Engine controls/scantool experts pls

[kevm14]

I thought I'd make a new thread since this is a more specific topic than my other one.

I replaced the thermostat and water pump and flushed the heater core recently due to what I thought was a water pump leak. I think it was actually a T-stat housing leak but at least those are replaced now. I used a brand new Napa pump and a Motorcraft thermostat. I have been experiencing poor mileage and since the dash gauge reports a seemingly low temp, I bought a new ECT sensor and gauge sender, as well as an O2 sensor to compliment the thermostat and make sure everything is working correctly.

I also got lucky and found an EEC-IV interface cable for my Autoxray XP240 (that I bought refurbished for $90 a million years ago for use with my OBD-I GM). Tonight I hooked it up and scanned the truck (it's a 91 2.3L btw) and I want to discuss my findings. Here is a list of what I saw as well as questions:

1. The dash temp gauge doesn't seem to indicate any hotter since replacing the thermostat indicating that engine coolant temp is about the same as it was before (i.e. prior stat was operating). It indicates right on the left line of the normal range when up to temp, which I interpreted as "too cold," spawning my cooling system maintenance episode. I used a Motorcraft RT-1140 if anyone wants to double check. I believe it should be stock temperature (~190+ opening temp).

2. After full warmup, and while driving on the highway, engine temp never climbs above ~171°F as reported by the ECM (from the ECT sensor). It fluctuates between 165 and 173. Since the dash gauge reports the same (low) temp as before I replaced the stat, I assume the engine was running at this temp on the prior stat. I may still replace both the ECT and temp sender just to be sure. Perhaps the thermostat housing is corroded enough that the rubber gasket on the thermostat can't fully seal because the surface is rough, causing internal coolant blow-by to the radiator. Is this a common problem? I'm talking about this guy: http://info.rockauto.com/getimage/g...http://info.rockauto.com/Motorcraft/rh-35.gif

3. Possibly due to the low reported temp (which may or may not be true coolant temp), I did not see any EGR operation and the ECM seemed to fall out of closed loop, even while driving down the highway. One or both of these (plus the enrichment from a reported cooler engine) could easily be the cause of the poor fuel economy.

4. The scantool shows me intake air temp in volts, not degrees, so does anybody have a conversion chart to determine if that sensor is working correctly? It seemed to run about 3 volts while driving down the highway and I have NO idea if this is even warm or cold. I did notice that when I sat idling the voltage decreased, suggesting that higher volts is cooler, which makes sense. The voltage was about 3.2 at startup which means the factory intake does a decent job getting cool air in there. Or this sensor is also malfunctioning, which would lead to possible false fuel enrichment.

5. TPS actually seems to be working properly, reporting 0.9V at closed throttle, which I think is right. However, the previous owner told me the A/C stopped working while he owned it and one of the things the scantool tells me is the state of the Wide Open Throttle A/C Cutout. Interestingly, it said "Yes" and never said anything else no matter what I did. I would have thought the ECM would control the A/C compressor relay based on TPS voltage but perhaps there is an independent switch? If so, where is it? That could be the only reason the A/C doesn't work, which would be neat.

6. I can't tell the state of the O2 sensor since the update rate from the ECM data stream is too slow but I'm going to replace it with a Motorcraft just to be sure. I wish they would have outputted long and short term fuel trims, not just O2 voltage. Even with a faster data stream update rate, O2 voltage by itself is of limited use.

So that's it I guess. If anyone can help me address these I would greatly appreciate it. I'm an engineer and I have to have this thing running at its optimum!

 

[kevm14]

Ok, status update. Maybe this will help generate some replies. Honestly guys, if there is another forum that would be better suited to my questions, please let me know so I don't waste everyone's time. I know somebody has some of these answers.

I CANNOT find the temp gauge sending unit anywhere. I was able to locate the intake air sensor and ECT sensor, as well as the oil pressure sender. The temp gauge sender is MIA. I am wondering if the temp guage is actually tied in with the ECT sensor. Is that possible or is it buried under everything?

