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Jon
06-09-2000, 09:40 PM
Under acceleration I have a surging problem. It's like something is holding the car back and letting it go over and over. If I accelerate hard(wide open from dead stop)it does not seem to be there. I have checked the following.<br>1)Fuel Pressure(42 psi at idle pinch hose pegs 75psi gauge).<br>2)Air box(AFM)for sticking, seems to be free no sticking.<br>3)Changed air filter.<br>4)Changed fuel Filter.<br>5)Checked for vacuum leaks.<br>6)checked for fuel leaks.<br>I did not check fuel pressure under a load. I have also noticed that my fuel mileage has gone from 22mpg down to about 16mpg. I do not know if the two are related but I have a 300 mile trip tomorrow and will check it again.<p>Can any body think of anything else to try. It just seems to also have a general lack of power.<br>Thanks<br>Jon

Bob
06-11-2000, 01:50 AM
<i>: Under acceleration I have a surging problem. It's like something is holding the car back and letting it go over and over. If I accelerate hard(wide open from dead stop)it does not seem to be there. I have checked the following.<br>: 1)Fuel Pressure(42 psi at idle pinch hose pegs 75psi gauge).<br>: 2)Air box(AFM)for sticking, seems to be free no sticking.<br>: 3)Changed air filter.<br>: 4)Changed fuel Filter.<br>: 5)Checked for vacuum leaks.<br>: 6)checked for fuel leaks.<br>: I did not check fuel pressure under a load. I have also noticed that my fuel mileage has gone from 22mpg down to about 16mpg. I do not know if the two are related but I have a 300 mile trip tomorrow and will check it again.<p>: Can any body think of anything else to try. It just seems to also have a general lack of power.<br>: Thanks<br>: Jon<p></i>---------------------------------------<br>What you are experiencing (like the car is hooked to a rubber band when you accelerate) is the DME operating in closed loop with the O2 sensor and the DME oscillating from rich to lean.<p>The reason it doesn't do it full throttle is that the TPS (throttle pos'n switch) tells the computer to add fuel (richen mixture0 and to ignore the O2 sensor (puts the DME into open loop operation).<p>You could try disconnecting the O2 sensor (most models it's a green single wire connector on right side firewall). That will put the computer into open loop with an internal nominal value substituted for the O2 sensor output.<p>If the surging goes away, you could:<br>1)Replace the O2 sensor<br>(which MAY cure the problem)<br>2)Check your fuel mileage and drivability w/ it disconnected. If it gets good mileage and runs well, just leave it off until your smog check. It will NOT harm a thing.<p>Bob<p>

Jon
06-11-2000, 09:19 AM
gentelman, and there are damn few of us left. I disconnected it(three wire type) and it runs fine no more yo yo. We don't have emissions testing here in Mississippi so if I leave it off it will be for a long time. I will moniter my fuel milage and see how it goes. I hope it does not hurt anything to leave it off for prolonged periods of time. If you think it could let me know!!<p>Again a million thanks as this has been going on awhile.<br>Jon Jacobs

Grant
06-11-2000, 04:56 PM
<i>: gentelman, and there are damn few of us left. I disconnected it(three wire type) and it runs fine no more yo yo. We don't have emissions testing here in Mississippi so if I leave it off it will be for a long time. I will moniter my fuel milage and see how it goes. I hope it does not hurt anything to leave it off for prolonged periods of time. If you think it could let me know!!<p>: Again a million thanks as this has been going on awhile.<br>: Jon Jacobs<p></i><p>since the problem seems to be the O2 sensor, why don't you replace it and then you will use the sensor for mixture rather than the fixed program that is used when the sensor is out of the loop. There not that expensive and easy to change.

Jon
06-11-2000, 08:33 PM
right. I can smell the exhaust and it seems to smell like it's running rich. I saw the post above about the O2 sensor but I don't know the part number of the 86 735i O2 sensor. I don't think I will run it long this way as I thought that it might ruin the converter.<p>Thanks ya'll for all the help!!!!<p>Jon

Bob
06-12-2000, 02:31 PM
<i>: right. I can smell the exhaust and it seems to smell like it's running rich. I saw the post above about the O2 sensor but I don't know the part number of the 86 735i O2 sensor. I don't think I will run it long this way as I thought that it might ruin the converter.<p>: Thanks ya'll for all the help!!!!<p>: Jon<p></i>-------------------------------------------<br>The "default" mixture value is calibrated NOT to harm anything. After all, it would be very bad PR if every time an O2 sensor failed, the CAT burned up to the tune of $500.<p>Earlier Bosch Jetronic did not even have the Lambda (O2) sensor.<p>Most Motronics end up running too lean after a few years. The injectors become less responsive to pull-in current, so eventhough the pulse from the DME is the right length according to the calibration mapping (which was done on new engines), the amount of fuel delivered is less per pulse.<p>If a new O2 sensor makes it run fine, great. I have just seen too many cases where it didn't help, so don't get discouraged if it does act the same with a new sensor. There is not a serious problem with your car.<p>Bob


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