Coolant temp sensor/sender causing truck to run rich?

[hfitzgerald]

Hello all,

I've been trying to rectify poor idling and reduced power on my 1999 Ford Ranger xlt (3.0 flex fuel) for months now. As of now, the truck won't throw a code but long term fuel trim banks are hovering around -7.5, and it remains in a closed loop no matter how long I drive it. Additionally, the dashboard coolant temp indicator is not working.

This is where I'm wondering if the thermostat is bad and I need to replace it, or if I should replace both of the sensors attached to it, maybe the whole damn assembly?

Also, I'm having one hell of a time finding part numbers for any of these items. How do I look these up? They aren't listed in the Haynes manual anywhere and internet part finders are often wrong (especially Amazon's).

Thanks in advance for any advice you can give.



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[RonD]

Welcome to TRS

Closed loop is what you want after 3 to 8 minutes of running from cold start

Open loop is set when computer boots up, after startup computer check coolant temp to see if it is under 150degF, if so then computer runs pre-set air:fuel mix based on temp, i.e. Choke mode, higher idle and richer fuel mix
As coolant warms up computer Chokes engine less and less, so idle starts to come down and mix gets leaner

The O2 sensors can not work to detect Oxygen until they are heated above 600degF
Computer watches their voltage, when voltage starts to switch very fast the computer goes into Closed Loop.
Closed loop means the computer is getting feedback from O2 sensors so can adjust air:fuel mix on the fly
Fuel Trim is the Open Time for injectors
Computer calculates Open time for the injectors, and that is 0 Fuel Trim
If O2 sensor shows Lean then computer opens injectors longer +2 fuel trim
If O2 sensor shows Rich then computer shortens open time -2 fuel trim

The fuel trim is a %, say 0 fuel trim was open time of 100milliseconds, -2 fuel trim would be 98ms, so 2% lower open time

When computer sees fuel trims off by +/- 15% or more for a length of time, it will set a code

The temp sensor on the engine is only used by the computer, no connection to dash board temp gauge, and the computer has a "timer" for this ECT sensor, temp needs to change after startup or it will set a code, but.............
ECT sensor could fail "warm", so it always shows say 180degF even when engine is cold.


Dash board temp gauge uses an ECT SENDER<<< sender not sensor, same type of sender has been used since temp gauges were installed on engines back in the 1930's
This run on 12volts

ECT sensor uses 5volts from computer, so not the same and not interchangeable.

If your engine is actually running cold, under 160degF, then it will run Rich, but you should also get a code.

First thing is to check upper rad hose
Start cold engine and feel upper rad hose, should be stone cold of course, now feel heater hose, should be cold as well

Let engine idle for 2 minutes or so
Then feel upper rad hose and heater hose again, heater hose should be warming up, upper rad hose should still be stone cold
If upper rad hose is warming up like heater hose then thermostat is stuck open and need to be replaced, use only 190 to 195degF thermostats

 

[hfitzgerald]

Ron,

Thanks for the welcome and especially for distilling all of that information down into a dummies version. I'm going to preform the check that you recommended asap.

Would you recommend purchasing an ect sender since that gauge isn't working or is it possible that a stuck open thermostat could cause that issue?

Thanks a ton,
-Herb

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[RonD]

Temp gauge should move a bit even if thermostat is stuck open, once engine is warmed up a little

On the engine near thermostat housing will be BOTH the ECT sensor and ECT sender

Sender will have a Red/white stripe wire, and maybe a black wire
It will be to the left of upper rad hose on a "elbow" that threads into lower intake
Often has a Red plugin

The Senders Red wire runs to the dash temp gauge, you can test the wire and gauge by GROUNDING this red wire to engine, and turning on the key
Temp gauge should go up to HOT if wire and gauge are OK

So pull off the connector use a jumper wire on Red wire to a ground, battery negative is OK as well, then turn on the key, its not a short, thats how this sender works

Replace sender if gauge goes to HOT

And make SURE you get a sender, most auto parts guys will give you a sensor 9 times out of 10


ECT sensor is to the right of upper rad hose and on the lower intake, Light green/red wire and Grey/red wire


Found this picture: https://www.ranger-forums.com/attac...coolant-temp-sensor-issues-coolant-sensor.jpg

 

[hfitzgerald]

Sorry for taking so long to reply, this truck is just feeling like a bag of Gremlins lately. I felt the hoses as you suggested, and seemed to pass that test.

