I have a 1988 W100 w/ 318 FI engine. The trouble I am having with it is a low idle. I checked the ISC. The motor was inop. so I replaced it. The problem is when I turn the key on, the engine retracts the pindle all the way in away from the throttle. It keeps trying to retract as long as the key is on. When the key is turned to the off position, the pindle should extend to push the throttle open. It does not move. Upon a voltage check, the motor has 12 volt & ground with the key in the on position. Upon start, the motor has 12 volt pulses. When the key is turned off, the opposite circuit shows 12 volt pulsed but the other wire stays at 12 volt. It should go to ground to reverse the motor. Any ideas what causes this? Can this be caused by a bad reading of another sensor? The only codes I am getting is 12, 25, 31 & 37.

 

ISC operational voltage should be between 2-6 volts and is directly controlled by the SMEC. Any one of a bunch of bad sensors can effect the signal to the ISC from the SMEC but I do not believe any sensor would cause what you are describing. There is no switching of grounds. There is only one constant ground wire. The SMEC pulses voltage to the ISC on different wires that move the pintle in and out. It sounds like you either have a shorted wire somewhere or the SMEC is bad.

Has the connector at the SMEC been disconnected? Have you checked the pins at the SMEC connector? I have seen pins bent over in the connector that can create very strange problems.

 

Found a bad Temperature sensor. Replaced it. Also checked all vacuum lines for leaks & proper placement. All we fine. Cleaned all connectors & plugs. Also ran a continuity check between the Idle Control & ECM. All OK with no shorts. Still same problem. I still have Codes 12 (normal),Code 25 (Idle air controller), Code 31 (Evap. Solenoid), Code 37 (Lock up torque converter (which I don't have) & Code 55. Evap. Solenoid works properly. Maybe a wiring problem? Also I researched the Idle Control Motor. According to Wells (Who makes the replacement) it has 4 wires. 2 of them are for the switch to tell the computer when the engine is idle. The other 2 wires are motor control. It works the motor one direction then reverses polarity to reverse the motor. In later engines, the Idle Control motor that is inserted into the Fuel injection system has a different type of motor that can not be operated by standard 12 volt power. I will have to get the Evap. Solenoid fixed, then see if the Idle Control Motor takes care of it's self? I hope so. I will keep posting until it is fixed.

 

Well, still working on it. Checked all grounds from ECM & they are good. Drives OK. Some hesitation but not bad. Drove 50 miles & used up almost 1/2 tank gas. Still running rich. Purchased an Autoxray XP240 scanner to get live data. I guess you can't get live data from an '88. Only gave me codes. It has to be the ECM that is causing all my problems. Just a guess. When I get my expenses all caught up, I will get an ECM from a Junk Yard. It is worth a try......

 

The main source of my problem was the ECM. I purchased a reman. from Advance Auto Parts. It was a BWD reman. As soon as I installed it, All codes were gone & it runs perfect. I did have to change the oil again. It was diluted with gasoline. I did check my MPG during the trouble. It was 5.8 MPG. I will recheck after I put about 200 miles on computer.

 

Glad you got it fixed. Thanks for reporting on the repair so others might learn from your experience.

 

I put over 250 miles on engine. It runs good. Just a little of hesitation when I first accelerate. I ordered a new TPS sensor to see if that takes care of that problem. I am still getting terrible gas mileage. I am getting no error codes from the O2 sensor but can it still be bad? I am going to check timing & what shape the spark plugs are in (They have less that 10,000 miles on them). Any other suggestions?

 

Still working on this. Replaced O2 sensor, spark plugs, wires, distributor cap & rotor. Made a big difference. Back up to 13 MPG. Still has hesitation after idle. If I disconnect EGR valve, hesitation goes away. Need to replace EGR I guess. I did replace valve cover gaskets. They were leaking. They were old & brittle.
Also now wipers only have low speed only. High & intermittent does not work. tried & replaced circuit board with another. Same thing. I need to pull wiper motor & check the ground wiring. I'll let you know when I get more items fixed.

 

OK. I have the engine running smooth. The timing mark has moved in the harmonic balancer. It needs replaced. I removed the spark plugs & set the #1 cylinder to TDC. I marked the harmonic balancer with that as a set point for timing. Set the timing & it ran much better. Still hesitates. I replaced both the rubber grommet & the PCV valve. That also helped. Now the engine surges at low speed. This was a head scratcher. My son recommended to reset the computer just to see what that would do. I disconnected the battery cable to reset the computer. It hasn't been done since I installed it. That did it. Now runs good. I also replaced the muffler (Single 2.5" inlet/2" outlet) with a 2.5" single inlet/2.5" dual outlet Thrush welded muffler with duals. Runs good w/ a good sound. Still have wiper problem with the delay module but works fine in low & high so really not worried too much about it. Paint & graphics have been completed. Now off to some car shows. It turned 25 this year so I had it registered as a historical vehicle.