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Wolfgang, Master Auto Tech
Category: Ford
Satisfied Customers: 4149
Experience:  Master, L1, L2, ASE 15yrs and running.
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1994 Thunderbird 4.6 remains in closed loop even when fully

Customer Question

1994 Thunderbird 4.6 remains in closed loop even when fully warmed up according to my Bosch OBD2 reader the LT fuel trim remains at 0. ST fuel trim remains below -20 until fully warm.


The problem I'm trying to resolve is that it runs beautifully and then sometimes within minutes or sometimes within about 15 minutes it suddenly starts missing, stumbling and dies at idle.  Then it will start but won't stay running.  If the accelerator is floored it will continue running a few seconds longer but when it dies the plugs are sooted up and sometimes gas fouled. 


My working theory is that something causes it to suddenly run rich, choking it out as it will start again after sitting (and the plugs burn off the soot) or will start again with clean plugs.


I have taken readings with my Bosch 1050 OBD2 reader at various coolant temps and RPMs and have listed the data in Excel


Submitted: 1 month ago.
Category: Ford
Expert:  Juan Crespo replied 1 month ago.

Hi there.

That it goes into closed loop is actually a good sign. However, we'd need more data in order to offer an accurate diagnosis; can you measure vacuum when engine cold at idle and again when hot? Also, can you measure Oxygen sensor voltage when the engine is idling hot?

One more thing, could you check and see if there are any KOEO or KEOR codes?

Customer: replied 1 month ago.

I don't have a vac gauge so will have to rent one which will take some time. However, it has never had a MIL or code of any kind. Regarding voltage to the O2 sensors, if you mean data from the my OBD2 tool this is what I've captured at various temps and RPM. I took pictures of the data and entered them into the attached file as below. I also noted that the FUEL SYS 1 data changed from CLSD to CLSD1 at 201 degrees engine temp. Also, why does FUEL SYS 2 = N/A?

Expert:  Juan Crespo replied 1 month ago.

The key to a successful diagnosis is to concentrate on the data that pertains to the symptoms. In this particular case we need to look at O2 sensor voltages plus engine temperature and vacuum.

Your spreadsheet doesn't reflect the time between data changes; for example, knowing how long it takes for the engine to warm up from cold to normal operating temperature can help us rule out a stuck-open thermostat, which could cause the Rich condition being indicated by the O2S1 voltages on the spreadsheet - normally, the thermostat will open after 2 minutes of fast idle.

Sometimes we can get too much information that can lead us away from the correct diagnostic path. Also, how we get that information and its source can be determining factors; we need to remember that any data we get from a scan tool is just that tool's interpretation and not the actual signals - those we can only get by measuring the circuits.

I strongly suggest you focus on vacuum readings and engine temperature changes as related to O2S1 voltages keeping the engine at idle so you can establish a baseline.

For what is worth, the PID that look off on your sheet are:

  • Coolant temp should've not dropped suddenly from 205 to 126
  • IGN ADV - should be less at idle - unless the RPM were increased with the vehicle in Park or Neutral, in which case the readings have no diagnostic value
  • MAF - same as above
  • O2S11 - should not have changed from 0.445 to 0.830 due to a 12 RPM increase

Once again, I do believe the best course of action is to get those vacuum readings so we can then move forward - by the way, if you need to, you can click here to see an interesting video by Eric the Car Guy on how to check for exhaust system restrictions using a vacuum gauge.

Please let me know what you find.

Customer: replied 1 month ago.

I have to apologize for inadvertently giving you misleading information on the sheet. The first eight columns were consecutive readings from a single session starting at 8:20 pm and ending at 8:30 pm when the coolant was 205. I've added the time to the bottom of each column in the sheet attached below. The additional columns were samples from another session because were unusual in that they displayed the LT fuel mapping and happened just before the car acted up and died, almost as if they were the cause of it. I did watch the video on vacuum readings and will get some vacuum readings as you suggested.

Expert:  Juan Crespo replied 1 month ago.

OK. We can continue as soon as we get those vacuum readings.

By the way, you are working on an OBD-I system, not OBD-II ;-)

Customer: replied 1 month ago.

This is what I'm working with on my 1994 Thunderbird 4.6.

Expert:  Juan Crespo replied 1 month ago.

My only goal here was to help you solve your issue, not argue with you. However, since you seem to be focused on a diagnostic path that is different than what I recommend, I'll opt out and let others try to help you. Please do not rate or reply until another expert responds or the site will cycle this back to me.

Customer: replied 1 month ago.

Sorry, I was just trying to clarify what I'm working with. To my knowledge this is an OBD2 port.

Expert:  Wolfgang replied 1 month ago.

Thanks for choosing Just Answer . I'm Wolfgang here to help!!

I am confident we can resolve your problem together. Please
understand that I don't know your skill level. In most cases when you have a problem
with the way the vehicle performs, specialized tooling is required to acquire computer
data/information or engine testing results. We didn’t build these complicated vehicles; we
just diagnose and fix them. Also understand that I can't see, smell, hear, or touch the
vehicle either, so it may take a few replies to get to satisfactory results. Note there may be an issue where the complexity of the issue will require a Premium Service. This service will be an addition to the normal question cost. Usually it does not come down to this but keep in mind!! I am also going to direct you with the fastest, easiest and most common things to check first. Please be
patient and permit me to do what I do best.
I also try to give all of the information you may need, expecting a “positive rating” in
return. I will also continue assisting you after you have rated me, if necessary. If there
is any reason I cannot get a positive rating, please let me know why “in advance”, so I
can go another direction for you. I am sorry you’re having this problem; I’ll help you
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Below is my first answer or request for more information. If you do not understand it, or you need more information from me, or need me to go in more detail.

did you have any codes stored before any repairs were done ? what were the codes ?

any codes stored now? please scan

note rinsing o2 sensors in gas may have contaminated the sensor so keep that in mind!!


Customer: replied 1 month ago.

No, it didn't have a code before I did the listed actions and replacements and it still doesn't.

Expert:  Wolfgang replied 1 month ago.

on the older cars like yours you need to do a EEC test of key on engine off and key on engine running . did you do that procedure?

Customer: replied 1 month ago.

Yes, I did the key on engine off to begin the test and when there were no codes I started it and observed the data. I think it's odd though that this engine has never thrown a code. My understanding is that 1994 is the first year of the 4.6 engine and Ford set it up for OBD2 but because OBD2 wasn't required until 1996 I wonder how functional the system was.

Expert:  Wolfgang replied 30 days ago.

what about the KOER test where the scan tool will do a diagnostic while running

Customer: replied 29 days ago.

I had not heard of the KOER test but found how it's done on line. I can't do it until sometime next week as I'm doing my regular job the next four days but will give it a try then.

Expert:  Wolfgang replied 29 days ago.

please do and keep me in the loop