All right guys, I have a puzzling issue with my 1991 Toyota pickup 3VZE. I parked it for two weeks and when I came out there was a no spark no check engine light condition with the IGN fuse blown. I replaced the distributor, rotor, distributor cap, spark plugs, wires, ignition control module and the ignition coil. Still no spark no CEL. So I went down the list and tested everything according to the factory service manual and narrowed it down to the ECU. Popped it open and found a blown capacitor with a blown trace.

So, whatever blew the IGN fuse mustve blown the ECU right?

I replaced the capacitor and soldered a jumper for the busted trace.

Current issue:
Starts and idles on all 6cyl but wont go above 1000 rpm. It begins to misfire terribly if I rev or turn the distributor.

Things I've tried:
Run motor with unplugged TPS sensor.
I put all the old hardware back in and still the same issue.
I've tried a different ECU, same deal. It also blows the IGN fuse within 10-15min of running.

 

My first guess would be some kind of electrical short in the harness, possibly from rodent damage after sitting a couple of weeks. If possible, follow the electrical pathway from the blown capacitor and see if it traces back to a specific pin on the blown ECU, which might give you a clue as to what circuit to look at. Check that all the other electrical accessories and gauges work on the truck. If one does not, that might point to a more specific location for harness damage. Other than that, you probably need to check every sensor and every circuit coming out of the ECU to see if there is a short to ground or a 12 volt circuit shorting to a wire that is suppose to carry low voltage, like a sensor signal wire. Test every relay that the ECU directly controls, also. Any relay or sensor that has significantly lower resistance than what it is suppose to have could also be drawing too much current from the ECU. A good pinout diagram for the ECU and good wiring diagram will be helpful here.

The fact it will not run above 1000 RPM makes me think to check the AFM and the AFM circuit first, and anything related to fuel delivery (fuel injectors, pressure, pump, relay, etc). The TPS circuit back to the ECU might be worth checking next. Any wires close to a hot exhaust (o2 sensor) or engine (fuel injector circuits, knock sensor, etc) would also be high on the list for a short. Also make sure all of your grounds, especially for the ECU and sensors, are in good shape. A broken ground in the wrong place could possibly be causing some stray voltage to ground through the ECU. Good luck!

 

Looks like I found it.

Ended up being that I had the ignition timing wrong causing it not to go above 1k, so I set that with a timing light.

I installed a faulty EFI relay from amazon causing the 7.5a IGN fuse which supplies power to the relay to short out burning it up every 5min during my rampage of replacing before diagnosing.

Along with that, I installed a new ECU which is picking up code 27 - a faulty o2 sensor causing a rough idle.