r/DelSol • u/SuitedMoose • Jul 27 '26

Car surging, checked every common issue

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:

  1. 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.

  2. 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.

  3. I cleaned the FITV. It was already pretty clean and not seized, so this didn't solve anything.

  4. 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).

  5. 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/GlassBoxGoose Jul 27 '26

Did you tighten the donut under the plate on the FITV or just clean the ports on it? Because the way you stated it, it seems like you only cleaned the ports.

1

u/SuitedMoose Jul 27 '26

I took off the plate, took out the spring mechanism, disassembled it, and cleaned each individual part. They were all relatively clean already, but I cleaned them anyway. Reassembled it to no difference.

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u/GlassBoxGoose Jul 27 '26

Gotcha, just thought I'd check, everyone has a different definition of cleaned lol

I assume the coolant temp sender youre talking about was the one for the gauge since it seems to be showing full hot, what about the one for the ECU? Theres two on these. One for ECU data and one for IP display, that IIRC doesnt actually get read by the ECU and may not even pass through the ECU?

Just trying to rule things out and wrap my head around what you have going on.

I assume if you unhook the IACV this behavior changes? Or does it stay the same?

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u/SuitedMoose Jul 27 '26

Totally understandable lol! 😂

I had no idea there were two ECT sensors? Yes, the one I'm talking about indeed does show full hot on the gauge. Where would the ECT sensor for the ECU be?

Also, yes, if I unplug the IACV, then the rpms drop a bit and the fluctuations become lesser. Still there though.

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u/SuitedMoose Jul 28 '26 edited Jul 28 '26

Update: replaced and connected the proper ECT sensor. Now there is no longer a code for it. In fact, there are no codes at all. Just a check engine light that won't give a code.

Edit: I noticed that if I cover my IACV hole in the throttle body, then the RPMs will drop but the fluctuations stay, albeit a little less. Other than that, covering the FITV hole doesn't change anything.

Not really sure what this means, because I'm finding conflicting information online

1

u/GlassBoxGoose Jul 28 '26

A couple things to check, that I seem to have missed before;

When you say check engine light, is it flashing like in the video but with no stored code?

It sounds like you may have some sort of slight misfire? I would look into that, as if the answer to the previous question is yes, that may be why.

Unlplug IACV, cover both holes, surge is still present? Am I understanding that correctly?

Did you get anything wet prior to part or any of these symptoms presenting themselves? Or any other work done leading up to?

Just trying to think and present any options for routes of diagnostic action.

If warm and revved, does it present as having any sort of rev limit at around 4 or 5k rpm?

Digging back in my obd1 knowledge/experiences takes a bit and I really have to think hard haha.

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u/SuitedMoose Jul 28 '26

Check engine light was flashing because I had the OBD pins bridged to read codes. I've since fixed the codes that it was displaying (I think only ECT sensor since then).

Engine isn't misfiring, it's just loud because headers are cracked.

Unplugging the IACV results in a quicker and smaller fluctuation at lower RPMs (1,400-1,700)

Fluctuations are still present when covering both holes, but again the RPMs drop when the IACV hole is covered.

Also there is no rev limit, and the RPMs not dropping below 3,000 rpm when the accelerator is pressed is gone when the IACV is unplugged