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86 2.9 Runs SUPER rich when cold/open loop, runs amazing when operating temp/closed loop


rang-a-stang

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My truck runs awesome after about 30 seconds of driving but before that, it runs like it is SUUUppppper rich. When I run KOEO or KOER I get no abnormal codes. I am not even sure where to start with this... It has a new fuel pressure regulator, just passed CA smog check about 2 months ago, and I am just not sure why it would be so bad...
 


RonD

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The computer uses the ECT(engine coolant temp) sensor to set Choke level needed for cold engine start up and driving until engine coolant gets up above 140degF

The colder the engine temp the more Choke is needed
Choking an engine means Rich fuel mix and high idle, the colder it is the Richer it needs to be

So I would change the ECT sensor just on speculation, to see if that helps
You can test ECT sensor with ohm meter and pan of water on the stove to heat it up
Google: Ford ECT sensor graph

There are TWO temp readers on the engine
5 volt ECT sensor, always has 2 wires, only used by the computer

12volt temp Sender, 1 wire, sometimes 2 after 1998 or so, only used by dash board temp gauge


What the Choke is for
Liquid Gasoline can not be ignited by a spark, yes the movie guys take liberties with that fact, lol
Only gasoline Vapor can be ignited by a spark
Gasoline Vapor is what you smell
The warmer the gasoline the more it smells, so the colder the gasoline the less it smells, so the less Vapor there is

An engine needs about 30% Vapor in the cylinder for a spark to ignite the air/fuel mix
A Choke Plate on a carb restricts air flow into the top of carb so more gasoline would be sucked out of the Jets
If the cold gasoline was only 15% vapor then you would need twice as much gasoline to get to 30% vapor for start up, so Rich mix

Computer does the same thing, temp of cold engine will be gasoline's temp, so it calculates how much extra gasoline is needed to get the 30%, based on ECT sensor temp, and opens the injectors longer to get the Rich mix
If ECT sensor shows 40degF when its really 60degF, then too Rich based on actual temp
Computer also has an air temp sensor but ECT is the main sensor in open loop/Choke operation
 
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rang-a-stang

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That makes 100% sense! I will be checking it tonight after work and after the engine cools down! I am allllll excited about this now!
 

franklin2

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In the video below around 39:00 he shows how he found a temp sensor where the pins were too short and not making good contact.

 

rang-a-stang

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I only had a few mins to tinker last night. My connector is pretty gooked up and looks original, the wires look well connected and not split or compromised. I decided I wanted to measure the resistance from the computer pin to ground since that is what my computer would see anyway. I opened up my electrical and vacuum book and it said that the ECT comes in on Pin 7. My pin 7 does not have a wire. I had to button the truck back up (like I said I only had a few mins). I'll do some searching today to verify which pin that signal should be coming from.

Why measure at the computer? Because so far, almost every connector under my hood, that I have touched, has been so brittle it usually brakes when I move it and I end up having to replace it, too. If the ECT is not my problem, I really don't feel like replacing that connector just because. Also, if I have a bad ground, this test will check that too.

Logically thinking this through, and looking at the resistance graph, if the sensor is open, or lost the ground, or bad wire, the computer would see the engine as STONE cold until it kicks into closed loop, right? Hence stupid rich? As opposed to a short, it would thing the engine is scalding hot and probably lean it out?
 

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The ECT like most sensors uses a pull down circuit(or pull up, lol) using 5 volts

This is why the voltage to resistance is reversed
At 60deg ECT should be at 58k ohms, and 3.5volts
At 180deg its at 4k ohms and 0.7volt

WTF!!!
Volts should go UP when resistance goes down, don't need an electrical degree to know that
But to understand a pull up, or pull down circuit you do, lol
I understand what it does but not enough to explain it, lol

Einstein once said, paraphrased, "if you can't explain something in simple terms to your Dear Mother then YOU don't understand it well enough"

In any case the computers circuit for the ECT sensor is not a simple voltage out/voltage in circuit
Pin 7 is the one for ECT, not sure why you don't see a wire there

2.9l wiring diagram here: https://www.therangerstation.com/tech_library/EDiagrams/files/Diagrams_ElectronciEngControls2_9_2of3.JPG

Computer outputs 5volts from pin 26, and that is shared by almost all the engine sensors
At the ECT sensor it shows that 5volt wire as the black/white wire but can be another color as you can see its spliced and changes to a few other colors, lol

You can use a sewing needle to pierce a wire to test voltage, doesn't hurt the wire
So you can test for 5volt, but it should be fine since all the other sensors use that 5v but that one splice to ECT could be bad

Then test light green wire(out to computer) to see its voltage with engine cold and then warmed up
 

franklin2

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They use a voltage divider circuit on the input to the computer. The circuit is below.



Vin is the internal 5v supply inside the computer. Z1 is an internal resistor inside the computer also. Z2 is the temp sensor.

In this circuit design, the Vout is the input to the computer for the temp sensor. This input requires a certain amount of current to work. For the divider to work and be stable, the divider circuit (z1 and z2 to ground) needs to be 10 times the current needed for Vout.

