How do I know when ECU goes closed loop?

[Josh]

I have a 91 X and Im getting realy bad gas milage. There is no CEL so the ECU seems to be happy. I have heard that staying in open loop will yeild bad gas milage and my truck runs cool, usualy the temp guage is on the N and on a hot day the O. The T-stat is a new 195 unit. I used to get between 14 and 15 in town and 18 on the highway. Now between 10 and 13 and 15 highway and nothing has changed.

 

[Josh]

Anybody

 

[spindlecone]

You need to get hooked up to a scanner to see when it happens.

 

[Josh]

Well yesterday I measured my coolant and at idle after driving for 30 minutes and idling for about 20 it stays around 153. I don't think that is hot enough to go closed loop. All the reading on OBD-I stuff suggests 170 minimum. I changed the T-stat about a month ago and no joy. It was at 182 when I pulled in the driveway, but quickly cooled. The t-stat is in the right way with the bridge facing out and the spring holdong the valve facing in. I guess a bad T-stat is possible. And yes its a 195.

 

[spindlecone]

I believe going into closed loop is determined by your 02 sensors, i have heated sensors in my 03, I go into closed loop after about 5 secs after cold startup

 

[Josh]

Here is a chart I pulled. Its not Explorer only just a general OBD-I. I think the motor uses the coolant temp to determine open or closed loop.

 

[shamaal]

I'm doing around 16 in town with the same temp readings. If it is indeed an open loop problem from being to cold, the Engine Coolant Sensor could be out of calibration. The best bet, as Spindlecone recommends, is hook up to a scanner. I'm too cheap to have one, so I can't help with anything in that area.
Another cause of bad milage is a vacuum leak.
Let us know if you learn anything.

 

[Josh]

I think the scanner he is referring to is one of those high dollar ones you see at the dealer. I don't know if I will ever do that or not. I don't think its the sensor because I measured the temp using a thermometer stuck in the radiator via the fill hole. Also, dont forget that the reading you see in the cab is not coming from the same sensor.

 

[spindlecone]

ck out www.scangauge.come

 

[Josh]

Would be nice but its OBD-II. I have been reading about this problem on a few other threads in reguards to the running cool problem. I think what I realy need is for someone with an OHV motor to read the resistance of the ECT sensor at normal operating temperature and I could base my readings off that. My other motor always ran a little warm as well. One thing that just came to mins is the fact that I am taking a measurement at the radiator where the temps will most likely be cooler anyway. I have been wanting to install an aftermarket sensor so maybe I will do that.

 

[peties3]

Very insterested in what you find out too!!! I think the sensor that computer uses is located close to the temp sending unit in the cab.

Mark

 

[Josh]

Yes, the 2 sensors are right next to each other. I believe the one wire sensor is the guage and the 2 wire is the ECU input.

 

[savage120]

I have sort of the same problem. My temp guage reads usually around the N or Below it, sometimes it gets up to O or R. I had it at the dealership a while back, and the did a full coolant system overhaul.

 

[Josh]

Yesterday was in the 90's and with the ac running it would occationaly get to the R but would drop back down. I think the only way to tell for sure is to buy an aftermarket temp guage and go from there.

 

[shamaal]

The location of the ECU sensor is as Josh stated previously and in the pic below.
A new gauge will tell YOU the temp of the water. That doesn't necessarily mean that's what the ECU is seeing. IF there's an open loop problem because of temperature, the sensor becomes suspect also. It's one of those logic problems:
1. If the temp is correct as verified, you don't know if the ECU sees it correctly.
2. If the temp is incorrect as verified, that must be fixed, then hope the ECU sees it correctly.
It may be cheaper to replace the sensor than buy a scanner. From what I understand, a reader doesn't provide enough info. But the gauge package looks cool.

 

[MarksSC94]

I am very interested in the results of this. I would also be curious to find out if the what the ECT resistance is when the engine is cool and what the ECT resistance is when the engine is at operating temperature. One might be able to fashion a resistor into the circuit to derive some interesteding results. There is also something with the O2 sensors and exhaust temperatures. I know the system will stay open loop until the sensors are something like 600 degrees, but I will have to check out my book real quick to find out.

 

[Josh]

I don't think the system knows when the O2 sensors are at their operating temp. I was under the impression that the ECU goes closed loop when the ECT sensor reads a temp above 170 f. I think my problem might be with the sender for the gauge not the ECT. My truck was running on the N and sometimes O on a 90 degree day with the A/C running. I find it hard to believe that my truck is running cold with those conditions. Another thing I noticed this weekend is I just replaced my TPS. Found a new one in the junkyard. Mine was cracked in half. Since replacing that, my truck feels a little stronger and on the highway, I have gone almost 220 miles on about 12 gallons of gas! Thats roughly 18.3 miles to the gallon I have also been using acetone so I don't know if that i helping or not. Not bad milage considering I had about 300 extra pounds of crap in the back and 33 inch tires. If one of you has a truck that is running at normal temps and would like to check that resistance, that would be cool. The only other way is to remove the sensors and heat up some water with a thermometer and check that way. Another thing I didnt think about was tire size and my odometer. It would be off by a little since that has never been corrected so my milage would actualy be a little bit more. I should have used my GPS to see what the differance would be.

 

[shamaal]

I've copied the temp-f degree to equivalent resistance readings (K Ohms) from Haynes below. Of course the trick is in determining the coolant temp when hot. If the ECT reading on a warmed up engine is <3840 ohms, the ECU should believe the coolant is > 176 deg F. According to Haynes this table works for the ECT and the IAT sensors.

Temp(F)/Resistance (K Ohms)
248/1.18
230/1.56
212/2.07
194/2.80
176/3.84
158/5.37
140/7.70
122/10.97
104/16.15
86/24.27
68/37.30
50/58.75
When measuring the resistance, make sure the engine is off and exercise caution around hot parts.
There was another thread recently where the ECT was bad and caused the car not to start. Something about too much gas flooding the engine.

 

[Josh]

Huh, never saw that in my book. Thanks. Of cousre my book took a dive into a bucket of antifreeze a few weeks back. The ford CD I have does not seem to explain it either unless Im not using it right. I might mess with it tomorrow. Thanks again!

 

[BrooklynBay]

Shamaal, is this the thread that you were referring to: http://www.explorerforum.com/forums/showthread.php?t=161936