87 Bronco II runs bad when warm

[bobbm1]

Have an 87 bronco II with I think a 3.8, auto trans 2wd. It starts up and runs great when cold but as soon as it gets to operating temp it starts bucking and stumbling all over the place. So bad you can't drive it.

Have replaced
fuel pressure regulator
distributer
ign modual
fuel tank with pump
cleaned the iac
tried a couple different computers with no change
coil
fuel filter
temp sensor

There may be more but have been working on this so long I can't remember everything off the top of my head

any ideas?

thanks

 

[Rhett]

That's a 2.8 engine you have most likely. I'm not sure if 87 BroncoII's were carbed or had fuel injection, or TBI. Probably fuel injected, but my experience starts with 89 BII's. First thing I would do is run a fuel pressure test when a) it's cold and running fine, and b) when it's warmed up and running poorly.

 

[bobbm1]

Yes you are right it is a 2.8 and it does have fuel injection. Thats what I get for posting at 2:00 AM. The bronco belongs to my father in law but I will suggest to him that he do the pressure test.

thanks

 

[Rhett]

Check the O2 sensor. If it's bad, the truck will run poorly when warmed up.

When the truck is warmed up the O2 tells the computer to lean out the air/fuel mixture for better fuel economy and emissions--in your case it could be leaning it too much--thus starving the engine and making it run poorly.

Since the O2 sensor is effectively turned off when the truck is cold, that could explain it running well when cold...

 

[MrShorty]

It's actually a 2.9 L multi-port fuel injected engine (the precursor in the cologne family to the 4.0 in Explorers). 1st thing to do, since I don't see it mentioned, is to run the computer self-tests (see www.dalidesign.com/hbook/eectest.html if you want to do this yourself). This is often a good first step, but it can be particularly important on an '87. They don't have a functional "check engine light" by which the computer can signal to you when it sees a fault. It, therefore, becomes important to take the initiative and "ask" it if it has seen/can see any faults.

 

[Rhett]

It's actually a 2.9 L multi-port fuel injected engine (the precursor in the cologne family to the 4.0 in Explorers).

When did the Bronco II's switch from the 2.8 L to the 2.9 L?
89's had the 2.9 L, but I my friends 84 had a 2.8 L.

 

[410Fortune]

1986 was the switch from the carbed 2.8L to the EFI 2.9L

With the 2.9L first:

check the codes
FUll tune up, plugs, fuel filter, clean the throttle body, IAC, air filter
Cap and rotor

wires and a coil dont hurt either

Listen to BOTH fuel pumps, check the fuel pressure at the rail

02 sensor is a good idea as well depending on mileage.

The wiring on those 2.9L's gets pretty old and cen get brittle and melted on the exhaust (because the old clips fail) so its not a bad idea to do a visual inspection on the harness..

Pulling the codes is the best place to start because 90% of the time it will point you in the right direction.

TFI module and also the timing should be checked out as well...

 

[410Fortune]

OH sorry I see the list of stuff you tried already, dont forget that truck has two fuel pumps, the one in the tank is the boost pump, the rail mounted pump is the one with all the duty.

I second the 02 sesnor is suspect since its when its warm (open loop operation)Also the fuel filter can cause problems

 

[bobbm1]

Just checked witht he F-I-L He had it hooked up to a computer to check the codes and it came back with MAP sensor which he replaced. No other codes. I didn't know about the second fuel pump will have to check that. Wouldn't the O2 sensor throw a code?

Thanks for all the ideas

 

[410Fortune]

How long has it been since it did run good?

 

[bobbm1]

to be honest with you we were having trouble with it and started working on it when he had a heart attack. That was almost 2 years ago. He is just now getting to the point where he is able to work on it again. That is why I am having a little trouble remembering what all we did to it. It is just kind of a project car that is getting on our nerves.

 

[410Fortune]

Sounds like a fuel problem, check the pressure at the fuel rail. listen to the hig pressure pump, you can OHM out the 02 sensor, or just replace it if its been a long time.
Check the wiring to the 02 sensor and you have to check the base timing setting...

 

[NusenseSD]

Can't say if this will fix your problem or not but I had a 87 that did the exact same thing.. Do you have a code reader? If you don't get one they are $30 at a any auto parts store..

When I got the code reader it came back saying the TPS (Throttle Position Sensor) voltage was high.. So I checked a whole bunch of things tried replacing the TPS .. Still same problem. Then I got a little inquisitive so I started check vacuum lines and that still wasn't the problem..

Finally I started to check the wires.. Well one of the wires coming off of the TPS was frayed. I put some electrical tape over it and guess what everything worked fine after that.. Check your wires that come from the TPS (its at the front of the intake) I bet one of them is frayed..

Of course the stupid thing decided to leak from the rear main and after wasting a grand on brand new heads I got frustrated at it and traded it in on my 02 Explorer..