Stall under hard braking and MAP line is clear - ZR-1 Net Registry Forums

1991 Corvette ZR-1 · 03-16-2023

So I was thinking this is either the MAP and associated tube or the brake booster assembly. Out for a drive recently, I used maybe 3/4 of what one would use aggressively braking, so not quite standing on the brakes and the car stalled.

I pulled the MAP sensor hard line and it was clean. Years ago when applying any level of braking at low speeds even gently would cause a stall condition. A poor choice in replacement tubing by someone from plenum to MAP was the culprit then.

I have no way to check if this is the actual MAP sensor directly causing an issue. So the next place my thought process goes is to the booster having a vacuum leak. The brakes on this car have never been anything to write home about. Even after replacing all four rotors and pads with a Stoptech kit and full fluid flush, the braking felt the same as the old tired rotors and pads. So I am wondering what if my booster has been bad from the start.

This would also explain why my idle jumps 75-100 rpm from stoplight to stoplight. This has always bugged me how could the car could have a vacuum leak that just decide to not leak until 15 minutes into a drive, then start, then stop again and repeat several times through the drive. Before anyone asks, Marc had already rebuild my TB, so I know that?s not causing the idle issue.

So this is a ZR-1 specific booster with dual diaphragms compared to the single diaphragm version used on standard C4s. How screwed am if the booster is indeed bad? I was looking into a way to bench test it.

Why is this a dual diaphragm? Is this related to the vacuum required for the secondaries? The FSM does not go into the details of why there are different boosters.

XfireZ51 · 03-17-2023

If the motor is stock and you are getting idle wander, I would check the PCV connections. Also, when the TB was rebuilt by Marc, was it separated from the plenum? Did u stall even after having the TB rebuilt?
U should log the motor capturing data when the motor stalls out.
How many miles on motor? Ever clean the IAC pintle and orifice?

1991 Corvette ZR-1 · 03-18-2023

Quote:

Originally Posted by XfireZ51

If the motor is stock and you are getting idle wander, I would check the PCV connections. Also, when the TB was rebuilt by Marc, was it separated from the plenum? Did u stall even after having the TB rebuilt?
U should log the motor capturing data when the motor stalls out.
How many miles on motor? Ever clean the IAC pintle and orifice?

Appreciate the response, Xfire. Motor is not completely stock. TB went by itself to Marc a few years ago for rebuild. No stalling before or after I got it back. This is the first stall in about five years of driving. I believe it?s linked to the booster as I was pretty hard on the brakes and that was what killed the engine. Restarted no problem, idled and operated just fine after.

97K for mileage on the LT5. I will check the IAC but it?s a new component as well. Unfortunately, I do not have a data logger. The check valve on the booster appears to be functioning correctly. Are there any brake booster options?

Anyone ever bench test one of these?

secondchance · 03-20-2023

1991 Corvette ZR-1,

I too had Marc rebuild and bore out the TB to 63 mm since my motor was modified a bit. The car ran fine for a while but started cutting out after deceleration despite IAC being clean and working fine.
Solution was adjust throttle blade stop by turning an adjustment screw. Marc typically replaces TB stop adjustment with a hex head making the adjustment much easier. Factory adjustment bolt, I believe, is an allen head.
All these years, I never heard of brake booster failure although anything is possible.

secondchance · 03-20-2023

https://www.zr1.net/forum/showthread...ade+adjustment

WARP TEN · 03-20-2023

Quote:

Originally Posted by secondchance

1991 Corvette ZR-1,

I too had Marc rebuild and bore out the TB to 63 mm since my motor was modified a bit. The car ran fine for a while but started cutting out after deceleration despite IAC being clean and working fine.
Solution was adjust throttle blade stop by turning an adjustment screw. Marc typically replaces TB stop adjustment with a hex head making the adjustment much easier. Factory adjustment bolt, I believe, is an allen head.
All these years, I never heard of brake booster failure although anything is possible.

A month or so after Marc did his 510 package with its 63mm throttle body on my '95, I also experienced this stalling at stops. Marc did exactly what you mention, he adjusted the stop screw. Never had a stalling problem after that. --Bob

secondchance · 03-20-2023

It could be after a TB rebuild TB blade may get seated better with repeated movement to a point where final seating position seals more effectively making idle air coming through the IAC insufficient.

1991 Corvette ZR-1 · 04-02-2023

Just a quick follow up that this is still in process. I fabricated a harness to plug in between the TPS and it?s factory connector so I did not have to tear into the factory wiring. So with alligator clips from the test harness on the leads of the multimeter, I show .54 to .55 with just key on. So not sure there is much room there for improvement? Marc rebuilt this throttle body for me so it has the upgraded hex bolt option.

