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Registered
1994 S500
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50 Posts
Discussion Starter #23
Changed the fuel filter and spark plugs, yet no change.

Growing desperate here. I am really considering selling it....

Wish i had the star diagnostics system. They charge $300+ to run a diagnostic....
 

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99 CL500
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41 Posts
This still sounds like a mechanical problem that no diagnosing will really cure...yet.

When u checked the plugs how did #8 look? Have you checked the fuel pressure regulator? DIY


Vaccum leaks can be an pain to chase down, but are very telling of an engine that wont throw codes but will adjust (for the worse) to maintain it's running condition.
 

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Registered
1994 S500
Joined
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50 Posts
Discussion Starter #25
This still sounds like a mechanical problem that no diagnosing will really cure...yet.

When u checked the plugs how did #8 look? Have you checked the fuel pressure regulator? DIY


Vaccum leaks can be an pain to chase down, but are very telling of an engine that wont throw codes but will adjust (for the worse) to maintain it's running condition.

Thank you.

The plugs tips were fine but plug itself was kinda black. Anyway, I've looked it up and its about $35 for the fuel regulator. so i will change it just for the heck of it. i will get back to you with results when I get the part.
 

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Registered
1994 s600 coupe
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684 Posts
you need to connect a scan tool, and read live data, read ALL the inputs, and find the ones that are off.
make sure the air filters aren't blocked.
if it's getting that kind of mileage, the car is staying in OPEN LOOP,
meaning it's never going into closed loop and trimming the fuel upon full warmup.
make sure the thermostat is working, that's what heats up the coolant temp sensors,
and puts the o2 sensors into closed loop mode.
also, the MAF sensors not working correctly, will default the engine to open loop,
and default to speed density i.e. MAP sensor control,
that will kill mileage.
when they're going into closed loop, they get surprisingly GOOD mileage, for a 5000 lb. car.....
another thing that hurts mileage is EGR valves staying closed, or blocked.
now, I just blocked the 2 EGR valves on my car, to gain pure power and mixture,
but that's the price I'm willing to pay.
my car did get better mileage with both EGR's locked full time open.
reason being it's reburning the exhaust gases 10-15% volume on average,
so it leans out the mixture, uses less fuel, pollutes less, cools the combustion slightly.
what that does is raises the knock point of any given fuel used,
so the computer adds more timing without it knocking,
and without the knock sensors retarding the timing.
therefore, having said that...if mileage is what you're after,
make sure your o2 sensors, MAF sensors are working correctly,
the car is reaching op temperature,
and it's going into closed loop,
and the EGR is functioning.
 
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