That said, I am starting to believe that the dash temp guage and ECT sensor are reporting accurately, which means I really do have cool temps.

I put the thermostat in a pot of water with a meat thermometer and verified that it opens right at 190, is probably fully open by 200 and fully closes by 185 or 180 at the lowest. About what I'd expect to pronounce my brand new Motorcraft thermostat with a clean bill of health.

When I reinstalled it I was extra, extra anal about seating it in the housing, to rule out the possibility of the gasket letting coolant slip by. Well, it was apparently all a waste of time because nothing has changed. I am still getting high 160s to low 170s driving down the highway in 34°F air temp. To me, this is too cold considering where the thermostat opens.

I could still try replacing the ECT sensor but my hand on the upper hose tells me the ECT is probably accurate. The hose is not too hot to touch the way it would be at 195+.

I will probably just keep driving it this way until I can think this through. I may still try another thermostat. I will also replace the O2 and see if that helps my mileage. If the mileage was acceptable and the heat is good (it is good now), I wouldn't care as much, I guess.

Finally, how do I diagnose this WOT A/C Cutout relay thing? Is the relay stuck or something? Where is it? Does the WOT A/C Cutout flag in the ECM data stream indicate a command or a state? In other words, is it telling me "Yes, the ECM is trying to cancel A/C operation due to being at wide open throttle," or is it saying "the WOT cutout relay is open and I'm just letting you know"? A/C is a warm weather thing, of course, but I am curious about this flag from the computer.

 

[squishyss]

I have a 91 2.3 liter 5spd ranger that is doing basicly the same thing. runs about 170 degrees. goes into open loop on a long accel (highway). i'm getting about 17 mpg city or highway. seems low on power most of the time. I also have a bad ignition module on mine. the tach doesn't work and the timing is all over the place. I live in a climate where it is cold right now. about 30-45 degrees and think that may have something to do with it. maybe we will be able to help each other figure this thing out. I also have a new water pump and t-stat on my truck, but still no change.

 

[onimitsu]

I have a 91 2.3L 5-speed; I have the same problem with my temp gauge, and my tach I jumping all over the place! I have changed my ignition modual twice with used ones from the junk yard and they work for about a couple of months then it goes back to the same thing! I really have not had time to try to figure out why the temp gauge is not working because it is about 2 degrees out, but for were the temp sensor for the gauge it is the one on the right side of the thermostat/intake, (as you look at the front of the truck) should be right above the oil pressure sensor!

 

[kevm14]

I put a NAPA 195° stat in it and, no surprise, it's doing the same thing. I also changed the ECT and it may be a little hotter, but still below 180. The old ECT had quite a bit of nasty white corrosion on the pins so maybe the harness needs to be cleaned out. The corrosion would add extra resistance to the circuit, and the more resistance, the cooler the computer reads the temperature. Replacing the O2 did nothing for the open loop issue.

Also figured out the A/C thing. Apparently that flag in the data stream is essentially a "compressor clutch engaged" flag, which means it is misnamed. I believe the refrigerant is low and nothing more.

I don't have a bad ignition module but we all seem to share the temperature issue. How do you guys know it's in the 170s? Do you have a scantool?

The ignition module, if it's like my GMs, gets hot during use and needs to be attached to a metal object of mass, or a heatsink. There should be thermal paste between the ignition module and wherever it's attached. Perhaps it is overheating and failing.

I looked really hard for the dash temp gauge sending unit and still nothing. I found the oil pressure sender/switch. It's below the valve cover at the back of the head on the driver's side, but I do not see, or feel, any other sender in that area.

 

[TooManyRangers]

Is your in cab heater on or off when you are testing?
have you checked the grounds on the engine, cassis, and ecu with an ohm meter?
try adding a temporary extra ground.

 

[kevm14]

I've tried both heater on and heater off. I don't get the feeling that the heater alone will prevent full operating temperature, even at idle, in 30+ degree conditions, especially at 70mph.

Have not tried tracking down any grounds yet. I was going to start at the ECT harness plug and take it from there.