I was unable to get the gauge to go hot by grounding it/following the procedure you specified. I'm beginning to wonder if there's an electrical issue somewhere. Anyway, today as I was doing these tests, the engine was really vibrating and just doesn't seem healthy. I'm going to attach some photos from my scan tool, I think there is probably something obvious going on here that I'm missing.

The photos are live data taken while the truck was idling, it was running for about 15 minutes before I captured the photos.

Thanks

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[cbxer55]

Been running a 180 in mine for as long as I can remember. Never been a problem, even on the coldest single digit nights.

My 04 Lightning came factory stock with a 180, and has had a 170 for as long as I can remember. Same story.

And yes, the needle on the temp. gauges doesn't go as high up the gauge as they used to.

But whatever, run what you want.

 

[Craig0320]

I have a 180 in my 5.0. Mine goes into closed loop withing a few minutes of startup.

 

[hfitzgerald]

Sorry, I'm still a newcomer at all of this but by 180 do you mean thermostat degree spec? Also the temp gauge is completely non-functional. Did you spots something that non-standard in those readings. This truck was my dad's and I'm currently trying to fix it up. He is suffering from dementia and cannot recall the work he did to the truck over the years.

Also isn't a 40% load pretty high while the truck is in park, warm, idling at 890-920 rpm?

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[alwaysFlOoReD]

180 is the degrees the thermostat opens at. There are different temp thermostats available. That won't have anything to do with your problem. I think the in dash gauge needs replacing if it didn't move when you grounded out the wire. I can't help with the readings, that's above my pay grade, sorry. Someone else should be by to help for that.

 

[RonD]

Coolant sensor is showing 185degF so thermostat is working like it should
It should be rated at 190-195, so engine will run at 185 to 200 deg

180deg T-stat just wastes fuel, lol.

Yes, your wire from temp SENDER to the dash is not connected to the dash or the gauge in the dash has failed

900RPM idle is high after warm up, i.e. temp sensor showing above 150degF

ST fuel trims are showing high voltage at O2s
.1 is lean
.9 is rich

so .7 and .8 are on the rich side, but..............that is normal at idle, which is rich, O2s are not used at idle or WOT(wide open throttle)

While cruising along you should see .3-.5 average

 

[Bondo]

Did you ever get this issue figured out?

I ask because Im going through something similar.

 

[91stranger]

If you are concerned about the sensor and the sender then just replace them. They are pretty cheap $10 each roughly... I think one was $8-10 and the other was $10-12. Either way you are looking at $20 and about 10 minute to swap them out. Also be sure to leave some of the threads clean so it can ground properly. I used thread tape on mine and left about 3 threads left and mine works like it should. I was having a lot of issues with stalling and the CEL was telling me squat so I just start replacing the basics until it goes away. I remember I did the t-stat, sensor and sender on my lunch break and that includes driving to the parts store, working on it in their parking lot, getting a bite to eat from Taco Bell and going back to work. Do like the other guy said and be sure they get you the correct part. he's right that 9 times out of 10 they will give you the sensor instead of the sender.

 

[hfitzgerald]

Sorry for the late reply here, I was frustratedly trying to solve this problem for quite a while.

Finally, I noticed that the shortest spark plug wire was used to span the longest length and traced the sequencing. It was wired in ascending order, coil 1->cylinder 1 all the way to six. So yeah, incorrect firing order. I fixed that, and after about 30 minutes of the engine relearning timing its running much better.

I'm definitely going to replace those sensors now, regardless. I figured it'd cost me alot more than 20$.

Also, I cleaned battery terminals and I'm going to replace ground connections when I can. There's been a decade worth of half a**ed "fixes" done to this truck that I need to clean up. Also need to address that annoying squeal/squeak (not coming from the belt or pulleys).

Thanks for all the help, everyone.

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