Hey guys, I want to adjust my compression braking...

I believe it’s too strong at the moment and contributes to increased fuel usage. I can’t really coast in gear, even down a hill, it slows me down way too quickly.

This means I have to always keep my foot ever so slightly on the gas (even down a hill), or coast in neutral which also isn’t good for fuel economy

I recall seeing (I think it was in the FSM somewhere), that you can adjust the engine braking to increase fuel economy but I can’t for the life of me find where it is...

Can anyone help? I’ve tried searching, and did a scan through the sections of the FSM I thought I may have come under and haven’t been able to bring it up...

cheers

 

You need to tighten up the muffler bearings .

Seriously, engine braking is purely a mechanical thing. There is no adjustment for it. The engine is a big air pump, you're letting the wheels turn the air pump and the drag it creates slows the engine down.

The fact that you have a lot of engine braking means you probably have a good, healthy engine that's tight with good compression. Pop an apex seal and that problem will go away .

Of course, this is the wrong car to worry about fuel economy. You can also try engine braking in a different gear to see if it makes any difference. Even if the revs are up, if your foot is off the gas and you're coasting, the engine isn't injecting any fuel.

Dale

 

Is your car an automatic by any chance?

 

No its not an automatic

And i know it sounds crazy, but i saw in a Mazda service doc recently that it can indeed be adjusted. Im 100% positive on this. I know it initiates fuel cut when coasting in gear, but maybe if the butterflies are kept open a bit more, it would pull air through easier while fuel cut is still activated?

I have been scouring the FSM. Section F; Deceleration control system may be what I need, but its deffinitly not what i saw previously or worded how I remember it... Maybe my dash pot is set incorrectly?

Im sure compression is great, always starts within a second hot or cold

 

My thoughts exactly

Either you misread what it said or someone is trying to take you for a ride....

 

Already changed the muffler bearings, same problem Ill try turning the tyres inside out?

Ill keep looking though, this is gonna drive me crazy.
I thought i had seen it in an actual mazda service document.

 

Engine compression braking, as Dale stated, is a function of the resistance of the air pumping ability of the engine. Believe me, there is NO valve or adjustment or anything remotely like that on this car or any gasoline powered vehicle that is mass produced.

Thats like someone asking you to go get a left handed screwdriver.

Compression is built internally in the combustion chambers. You cannot bleed that off or alter it unless you blow a seal.

 

IF the OP had previously installed a lightened flywheel, and then re-installed a stock FW it might improve what he described....wouldn't it?

 

I guess the throttle plates could be adjusted to change engine braking. If you don't let them fully close it would lessen the restriction, and restriction is what is causing the braking.

But, you'd then have a high idle. So that's kind of moot.

Dale

 

You are thinking of a diesel exhaust brake.

 

If it's not an automatic (in which case, the transmission itself might not be coasting correctly), I think you're mistaken, sorry.

The gear you're in (in a manual) will affect engine braking, but I'm pretty sure there's no adjustment for it. The only thing that this makes me even vaguely think of is the air bleed adjustment on the throttle body which will affect idle and the vacuum you pull at idle, but that would in no way affect engine braking.

 

Like Stated above I think you probably missread that document . and what gear do you usually coast down in ? what RPM ?

The less the RPM the less the engine braking because the engine's air brake of sorts is spinning much slower .

 

This is something that most people don't think about. My guess is you have small tiers or low gearing. Leaving the throttle blade open is the only way I know to reduce engine brake but it will idle fast.


If you're using a stand alone retarding timing at idle will return a normal idle.

 

Um, correct me if I'm wrong here but wouldn't having the throttle plates more open would INCREASE engine braking, not decrease it. More air = more compression = more engine braking.

Try experimenting, pull the EGI fuse out and just crank and listen to the engine. Now go WOT and hear the engine speed change due to increased resistance AIR resistance from compression. Engine braking is from compression, not air restriction.

thewird

 

Not quite. The engine braking phenomenon is because of the suction created against the throttle butterflies. Think about it, if opening the butterflies created more engine braking, how would you ever make power from the engine by hitting the gas pedal (which opens the butterflies)?

 

I think what you refer to the decel fuel cut that the ECU controls. It's a fuel economy measure that Mazda incorporated on the FD.

When the throttle is fully closed, the ECU cuts fuel to each rotor during deceleration. This occurs at a different RPM for each rotor, and fuel is reintroduced again at either 1000rpm or 1500rpm (depending on the rotor) to prevent stalling. At half throttle, fuel is cut to only one rotor.

This decel fuel cut is purely an ECU function and can't be adjusted. If however you have a PFC, then you don't have the OEM ECU fuel cut function....because you've removed the OEM ECU .

 

PFC and most other standalone ECUs have a deccel fuel cut. It can be adjusted. Think about it: when you let off the ecu will cut fuel, otherwise the car would continue to accelerate. You can monitor injector duty cycle on most ecus and you will see it drops to 0% on deccel.

 

The OP was talking about an OEM adjustment. He can't do that with the OEM ECU, and he loses that OEM decel function if he removes the OEM ECU.

The OEM ECU also stages the fuel cut, cutting fuel to individual rotors at various times during the decel....I doubt the PFC does that.