1999 5.9

Did the plenum change.

Anyway it will idle fine for about 3 minutes then the rpms begin to drop and it sounds like a vacuum leak (loud hissing sound) at the back of the engine but i cant find any signs of leakage and I thought a vacuum leak would cause RPMs to go up not down.

THANKS!!!!!

 

IAC may be faulty.
Is the throttle body torqued down? Is the gasket intact? Is the gasket for the airbox there? Spray around the intake with carb cleaner, if rpms jump then you've found the location of the leak.

 

Tell more about the plenum. When did you do it/have it done? Has it been doing this ever since then? Rear gasket on intake manifiold will make a pretty good hissing sound also and have adverse effect on idle.

 

Three words: Upstream oxygen sensor. Two more: Replace it.

Use only NTK or Denso brands.

 

That could cause the idle issue...but what about the vacuum leak sound? There may be more than one problem.....

 

Wow guys thanks for all the responses!!!!

I did this plenum change over a year ago. Its not something thats constant ie when I crank it up its fine and if I start driving its fine. It only happens after the initial start while in park and idling. Then the rpms start to drop and the hissing begins.

By the way the exhaust starts to stink really bad when this happens.

I will take yalls responses and see what i can come up with!

 

I am having the exact same problem!

 

Symptoms include a loud hissing noise (sound is traced back to the air filter), engine choking / dying at first instance of throttle pressure being applied, backfiring through exhaust, pre-detonation / pinging under throttle, and if and when the engine dies; smoke emanating back through the air filter.

The loud hissing noise is traveling through the throttle body housing and not through any broken or loosely fastened gaskets. Apparently, the engine is oxygen starved in some way and the, (IAC), intake air control valve is being actuated to compensate for the lack of oxygen by opeing up to increase the flow of air through the idle bypass port cast into the throttle body.

Recently I installed new oxygen sensors into the truck for hopes of better performance to no avail. This event took place before the issue occurred with the hissing noise, stalling, backfiring etcetera problem. Since my pre-cat O2 sensor is setup in such a tight spot, I thought that I may have broke the wires somehow during the installation process. I decided that after this problem occurred that I should try to switch back to my original O2 sensors; I incurred no beneficial results...

I started to think that my timing chain was to blame, close to 100,000 miles on the odometer, but found that the timing sprockets were still in perfect syncronization, yet, after inspection, I found 10 degrees worth of slack in the chain. The sprockets and chain were replaced, checked, rechecked, and found later that, that too was not the main issue.

I checked my spark plugs on the side of the engine that was connected to my carbon fouled catalytic converter; results found were no oil, no traces of foreign material, and nothing other then carbon deposits on the tip of the plugs especially towards the rear of the engine block. Carbon deposits were coating the entire surface of the plugs except for on the actual electrode and ground electrode surface of the plugs exposed to electrical discharges. At first glance the carbon deposits didn't worry me, but after a compression check on all cylinders, I found abnormally high levels of compression.

Compression results ranged from 145psi to 165psi. Most common reading was 155psi. Considering the amount of atmospheric pressure at around 1000 feet elevation as well as the compression ratio of my engine being set at the factory near 8.5 to 1, common sense suggests that the carbon deposits within the cylinders is so high that the compression ratio has increased greatly past the appropriate range.

Average compression ratio was 155psi. 155psi divided by 13psi atmospheric pressure equals compression ratio at 11.875 to 1.

Octane rating of fuel coming in at 87 definatly does not suit this level of compression and thus would explain for the backfiring, hesitation under throttle, smoke from the intake condition my truck is having.

Plan of attack at this moment is to find grenade, throw it under the hood and pray that the vibrations created will shake the carbon off of my cylinder walls, pistons, and combustion chambers enough to the point that upon ignition the stuff will get thrown up and out the exhaust valves.

If that doesn't help, I will attempt to find a chemical solution to my carbon deposit problem, also I plan to find spark plugs of a higher heat range to help keep my plugs a bit cleaner, and most importantly to change from the cheapest gas I can find to a higher quality brand of gas that has less tendency to leave my cylinders looking like the inside of a chimney.

Anybody have anything to add? As always, I am open to suggestions...

 

The hissing is almost universal when that sensor fails. It's caused by the IAC being fully open as the PCM tries its best to get the idle speed back up to normal.

In most cases it starts out as a stumble that lasts just a few seconds when the PCM goes into closed loop mode. Over time the stumble becomes more pronounced and lasts longer, then eventually the failure progresses to extremely low idle speed and hissing intake upon transition to closed loop. Let it go still further and the engine will intermittently lose power on the highway at cruising speed while the truck is unladen. Until just about the very end, you can open the throttle to get the engine speed up so more hot exhaust gas flows over the sensor to heat it and get it functioning more or less normally.

Y'see, an end of life oxygen sensor reads rich, and the more it decays the more falsely rich it reads. So at the appointed time the PCM goes closed loop and starts paying attention to the oxygen sensor, and it sees a false rich condition. It narrows the injector pulse width to compensate, which lowers the idle speed, and when the PCM sees the RPM drop it starts stepping out the IAC to compensate for that. This drives the mix still leaner, but until the PCM sees that via the O2 sensor it doesn't react because the TPS is not giving an indication that the throttle is opening -- thus the transition stumble. In the early stage of the failure it'll find a happy medium pretty quickly, but as the decay progresses the PCM is getting an increasingly inaccurate picture of the world. Eventually the problem reaches the point at which the PCM goes closed loop to find what looks like a gonzo rich condition that calls for the shortest injector pulses it knows how to throw, which requires the IAC to be stepped out to wide open. Hisssssssss....

The easy test: Unplug the sensor to force the engine to stay in open loop. If the symptoms stop, the O2 sensor is almost certainly bad because that's the one big difference between open and closed loop: The PCM starts paying attention to that sensor.

This is the time of year when we'll start seeing lots and lots of failing oxygen sensors because the weather is cooling. Fall and early winter are oxygen sensor and heater core season.

 

So the plenum was replaced a year and a half ago? I agree with o2 sensor, but what has been done about checking or replacing the clogged/clogging cat 1 1/2 years ago?