Better gas mileage with EHA off, Why?
 

I disconnected the wiring to the EHA on my 560sl because I was getting a pulse on decel and I was told it was the EHA limiting the gas (if I understood correctly). I was expecting gas mileage to suffer since there is apparently no longer fuel control on decel with no EHA.

Although I haven't "run the numbers" it is apparent to me that after a tankful of gas over a weeks time, the mileage has actually improved slightly, certainly hasn't gotten worse. Plus, the idle has improved slightly and the aforementioned pulse on decel is gone.

Now I'm really confused.

J. Boggs

Look at the following. See what Steve Brotherton had to say.

http://www.peachparts.com/shopforum/tech-help/42382-can-you-test-eha.html?highlight=eha

"More current, more fuel passing".

Since it's been disconnected, there's no current and I would suspect less fuel passing, therefore better gas mileage.

I am guessing. Your O2 sensor is bad and it is causing the the engine to run rich in closed loop. When the wire is off the engine runs in open loop preprogramed mode slightly rich but better than with bad O2 sensor.

Plus, you're not getting any enrichment on acceleration or cold warm-up, so that would improve the mileage but at the cost of some performance, or so one would think.

Still don't get it.
 

The reason I uplugged the EHA was to test the theory that is causing a pulsing drag (like very light tapping the brake) on decel. With the EHA off, it does not drag.

The performance has been fine just by feel, plenty of accelertation, idle seems a little high but smooth. I suppose I could drive it like this forever but what are the consequences?

Thanks.

J. Boggs

Three things:

First, your engine SHOULD drag when you take your foot off the accelerator. If it does not, it means the injectors are still passing fuel, and wasting it.

Second, the electronic engine management will cause a very small improvement in fuel mileage, and a BIG one in emissions, if it is working properly. If it is NOT working properly, it will screw up mileage in proportion to the severity of the malfunction, and this can be in a major way.

Thirdly, the KE system in proper mechanical adjustment should get nearly as good mileage with computer controls disabled as enabled. If electronics are disabled, you will likely not pass emissions anywhere, and driveability in various conditions will suffer.

Do not compare your car's behavior with malfunctioning computer controls to what it will do with the problem(s) fixed.

Steve