Thanks again to all at SITM who attempted to help get my blue '89 AT right !

To recap: drove the first 200 miles or so to SITM with minimal issues except very poor gas mileage (2-10). The car was running very very rich. As I got closer to SITM, car would go into limp mode. I could solve this by pulling off to the side of the road and turning off the ignition and immediately restarting the engine. Voila, it would run on all 8 cylinders for a period of time to be solely determined by the car itself ( sometimes 2 miles, sometimes 100 miles) By the end of the trip to SITM, when I would switch off the ignition, the tach would stay at 3000 and then slowly drift back down to 0 (It hadn't been doing that initially). The original MAF and LH were just rebuilt. The plug wires are 6 months new. O2 sensor was replaced at SITM. New plugs and even a new set installed on the road to SITM. LH, EZK, and MAF were replaced at SITM (THANK YOU GARY KNOX) All the plugs were black and sooty as you might expect. Intake was rebuilt and everything under the intake was replaced (except WOT switch). I ended up disconnecting the O2 sensor to make the drive back to DFW, which mostly worked (10-16 MPG) and only 2 times needing to reset out of limp mode.

Since I have owned the car, I have gotten a fault on both the LH and EZK : "ECTS-2 fault to ground"
I am pretty sure that that is a TEMP2 fault, but the sensor tests okay, and the wires back to the EZK and the LH test okay: no short and no grounds.

I talked to DocMirror the other day and he said something that made me think about this: I believe that the TEMP2 sensor is a thermocouple type sensor, and not just a resistance type sensor. If it was somehow plugged up backward, would that, could that give these issues? I'm thinking that maybe the plug was replaced and put on backward because they are directional plugs.

Any thoughts on this or anything else come to anybody's mind about these symptoms?

Thanks in advance

 

John, the ECTS-2 stands for Engine Coolant Temperature Sensor -2, so if you are getting a fault, that would indicate that either the lines between the sensor and the LH/EZK are shorted (your harness didn't look pretty) or you have a ground problem as this sensor measures the resistence between the sensor and ground. But I don't think this is your issue.

If you made it home with the O2 sensor unplugged - and it ran better, then I would tend to suspect the O2 sensor. This is where I would start. Make sure you crimp the connections - do not solder them.

Also, make sure you check all of your gounds.

 

Thanks for the thoughts Andrew. I unplugged the O2 sensor to try to overide the LH attempting to provide more and more fuel to the engine. My thought was that the LH was thinking that the engine was running too lean and therefore just kept adding more and more fuel to it until it finally would not run, hence the limp mode. It obviously ran rich on the way home, but not crazy rich like it did on the way there.
I do have to pull my intake again since you found the little flappy spring retainer in the bottom of my V, so I will be checking the grounds under there again.

The O2 sensor plugs right in at the Central fuse panel.

Sterling,Jim, Roger, Mike and I checked the wires for the Temp2 sensor from the plug all the way back to the LH and EZK plugs. They had no grounds and no shorts between them and appeared to be contiguous.

Thanks again

 

Why not change the Temp II sensor? Weirder things have happened.

 

I would also suspect the throttle position switch, although i believe you tested that.

I had a similar issue where my car went pig-rich and it was caused by a crispy LH wiring harness. Hopefully that's not your issue.

If I think of any other culprits I'll throw them out there.

 

The Temp II sensor consists of two independent thermistors - That is, Negative Temperature Coefficient (NTC) resistors, whose resistance decreases as their temperature increases. The NTC resistors are essentially identical - one is hooked to the ignition ECU, one to the injection ECU. That is why you always test the TEMP II sensor pins from pin to ground, never from pin-to-pin.

If the sensor feeding the LH ECU shorts to ground, it tells the ECU that the engine coolant is very, very cold, and the ECU will respond by enrichening the air/fuel ratio.

Given the symptoms, and given that the diagnostic told you that the Temp II sensor was shorted, looks like a smoking gun to me...

