Hoping for some help from 3FE gurus, and/or early Denso (Bosch) fuel injection gurus. I've attached a .pdf of the 3FE engine control wiring diagram for reference.
The Long Version: (Short Version to follow in next post)
Basically, my 3FE powered 80 had an intermittent running issue when I bought it where it would hesitate like it was lean in the mid-range, then it would surge ahead if you stomped on the pedal. It also had a low and rough idle, where the RPM would bounce significantly with the signal lights
. A couple of times it also bogged down, popped and crackled and wouldn't accelerate. I had originally thought it was vapour locking due to heat from the header, as it seemed to get better in colder weather.
Eventually I found a big vacuum hose to the charcoal canister had popped off, putting that back fixed the poor idle, most of the fluctuation with signal lights and made it seem to run decent, but with poor fuel economy based on what I'm used to for a 3FE.
I started having issues again (which I later found out were due to that vacuum hose popping off again), but I figured out through trial and error that the hesitation would consistently go away above 2800-ish RPM or at/near Wide Open Throttle. I came to learn that these are some of the conditions where the computer goes from "closed loop" to "open loop" for acceleration enrichment etc.. With the vacuum line put back it ran pretty good, again with poor fuel economy.
Had a look underneath, and low and behold the custom exhaust had no O2 sensor bungs at all, so I had been running without O2 correction this whole time. No Check Engine Light. Welded in some O2 bungs and installed some new Denso O2 sensors. Now consistent hesitation/lean-feeling in closed loop, rough idle (600rpm-ish).
Did a long afternoon of troubleshooting. Pulled the glovebox, found the CEL wire coming from the ECU was clipped. Re-attached and there was indeed a Check Engine Light, codes 21 and 28 (open circuit/deterioration of O2 sensors 1+2). Got out the multimeter and verified continuity between OX1/OX2 at the ECU, check connector under the hood, and the plugs underneath. Verified +12V getting from ECU to O2 sensor plugs. Issues still present, code 21+28 still present. Thought the O2 sensor connectors might be corroded and dirty from dangling under the vehicle for God knows how long, tried cleaning them. No luck. Also wasn't sure if I was getting a reference signal from the sensors via OX1/OX2 (I know now that its a 0-1V signal, I didn't know that then so I can't say 100% that I was getting a signal or not) Got frustrated, unplugged the sensors.
On a road trip, I had issues with various other things, high beams, power windows, etc that I traced to the ground in the driver's side footwell. It also appeared that the shielding(?) for the OX1/OX2 signal wires were grounded there? Not sure how that works.
Another day of troubleshooting. Got somewhat reliable voltage signals at OX1/OX2 at the connector now. I managed to get code 21 to go away, but code 28 remained persistent. I tried swapping out for another 3FE ECU I had and code 28 went away. I'm guessing that circuit in the ECU was just tortured? I opened up the ECU case but saw nothing obvious. It still runs the same. Replaced all the vacuum lines, as well as both intake boots. Pulling 16-17inHg of vacuum at idle, which seems a little weak. Sprayed brakekleen and quick start all around the intake manifold joints and anywhere I could think of, no luck on any other vacuum leaks. Verified the TPS function as per the FSM, passed all tests. Checked fuel pressure and got 43psi (supposed to be 37-46psi). This 3FE was de-smogged and they deleted the fuel pressure-up VSV, so it sees the full pressure all the time. Going to put that VSV back once I figure this issue out. I connected the FPR hose to manifold vacuum and got 39psi (supposed to be 33-37psi).
I feel like I'm getting closer but I was hoping the TPS was it. Short version coming next post.
The Long Version: (Short Version to follow in next post)
Basically, my 3FE powered 80 had an intermittent running issue when I bought it where it would hesitate like it was lean in the mid-range, then it would surge ahead if you stomped on the pedal. It also had a low and rough idle, where the RPM would bounce significantly with the signal lights
. A couple of times it also bogged down, popped and crackled and wouldn't accelerate. I had originally thought it was vapour locking due to heat from the header, as it seemed to get better in colder weather.Eventually I found a big vacuum hose to the charcoal canister had popped off, putting that back fixed the poor idle, most of the fluctuation with signal lights and made it seem to run decent, but with poor fuel economy based on what I'm used to for a 3FE.
I started having issues again (which I later found out were due to that vacuum hose popping off again), but I figured out through trial and error that the hesitation would consistently go away above 2800-ish RPM or at/near Wide Open Throttle. I came to learn that these are some of the conditions where the computer goes from "closed loop" to "open loop" for acceleration enrichment etc.. With the vacuum line put back it ran pretty good, again with poor fuel economy.
Had a look underneath, and low and behold the custom exhaust had no O2 sensor bungs at all, so I had been running without O2 correction this whole time. No Check Engine Light. Welded in some O2 bungs and installed some new Denso O2 sensors. Now consistent hesitation/lean-feeling in closed loop, rough idle (600rpm-ish).
Did a long afternoon of troubleshooting. Pulled the glovebox, found the CEL wire coming from the ECU was clipped. Re-attached and there was indeed a Check Engine Light, codes 21 and 28 (open circuit/deterioration of O2 sensors 1+2). Got out the multimeter and verified continuity between OX1/OX2 at the ECU, check connector under the hood, and the plugs underneath. Verified +12V getting from ECU to O2 sensor plugs. Issues still present, code 21+28 still present. Thought the O2 sensor connectors might be corroded and dirty from dangling under the vehicle for God knows how long, tried cleaning them. No luck. Also wasn't sure if I was getting a reference signal from the sensors via OX1/OX2 (I know now that its a 0-1V signal, I didn't know that then so I can't say 100% that I was getting a signal or not) Got frustrated, unplugged the sensors.
On a road trip, I had issues with various other things, high beams, power windows, etc that I traced to the ground in the driver's side footwell. It also appeared that the shielding(?) for the OX1/OX2 signal wires were grounded there? Not sure how that works.
Another day of troubleshooting. Got somewhat reliable voltage signals at OX1/OX2 at the connector now. I managed to get code 21 to go away, but code 28 remained persistent. I tried swapping out for another 3FE ECU I had and code 28 went away. I'm guessing that circuit in the ECU was just tortured? I opened up the ECU case but saw nothing obvious. It still runs the same. Replaced all the vacuum lines, as well as both intake boots. Pulling 16-17inHg of vacuum at idle, which seems a little weak. Sprayed brakekleen and quick start all around the intake manifold joints and anywhere I could think of, no luck on any other vacuum leaks. Verified the TPS function as per the FSM, passed all tests. Checked fuel pressure and got 43psi (supposed to be 37-46psi). This 3FE was de-smogged and they deleted the fuel pressure-up VSV, so it sees the full pressure all the time. Going to put that VSV back once I figure this issue out. I connected the FPR hose to manifold vacuum and got 39psi (supposed to be 33-37psi).
I feel like I'm getting closer but I was hoping the TPS was it. Short version coming next post.
