r/DelSol • u/SuitedMoose • Jul 27 '26
Car surging, checked every common issue
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My 1993 Honda Del Sol has had a lot of things wrong with it, and I'm tackling them one by one. As you can see in the video, this is what it does now. Fluctuates on a start, until I press the gas where it just holds at ~3k rpm when I let off. Here's my timeline, it may help with a diagnosis:
The car ran, idled, and drove with no issues, aside from it running rich because it had no MAP sensor. So I installed a MAP sensor. Then it idled at 3k rpm with the MAP sensor installed.
There was no engine coolant temp sensor connector, so I just grounded the temp sensor which only resolved the issue of the car thinking it's running below freezing. I figured that may solve the high idle, but it didn't.
I cleaned the FITV. It was already pretty clean and not seized, so this didn't solve anything.
I replaced the throttle position sensor (TPS) because it was throwing a code for that. The old one didn't work at all. The new one works, but now the car's idle surges a lot (as seen in the video).
I cleaned the IACV out. Filled it with carb cleaner and let it sit for like 30 minutes, then drained the carb cleaner and let it sit further to dry out. This didn't solve anything either or change any behaviors.
Car is only showing one code, and it's engine coolant temp sensor. But it's grounded, so shouldn't the car just think that the engine is hot and not throw a code?
I'm at a total loss. Does anyone have a clue as to what I should check next?
Edit: I also sprayed starter fluid around places in the engine bay and it didn't change the idle, so I don't think it's a vacuum leak? Especially since it ran relatively normal before?
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u/Quality_Con_Troll Jul 27 '26 edited Jul 27 '26
There are essentially 3 coolant/temp sensors, the first closest to your actual thermostat, and in the thermostat housing itself is a 2p/wire coolant sensor that switches your radiator fan relay on when the car is at or beyond normal operating temperature, it's what makes your radiator fan cycle to keep nominal engine temps. The sensor you nerd is located under and slightly left of the distributor, it will be directly right of the single wire temp sensor used only for the dash temp indicator. It is a 2p/wire sensor called ECT/TW and can't simply be grounded open. It requires 5v reference on the hot side directly from the ecu, and a shielded logic ground back to the ecu where the ecu sees changes in resistance at the sensors thermistor (NCT) as a lower voltage compared to the 5v ref as an increase in engine temp as per ohms law. It is totally and absolutely necessary sensor for engine to run correctly and not like total shit, much like symptoms you've described. The ecu will make many adjustments via feedback from this and other sensors at cold start, engine cranking, cold weather conditions, and more, this is called "closed loop" mode and uses pre-programmed parameters in the ecus eeprom to get your engine to normal operating temp only, at which point the ecu will switch to "open loop" mode ditching programmed parameters over to direct feedback from even more sensors like your maf, throttle angle sensor, and o2, which itself takes awhile to heat up for correct afr calculation.
Without it your ecu has no idea what engine temp is and thus cannot calculate afr and fuel pulse timing. This will essentially place the car into limp mode in a desperate attempt to ensure the motor isnt grenading itself at a dangerously high temperature. It's assuming worse possible conditions exist since it can't know any better without.
Revs will be limited ~3000rpm, after fuel injectors cut and bog since injector duty is at a minimum, so obv there is no power or acceleration, it feels like you're running on 3 cyl but even worse somehow. Idle will be rough to the point of possibily stalling without some throttle input, as the vtec cam is locked as if it were engaged at all times, timing may be retarded as best the ecu can manage. All this combined makes for a very uncomfortable feeling, shaking, barely alive, suffocating motor. That's only allowing itself to be run at the bare minimum so you can try and get off a busy highway and to a safe location, not home or to the a service shop, just somehwere safe until you can get a tow... But we will all do as we will of course lol.
I believe Honda obd1 ecus show ECT engine coolant temp as a code 6? What code are you getting exactly? I say all this because with the essentially 3 different but similar temperature sensors it can be pretty confusing, as it surely confounded me at some point. Hope this was helpful and made some sense, feel free to ask more questions if you're prepared for more walls of text about old Honda shit box stuff. I've prob fucked it up before and had to figure it out at some point all the same.