88 Bronco 2 Looses Power once warmed up

OK so this isnt quite an emergency but it is a big deal and is probably making me burn more gas then i need to but lets get down to business

I have a 1988 Ford Bronco 2 Eddie Bauer Edition 2.9l Fuel Injected Automatic with the manual 4X4 on the floor

When my truck first starts up id say it runs pretty good for a 26ish year old truck [other then the fact the tranny probably needs atleast a flush and fresh fluid (or to be junked)] but after it starts getting warmed up it tends to run like shit still drive able but the acceleration and general power is horrid but like i said still drive able. like for ex. when i push on the gas to get going from a stop it takes a while (say 0-25 in like 25 seconds with the gas pedal push about half way down)and has a deeper sounding growl to it then the higher pitched roar when it first starts up and this problem becomes very noticeable when towing a 4x8 trailer with a small riding lawn mower a push mower etc. it fells like i'm trying to pull a semi which i know shouldn't be as much of a problem as it is. so any ideas would be greatly appreciated.


things ive already done
New Plugs and Wires
New distributor cap and stator or what ever the **** that think that spins is called
New Coil
New TFI Module + Relocation kit with heat sink and added fan (due to stalling after 20-30 minutes of driving and not starting till cooled)
All new fuel system (except for lines and injectors did run some injector cleaner threw it though doubt it did anything)
Punched a hole threw the CAT with a pipe (don't tell the cops)
Put a glass pack on (not to fix the problem but to replace the fact that the muffler was shot and didn't have the extra 10 bucks for a new muffler so glass pack)
lots more parts but none that should affect this issue




By new fuel system i mean 2 Fuel pumps 1in tank one on frame rail new gas tank and fuel sending unit All 3 fillters replaced and plenty of fuel pressure at the rail 35-40psi

Here Are The Videos Of The Issue

Here Are the videos

I assume the Check Engine light is working, comes on with key then goes of??

So no codes are pending or in history

Normal cold start should be like this:
Engine starts and idle jumps up to 1,500rpms then drops to about 1,100
As you drive and engine warms up the idle should drop relative to coolant temperature.
Temp goes up idle goes down.
Target warm idle for automatic is 800rpms, coolant at 200degF

Fuel injected engines can not use a Choke Plate for cold starts, no jets to suck extra fuel from.
So they use an ECT(engine coolant temp) sensor, this is a 2 wire sensor only used by the computer(dash board temp gauge uses a 1 wire sender)
When you turn on the key the first thing the computer does is read the engine temp using ECT sensor.
If ECT sensor shows 60degF then computer opens injectors longer(richer mix), raise idle above 1,000, and advances spark timing, exactly the same as a choke did on a carb.

If your cold start isn't as described above then your ECT sensor could be reporting wrong temps to computer, your warmed up engine could be running too rich, check a spark plug.
You can test ECT sensor with volt meter and sewing pin.
Computer sends ECT sensor 5voltsDC on 1 wire
Then reads the return volts on the other wire
So ground the volt meter, turn on key
push sewing pin into 1 wire and test voltage
If 4.7-5.3volts that's the power from computer, test other wire.

ECT volts by temp, these are approx.
30degF 4volts
70degF 3volts
120degF 2volts
200degF .5volts

'88 2.9l shouldn't have an EGR valve, if it does that could also cause similar problem
EGR systems are not used on cold engines, if it was malfunctioning it would only effect engine after warm up.

Same with O2 sensors, they can not work until above 650degF, so computer doesn't use them until engine is warmed up.

Computer itself could be the issue.
Computers have 2 modes of operation.
Open Loop is when engine is cold, computer uses preset air/fuel tables in memory.
Closed loop is when engine has warmed up and computer calculates air/fuel mix based on sensor data, more variables in this mode so more opportunities for a problem.
There is no real test for computer problem, you can remove the top cover and have a look inside, often capacitors will leak over time and effect circuit board voltages, which changes data computer is using for calculations.

Clean computer board can be seen here: http://www.auto-diagnostics.info/ford_eec_iv

Check Engine light works fine

Upon Cold start the truck revs up to about 1500 to 2000 i think it goes Start 2000 couple seconds 1800 couple seconds 1500 then kinda stays there i never really let the truck warm up cause it runs like shit once it is. the idle is around 750 - 900
{Doesnt seem like it matter other then to show why rpms are high. but i adjusted the throttle stop screw to keep the throttle open and rpms up a little to compensate for the hi-drain electronics i have fog lights, subs, power inverter along with head lights wipers and heater the lights dim quite a bit but the battery light doesnt come one
the truck also sometimes stall if i let it sit running in park for a while and then hop in and throw it in drive but it always starts back up }

i haven't searched at all cause im about to head off to bed but do you know if there is a way i can force it to stay in the open mode? cause i think that would really help with my gas mileage
and what do you think i should look at or replace first like most likely problem cause every problem ive had with this truck is very common on all other bronco 2's like the tfi module dieing and leaving you stranded had that happen like 6 times in the first month i had it running

oh and thanks for your help whether we figure out this things issue or not

So, what we KNOW is it runs poorly when in computer-controlled, closed loop.

Likely to be a SENSOR out of spec / giving bad info to the computer, just not bad enough for the computer to flag the sensor as bad. That can happen if you disconnect/reconnect the battery repeatedly. See Page 45 Adaptive Control

Go to FordFuelInjection.com and look at how you can test each of the sensors, like the IAT, ECT, TPS. MAP and O2 sensor are trickier to figure out, but if your O2 Sensor is old, replacing it may help. They only tend to last 60-100k miles.

