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Old 09-05-2020, 03:55 PM   #1
BuddeTheBlueBrick
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Default Runs Poorly After Unplugging Battery

I'm curious if anyone else has any experience with this happening to them. I've seen mixed but no definitive answers on whether it's normal for a 240 to buck and shake at idle for a few minutes after starting it after the battery has been unplugged. From what I understand, the computer has to relearn its settings and adjust itself but I'm wondering how long it takes for it to do that. After it stops shaking, the idle climbs fairly high and stays that way until I shut it off. I can immediately start it again and it idles just fine. Also if I drive while it's seemingly misbehaving, the idle shoots up as soon as I stop and put it in park. On my way home it behaves normally as long as I'm not gone too long. I had this issue in late winter/early spring and replaced some things, namely the intake gasket and the coolant temp sensor among some other routine maintenance. I don't think it really changed much but it didn't give me any problems again until my battery died last week and I had to unplug and recharge it. It died again yesterday after I hadn't driven it for a week. I couldn't find a parasitic draw anywhere so I guess I need a new battery. This one is 8 years old.

Anyway I'm just curious if it's normal to experience erratic idle after disconnecting the battery and how long does it take to relearn idle and go back to normal?
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Old 09-05-2020, 04:29 PM   #2
142 guy
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Without knowing the specifics of your ECU, most ECUs learn fuel trim and some have an idle setting learning procedure. When you disconnect the ECU you typically erase these learned settings, that is the reset procedure which also deletes any stored error codes.

If your particular year of ECU uses a global long term fuel trim value, typical of pre 2000 ECUs and your car happens to be running with a lot of fuel trim then the engine may operate poorly at idle or other operating conditions until that long term fuel trim is restored in memory. A lot of long term fuel trim is usually a sign of other underlying issues (fuel pressure out of spec, leaking / dirty injectors, O2 sensors and on and on...). It might be worthwhile to see if you have any pending error codes for fuel mixture out of range which would indicate that something is off. An engine / fuel system that is in good condition will typically not accumulate a lot of fuel trim and won't suffer any operability problems as a result of having the fuel trims erased by disconnecting the battery.
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Old 09-05-2020, 04:55 PM   #3
BuddeTheBlueBrick
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It's a 951 ECU if that helps at all. I haven't checked codes in a while but in the spring I did have code 2-3-1 (Fuel trim too lean or too rich at part load), I think. It didn't throw a check engine light though. I cleaned my injectors in April while I had the manifold off, and the O2 sensor is new. It does smell pretty rich when it's bucking around like that but not to where there's black smoke or anything. I do have an exhaust leak at the downpipe to manifold flange that I've been putting off fixing because one of the nuts is rounded off. That would be pre-O2 sensor and is the only thing I can think of that would be affecting anything right now.
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Old 09-05-2020, 06:51 PM   #4
iamrolling
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Check the coolant temperature sensor. It can also cause that error code if it fails. I just finished checking mine with the results in this thread. The connector can also cause a problem if it corrodes. Deoxit was recommended to me to clean the contacts, but I couldn't find any and used CRC 2-26 which seems to work well enough. Unless your downpipe leak is massive it won't trigger an error condition.
To test it, pull out the Jet box and use the numbers on the box side connector to find pins 5 and 13 on the harness side connector. Connect the black lead of a multimeter to pin 5 and the red lead to pin 13. Do it both with the engine "stone cold" and after warming it up with a good long drive. If it's warm out where you are, the 2000 ohm range on the meter will probably cover both parts of the test. I'd recommend pulling the engine control fuse first just to be safe.
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Old 09-06-2020, 11:49 AM   #5
BuddeTheBlueBrick
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the CTS tests check out okay and like I said, I had replaced it in April. The connector too since the plastic had broke on it so it didn't click in all the way. It's just the battery seems to be the what starts the issue over again. I'm leaving it unplugged for a few days and see if it somehow drains by itself, since I couldn't find a parasitic draw from it. It went from 12.6v to 10.2v in 6 days last week with it plugged in and sitting. The downpipe leak is decent. I can feel hot air blowing out of the flange if I put my hand behind it and it's fairly loud. So many little things keep popping up with this car but I love it a lot. I don't know if I'm dumb for still trying to keep it on the road or what.
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Old 09-06-2020, 12:31 PM   #6
iamrolling
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Yeah, a leak that increases the noise level can cause that issue. Happened when my exhaust manifold split open. If it's naturally aspirated, your exhaust manifold is probably cheap. Get a used one, that way if you bust the stud with the rounded nut you can just put that one on and take your time fighting it out of the old manifold. I agree it's worth fighting with the car to get it working, no matter how much of a pain it seems like at times.
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Old 09-06-2020, 01:15 PM   #7
142 guy
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Quote:
Originally Posted by BuddeTheBlueBrick View Post
It's a 951 ECU if that helps at all. I haven't checked codes in a while but in the spring I did have code 2-3-1 (Fuel trim too lean or too rich at part load), I think. It didn't throw a check engine light though. I cleaned my injectors in April while I had the manifold off, and the O2 sensor is new. It does smell pretty rich when it's bucking around like that but not to where there's black smoke or anything. I do have an exhaust leak at the downpipe to manifold flange that I've been putting off fixing because one of the nuts is rounded off. That would be pre-O2 sensor and is the only thing I can think of that would be affecting anything right now.
I am not familiar with the Bosch pre OBDII error codes and how Bosch manages pending error codes. That said, if you had a stored fuel trim error code that suggests that the ECU is likely accumulating long term fuel trim and when you disconnected the ECU you erased the long term trim resulting in the rough running.

