I've noticed this lately, a tad more than usual, but if I simply let off the fuel at say 40-45 mph @ 1,800-2,000 rpm...

The truck starts a normal deceleration, then after a 100-300 rpm drop, goes BERRRRRRRRRRRRR and literally makes the truck slow down FASTER. (Imagine an extremely "Light" sounding Jacobs Engine Brake sound)

After the rpm's fall to 1,200-1,300...And I downshift, it'll do the same thing..All the way to a complete stop.


I honestly don't understand it. Aside from a Code 522, 327, and 329 (NSS is disabled, and the last 2 codes are EGR related) the truck runs flawlessly; Except this lil exception..

I mean, it sounds cool, etc...Just curious as to the actual cause.

SMOG pump is being used an Idler Pulley, and everything else, vaccum lines, etc have been capped off.

The A.I.R Injection rail has a break in it, where the #1 port T's in..Causing a slightly loud "WHOOSHING" sound when you rev the engine, but it doesn't affect Idling, driving, etc.

Mpg bounces around between 15-19 mpg in city/mixed/rural driving; So it isn't an issue there...Just am curious as hell about this.

Truck is a 1992 F-150, 4.9l/5 speed, with 203,000 mi.

 

i wonder if what you are hearing is the exhaust heat shield rattling around under the truck when the rpm's get in a proper harmonics range?

my nephews 88 4.9 C6 would do the same thing until i took what was left of the heat shield off.

 

No heat shields here...Exhaust is completely new.

Closest thing to one of those left on the truck is on the bottom of the cab, but I've checked it already and it's solid. No rattling, etc.

I might make a video to see if I can pick the sound up on camera.

But you can literally feel the engine braking and slowing the truck..

Which makes no effin sense; Given the fact I KNOW it doesn't have anything even CLOSE to an Engine Brake/Exhaust Brake on the truck.

If you give it gas and let off, it has a slight "Hesitation" in the time you let off the fuel and when the "Braking" comes into play.

Actual engine running, etc is fine...No hesitation/stuttering...

It's just like clockwork...To be honest, I use the "Braking" of the engine or whatever in the world it is, to slow down....Works like a charm.

I have noticed if you do a really gradual deceleration that doesn't involve simply letting your foot off the pedal, it doesn't seem to do it...Just like a full sized rig/Engine Brake...

I'm not saying it has an engine/exhaust brake of any sort (That's just ludicris)...I'm just trying to figure it out. lol

I'm not complaining, just trying to wrap my head around it.

 

Did your truck just recently start doing this? There should be some engine braking action, but it sounds like yours might be excessive.

 

Hence..The name of the thread...

It doesn't seem to be affecting anything at all.. Just curious as to what would cause it..

You can literally feel it "Grab hold and let go" between certain rpm levels.

 

clutch related? just throwing it out there

 

How new is the exhaust? Is that when it seemed to change? Maybe new exhaust changed the back pressure characteristics. Not that that's a bad thing. It sounds like your mileage is fine so performance is good. Just a thought

 

New Sachs brand Clutch, Slave Cylinder, Pressure Plate, etc...Less than 30,000 miles on it.

Less than a year old...

It's 2.25" 14g aluminized steel, welded to each Manifold, forming a Y, then on to a Dynomax Turbo Muffler, and ending in a 3" x 13"l Stainless Tip...

Here's a pic of the exhaust.

 

It could be an implementation of DFCO (deceleration fuel cut-off). My '03 Focus does the same thing in 5th and 4th gear after coasting for about 2 seconds with no throttle until the RPM drops below 1900 and 3rd gear when it's above 3000 RPM.

From what I've read other places, newer vehicles are starting to use DFCO for fuel economy purposes, but traditionally it was used to protect the catalytic converter. Early EFI systems weren't always great at metering fuel, and while coasting under "high" RPMs the system couldn't maintain accurate AFR for the catalytic converter to operate properly, so it was easier to just assume 100% air rather than a varying AFR. It also helps mitigate the "backfiring" pops and cackles that you hear when going closed throttle from high RPM into an engine braking situation. Apparently you can disable the DFCO code with a Tweecer/Quarterhorse and it makes for some loud deceleration at high RPM.

As for the sound, I don't know what causes it, or why it seems to be changing recently, but my Focus's exhaust tone definitely changes under DFCO. Maybe you're just more aware of it now than you were before?

 

-Sounds about right...If I do what I call a "Hard deceleration" I.E Completely letting off the fuel pedal all at once, it does the engine braking sound.

-My exhaust was put on hold, and for about a week, I had to run open manifolds (NOT fun). Sounded like a pissed off weedeater on a beercan....With intermittent .22 rifle shots tossed in the mix...lol


-I think you're right to be honest...I'm becoming more and more aware of these lil nuisance squeaks, rattles, etc. Also, given that my theory is correct, finishing up plugging the A.I.R Injection Rail Ports with bolts, *SHOULD* quieten the sound down. Since the Rail is broken on the #1 port, it allows it to have an added sound similar to bicycle being ridden with a baseball card in the spokes. Which, in the cab, would probably account for the "Engine Braking sound".


My truck is a 1992, which mean OBD-I/ Speed Density computer system an whatnot.

I think we can consider this a partially resolved issue; The unresolved portion being, "Why does it engage and disengage the engine braking so aggressively?"

It's not an issue...Just curious if it's something that can be "Tuned" or adjusted for more or less E.B while driving.

 

I agree that the unplugged A.I.R. injection rail ports aren't a problem, but it makes a huge difference in sound when they're plugged.

It's not a great picture (since it wasn't the focal point of the picture), but you can see to the passenger side of the upper intake, 3 hoses wrapped in black electrical tape with white ends. When I removed the A.I.R. system, I left the tubes and plugged them with PVC TAs and end caps to stop the leak.

Attachment 293489

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Originally Posted by SideWinder4.9l

The unresolved portion being, "Why does it engage and disengage the engine braking so aggressively?"

It's just the nature of how the DFCO works. It's entirely shutting off fuel injection so it's only the momentum of the truck turning to engine. When the fuel kicks back in, the engine is once again producing power to turn itself and some of the rotating mass of the drivetrain. You can't gradually lean out the fuel delivery as it would cause detonation with a lean AFR.

The RPM engagement/disengagement points can be tuned with a Tweecer or Quarterhorse unit, but not the "aggressiveness". You can also avoid the issue by shifting into neutral which requires fuel to keep the engine idling. On the automatic trucks, the DFCO only engages when the RPM is high, the throttle is closed, and the torque converter is locked, which under normal coasting unlocks to mitigate engine braking.

 

Wes, when did you notice the effective increase in engine braking? You never did answer that.

 

In the past few months or so...I think it's just me noticing it all more than before..

 

If an answer is ever found for this, I'd like to know. My truck does the same thing and it's really obnoxious. However, it is kinda sweet when pulling a load.

 

If I keep my rpm's around 1,5oo and down...It gets a REALLY deep diesel-esque sound to it...

I just call it my wannabe Cummins and walk on...