I'm pretty sure if you disconnect the O2 Sensor, it'll HAVE to run in open loop. See Page 38:

http://www.therangerstation.com/tech_library/pdf_documents/Ford_Fuel_Injection_Book.pdf

Good luck with your truck and Welcome to The Ranger Station!

You shouldn't adjust your throttle screw, the computer will keep the idle up itself even with increased load on the alternator. You might consider a larger alternator at some point, but that's not relevant to your current problem.

You cannot "force the computer to stay in open loop mode," because that requires the computer to read the O2 sensor, which requires the O2 sensor to be above 650 degrees. Open loop mode cannot work when the engine is cold. Once the engine warms up enough, the idle drops to about 800 rpm, and the computer enters open loop mode, and should stay in open loop mode until you shut it off and the engine cools down.

RonD is suggesting that your computer may be misbehaving when it's in open loop mode.

Spott, you have it backwards
Open loop is when computer uses factory presets instead of sensors, so cold starts.
Closed Loop is after warm up and computer runs from it's own calculations based on sensor data.

crazyjp, the ECT sensor is working or your idle wouldn't change as engine temp warms up.

Throttle screw is to adjust TPS's(throttle position sensor's) lower voltage, which should be between .7 and .95volts.
With engine warmed up and IAC Valve unplugged idle would usually be about 500rpms or engine may even stall, either is correct.
So it isn't an idle screw, for the IAC valve and computer to do its job throttle plate must be closed enough to cause idle below Target idle, 800rpms in your case.

I understand you wanting to drive the vehicle but it will make it harder for you to track down the problem.

On the firewall you will find the MAP sensor, passenger side center, this is the main sensor the computer uses to calculate fuel amount to be added to the air in the intake.
O2 sensors are only for fine tuning that mix.
MAP sensor will have a vacuum hose attached coming directly from the upper intake manifold.
Remove this vacuum hose and make sure it is air tight, also disconnect MAP's wiring and check connector for corrosion.

Also just to check if computer can turn on CEL and store codes unplug MAP sensor wires when engine is running, CEL should come on as soon as you do that, engine may stall but might not.

(Tried to post this earlier but either the site or my internet ****ed up probably the internet think the modem is going)
OK im gonna go at this as you guys posted

to earl im gonna go threw all that tomorrow. i downloaded the god book last night and thank you fro the welcome ive been finding stuff that was use full for awhile just didn't join up cause of email issues and being lazy

and spot i turned it about 1 or 2 turns in just to keep the idle up when the truck in gear and foots on the brakes cause. other wise im afraind it could stall. And sure the computer is supposed to do it on its own but it doesn't or atleast not to my liking, it idles at the same rpm just the higher the load the lower the rpms. the other reason i adjusted it is because i have what autozone says it a bad battery but its 250cca higher then what my truck requires figured it would be better due to lots of electrical shit but idk adjusting the throttle screw im pretty sure didnt cause my problem since ive had this problem for about 6months and i did it just a couple of weeks ago due to letting it idle for 10-15 minutes and then when id put it in it would stall so i turned it in a few turns problem solved

and last but not least ron i will combine your info with earls and test everything

but for now i tested some shit last night again before any of you posted anything i unplugged the map sensor if i remember correctly the truck seemed to stay the same but CEL (wow that's weird typing it like that) came on like it should i also unplugged the TPS sensor truck went to about 500rmp and went up and down till it finally stalled

oh and to wanting to drive the vehicle making it harder to track down the issue is one of those if i don't drive it i have to walk and my lazy ass don't wanna walk since the weather is like 40F - 10F pretty much that last 4 weeks

My guess is its probably the O2 sensor and maybe something else cause i know the truck burns oil which can cause premature failure. but it doesnt burn it bad in my opinion, by the time it needs an oil change (3000Miles is my preference new filter every time) I'm about 2quarts low oil light comes on about 1 1/2 quarts low i usually just leave it just waiting for a reason to do an engine swap also im pretty sure none of the sensors have been replaced and im pretty sure this truck has at least 300,000 miles on it

but to end this long post i will post with results later tomorrow

You might have a vacume leak somewhere. broke/ cracked line? leaking/bad brake booster?
Does it try to die if you stomp on the brakes after it is warmed up?

I don't think they were on a 88 but Does it have a EGR valve?

Yes, Ron, now that you pointed it out, I can see my mistake. Closed Loop means it's using the O2 sensor data to *complete the feedback loop,* which it does once the engine and exhaust are warmed up.

That's what I get for posting first thing in the morning, before my brain is firing on all cylinders.

well the try to die part idk its never died on me yet i like to do lots of un necessary brake checks when people decide not to wear there seat belt and as far as i remember its never died the only time i can remember it dieing other then after a real long idle (as in 10-15 minutes long in my book i never let it idle for more then a minute due to this issue) is when i whipped the truck around in drive on icy snow and got it to go backwards in drive idk but like i said ill read up on the tests in the morning and see where i can go from there

oh just thought of this not sure if it could really cause my issue but after my truck caught on fire oh yeah did i mention I GOT MY TRUCK TO START ON FIRE not the engine but the ass end.

the lines and wires by the gas tank were all melted i reconnected 2 of the 3 lines that went to the gas tank and all of the wires i just couldn't find a line to reconnect the vent valve i don't think that it would cause this issue but figured id throw it out there i mean if some one thinks this could be the issue ill go find some small tubing now that i have a little money and reconnect it

probably should note that the gas tank caught on fire and just burned a hole in my carpet cause i cut a hole in the floor yada yada yada
only thing damaged was the fuel lines/wires, my carpet, my subs and some car batteries i had in the back of the truck

so to make a long story short its my fault the truck started on fire and even with everything melted off the truck still started the next day and ran for about 30 seconds