Hello, I have a f250 5.4L V8. I have been having some issues with it, it is making some hissing sounds when it is underload. I have noticed that It is worse when I am pulling a trailer, my RPM's are about 3500 and then instantly drops 500 rpms when the hissing starts.

I am not sure if a vacuum leak could cause that or in that sense really where to start looking for the problem. I would like to diagnose it myself without having to take it in. If there is anyone that has any input that would be great.

 

yes most likely you're right its a vacuum leak.

1st suspect area is a elbow of the PCV line near the firewall that connects to the throttle body.

Vacuum leaks can be anywhere though...so check things over closely

does it hesitate or shake during this? or just drop RPM?

 

Just drop in rpm along with a loss of power. It hisses more, like I said earlier, when it is under load, like when I am towing something up hill. You can hear it now and then when I am just driving, but it is more pronounced when I am pulling. I will check the vacuum system for leaks. Thanks.

 

what year is your f2fiddy (F250)? Auto tranny or manual?

when it is the most "pronounced" can you still change where the air is coming out?? Defroster to the heat, or heat to the dash output of air?

 

It is a 2001, with an auto transmission. It is most pronounced at around 3500 rpm and while it is under load. Yes I can still change the air on my dash to whatever I need. I know that is one of the main ways of knowing you have a vacuum leak on fords. My brother just had a leak and he could only use his defrost, it was a line that was loose behind the battery.

I used some carb cleaner around my engine and couldn't find a leak. I did take a line off of the vacuum box behind the battery and sprayed the cleaner directly on the elbow line and there was no change in the idle of my truck.

I did notice though as I was sitting there listening to the engine it would rev a little bit and then let off. Almost as if the ac was on but I had turned off the ac and the air. So I don't know if that could be another sign of a leak, an intermittent idle. Now it was not cutting in and out only a slight change in the rpm.

Let me know, thanks.

 

how many miles are on this unit?
Have you had the codes scanned?...there can be stored codes without tripping the CEL (Check Engine Light or Service Engine Soon) on the dash.

Prolly got more than one gremlin in there giving you fits.
A fluctuating idle, hints that the IAC is need of some cleaning, maybe replacement, but shouldn't come into play around the 3500 rpm and load situations.

If you have some higher mileage on this...then the best way to clean and or replace the IAC is to remove the whole throttle body housing. This not only will get to the IAC but allow you to check/clean the internal EGR ports that are inside the housing too. also give you a real good look at the elbow on the PCV line ta boot.

Once you have the Throttle body housing off then remove the IAC and clean it...small plunger inside that needs to move freely, and any built up carbon good ole WD40 works pretty well for that.

then clean the throttle body housing and back side of the throttle butterfly plate, carb cleaner can used here. IF the EGR ports are pretty well clogged up...then you turn your attention to the EGR itself & its plumbing

 

Could the MAF have any bearing on this condition?

 

possibly...but would that explain the hissing The OP originally brought to light? ( Not trying to be smart A$$ here)

Non the less you make a good point JWA as to cleaning it (MAF sensor). Certainly could not hurt to clean it

 

No No No No--absolutely NOT being a smart azz----'tis all good!

Also odd is how those "lights" can sometimes make all us somewhat dim bulbs huh?

 

LOL no worries....thought I was missing something