooded, but each time afterward it seemed to catch a bit easier, as if it was slowly forgetting the offense, and within a few days it was completely back to normal, starting readily on the first hit. That was many months ago and I've had no hint of trouble since, but now, whenever I move the car, I let her run for a minute or two to collect her thoughts.

dcb
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Old 09-28-2004, 04:00 PM
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Quote:
Originally posted by dcarrb
I believe the ECU can indeed take a temporary trip around the bend. In my case, I'd started the car just long enough to back it out of the garage to wash it, then again to pull it back inside.
There may be something to that. When this problem occurred, I started the car once before I put it away for the night just to be sure the battery had enough juice to start in the morning. I let it run for about three minutes and then shut it off. ...but again, the battery was pretty low. (Actually, I've got two batteries; One deep-cycle Optima in the trunk and the factory battery, soon to be another deep-cycle Optima.)
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Old 09-28-2004, 04:34 PM
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ECU voodoo.

If the ECU has a sensor go bad, it will default to a base setting to continue running. You can bet that the setting won't be the setting necessary for coldstarting. When the code is cleared, it returnes to the normal inputs.

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Old 09-29-2004, 12:26 AM
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What makes this so strange is that the engine was stone cold and the code appeared immediately upon starting in the morning; and the engine was stone cold when it was started the night before and only ran for a couple minutes before shutting down. The engine was never warm before or after. The only codes were for a temperature sensor and EGR solenoid. Neither of these things would have any reason to function, (or malfunction,) on a cold engine. Moreover, I haven't fixed anything, yet the problem went away by shaking the loose the ECU's marbles.

When both fans came on when the engine was cold, that pretty much tipped me off that the engine was in some sort of default mode. You'd think it'd eventually figure out that the sensors were just fine. Computers in cars. What'll they think of next?

Moving at the pace of technolo... ERROR: UNEXPECTED ARGUMENT AT LINE:517,12 REF:CRASHFUNCTION("SVX");
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Old 09-29-2004, 06:37 AM
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I should have mentioned that my engine was also cold, and I never had a CHECK ENGINE light. The battery was (and remains) strong. Honestly, I know next to nothing about engine management systems, so my rationalization may be way off the mark, but since the trouble went away so readily, I concluded that the ECU got its signals from the assorted engine sensors scrambled by the brief on/off cycle, and it took a few days of routine operation for it to find its way back to reality. Or maybe some sensor just got a temporary twitch.

dcb
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Old 09-29-2004, 12:50 PM
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The ECU should always be monitoring the engine temperature regardless if the engine is cold or not. If it's getting an incorrect reading at any time it will switch into limp mode.
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Old 09-30-2004, 11:50 AM
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I'm not sure how the ECU would detect an incorrect reading. Unless the reading is out of range, it would never know if the sensor was wrong. ...would it? What would it contrast the signal against? Perhaps it has a duty-cycle timer, but that'd be pretty far-fetched. Unless it was out of range, I would think it'd just try to run like a cold, (or hot,) engine.
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Old 09-30-2004, 12:55 PM
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If there was no reading (or maybe spikes in the readings) that may trigger limp mode. It basically treats everything as if the engine was overheating.
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