 

[sstoli123]

I have the same heating temp and replaced thermostat and flushed the coolant and all that but no change here either. I hope we can figure this out together

 

[kevm14]

I have the same heating temp and replaced thermostat and flushed the coolant and all that but no change here either. I hope we can figure this out together

You're only getting to mid 170s as reported by a scantool?? Aw crap. I was assuming that the engine was really up to temperature but now I'm not sure. I will clean the harness pin recepticles tonight.

 

[sstoli123]

Sorry about that I have not verified my low engine temp with a hand tool!
My symptoms are...
1. Low cabin heat
2. Big decrease in gas mileage
3. Temp gauge on dash reading very low
I pit in a higher temp 195 Deg. thermo and made very sure it was seated correct and also replaced the rusted housing! None of which helped but the coolant needed changed so all was not lost
Sorry if I threw you off
Steve

 

[kevm14]

Alright, I have an update that might help get someone on the right track. I believe I am working towards the solution. I found this:

I then plotted the data points in Excel so I could more easily pick out the values between the data points, which looks like this:
ECT resistance vs temp

I unplugged the ECT sensor harness from my brand new Motorcraft sensor and measured the resistance with my digital volt meter. Once I got a reading (be patient, you are working blind), I quickly plugged the harness back in, got in the truck, turned the ignition on, and looked at the readout of engine coolant temp from my scantool.

The reading I took was 10.2k-ohms. On this graph, that corresponds to about 125.5°F. However, the scantool said about 121. Going to the graph, I notice that at 121 the resistance of the ECT sensor is actually about 11.2k-ohms, or exactly 1k-ohms higher, which is what the computer is seeing. This is only a few degrees and doesn't sound like much of an issue but let's assume that 1000 ohms is a static resistance somewhere in the circuit (possibly a ground to the computer). What does 1000 ohms do at operating temperature?

At 195°F, I see about 2.8k-ohms (194 is actually a data point, which is nice). Adding 1k-ohm to that, 3.8k-ohms, the temperature is significantly cooler, at about 177.5°F. This is 17.5° cooler than actual, which is significant. More importantly, that's right in the ballpark of where the truck seems to run when driving down the highway (maybe a bit cooler). This means that I probably have a ground issue somewhere (or at least a bad connection, causing about 1000 ohms of extra resistance on that circuit, which is a lot). So my next step is to try hunting down the bad connection.

 

[Earl43P]

Kev, you're doing a bang-up job with the electrical troubleshooting.

But, I think your radiator fan clutch is stuck.

It has a viscous fluid that is temperature sensitive. The fan SHOULD spin freely after reaching normal operating temperature. I'm guessing yours stays "coupled", driving the fan at full speed, cooling your coolant too much.

It's cold enough outside that you could run the engine with the fan removed, on a test drive. Just watch your temp guage while sitting still. If it starts getting too hot, turn the heater on high/hot to bring it down.

Good luck!

 

[kevm14]

Kev, you're doing a bang-up job with the electrical troubleshooting.

But, I think your radiator fan clutch is stuck.

It has a viscous fluid that is temperature sensitive. The fan SHOULD spin freely after reaching normal operating temperature. I'm guessing yours stays "coupled", driving the fan at full speed, cooling your coolant too much.

It's cold enough outside that you could run the engine with the fan removed, on a test drive. Just watch your temp guage while sitting still. If it starts getting too hot, turn the heater on high/hot to bring it down.

Good luck!

Thanks for the idea, but I already checked that. It spins quite freely when everything's warmed up. On cold start, you can hear the fan in partial lock up before the viscous fluid thins out, and after a couple minutes of idling, it thins out and the fan goes into freewheel mode, which I assume is exactly as designed.

Don't take this as argumentative, but even if the fan WAS stuck (which I would notice anyway between the sound and power loss), I would think the thermostat would stay shut and keep coolant out of the radiator. If coolant can't enter the radiator, it can't cool down (other than the heater and any limited air cooling of the block/head).