If you unplug the Temp II sensor, it tells the ECUs that the engine is very hot, and it might give you another indication of whether or not that is the problem.

 

Thank you Wally. When I was speaking with John on the phone, didn't have access to the prints, and couldn't remember which type it was. Now that I think about it, we tested a Temp II sensor at Roger's house with a hairdryer and ohm-meter.

The wiring to the LH and EZK from the sensor must be checked carefully. Locate the wire at the entry to the LH from the Temp II. I believe it's pin 13, but it's hard to read. Set your meter for ohms, low scale, and measure the resistance to a local ground point. As you keep the meter on the pin, use a hairdryer to heat the area of the Temp II sensor, and watch the resistance change fairly linearly. I can't make out the pin to the EZK but it at L-85 grid of sheet 10. Test is the same as the LH.

I also advised John, although he didn't mention it that the fault condition MUST be cured prior to solving this problem. It may fix the issue. Additionally, the ground point for both the LH and EZK are at MP IX. A ground loop at this point can cause problems with resistance changing with temp. The Temp II is grounded by the case into the engine block, so not an issue.

 

John,
as you have been in touch with some of the 928 owners you may also have access to a 928 diagnostic tool. Maybe borrow it. Both EZK and LH have a temp sensor that can be read from the ecu using the diagnostics. Not just the value at the connector, but the value that the ecu thinks it sees. So the digital value after A/D conversion of the signal. If that shows inconsistent values with the actual temperature of the engine., this is where you focus on. As soon as that is all right, the other bits come into play.
Don't just replace things and hope for the best....
regards,

 

Aryan, I did replace the temp2 sensor.

Andrew, I replaced the throttle position switch which had gone bad after I had R & R'ed my intake. Truthfully, at this point, I am just kinda hoping for a resolution, so IF it is the LH harness, at least then I'll know WHAT it is. Thanks for the thoughts.

Wally, Thanks for the explanation of the sensor. So if I understand correctly, it will not matter if the sensor is somehow plugged in backwards? I did unplug the sensor at SITM (well, at the hotel) and it did not seem to make any difference in the way it ran. I don't remember if I disconnected the battery to reset the LH, though, when I did disconnect the Temp2 sensor. Would that be an issue?

Doc, While at SITM, we disconnected the temp2 sensor and checked the wires back to the LH and back to the EZK. Sterling was running the multimeter and said there was no ground and no short on either of the two wires back to the plugs of the LH and EZK. I will check these again, but that is why I was asking if the wires plugged in backwards might make a difference.

Theo, The problems could be multiple which could be masking what the real problem(s) are. I had already replaced some of the bits as WYAIT while doing the intake R & R. I would prefer not to replace bits that are not part of the problem, but in the interest of eliminating potential problems or solving the problems, I have replaced some parts.

Thanks for everyone's input so far.

 

John,

I know that we kind of ruled the ignition monitoring relay out because all of your plugs were black, but I would check it for a possible explanation to the limp mode part of your problem. I think you still might have multiple things going on causing all plugs to be sooty/black. I know this relay is expensive. There has got to be a way to test it.

 

Mike, I'm not ruling anything out at this point!

 

At this point, I'm gonna have to go with full out demonic posession for my diagnosis...........an exorcism is in order

 

Angela needs to do the naked exorcism dance. I hear it take over an hour! Please tape it.

 

We can't just scare the demon, we need to get rid of it!

I was just thinking we could douse it with some of Roger's boxed wine, that ought to kill it

OK, sorry for the derailment John, I'm still sticking to my original diagnosis though

 

When we checked the temp II sensor at SITM we did it both hot and cold. With the engine at room temp. we had 3k ohms, with the engine hot we had 300 Ohms. Both of these are within limits per the WSM. The O2 sensor we got from Dave and Jeannie was a used one but unless it had the same failure as the original I don't see it as the problem.

I'm leaning towards a harness problem.