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I guess the time has come to remove the ECU and inspect the capacitors and such inside....
Great thing to do on a vehicle this old! Unless the capacitors look new, it would be good to replace them no matter what. Preventative maintenance for it!
Post pics of the board.
Last edited by 90project5.0; Apr 30, 2026 at 06:04 AM.
I will...for sure. the ONLY thing I'm chasing from a drivability standpoint is the fuel usage. starts 1st crack every time, idles down as it should after 90 seconds
has good acceleration, etc. just want to resolve this last issue if I can.
Anyone know which fuse controls this? From the fuse panel diagram there is nothing specific about it.
The O2 sensor heater circuit gets power from the ignition switch on Circuit 687 via Splice S36A when it is in the Run position.
Ignition Switch Part 3 of 3
Engine Control diagram Part 2 (shows the O2 sensor and reference to the heater supply circuit 687).
I do not have a good diagram for a 1987 model year 5L gas engine truck to show the complete power path to the O2 sensor heater. My 1988 EVTM shows the O2 heater is fused by Fuse Link X (left hand side of the engine compartment near connector C175) after power leaves the ignition switch. No idea if that translates to the same place or naming convention for a 1987 model year truck.
You can test for power at the O2 sensor connector, key in the Run position.
My go to method to see if the O2 sensor is causing an issue is to unplug it. The only purpose of the sensor is to make certain the fuel mixture is perfect. The control unit does a really good job of sending out the right signal to the injectors to keep it running right.
The issue I'm having is a very rich condition and poor fuel mileage as a result. Truck starts and runs fine strangely. I doubt that unplugging it will do much of anything but will give it a try.
Deaiing with 2 broken vacuum lines this morning as well...so that is slowing me down.
when testing the heater circuit....after unplugging the O2 connector from the harness, turning key on...I put the multimeter into DC volts (20 range) and put the red probe onto one of the white connector pins
and ground the black multimeter wire to a solid metal surface.....do this for each of the 2 white connections....is that correct?
Most ECU's control the ground circuit, saves on burned up circuit boards if the power wire is shorted.
That does not happen on these trucks. On the 1987 model year truck the OP has the O2 heater ground is shared with EEC relay coil ground. Not a contact, the relay coil.
Here is the ECU from the truck. ended up breaking 2 of the screws off ....grrrr...will have to drill them out and re-thread them.
nothing is easy!!
but there is ZERO corrosion or leaking going on. The conformal coating is all there. I cleaned the pins on the connector off
good. should I put some new dialectic grease on them when I put it back in the truck?
pics below...
Last edited by mackendw; Apr 30, 2026 at 03:23 PM.
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