An up-stream leak in the exhaust manifold can admit O2 which may cause the O2 sensor to read a lean mixture and add fuel trim to add fuel to correct what it is reading. However, that fuel trim should actually make the engine run worse because the 'engine' isn't actually running lean. If the O2 sensor is being skewed lean by the exhaust leak the ECU will accumulate fuel trim which causes the engine to run rich; but, the sensor still sees the free O2 from the exhaust leak so things get really screwed up. However, my gut reaction is that if the manifold leak is causing the sensor to read lean, when you erased the fuel trims the engine should initially run better. I think you might have another problem in addition to the manifold leak.

On OBDII cars the upper limit on long term trim is somewhere around a +/- 15-20% fuel pulse width adder. Once it reaches that limit it usually throws an error code and is no longer able to correct for further drift in the AFR at which point you may start smelling something. If the fuel trim is negative because the system is running rich, once the limit is reached you will start running higher levels of HC in your exhaust which will cause your catalytic converter to run hotter than normal, sometimes really hot. If it is bad enough it will soften the undercoating on the underbody which is what may be smelling. Persistent high temperature operation will destroy the internal structure of the catalytic converter and sometimes light the car on fire!

Last edited by 142 guy; 09-06-2020 at 01:26 PM..
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Old 09-06-2020, 02:13 PM   #8
BuddeTheBlueBrick
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Quote:
Originally Posted by 142 guy View Post
However, my gut reaction is that if the manifold leak is causing the sensor to read lean, when you erased the fuel trims the engine should initially run better. I think you might have another problem in addition to the manifold leak.
Man don't tell me there's another problem I have no idea what else it could be. But it doesn't run poorly after it's relearned how to idle. Just a little loud

Quote:
Originally Posted by 142 guy View Post
If it is bad enough it will soften the undercoating on the underbody which is what may be smelling. Persistent high temperature operation will destroy the internal structure of the catalytic converter and sometimes light the car on fire!
The smell is right after start up so I don't think it's getting that hot that quickly. It's mainly a gassy smell to me. My temp gauge sits at about 10:00 if that means anything. I had removed the compensator board after it had acted up. I wish I had a infrared thermometer to really check.


Anyway I guess a new battery and fixing that leak is in order before deciding if and what to do next. And fix the leaky windshield, rusty wheel well and figure out if I need to replace my brake junction box since the brake failure light is on. Ah I love this car.
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Old 09-06-2020, 04:43 PM   #9
142 guy
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Quote:
Originally Posted by BuddeTheBlueBrick View Post
The smell is right after start up so I don't think it's getting that hot that quickly. It's mainly a gassy smell to me. My temp gauge sits at about 10:00 if that means anything. I had removed the compensator board after it had acted up. I wish I had a infrared thermometer to really check.


Anyway I guess a new battery and fixing that leak is in order before deciding if and what to do next. And fix the leaky windshield, rusty wheel well and figure out if I need to replace my brake junction box since the brake failure light is on. Ah I love this car.
The engine temperature gauge has no value in terms of what may or may not be happening to the catalytic converter.

Fixing the exhaust leak which is likely skewing your O2 sensor reading is a logical place to start. With luck, maybe that is where the repairs stop.
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Old 09-10-2020, 11:23 PM   #10
84B23F
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Quote:
Originally Posted by BuddeTheBlueBrick View Post
Man don't tell me there's another problem
When a battery is removed, ECU will go back to its factory programm.

Translation - If engine has issues (vacuum and exhaust leaks, cylinder/valve issues, etc), then engine will not run smoothly.

Installing a new O2 can assist, while taking vehicle out for a long cruise in different driving conditions.

RE: buck and shake at idle

Suggests a lean mixture....intake manifold, injectors, brake booster, vacuum lines could be adding extra air. AMM is defective or leaks in its tubing to throttle body. FPR not fit for duty...and/or fuel pump not up to full pressure
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Old 09-23-2020, 03:15 PM   #11
BuddeTheBlueBrick
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Well replacing the downpipe/manifold gasket didn't work, it still leaks somehow. Old one was pretty well burnt. Not sure what to do about that anymore. Still starts and idles funky. Also my rear most muffler rusted off so there's that too. Every time I get under the car and see the crust I want to cry but that comes with living in the salt belt hell. There's a spare FPR in the trunk that came with the car that I could try. It's dented a little but I think it's useable. I'm not sure if an FPR would not work when the car is cold and work better when it's warmed up. I thought it was the opposite. idk it's just frustrating
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