Gentlemen. My 1999 Ranger with 4.0 EFI V6 engine makes a "sucking sound" occasionally. At first I thought it was a belt slipping, but now I believe it is some kind of intake air leak. If I stop the truck while the sound is on, I can open my driver's door and get to about the front of the hood before the sound stops. I have never been able to observe the engine with the "sucking sound" on. Sometimes the engine will kill when the "sucking sound" stops, if the engine is idling and in gear. If I just keep driving normally, there does not seem to be any permanent problem. Any ideas as to what the problem is?

 

I would inspect your air intake system to see if one of the plastic tubes is partially collapsed when you hear the sound. You might be able to see some indication by looking at the plumbing. I can't remember if your model has the conical air cleaner, but if so, there is a tube inside the fender outboard of the cir cleaner to check also. I had a diesel Ranger that sometimes would collapse the air induction plumbing trying to draw more air.

 

When I wrote the initial post, I did not write very clearly. When I said I "stop the truck" to investigate the sound, I meant that I leave the engine running and run around to the front to lift the hood, but the sound stops before I can get the hood up. You are exactly correct that a collapsing of a tube under suction could be causing the "sucking sound". I am surprised that there is not an accompanying "driveability issue" when the sucking sound comes on. I will have to work harder to find a way to look under the hood while the sound is being made.

 

Maybe you have a critter in the air intake plugging up the hose somewhere...

 

Today, the sound came on just as I pulled up to the post office. I was able to jump out and lift the hood. I found that the rectangular air cleaner box was vibrating from the sound. I was able to release the two hold-down clips and lift the cover from the box with the air intake hose from the air cleaner to the engine still attached. The noise immediately became loud, and I could look up inside the hose. The noise then stopped, but I could see what appears to be the air flow sensor. I have not had time to disassemble the hose and sensor, but it seems clear that something related to the sensor is making the sound. Any ideas?