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I recently replaced the intake gasket on my 1985 L98 with 4+3 manual and now have a Check Engine light on all the time. While replacing the gasket I also replaced the fuel injectors, oil pressure switch, distributor rotor and cap, plugs, wires, thermostat, and pulled the valve covers and repainted them. Also, my cold start injector got bypassed because I stupidly put the runners back on before remembering to install it. Now I'm getting error codes 15 and 23 when I check with the scanner.
From reading the scanner manual 15 is "Coolant temperature sensor or circuit fault - signal voltage high." That said, the temp gauge appears to be working, reading up to about 180 on my 180 degree thermostat.
Code 23 has a few different things it could be according to the scanner book:
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It is also throwing code 44, which is the O2 sensor. I've got a new 3 wire sensor on the way to see if that helps.
The car is generally running okay except for 3 things.
When I pull up to a stop light and go to idle, the idle pulsates. Sometimes it does it, sometimes it doesn't.
Sometimes after startup the idle is high when I push in the clutch pedal. It will be up around 1700 RPM for a bit and then will go back to normal. This doesn't happen every time.
On the last tank, my fuel mileage was about 25-30% worse than normal (10.5 MPG last tank). Could be because I had my AC on more and had to drive more in traffic than usual on that tank but it seems really bad.
I'm looking for suggestions on what to check regarding codes 15 and 23. I'm hoping it's just a loose connection from something getting pulled around while doing the intake but I'm not sure how to check other than just feeling around. Is there a common ground that might cause these codes to come up. Any advice or tips would be greatly appreciated!
The gauge and the ecm sensor are two different sensors. I believe the coolant sensor for the ecm is located in the front of the intake manifold. The dash gauge sensor is in the cylinder head. Code 23 is the MAT.
I unhooked the sensors last night and cleaned them with electrical cleaner spray. That seemed to help with code 23. The code went away and the idle is now steady.
Error code 15 is still present. This code is the one that deals with the temperature sensor. I know where the sensor is, undid it, cleaned it with spray, and hooked it back up. The thermometer is reading what appears to be normal, although the car doesn't seem to go above 180º even in traffic (it has a 180º thermostat).
Any suggestions on what to check next with Error code 15?
as yd328 said the ecm sensor and the gauge sensor are two separate things. Code 15 is a high voltage signal meaning the ecm sees a temperature of -45F or so. Can be the sensor or wiring to the ecm, you need to follow the diagnostics in the FSM. The sensor connection is a bear to get at but I'd make sure its plugged in OK, it can be difficult getting it connected after doing the intake.
Thanks Tod. I messed with it more last night but still am getting Error Code 15. I was able to get rid of the other codes though, so that's good. Of course, this problem is on a sensor that's tough to reach and mess with. I think all the sensors might be tough to get to though.
Any of the experts know which sensor is the coolant temp sensor for the computer? I was working on cleaning the one with the green arrow (passenger side), but now I'm wondering if it's the orange arrow (driver's side). The one with the green arrow was difficult to get undone because it had a wire clip around 3 of the 4 sides. This pic would be from the front of the car on the driver's side when installed. Thermostat is near the bottom of the pic. Also, it's much much cleaner now!
I believe the orange is the cts. Either a scanner or a fsm will help you diagnose it. With a scanner that has live data you will be able to see what the ecm is seeing. A fsm with a dvom will let you know what the sensor values should be at certain temps. You would be able to see what the resistance of the sensor should be at a given temp, like before you run it and know what the outside temp is. The coolant temp will be the same temp at that point and you look at a chart in a fsm to see what the value should be at that temp.
UPDATE: I got the car buttoned up and now no codes are showing! In the pic above, the orange arrow points to the Coolant Temperature Sensor that goes to the computer.
When I was checking things the other night, I was messing with the sensor with the green arrow. I had to search around a ton to find the right sensor in the FSM. After I found which one was right I looked and the plastic connector was broken. I went to the Zone, got a new one, wired it up, reconnected everything, and no more codes... for now. Just thought I'd give an update for anyone that might have a problem with this sensor in the future.
The green one with the clip on it is the cold start sensor. It only operates the cold start injector below 95F. The orange arrow points to the ecm temperature sensor, the one you are interested in.
Just saw your update. Terrific!
Last edited by Tod Stiles; Jun 14, 2017 at 10:58 PM.
Error codes 15 and 23
