940 computer woes?

  #1  
Old 04-16-2019, 06:14 PM
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Default 940 computer woes?

I am wondering if anyone else has run into this situation. My son and I are installing a stereo in our 1994 940 wagon. I have noticed that every time the battery is disconnected the car barely runs on start up when cold. Go to accelerate and it bogs down, then if you release the gas and get back on it again there is a brief surge then bogs again. After a while this goes away but when it is cold it goes through the routine all over again. My thinking is that when the battery is disconnected the computer resets and isn't processing information correctly for awhile. Any advice, hints, and comments as usual are greatly appreciated.
 
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Old 04-17-2019, 05:17 AM
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no, it doesn't work like you hypothesis. yes, when the battery is disconnected, the ECU forgets its learned fine tuning, but those are far subtler and will not result in any sort of behavior like you describe, the 'learning' on LH 2.4 is quite limited, and just relates to fine tuning the lamba balance of the fuel-air mixture.

you might have any number of potential problems its pretty hard to guess from here without running a series of tests and diagnostics. air leaks, wiring issues, a faulty engine sensor, etc.
 
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Old 04-17-2019, 07:08 PM
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Ah , well thanks for saving me the money for a fresh computer. After a few days the situation goes away and it drives fine but as soon as the battery gets disconnected the same scenario starts all over again. I was hoping someone might have had a very similar situation going "ah ha. I remember going through that".
 
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Old 04-17-2019, 07:53 PM
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is this a pulsair system, or a regina/rex system, or a regular LH 2.4 ? The B230F engines could be had all three ways...

pulsair had a pair of funny "F" shaped air hoses that go over the front of the camshaft/valve cover and head down to a set of air injection valves between the exhaust ports.

Regina/Rex has a MAP rather than a Bosch AMM/MAF, and has a square 'transfomer' like ignition coil rather than a conventional 'baby bottle' sized coil.
 
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Old 04-17-2019, 10:43 PM
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I am going with a regular LH 2.4. I don't see those odd "F" shaped hoses and I do see a "baby bottle" coil.

 
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Old 04-17-2019, 11:44 PM
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OH, its a turbo. turbos are all LH2.4 from 1990 on (meaning, ALL 940 T's)

the Bosch AMM/MAF thats the dead giveaway to the LH'ness is that thing just above the airfilter box on the airhose from the filter to the turbo. on a non-turbo it would be on the other side.
 
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Old 04-18-2019, 07:36 PM
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So, are you having a problem every time when it is cold, meaning cold weather or cold engine, or after a battery disconnect regardless of weather and or engine temperature, regardless of battery disconnect?
 
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Old 04-19-2019, 11:56 AM
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Whenever the battery is disconnected and the car is cold. For a few days to a week it will bog down, stutter, back fire, then get going. When it warms up it drives fine. After a few days to a week all that goes away and it is good for a few months then it will start to surge at idle at stop lights. Disconnect the battery and the scenario repeats. The surging at stop lights goes away and it drives good after the initial few days to a week of poor cold start performance. There must be something that is affected by the power being cut off. It will do this routine without fail every time the battery is disconnected.
 
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Old 04-19-2019, 03:43 PM
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Originally Posted by jandm View Post
Whenever the battery is disconnected and the car is cold. For a few days to a week it will bog down, stutter, back fire, then get going. When it warms up it drives fine. After a few days to a week all that goes away and it is good for a few months then it will start to surge at idle at stop lights. Disconnect the battery and the scenario repeats. The surging at stop lights goes away and it drives good after the initial few days to a week of poor cold start performance. There must be something that is affected by the power being cut off. It will do this routine without fail every time the battery is disconnected.
Maybe if you stop disconnecting the battery it will fail and you will be able to find the problem.
 
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Old 04-19-2019, 06:38 PM
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My initial problem was the surging at idle. Ran it like that for about 6 months with no change. When I disconnected the battery while cleaning the throttle body did I find this out. It cures the surging at idle but I have to go through the scenario I described. So it will run fine for about 4 months or so then the surging at idle comes back. It's got me scratching my head.
 
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Old 04-19-2019, 09:00 PM
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What is your definition of surging. Does it stall or does the idle vary in rpm. Being you can't find the issue maybe it's time for professional help. You can also run it until it dies and it will be easier to find the problem. Now you are just running a patch but if it suits your need then keep doing it.
 
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Old 04-20-2019, 11:39 AM
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By surging I mean the rpm going up and down when sitting at a stop. As soon as I put my other vehicle on the road for the summer I am going to drop it at the shop that does work on it for me. Interested to see what it is all about.
 
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Old 04-20-2019, 11:53 AM
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Originally Posted by jandm View Post
By surging I mean the rpm going up and down when sitting at a stop. As soon as I put my other vehicle on the road for the summer I am going to drop it at the shop that does work on it for me. Interested to see what it is all about.
The IAC controls the idle speed . Did you ever clean the IAC valve and test it to see if it is functioning correctly. Also are you sure there aren't any air leaks in the hose to the throttle body. Next did you remove and clean the throttle body and reset the base idle (500 rpm with IAC detached and plugged) and 650 rpm when IAC reconnected. Also need to check/set the TPS.
 
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Old 04-21-2019, 02:08 PM
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Replaced all the vacuum lines, cleaned the throttle body.It idles nicely for a while, a few months at a time. What does IAC and TPS mean?
 
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Old 04-21-2019, 03:39 PM
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Idle control valve, throttle position sensor. May the throttle body gets fouled up every few months? Any symptom that doesn't show up for a few months, I wouldn't worry about too much...

How's the rest of the "tune up" items, plugs, wires, rotor, etc?
 
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Old 04-21-2019, 09:22 PM
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Originally Posted by jandm View Post
Replaced all the vacuum lines, cleaned the throttle body.It idles nicely for a while, a few months at a time. What does IAC and TPS mean?
IAC is Idle Air Control valve which determines the idle quality by adjusting the idle by adding or restricting air into the throttle. It's input comes from the ECU. There is a procedure for adjusting the idle to the base 500 rpm and then to the running rpm of 650 with the IAC connected. The TPS or Throttle Position Sensor is a switch mounted on the throttle body and tells the ECU when the throttle is off idle. Also if you have to clean the throttle body every few months then you are getting a lot of blow by oil going into the throttle body. There are methods to test the IAC and set the throttle so I would suggest you look them up and try them out.
 
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Old 04-22-2019, 12:24 AM
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I don't have to clean the throttle body every few months. That was one of the things to do on my check list when I was trying to figure out the "surging" idle. All the other tuneup stuff is all fresh for the same reason. It's not a huge deal I could just unhook the battery for a couple of minutes every 4 months or so then put up with the poor running when cold for a few days. I''m just interested to know what it is. I had a 740 before this one and there was nothing like that with that one.
 
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Old 04-26-2019, 12:04 AM
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Originally Posted by jagtoes View Post
IAC is Idle Air Control valve which determines the idle quality by adjusting the idle by adding or restricting air into the throttle. It's input comes from the ECU. There is a procedure for adjusting the idle to the base 500 rpm and then to the running rpm of 650 with the IAC connected. The TPS or Throttle Position Sensor is a switch mounted on the throttle body and tells the ECU when the throttle is off idle. Also if you have to clean the throttle body every few months then you are getting a lot of blow by oil going into the throttle body. There are methods to test the IAC and set the throttle so I would suggest you look them up and try them out.
that procedure is only for earlier LH 2.2 cars, all 940's are LH2.4 (or regina) and the idle speed is managed digitally and not adjustable.
 
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