86 Bronco II Running Rich

ox1969 · Apr 9, 2016

So here is the back story. Bronco would intermittently cut out while driving like it was starving for fuel, then would start running fine in about 20 seconds. Changed the fuel pump relay, and installed a new harness. Wired the harness incorrectly and popped a fuse-able link. Rewired harness, and replaced link.

Now when I start the truck it is running really rich, so I swapped out the computer with a new one thinking I may have smoked it when I wired the relay incorrectly. New computer, still doing the same thing. I have also put in a new MAP sensor, better but still running rich. I used a computer for an 88 Bronco II, manual trans so matches my specs. Disconnected and plugged vacuum lines for the EGR system as well.

Also checked fuel pressure, 40 psi KOEO, and 35 PSI KOER. Has been sitting for about 2 hours and still holding 40 PSI. So assuming the fuel pump system is fine.

Have not checked the temp sensor yet, but was working fine before I wired the fuel pump relay incorrectly. Potentially could have shorted it, but not sure.

Have checked all vacuum lines for leaks, and checked grounds for the computer, all good.

Sorry for the long explanation, just wanted to explain what I have done up to this point and that it was running fine other than the intermittent stalling out, until I screwed it up some how lol.

Thanks for the help in advance.

RonD · Apr 9, 2016

So it runs really rich cold, O2 sensors are not used for the first 5 minutes or so, so thats not the problem.

Can you check the vacuum with engine idling, 17" to 20" is expected, your system is a Speed Density system so relies on Vacuum level in the intake to adjust fuel mix, that's what the MAP(manifold absolute pressure) sensor is for.
As vacuum level drops computer can "assume" engine is under load so increases fuel mix.
Also unplug the TPS sensor, see if things change, throttle response will be sluggish.
TPS can be easily tested with Volt or OHM meter.

ox1969 · Apr 15, 2016

So in theory if I disconnect the temp sensor, it would essentially take that indication for the computer out of the loop, which would indicate a bad sensor correct?

Have not tried this yet, but was thinking about how the system works, and the sensor would tell the computer that the engine is cold, and needs more fuel to run correct.

I am kind at a loss on this, since it ran fine other than the intermittent stall issue that I thought was a fuel pump relay. I have not done anything with the vacuum system other than check for leaks and plug the EGR vacuum lines.

I would assume it is something electrical, or a sensor that shorted when I wired the fuel pump relay incorrectly.

I will test the TPS and the IAC valve to see if they are reading correctly and go from there. I hate throwing parts at problems, I want to understand what is causing the issue before I just change out parts.

Thanks for the advice.

RonD · Apr 15, 2016

If you unplug the ECT sensor then computer will run in Open Loop because it doesn't have any engine temp to base air/fuel mix on.
It does have the IAT(intake air temp) sensor which it will use but engine would run richer in Open Loop since that is the safer mix for an engine.

There are TWO coolant temp devices
ECT sensor, 2 wires, used only by the computer
ECT sender, 1 wire, used only by the dash board gauge

The ECT sensor will have 1 wire with 5volts, that comes from the computers internal 5v power supply, it sends this 5v power to ECT sensor, TPS, and IAT sensor.

And 2nd wire is the Return Voltage, this tell computer the engine/coolant temp.
The higher the voltage on this wire the colder the temp
3volts is about 70degF
2volts 110degF
1volt 160degF
.5volt 200degF operating temp

I use a sewing pin to pierce wires to test voltages while wires are connected.

The 5 volts is the Reference voltage the computer uses and monitors, if it is only 4.9v then computer knows that, if it is 5.1volts then computer knows that as well.
So the Return voltage from the ECT, TPS or IAT sensors is compared to the Reference voltage to get the difference.
This allows system to "age with grace" lol, so grain of salt when testing voltages, new and 20 years old will be different but accounted for.

Also check the Fuel Pressure regulator's vacuum hose, it's internal diaphragm can start to leak, and extra fuel is sucked into the engine.

ox1969 · Apr 15, 2016

RonD,

Thanks for the info, I will check those items. Great Idea about the sewing needle, had never thought of that.

ox1969 · Jul 5, 2016

So it appears the previous owner may have hot wired the fuel pump to the ignition switch as the pump does not shut off after a few seconds KOEO. I disconnected the Fuel Pump relay, and the EEC relay, pump still runs. Doubtful this is causing the rich running condition, but still need to fix this error. Also noticed that the previous owner has removed the inertia switch, so will need to repair that as well. But does explain the 87/95 codes I got from the code reader.

No solution yet, just wanted to give a bit of an update.

86 Bronco II Running Rich

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