SmokingRocks
hopelessly addicted to Cruisers
SOLVED: The problem traced back to an intermittent ground issue with the main engine harness, I built a temporary bypass harness for the fuel injectors until I can get a new main engine harness.
Read the entire thread for the diagnostic journey. Oscilloscope diagnostic techniques were used to track down this issue. I highly recommend getting one, this Picoscope 2204a is an excellent unit. Beware of the cheaper units, it's almost impossible to get a quality oscilloscope for under $100.
Rig
Heavily modified 1993 FZJ 80
50,000mi on fully rebuilt engine
250,000 on chassis
PAIR removed
EGR disabled
Issue:
Over the past half a year (2,000mi) I have noticed a hesitation, like a misfire, between 2000rpm and 4800rpm, under full load. It's more noticeable in the higher gears when the engine loaded and taking longer to get through that rev range. At first, I thought it was detonation so I added premium fuel and when home checked the timing. The timing was still at 8 degrees advanced (no change from rebuild).
Then I checked the codes (the CEL was not illuminated during normal mode):
13 RPM signal lost when the engine revs above 1000rpm
24 Intake Air Temp Sensor Fault | Open or short in THA circuit for more than .5 seconds
31 Volume Air Flow Meter | Open circuit between one of two circuits in the AFM below 1500rpm
So straight away codes 24 and 31 shouldn't affect off idle running based on my understanding of what these sensors are providing.
What I've done / Checked:
Really trying to figure this one out, so I'm calling on you guys to help me out!
Read the entire thread for the diagnostic journey. Oscilloscope diagnostic techniques were used to track down this issue. I highly recommend getting one, this Picoscope 2204a is an excellent unit. Beware of the cheaper units, it's almost impossible to get a quality oscilloscope for under $100.
Rig
Heavily modified 1993 FZJ 80
50,000mi on fully rebuilt engine
250,000 on chassis
PAIR removed
EGR disabled
Issue:
Over the past half a year (2,000mi) I have noticed a hesitation, like a misfire, between 2000rpm and 4800rpm, under full load. It's more noticeable in the higher gears when the engine loaded and taking longer to get through that rev range. At first, I thought it was detonation so I added premium fuel and when home checked the timing. The timing was still at 8 degrees advanced (no change from rebuild).
Then I checked the codes (the CEL was not illuminated during normal mode):
13 RPM signal lost when the engine revs above 1000rpm
24 Intake Air Temp Sensor Fault | Open or short in THA circuit for more than .5 seconds
31 Volume Air Flow Meter | Open circuit between one of two circuits in the AFM below 1500rpm
So straight away codes 24 and 31 shouldn't affect off idle running based on my understanding of what these sensors are providing.
What I've done / Checked:
- Removed the AFM, Cleaned all connectors thoroughly
- Bench tested AFM per the FSM, all tests passed except for the air temp test but it was JUST out of the ohm range. This wouldn't cause my issue because it still is providing a value to the FSM which is within range, so if anything it's just telling the ECU the temp is a bit off from what it actually is.
- Tested the resistance between the AFM connector and the ECU <0.4 Ω
- Cleaned all of the connectors on the distributor
- Tested the resistance between the RPM signal wire from the distributor to the ECU <0.9 Ω
- Replaced the distributor cap, rotor, wires, and plugs
- Replaced the ECU with my spare
- o2 Sensors? | They aren't that old and that would probably trigger a more direct code right?
- AFM is truly bad | Hard to believe
- Problem with Fuel Delivery | more likely than the first two, probably wouldn't trigger a CEL code, BUT after 4800rpm the issue clears up (or perhaps the speed of the engine begins to hide the symptom at this point) additionally if it's a delivery issue wouldn't it get worse at higher RPM?
- An issue with the ignition side/control | Low hanging fruit has already been replaced by denso/ngk stuff. Could be the Coil or control module???
Really trying to figure this one out, so I'm calling on you guys to help me out!
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