Posted August 14, 20222 yr comment_643906 Hi guys, My friend's 75 280 has always made this strange hissing from the intake: https://youtu.be/mjrzNhHvXL4. Any ideas what this could be? We took off the cone filter and when we did we could hear it from the afm, so we took off the afm, and could hear it through the throttlebody, when we took off the throttlebody the sound stopped (kept the engine running by pulling the temp sensor cable and left the tps switch in to run rich enough for it to not die). We've checked multiple times for vacuum leaks using both a smoker and a can of starter fluid on the manifold and there are no leaks. We also tried 2 other throttlebodies and both make that noise. I've never heard if this issue - has anyone encountered it? -chase Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/ Share on other sites More sharing options...
August 14, 20222 yr comment_643907 Is the PCV system intact? Might be pulling through the PCV valve on the bottom of the intake manifold. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643907 Share on other sites More sharing options...
August 14, 20222 yr Author comment_643908 Just now, Zed Head said: Is the PCV system intact? Might be pulling through the PCV valve on the bottom of the intake manifold. The pcv valve is new but I’ll have him check that Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643908 Share on other sites More sharing options...
August 14, 20222 yr comment_643912 If the system is not intact you can suck a lot of air through the PCV valve. How did you keep the engine from revving to 8000 RPM with no throttle blade? Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643912 Share on other sites More sharing options...
August 14, 20222 yr Author comment_643913 6 minutes ago, Zed Head said: If the system is not intact you can suck a lot of air through the PCV valve. How did you keep the engine from revving to 8000 RPM with no throttle blade? Intact in what sense? The hose is connected to the block and intake manifold with the new pcv screwed into the intake. Is there anything else I’d be missing? The valve cover hose isn’t plugged either. Oddly the car barely ran without the throttlebody on. I’d have thought the engine would freak out too. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643913 Share on other sites More sharing options...
August 14, 20222 yr comment_643914 That's what I meant. You didn't answer so I couldn't tell. With no throttle body there is effectively no vacuum being pulled on the intake manifold. So any leaks that would make noise with vacuum will stop making noise. Could be an injector seal, the gasket around the cold start valve, intake manifold gasket. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643914 Share on other sites More sharing options...
August 14, 20222 yr comment_643916 I guess you could start spraying areas with carburetor cleaner to see if the RPM's fluctuate. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643916 Share on other sites More sharing options...
August 14, 20222 yr comment_643917 Just now, Reptoid Overlords said: I guess you could start spraying areas with carburetor cleaner to see if the RPM's fluctuate. Nevermind I see where you guys already did that Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643917 Share on other sites More sharing options...
August 14, 20222 yr Author comment_643918 Just now, Reptoid Overlords said: I guess you could start spraying areas with carburetor cleaner to see if the RPM's fluctuate. We actually did that. Sprayed absolutely everywhere to no avail. I don’t think there are any leaks because the rpms drop drastically with the oil cap off Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643918 Share on other sites More sharing options...
August 14, 20222 yr comment_643919 I watched the video and don't hear the noise. Are all of the various vacuum ports on the intake manifold used or sealed? Charcoal canister, AC system control, brake booster, etc. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643919 Share on other sites More sharing options...
August 14, 20222 yr comment_643920 I'm wondering if that aftermarket air cleaner has something to do with it..but I run a similar setup and don't have that noise. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643920 Share on other sites More sharing options...
August 14, 20222 yr comment_643922 He said they took it off. Link to comment https://www.classiczcars.com/forums/topic/67750-strange-intake-noise/#findComment-643922 Share on other sites More sharing options...
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