Hi all,
Pardon a question that has probably been covered many times, but I'm unable to find anything with a quick search.

I've got an '83 928S 5 speed that I've had since '87 and absolutely love. One feature that I've always liked about the car is that when you take your foot off the gas in gear and decelerate, a moment later the engine goes into a mode that creates a lot of additional engine braking... and also makes a wonderful deep baritone exhaust sound.

What component makes this feature work, and what does it do? I don't think it's a deceleration valve (which would decrease vacuum and thus decrease engine braking). Is there a fuel cutoff system and if so how does it work? I've never had occasion to look into this.

However, for a few years the system has been a bit "sticky"... sometimes the engine won't go into this decel mode, and thus the car decelerates less than otherwise (very little engine braking). But if I quickly tap the gas and release, it'll engage. I'd like to fix whatever this is so it operates smoothly again.

Thanks!!
Rick

 

Gas engines don't have a "jake brake" like big diesels do. The deceleration is just the vacuum created by a fast-spinning air pump (your engine) being restricted by a closed throttle plate.

Maybe your throttle butterfly is sticking open a little from time-to-time?

 

Check the idle micro-switch and/or throttle linkage adjustment. ('80-'84 L-Jet has two discreet switches mounted on a bracket attached to the throttle body.)

The idle switch signals the fuel brain to shut off fuel above (????) rpm.

 

Good engine compression and no intake leaks. My stroker has nearly 11:1 compression, its engine braking is impressive.

 

bingo

in tuning the 944 it's been found that you can add a level of control to engine braking by tweaking the operation of the idle control valve in the DMe...

shut throttle above certain rpm cuts fuel, but then you can tell it to allow more air to bypass the throttle plate which helps to "brake" it..

 

There is a deceleration valve mounted to the side of the throttle body on 78-84 cars. It opens when there is a large amount of vacuum and allows some air to bypass the throttle body.

 

The idle switch signals fuel cutoff. BRILLIANT! Thanks Ken!! I know exactly what to check. I'll check linkages but I'm already guessing it's a slightly wonky microswitch. Still original!

I should have mentioned that this "deceleration mode" (fuel cutoff I now understand!) disengages at about 1100 rpm. If you're still in gear, it does create a small kick in the drivetrain. So, if I'm still in gear, cruising to a stop with clutch engaged as the RPMs wind down to idle, I've become accustomed to pushing in the clutch pedal at just the right moment (just before the fuel kicks back in) to make a smooth stop. Works great.

Thanks!!!!!
Rick

 

Oh, one more thing Ken -- what is the OTHER microswitch for? Just curious!

 

If you get that working and try to use it on wet or slick roads, you will swap ends and spin!

 

For my 1981 L-Jet, the second micro-switch is the full throttle indication. I believe that in full throttle (actually, about 3/4 throttle) mode, the computer ignores the oxygen sensor and significantly enriches mixture.

Your 1983 should be the same.

 

Ummm... No.

The switch signals the ECU to use a fixed amount of gas vs RPM when the throttle is closed as the AFM doesn't accurately measure very low airflow. People will disconnect or otherwise defeat the switch because shifting to that map can make the engine, and car, lurch.

What you're feeling is when the decel valve closes. It's held open for a moment to let the airflow, and mixture, decay more gently than just slamming the throttle closed. After that, engine braking is just engine braking.

Diesel engines, if you wonder, don't have a throttle. (A "throttle" is a restriction in fluid flow and not a pedal.) They vary the gas that gets squirted in and the high compression ignites that. Now, I'm sure some diesel, somewhere, has a throttle. The "Jake Brake" is like a throttle in that it restricts airflow into the engine to produce engine braking. Saves brakes and is darn noisy.

 

If the switch isn't adjusted correctly, you'll get popping from the exhaust on deceleration with your foot off the gas pedal. If you're not getting any popping, it's likely it's adjusted correctly.

As others have said, it shuts off fuel if the engine is turning over 1200 rpm with foot off the gas pedal.

Hugo

 

Sorry to revive an old thread, but been searching for a bit. The idle microswitch is part of my problem on the 82. When I got the car, the PO had both switches disconnected. I am not sure if I have the wires on the right switch, or if my switch is bad. I can check the switches with a meter easy enough....can anyone give me a a picture of the two switches with wires connected, and / or a wiring diagram? Car still won't run for crap with vacuum on the distributor, but want to get these switches sorted first.

Thanks!

 

Ron,

Per WSM wiring diagram, the "no load" switch gets the white-with-black-tracer on one contact, and one of the black wires on the other contact. The "full load" switch gets the white wire on one contact and the other black wire on the second contact.

ASSuming that the wiring diagram shows the switch contacts in the relaxed position (normal convention for such things...), the "no load" switch is normally-closed until the throttle is closed. The "full load" switch is normally-open until the throttle gets towards full-open, at which time it should close.

At the control unit, the white with black tracer from the no-load switch terminates at pin 2, if you want to include the wiring in your testing. The white from the "full load" switch terminates at pin 3. The shared black wire from the switches lands on pin 18 in that controller connector.

 

Thanks Doc.