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Discussion Starter · #1 ·
Recently my 99 Taurus will stall on startup when the engine is warm. The idle will get really rough and then it will eventually die. However, if the car does sit for 30mins it will fire right up. Unfortunately, there had not been any codes thrown to give any indication of what is wrong.

I noticed that in some other posts, members felt that there was something wrong with the closed loop of the system. I am unfamiliar with the difference between the closed loop and the open loop. I thought that a closed loop is what a car normally operates in, as it connects all the readings of the IAT, MAF, ECT, and O2 sensors. Is this incorrect?

In addition, I was wondering if a bad ECT sensor could cause stalling on startup because it would trick the PCM to think it was cold and run rich when it should not. Any thoughts? Thanks!
 

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Are you up to date on your maintenance? - Fuel Filter, Air filter, Oil changes, cleaning the MAF with MAF cleaner only?

If your plugs and wires are old that might be your issue. Motorcraft wires + Autolite plugs.

If so i would start by taking off the Idle Air Control valve and cleaning it with brake fluid and a tooth brush. Also cleaning the Throttle body with brake cleaner/toothbrush won't hurt. My 99 Vulcan got a lot of carbon build up in the TB and cleaning the IAC valve along with replacing my plugs and wires helped me a lot.
 

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Even when started hot, the PCM doesnt go into closed loop right away. There is a "hot restart timer" in the PCM software that doesnt allow closed loop for a short time. How long? I dont know exactly, but on the order of a minute or so I think.
 

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Discussion Starter · #4 ·
I am all up to date on my maintenance. I did plugs/wires, coolant, fuel filter, and a new DPFE sensor within the last 8k miles. I also recently cleaned the TB and the IAC when I started have DPFE type symptoms.

As for the closed loop, I really cannot say how long the timer is. When I have issues, it is whenever I run into a store for like 5 minutes and come back to start the car. I feel like it might be a heat soak issue or a sensor that is detecting engine temperature is sending the wrong signals. Is it possible for a bad ECT or a IAT sensors to cause these symptoms and not throw a code?
 

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Use a scan tool that can read PIDs and see what the ECT, IAT, etc is telling the PCM what the various temps are. Checking the fuel pressure PID might also be useful.
 

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Could the coolant temp sensor cause this? I know they are cheap and easy to replace
Yes, that could be the problem, or it could be 100 other things. Personally, I have always believed in properly troubleshooting the problem instead of throwing parts at it hoping to randomly fix it. Throwing parts at the problem gets real expensive real quick.
 

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Discussion Starter · #9 ·
Through trouble shooting I have found that my fuel line is loosing pressure because the fuel pump is not kicking on during a warm startup. Thus, I am loosing fuel pressure in the line when I go to start the engine. I had thought that it could be a bad PCM or fuel pump relay in the box on the front of the car but swapping in a new relay (while I had the engine reproduced the issue) did not solve the problem. However, if I do allow the car to sit 5-10min you can hear the fuel pump engage again and produce fuel pressure.

I have noticed that some members with similar car symptoms have been able to resolve their issues by replacing the fuel pump. However, before I put in a new fuel pump, I was wondering if someone could explain why the fuel pump does not engage only when the car is warm or if there is another relay I am missing. Any thoughts or comments are greatly appreciated!!!
 

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Discussion Starter · #10 ·
In addition, I noticed somewhere on another thread that a bad oil pressure sending unit can cause the PCM to cut power to the fuel pump. Is this correct or even a possibility as a cause of the fuel pump to not work when the engine is warm?

Thanks!!
 

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I would check the wiring near the pump for 12v when it should be on. It could very well be the pump failing, but corroded wiring will have higher resistance when hot. The pump needs full current just like any electric motor, or it will be damaged.
 

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Discussion Starter · #12 ·
Okay, I did the recommended test and I am finding the fuel pump is not getting power when the engine is warm. I feel like it must be a relay and I have tried replacing them in the main box on the front of the car with no avail. However, I did notice that there is a box directly to the right of the battery before the fender. It is full of different relays but I cannot find a map of what they are in the owners manual, haynes manual, the top of the box, or on all data. Does anyone have any idea what the relays are for?
 

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Please, if you find an i.d. legend let me know. I even tried a couple of dealerships and they could not(would not) i.d. the relays contained within. Ended up grabbing all of them @ a pick n pull to solve my wiper prob. I know the wiper-park, wiper hi-lo, and i think poss. an ac clutch relay are in there. searching for 98-99 tec. thanks, -sheila
 

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Discussion Starter · #14 ·
So, I pulled one relay at a time in the box next to the battery to see if any of them controlled the fuel pump. Unfortunately, none of the relays controlled the fuel pump, so now I am at a loss. Does anyone have any ideas on a sensor or a relay that controls the fuel pump and would be affected by a heat soak condition? Thanks!
 
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