Does anyone know how to bypass the computer in a 1987 Bronco? I want to completely disable it, junk the EFI and go to a carb with simple ignition!

For the past few months this POS has been driving me crazy, there is something electrical wrong and it does not want to be traced down. It started with a melted plug wire, resulting in a very rough running engine. Thaty was replaced, worked fine for a week and it started bucking, no matter what speed, like you were constantly and roughly pushing and letting up on the gas.

So plugs, wires, distributor, coil, injectors, the regulator valve on the throttle body (not the EGR the other one, cant remember its name), fuel pressure regulator all replaced. It runs better, but STILL bucks. Now it is mainly when holding a steady speed, does not do it on acceleration, but does do it on deceleration. Also when idling it constantly runs down to almost stall then throttles up high in an endless cycle. When coming off idle, it wants to stall for a second when first given fuel, then takes off.

This is NOT a stock Bronco motor, it was replaced 12 years ago with a 5.0L High Output mustang crate motor. Ran perfectly until recently.

I refuse to spend the rest of my life chasing electrical gremlins, so if you know what is wrong with it, I would like to know so that I can get it running right UNTIL I get all the parts to go to a carb.

Thanks in advance for any help you can provide

 

First off, have you ran a KOER, KOEO tests with a code reader to see if it gives you any problem codes?

Have you checked the actual computer for damage, as in opening it up? If it has damaged components that could cause some odd problems.

What about all of your vacuum lines?

Are you getting solid fuel pressure?

What about the MAP sensor, is that still good?

What kind of transmission are you running, have you eliminated potential problems with it, causing the bucking?

 

Yeah.. you're going about this all *** backwords, the first thing that should be done when troubshooting an EFI motor is to pull the codes, that might tell you something(give you a code) or it might eliminate the whole EFI system as the source of the problem(all clear code 11 or 111), either way you got something to work with.

That said I have seen this bucking at constant speed and traced it to air leaks in the smog(air injection) system or the exhaust system upstream of the O2 sensor, is that smog plumbing still present on your truck?

 

Tigerhawk,

I do not have a code reader nor does any shop within 50 miles of me that is capable of reading that computer. Heck, I don't even know what code reader to buy if it is still available.

No have not looked at the computer, however this problem is not constant, but intermittent, though it is a problem far more often than not. Seems to me that if it was the computer it would be always.

Vacuum lines are good as is fuel pressure, in fact vacuum lines were replaced less than a year ago.

Unknown on the MAP sensor, have not checked that in a few years.

Tranny is an AOD with a shift kit and that is not the problem. The bucking is very definitely a sudden loss of fuel, followed by a sudden return of fuel.

 

Conanski,

Have no way to pull codes, no shop anywhere near me has a reader for that old a computer. I would buy one, but have no clue which one to buy.

Smog injection system was removed and plugged when the original 302 was replaced with the high output 5.0 crate motor.

The exhaust system may have a small leak above the sensor, it is about due to be replaced.

 

Ah.. you're in luck. You don't need any special tools to pull codes.. well OK maybe a test light, but if your truck has a check engine light on the dash(some early vehicles didn't) then you don't need anything but a piece of wire or paper clip. What you do is put the jumper wire between the STI and SIG_RTN pins of the EEC test connector, turn the key to run and count the flashes on the check engine light or test light. Here's a video of doing it without the code reader..

But if you want to avoid the potential for mistakes just buy this code reader...

 

OK, so the first thing is I am ordering the code reader.

Now, everything is back to normal. I spent today going through EVERYTHING with a fine tooth comb and what I found was…. The bolts on the passenger side exhaust pipe, where it joins the manifold, were about 3/4 turn from completely tight, ( they loosen up all the time and I do not know why), and this is right above the O2 sensor that is mounted in the exhaust pipe, like by inches. There was no audible exhaust leak, but if you were in exactly the right position, you could smell it. So now that it is tight again, everything is back to working normally, without the bucking.

I REALLY HATE EFI, want a Holley Carb!

 

Yeah that will do it, a leak just upstream of the O2 like that is one of the worst places for a leak.

 

What I do not understand is that those bolts loosen up all the time. I have to tighten them at least once a month. Usually hearing the exhaust leak is the clue, yet NEVER in the past has that leak resulted in the engine bucking like that.

 

Get some high temp loktight. or LMC sells non-back-out header bolts, might work for manifolds too.

 

Thunderhorn,

The locking header bolts will only work with headers. Also t hey are for attaching headers to the block, not for attaching the exhaust pipe to the exhaust manifold. They work great for the right application, just not for mine.

 

Conanski,

Got the code reader in, but before I try to use it, I went over everything with a flea comb, because it was still doing it, just not anywhere near as bad. Then I started it up and at the cost of second burns to three finger tips, I found, while running, a barely perceptible pinhole leak on the top edge of the O2 sensor bung. SO without disassembling the exhaust, I barely got my mig gun within reach and made the UGLIEST weld I have ever made in my life, but the leak is sealed, another burned finger tip, and it is idling smoothly. So now on to the test drive.