Hissing sound near intake/injectors
05-07-2011, 05:43 PM
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Hissing sound near intake/injectors I just finished putting my rebuilt heads on my 99 vette but I'm hearing a hissing sound near the intake manifold and injectors. I can hear it on both sides and it seems to go on and off in patterns. The car seems to be running fine and hasn't thrown any codes yet. The car is basically stock except when I had the heads rebuilt I had the valves bumped up to 2.02 and 1.57. I'm not sure what it is? Vacuum leak? Gasket issue? Injectors? I don't see anything leaking.
05-07-2011, 08:41 PM
#5
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If an injector o-ring is screwed up, you'd probably know it. The gas smell is almost unmistakable, and o-rings in my experience fail pretty obviously because of the pressure they're under. Once they leak, you know it. Fire hazard! Careful.
It wouldn't take long to lift the intake and inspect mating surfaces, vacuum lines, gaskets, and installation of other miscellaneous parts. I'd start there... take that half-hour and carefully rule out a whole bunch of physical items. Who knows what you might have forgotten to check in a left-field moment. We all do it.
To check vacuum... I'd hook up a scanner if you have access and check the PID for the MAP sensor. If your MAP sensor goes high (2.5 < v < 5) and stays up while the engine is running, definitely start looking for a larger vacuum leak. The engine may idle high, or it may not if the PCM can compensate for a smaller sized leak. This would keep the MAP in range, so it wouldn't light the SES, but it would allow the vacuum leak to persist.
A vacuum gauge would be handy to verify the PID info, if you know what readings to expect for your setup. It varies, but typ. a stock car will have 17-22", a modded car with a cam could be all over the place, even less than 10" in some cases. You really kind of need to know your engine, and since this is a newer setup for you, you may not know what reading to expect. Just one more test to try if you have a mechanical gauge handy.
Still, the PID data on the scanner should be steady at idle (probably 1.0v or less). Blip the throttle and compare MAP to TPS voltage, they should be close. As TPS peaks, MAP should be somewhere close but will lag a tad bit behind. Vacuum drops as TPS increases. This signals engine load/demand, so the MAP responds to the drop in vacuum as the engine spins up. Higher voltages @ the MAP sensor (above 1.0v but probably less than 2.5v) at idle means a vacuum leak.
It wouldn't take long to lift the intake and inspect mating surfaces, vacuum lines, gaskets, and installation of other miscellaneous parts. I'd start there... take that half-hour and carefully rule out a whole bunch of physical items. Who knows what you might have forgotten to check in a left-field moment. We all do it.
To check vacuum... I'd hook up a scanner if you have access and check the PID for the MAP sensor. If your MAP sensor goes high (2.5 < v < 5) and stays up while the engine is running, definitely start looking for a larger vacuum leak. The engine may idle high, or it may not if the PCM can compensate for a smaller sized leak. This would keep the MAP in range, so it wouldn't light the SES, but it would allow the vacuum leak to persist.
A vacuum gauge would be handy to verify the PID info, if you know what readings to expect for your setup. It varies, but typ. a stock car will have 17-22", a modded car with a cam could be all over the place, even less than 10" in some cases. You really kind of need to know your engine, and since this is a newer setup for you, you may not know what reading to expect. Just one more test to try if you have a mechanical gauge handy.
Still, the PID data on the scanner should be steady at idle (probably 1.0v or less). Blip the throttle and compare MAP to TPS voltage, they should be close. As TPS peaks, MAP should be somewhere close but will lag a tad bit behind. Vacuum drops as TPS increases. This signals engine load/demand, so the MAP responds to the drop in vacuum as the engine spins up. Higher voltages @ the MAP sensor (above 1.0v but probably less than 2.5v) at idle means a vacuum leak.