Unfortunately, I do not have a device that can read the IAC activity. Since resolving that old issue years ago with the incorrect type of hose used to the MAP, stalling out has never been a thing. So coming of throttle to coast to a stop has never been an issue. This stall happened right off a hard brake event.

03-16-2023	#1
1991 Corvette ZR-1 Join Date: Sep 2018 Location: Florida Posts: 298	Stall under hard braking and MAP line is clear So I was thinking this is either the MAP and associated tube or the brake booster assembly. Out for a drive recently, I used maybe 3/4 of what one would use aggressively braking, so not quite standing on the brakes and the car stalled. I pulled the MAP sensor hard line and it was clean. Years ago when applying any level of braking at low speeds even gently would cause a stall condition. A poor choice in replacement tubing by someone from plenum to MAP was the culprit then. I have no way to check if this is the actual MAP sensor directly causing an issue. So the next place my thought process goes is to the booster having a vacuum leak. The brakes on this car have never been anything to write home about. Even after replacing all four rotors and pads with a Stoptech kit and full fluid flush, the braking felt the same as the old tired rotors and pads. So I am wondering what if my booster has been bad from the start. This would also explain why my idle jumps 75-100 rpm from stoplight to stoplight. This has always bugged me how could the car could have a vacuum leak that just decide to not leak until 15 minutes into a drive, then start, then stop again and repeat several times through the drive. Before anyone asks, Marc had already rebuild my TB, so I know that?s not causing the idle issue. So this is a ZR-1 specific booster with dual diaphragms compared to the single diaphragm version used on standard C4s. How screwed am if the booster is indeed bad? I was looking into a way to bench test it. Why is this a dual diaphragm? Is this related to the vacuum required for the secondaries? The FSM does not go into the details of why there are different boosters.

03-17-2023	#2
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,678	Re: Stall under hard braking and MAP line is clear If the motor is stock and you are getting idle wander, I would check the PCV connections. Also, when the TB was rebuilt by Marc, was it separated from the plenum? Did u stall even after having the TB rebuilt? U should log the motor capturing data when the motor stalls out. How many miles on motor? Ever clean the IAC pintle and orifice? __________________ [SIGPIC][/SIGPIC]Former Membership Chairman Former ZR-1 Registry - BOD 1972 Corvette 4speed base Coupe SOLD long time ago 1984 Corvette Z-51/4+3 SOLD 1992 Corvette ZR-1 Aqua/Gray #474 SOLD 1992 Corvette ZR-1 Black Rose/Cognac #458 2014 Honda VFR Interceptor DX

03-20-2023	#4
secondchance Join Date: Aug 2006 Location: McLean, VA Posts: 3,707	Re: Stall under hard braking and MAP line is clear 1991 Corvette ZR-1, I too had Marc rebuild and bore out the TB to 63 mm since my motor was modified a bit. The car ran fine for a while but started cutting out after deceleration despite IAC being clean and working fine. Solution was adjust throttle blade stop by turning an adjustment screw. Marc typically replaces TB stop adjustment with a hex head making the adjustment much easier. Factory adjustment bolt, I believe, is an allen head. All these years, I never heard of brake booster failure although anything is possible.

03-20-2023	#5
secondchance Join Date: Aug 2006 Location: McLean, VA Posts: 3,707	Re: Stall under hard braking and MAP line is clear https://www.zr1.net/forum/showthread...ade+adjustment

03-20-2023	#7
secondchance Join Date: Aug 2006 Location: McLean, VA Posts: 3,707	Re: Stall under hard braking and MAP line is clear It could be after a TB rebuild TB blade may get seated better with repeated movement to a point where final seating position seals more effectively making idle air coming through the IAC insufficient.

04-02-2023	#8
1991 Corvette ZR-1 Join Date: Sep 2018 Location: Florida Posts: 298	Re: Stall under hard braking and MAP line is clear Just a quick follow up that this is still in process. I fabricated a harness to plug in between the TPS and it?s factory connector so I did not have to tear into the factory wiring. So with alligator clips from the test harness on the leads of the multimeter, I show .54 to .55 with just key on. So not sure there is much room there for improvement? Marc rebuilt this throttle body for me so it has the upgraded hex bolt option. Unfortunately, I do not have a device that can read the IAC activity. Since resolving that old issue years ago with the incorrect type of hose used to the MAP, stalling out has never been a thing. So coming of throttle to coast to a stop has never been an issue. This stall happened right off a hard brake event.