Car has a weird combination of symptoms and I’m trying to narrow it down. When driving, if I clutch in to coast, the idle hangs around 1800 RPM instead of dropping to ~900. Once I come to a full stop it eventually settles back down, but while moving it stays high.



Warm idle issue: after the engine is fully up to temp, the idle starts to sputter between 1400–1800. I’ve already replaced the IACV and confirmed it’s reacting normally (blocking the bypass kills the engine, unplugging it drops idle). The sputter disappears cold and only returns warm. Sometimes after unplugging and replugging sensors it goes away for a bit, then comes back.



Additional symptoms:

• Fuel smell from FPR vacuum line
• Car never struggles to start
• MAP readings slightly off KOEO (around 2.8V on ref/green line)
• TPS sweep checked smooth (0.45V idle → 4.5V WOT)
• No major vacuum leak — blocking IACV bypass stalls the engine immediately
• Idle hang ONLY happens while rolling, not stationary

Engine pulls fine otherwise. No overheating, no coolant loss, oil clean, no misfires under load.



Given the fuel smell and the way the idle changes only when warm, I’m leaning toward a failing fuel pressure regulator diaphragm but want second opinions before throwing parts.



If anyone has dealt with a Y7 block / Y8 manifold combo doing this, let me know what direction to chase next — FPR, MAP port not drilled correctly, purge valve sticking, or TPS closed-throttle signal?

 

What ecu?

 

https://honda-tech.com/forums/honda-...motor-2451175/

Or, just stick a piece of gasket material between the manifold and IACV and just open the idle adjustment screw.

 

it’s a p2p ecu.

 

replaced the fpr, still the exact same. tested map again 2.8 key on engine off. started the engine and at operating temp i get 0.8v. also took off my map sensor to look inside and it seems it’s blocked off? but i see a tiny little hole on the wall. just looking confirmation if

this is how it’s supposed to be or not

 

The fact that your voltage reading with the engine on and at idle is .8v suggests that your MAP sensor is working just fine. The small hole that you see in the side of the opening is normal.

 

okay thank you for that information. this is going to be an update on what is happening then.
idle on cold start/while warming up is perfectly normal no issues at all. once reaching operating temp the idle surges up and down between 1200-2000.

clutching in while driving my rpm goes to 1800 and doesn’t settle until i’m at a complete stop.

letting it sit idle at operating temp for 3ish minutes, it’ll throw code po505 and when i press on the gas lightly and hold it, it’ll bounce between 1500-2000. when it gets like this there’s absolutely zero way to get the rpm’s between 1000-2000.

also for another piece of information letting off/applying throttle i feel a jerk.

anyone have any ideas?

 

thoughts?

 

My issue was at idle, but when my car would get to operating temp it would start misfiring. Culprit was the spark plug wires.

 

same symptoms i was having?

 

also just for clarification i have po505 code, replaced my iacv, iacv bypass in tb kills car when i plug hole.cold starts high ≈1800, normal idle when car is warm ≈1100 surges between 1200-2000, stutters like a two step when it’s operating temp and i lightly touch gas.

PROBLEMS ONLY HAPPEN WARM NOT COLD
which is why i believe it’s not a vac leak or timing issue. could possibly be spark or fueling.
any help appreciated

 

bump

 

I would try a "known good" IACV and check the TPS positioning making sure that the closed and full-throttle voltages are proper. Lastly, make sure that there are no air bubbles in the cooling system.

 

i’ve found my issue to be the idle set screw, idle with iacv unplugged was 900. set idle to 550 with iac unplugged and it runs like a charm now, no surges, no sputters.

 

Damn I was gonna suggest it might have been your idle just being set too high but I didn't have enough experience to be confident in that suggestion lol

 

Funny thing: The idle set screw is painted over from the factory and is PRESET. There is no reason why this is ever messed with, and yet, people do it every day from information provided over various forums on the interweb.

Glad you got it fixed.

 

me too! thank you guys for all the help