So you have a lot of current constantly running through z1 and z2. Vout just taps off this for the temp signal. If Z2 has a large resistance, then Vout will "float up" to Vin. If Z2 has a small resistance, then Vout will be pulled down to ground.
 

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Thanks, but..................

If testing pin 7, ECT sensor wire, to Pin 26, "5v wire", key on, what would you expect to see with sensor unplugged, testing the 2 wires at the ECT sensor?

And would there be a valid OHMs test, key off, to test computers internal circuit, pin 7 to pin 26?
Or between pin 7 and ground?
 

rang-a-stang

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I think the easiest way to go about this is pull the connector at the sensor. If it breaks, I'll just have to replace it. I should have 5V on one pin with Key on. Then I will measure across the ECT. If both those checks pass, I will shut the truck off, ground the green/yellow wire at the connector and check Pin 7 for a ground. If I have a ground on Pin 7, I will re-connect the connector at the sensor and measure for ground again (I should NOT have a ground anymore). If that passes or if I don't have a ground, I will have to poke around inside my ECM connector a little.

I did replace the ECM about 4 months ago so maybe when I installed it, it pushed the pin out on the connector side?
 

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I would check the ECT sensor's ohms with cold engine first, compare to outside temp ohms from graph, if its way off then sensor is the issue
 

rang-a-stang

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OK, I am a schmuck. I was reading the connector to the ECM backward. There is a wire on pin 7. Duh.

Went out to the connector at the ECT and pulled it. It looks like this:
72119

So that's trash. It's worse in person.

I have 4.57 volt on the black/LT Grey wire pin. I think that is because that connector is so bad, it's losing voltage. So I have ordered a new connector. There is NO WAY I could get my multimeter leads on the posts on the ECT Sesnor itself so for now, I am going to let it ride. If I am losing 10% of my reference voltage right off the bat, that may be the whole problem.

So when I get the connector replaced, if it still does this (runs super rich when cold) I will touch base with you all again.
 

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4.5 v ref is workable
 

rang-a-stang

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Last night we installed a new temp sender (now the temp gauge finally works), a wrecking yard acquired/Original Ford CTS, and the new CTS connector. I did not drive it but my son did and he said HUGE difference. I notice it seems to idle a little smoother at temp. We had disconnected the battery while we worked so it is hard to guess if the cold idle was any better.

When I clipped off the old connecter, I found three significant cracks in the insulation that were not in that above picture. That's all clean and good now.

Changing the CTS SUCKED because it is in such an awkward spot. We did not have a socket that could fit the CTS and clear the lower intake so we had to use a wrench. I only had crescent wrenches that would fit it so it was REALLY slow going. Took a solid 20 mins of mini-turns to get that sucker out (after removing the throttle body and T-Stat housing). Another 15 mins to get the replacement CTS in. I did measure across the pins of the old one once it was out and it was really low. Like 200 ohms. The replacement one was about 40k ohms.
 
Last edited:

bobbywalter

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Crescent wrench ...


That's some proud cave pig get er did....



👍
 

rang-a-stang

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Reviving this long dead thread.

In the last year, my son has put about 5k miles on his (this) truck. It still runs stupid rich when cold. My daily driver Mazda and the wife's Pathfinder both have black splotches on them from when he was parked next to us and started his truck cold (it spits wet carbon out when its cold). We have also moved from SoCal to Boise so this has really started to bother me.

More detailed symptoms:
1) It fires RIGHT UP when cold (or hot/normal operating temp). It is never super hard to start except if we start it, run it for about a couple minutes, then shut it down and try to restart it so like if the engine is warm but not operating temp. It never goes over normal operating temp when we drive it. We took it up a looooong uphill mountain road and it spent 45 minutes in 3rd and 2nd gear, mostly, WOT or close to it, and did not even get hot.
2) When it is cold, it idles high as it should but has an occasional, regular "miss". I call it a miss but I don't think its really a miss and it is consistent/regularly timed. I'll try to get a video when I get home next week. Its like "Vrooooooooooom-buh-Vrooooooooooom-buh-Vrooooooooooom-buh..."
3) It still runs amazing once it is at normal operating temp
4) I replaced the Coolant temp sensor and it's connector with new McParts parts and it BARELY made a difference. Maybe it didn't make a difference and it is just placebo affect?
5) Intake air temp sensor was replaced a while ago too, no change in symptoms.
6) when cold, it bucks and studders almost like a vacuum leak when we drive it. I have checked for vacuum leaks 3 different ways (measured each line with a vacuum pump, with brake cleaner, and with a vacuum gauge). I am not going to saying it does not have a vacuum leak, but if it does have one, I don't know where it is.

Maybe I have a small vacuum leak that only opens up with the engine is war and the computer has learned that when it is warm/closed loop, it needs more fuel, then when it is cold/open loop, it has extra fuel? I also installed a rebuilt ECM (during my last get through smog fight, the computer would not trigger the EGR valve so we had to swap the ECM) and swapping the ECM fixed the EGR valve problem but the ultra rich cold idle stayed the same on both ECMs).

Where should I go from here?
 

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