Brake booster
Thread Starter
Cruising
Joined: Feb 2023
Posts: 13
Likes: 1
Brake booster
Hello I have a 1988 corvette that’s been giving me a lot of issues like slow starting, misfiring, shaking and a bunch of other issues I’ve changed and replaced a lot including a new distributor and timed it and it’s finally sounding good, the egr code is on but I haven’t gotten around to change that but I went to start it and take it for a drive and when I pressed the brake at idle the engine sounded like it was going to stall. The brake has had trouble stopping the car and the pedal isn’t stiff I assumed it just needed pads but now idk when I disconnect the vacuum line to the booster the rpm’s jump up then calm down when press the brake then the rpm’s don’t change so would this be the booster going bad and could that be why the car takes forever to start?
Drifting
Joined: Feb 2016
Posts: 1,384
Likes: 339
From: Simi Valley CA
An easy test is too clamp vice grips on the vacuum hose going to the booster. If your idle and performance improve you know the booster is bad. Put tape on the vice grip teeth and open them a little to not damage the hose.
Instructor
Joined: Jul 2020
Posts: 227
Likes: 43
From: New Jersey
Hello I have a 1988 corvette that’s been giving me a lot of issues like slow starting, misfiring, shaking and a bunch of other issues I’ve changed and replaced a lot including a new distributor and timed it and it’s finally sounding good, the egr code is on but I haven’t gotten around to change that but I went to start it and take it for a drive and when I pressed the brake at idle the engine sounded like it was going to stall. The brake has had trouble stopping the car and the pedal isn’t stiff I assumed it just needed pads but now idk when I disconnect the vacuum line to the booster the rpm’s jump up then calm down when press the brake then the rpm’s don’t change so would this be the booster going bad and could that be why the car takes forever to start?
Thread Starter
Cruising
Joined: Feb 2023
Posts: 13
Likes: 1
the engine actually stalls when I plug the vacuum line
Instructor
Joined: Jul 2020
Posts: 227
Likes: 43
From: New Jersey
My son has a 1990 which has a "speed, density" system for the injectors, but I believe your 1988 has a MAF sensor. My guess would be that the computer has adjusted the IAC motor all that way closed to try to keep the idle down at a level it wants, but that means that the ONLY source of air for it to idle is what's leaking in through the vacuum leak in your booster. When you suddenly clamp it closed, you starve the engine of air quicker than the IAC can compensate for it. Someone else would have to chime in with whether that explanation is plausible for a MAF sensor engine. I know that in the speed density system, the computer doesn't really know or care where the combustion air is coming from, just uses some calculation based on manifold vacuum, temperature, and RPMs to figure out how much fuel to inject.
Thread Starter
Cruising
Joined: Feb 2023
Posts: 13
Likes: 1
My son has a 1990 which has a "speed, density" system for the injectors, but I believe your 1988 has a MAF sensor. My guess would be that the computer has adjusted the IAC motor all that way closed to try to keep the idle down at a level it wants, but that means that the ONLY source of air for it to idle is what's leaking in through the vacuum leak in your booster. When you suddenly clamp it closed, you starve the engine of air quicker than the IAC can compensate for it. Someone else would have to chime in with whether that explanation is plausible for a MAF sensor engine. I know that in the speed density system, the computer doesn't really know or care where the combustion air is coming from, just uses some calculation based on manifold vacuum, temperature, and RPMs to figure out how much fuel to inject.
Drifting
Joined: Feb 2016
Posts: 1,384
Likes: 339
From: Simi Valley CA
Once you replace the booster you should be able to reset the iac. There's a procedure in the FSM or search this forum. It is covered a lot. I have a 92 so I don't have the FSM for the L98 anymore.
Burning Brakes
Joined: Nov 2017
Posts: 1,060
Likes: 80
From: Fort Mill, SC
I think it is important you rule out the booster.. which I guess you could say you have.. however, I would think, and I know nothing about the older c4's but I would think, if you clamp it off and start the car and keep letting it try to get to idle that the system would clear itself out.. Just keep the car running and let it almost die, get it running again, etc.. over and over..
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Instructor
Joined: Jul 2020
Posts: 227
Likes: 43
From: New Jersey
I think it is important you rule out the booster.. which I guess you could say you have.. how
ever, I would think, and I know nothing about the older c4's but I would think, if you clamp it off and start the car and keep letting it try to get to idle that the system would clear itself out.. Just keep the car running and let it almost die, get it running again, etc.. over and over..
ever, I would think, and I know nothing about the older c4's but I would think, if you clamp it off and start the car and keep letting it try to get to idle that the system would clear itself out.. Just keep the car running and let it almost die, get it running again, etc.. over and over..
Thread Starter
Cruising
Joined: Feb 2023
Posts: 13
Likes: 1
I think it is important you rule out the booster.. which I guess you could say you have.. however, I would think, and I know nothing about the older c4's but I would think, if you clamp it off and start the car and keep letting it try to get to idle that the system would clear itself out.. Just keep the car running and let it almost die, get it running again, etc.. over and over..
2nd Gear
Joined: Sep 2023
Posts: 2
Likes: 0