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Discussion Starter #1
So I bought a 91 CRX with a DOHC ZC motor already swapped in it vehicle ran ok had slight hesitation when accelerating at low RPMs. Drive vehicle home and around town for about a week and master cylinder went out I replaced the master cylinder and decided to do a brake booster delete vehicle starts and runs fine but has a idle surge or hunting idle when I start slowing down and press in the clutch. When I stop the surge goes away. One other issue is if I plug the spot where the brake booster vacuum hose was normally hooked up at on the back of the intake manifold the vehicle shuts off. Does anyone have any ideas as to what’s going on? I have replaced the IACV and bleed the coolant it is fine. Throttle position sensor is set and within the .5v when sitting and 4.5v at wide open throttle ranges it should be at. Any help welcome and appreciated thanks in advance.
 

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BATSLOMAN GIVES NO FUCKS.
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sounds like a timing issue with the hesitation
 

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93 Civic HB SI
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Are you seriously running the car with the brake booster vacuum hose nipple on the intake open to atmosphere? There's your hunting idle, or at least one part to it haha.

Cap it, then try and start the car. Don't cap it while the engine is running and you have an active vacuum leak through that hose nipple.

The reason it kills the car when you cap it while the engine is running (at least this is the way you describe), is because the IAC is slammed shut at this point by the ECU trying to control idle through its air flow control mechanism. ECU notices idle is higher than it should be due to the vacuum leak, so it closes IAC as far as it needs to control idle, including all the way shut. Then the minute you plug the hose nipple, the engine is completely starved of air because the IAC is closed and hasn't had time to respond to open back up. The ECU needs time to dial the IAC back in to a desired air flow position to control idle.

The fact that it hunts at idle is due, in part, to your self induced vacuum leak via the open brake booster hose nipple. it's too much air through that wide open nipple to keep idle satisfactory.

The ECU only has two ways to control idle speed, the IAC air metering device, then a fallback level of control that cuts fuel to force RPM to fall if IAC control cannot properly maintain target idle.

When you have a hunting idle symptom, the ECU has defaulted to fuel cuts to control idle. The only time this happens is due to too much unmetered air flow entering the intake manifold beyond the typical air flow control devices.
 

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89 EF 4wd sedan
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I would think it would be revving at 4000 with the brake booster hose open?
I'd think the same thing, the brake booster line is huge.

Bouncing idle is likely caused by a minor air leak though if you've swapped with a known working IACV. Check the TB to intake gasket, intake to head mating surface gasket, and the 8 shapes gasket on the IACV if you reused it when swapping the IACV's.
 

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93 Civic HB SI
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If TPS is at 0%, or whatever that voltage is to the ECU, yet MAP signal indicates a pressure that correlates to a typical loaded position (regardless of how it reaches this pressure range, small vacuum leak or booster hose size), this fails an ECU plausibility check and the ECU will use whatever means necessary to prevent from redlining, usually fuel cut.

Try it lol, cut the key off, pull the booster hose and don't touch the throttle, then start. It will sit there and surge just like it will with a smaller vac leak.

The max RPM is ECU determined while in this failed plausibility range, and is typically 2000-2500RPM. The max RPM will hit, then fuel cut happens pulling rpm down to 500-600, then will re-engage fuel up to max limit again in this surge pattern, until TPS is added to a point where TPS and MAP ranges "agree" with each other, wherever Honda determines a range to be plausible.

This style of plausibility check is just a fail safe for most speed density vehicles to prevent over-revving the engine due to a vacuum leak or other false air leak.
 

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89 EF 4wd sedan
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If TPS is at 0%, or whatever that voltage is to the ECU, yet MAP signal indicates a pressure that correlates to a typical loaded position (regardless of how it reaches this pressure range, small vacuum leak or booster hose size), this fails an ECU plausibility check and the ECU will use whatever means necessary to prevent from redlining, usually fuel cut.

Try it lol, cut the key off, pull the booster hose and don't touch the throttle, then start. It will sit there and surge just like it will with a smaller vac leak.

The max RPM is ECU determined while in this failed plausibility range, and is typically 2000-2500RPM. The max RPM will hit, then fuel cut happens pulling rpm down to 500-600, then will re-engage fuel up to max limit again in this surge pattern, until TPS is added to a point where TPS and MAP ranges "agree" with each other, wherever Honda determines a range to be plausible.

This style of plausibility check is just a fail safe for most speed density vehicles to prevent over-revving the engine due to a vacuum leak or other false air leak.
It's 3k on mine. I driven around before by accident with my TPS not engaged properly and it drove fine until it hit 3k and stumbled until the revs dropped back below.

I'd still think it would be pretty wild to have the booster line open. Most bouncing idles i've noticed are slow lope closer to idle (800-1500 rpm ish)
 

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Discussion Starter #9
Are you seriously running the car with the brake booster vacuum hose nipple on the intake open to atmosphere? There's your hunting idle, or at least one part to it haha.

Cap it, then try and start the car. Don't cap it while the engine is running and you have an active vacuum leak through that hose nipple.

The reason it kills the car when you cap it while the engine is running (at least this is the way you describe), is because the IAC is slammed shut at this point by the ECU trying to control idle through its air flow control mechanism. ECU notices idle is higher than it should be due to the vacuum leak, so it closes IAC as far as it needs to control idle, including all the way shut. Then the minute you plug the hose nipple, the engine is completely starved of air because the IAC is closed and hasn't had time to respond to open back up. The ECU needs time to dial the IAC back in to a desired air flow position to control idle.

The fact that it hunts at idle is due, in part, to your self induced vacuum leak via the open brake booster hose nipple. it's too much air through that wide open nipple to keep idle satisfactory.

The ECU only has two ways to control idle speed, the IAC air metering device, then a fallback level of control that cuts fuel to force RPM to fall if IAC control cannot properly maintain target idle.

When you have a hunting idle symptom, the ECU has defaulted to fuel cuts to control idle. The only time this happens is due to too much unmetered air flow entering the intake manifold beyond the typical air flow control devices.
If I cap it with the vehicle off it will not start at all. So what would be your suggestion as to where I would run a hose from it to where?
 

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Discussion Starter #10
what ecu is in it?
It has a PM6 in it right now that’s what the previous owner had in it when they swapped it I know the PM7, PG7 are recommended ECU’s but I’ve found the PM6 works and shouldn’t be causing this issue.
 

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Discussion Starter #11
If TPS is at 0%, or whatever that voltage is to the ECU, yet MAP signal indicates a pressure that correlates to a typical loaded position (regardless of how it reaches this pressure range, small vacuum leak or booster hose size), this fails an ECU plausibility check and the ECU will use whatever means necessary to prevent from redlining, usually fuel cut.

Try it lol, cut the key off, pull the booster hose and don't touch the throttle, then start. It will sit there and surge just like it will with a smaller vac leak.

The max RPM is ECU determined while in this failed plausibility range, and is typically 2000-2500RPM. The max RPM will hit, then fuel cut happens pulling rpm down to 500-600, then will re-engage fuel up to max limit again in this surge pattern, until TPS is added to a point where TPS and MAP ranges "agree" with each other, wherever Honda determines a range to be plausible.

This style of plausibility check is just a fail safe for most speed density vehicles to prevent over-revving the engine due to a vacuum leak or other false air leak.
The vehicle does not hunt/surge at idle at all I can let it sit and run for days and does not hunt/surge that only happens when I’m drive and go to stop the vehicle then as soon as I stop the surge stops. And I’m not driving it around like this I am trying to fix it so it runs right before driving it was just trying to describe what happened when I drove it after the brake booster delete.
 
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