My friend recently came to me looking for help with his civic running a d16y7 that is getting **** poor gas milage. We discovered that his spark plugs were fowled as a result of his engine running overly rich. He mentioned that when he accelerates in a certain way, you can hear a strange sucking/hissing noise coming from the engine bay. I had a similar problem in my last civic with a b18c1 that I just attributed to a vacuum leak that I was too lazy to fix.

I tried searching the forums for advice on how to troubleshoot and determine if/where there is a vacuum leak, but all I found were a bunch of walkthroughs on how to clean an iacv for cars having idle issues. That being said, both of our vehicles experienced irregular idles and could likely have used an iacv cleaning, but that doesn't really explain the whistling sucking noise. Are there any suggestions on how to troubleshoot something like this?

He is getting about 16mpg at this point. Would a dirty iacv cause the engine to run that rich?

 

What year Civic matched with what engines?

Look for disconnected vacuum hoses, and check for intake air leaks from the TB and IM gaskets.

Why not clean the IACV as well, and the FITV if you have one?

 

Mine was a 96 civic dx with a b18c1, his is a 98 civic hatch (not sure of exact trim) with a d16y7.
I was trying to sell mine when I started noticing the problem, so I just told the buyer about it and didn't bother to fix it. I don't see any reason not to clean the IACV in his, but I don't think it would cause the noise we both experienced and am still unsure as to whether the IACV alone would cause such a severe decrease in fuel economy. Next time I see his car, I'll be sure to look for leaks in the gaskets and disconnected/damaged lines.

 

You must remove the IACV to clean it, yielding an opportunity to inspect the IACV rubber gasket. A leak there could also cause the hissing noise and idle issue.

 

I got ahold of a service manual:

Starting at page 11-192 (461 in the pdf), it goes through troubleshooting to determine if the iacv is gummed up and/or if there is a vacuum leak. Should have looked for a service manual sooner, I guess...

 

As Ron mentioned it could be a manifold or intake leak so do a google search on how to test.. otherwise look for loose, broken, or disconnected vacuum hoses. Also, the PCV valve under the intake may have come loose.

 

I took a closer look at his engine and his idle was set really low as well as his throttle cable having a lot of slack in it. I adjusted the throttle and brought the idle up and he says it's been running fine since then and hasn't made the noise since.
I think the issue on my b18 was the IACV, which I know needed cleaned. The throttle cable didn't have any slack and the idle had been adjusted properly, leaving the IACV as the last thing (according to the shop manual) to check.

Thanks for all of your help and sorry to open a thread for a problem that could have been resolved with a little more research on my part.

 

There should be some slack in the throttle cable. See service manual.

 

Some, yes. His had a LOT of slack. It was to the point where he was unable to open the throttle fully with the pedal and wasn't getting much response until it was about 1/4 of the way down.