Possible vacuum leak??
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Intermediate
Joined: Oct 2022
Posts: 39
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From: Ohio
Possible vacuum leak??
I'm currently in the process of diagnosing my vettes problem. Basically it has a very rough start/idle and running. On start up (hot) it stumbles from 300rpm up to 1500rpm and slowly goes down to 700rpm where it revs up to 900 and back to 700 and keeps doing that when idling. When driving it easily stalls out and I have to keep my foot on the gas or it easily dies at stop signs. Pretty alright at cruising speed but definitely funky. Already changed some vacuum lines. And I have one bad one that goes through the firewall below the break booster. Could that one be causing it?? It attaches to the cruise control unit aswell. For reference my car is an 1984 crossfire. Any advice is welcome. Also worth mentioning that almost EVERYTHING is orginal. Only changed lines not components that relate to vacuum so I'm thinking it could be that aswell.
Thread Starter
Intermediate
Joined: Oct 2022
Posts: 39
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From: Ohio
. Fuel pressure is good I know that but haven't checked the tps sensor. I fixed the vacuum line I mentioned which did help alot with bad idle but it still revs up to 2000 on start up but does come down over time to 800 and stays their. Noticed that when I press the brakes coming to a stop it drops down to 600 but comes back up after a couple of seconds.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
If the idle is fine until you step on the brake and then it drops it may be an internal leak in the brake booster…clamp off the brake booster hose and then step on the brake and see if the idle remains steady…start it up with it clamped off and see if it runs better.
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Intermediate
Joined: Oct 2022
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From: Ohio
Yeah, that's kinda what I was thinking myself. I'll have to test it at some point. Thinking I need a new one anyways because it's been leaking fluid and has no paint on it anymore lol. But I wouldn't be surprised if that's what it is because I can hear what sounds like a leak when I press on the brakes. Would I clamp off the big hose that's on it? Don't remember what size it is
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Joined: Sep 2013
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From: Cape Coral, Florida
Yeah, that's kinda what I was thinking myself. I'll have to test it at some point. Thinking I need a new one anyways because it's been leaking fluid and has no paint on it anymore lol. But I wouldn't be surprised if that's what it is because I can hear what sounds like a leak when I press on the brakes. Would I clamp off the big hose that's on it? Don't remember what size it is
Thread Starter
Intermediate
Joined: Oct 2022
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From: Ohio
OK so I did that and it helped somewhat so I checked some lines around it and fixed a ton and replaced my check valve. At cold start it revs up to 1700 now but goes to 1500 until car is up to tempature. Then idle evens out to 700-800. Doesn't try to die anymore when i break aswell. And overall just alot more stable. And doesn't backfire like crazy anymore which I forgot to mention. Is the high idle on start normal? Or is it slightly too high
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
All cars will idle higher when cold until the engine is warmed up a little…even my DD 4 Runner in Florida idles about 1200 RPM when first started in the morning…temps are in the low 70’s…normally there is not a cold idle value in service information.
Last edited by C5 Diag; May 23, 2023 at 08:30 PM.
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Thread Starter
Intermediate
Joined: Oct 2022
Posts: 39
Likes: 4
From: Ohio
