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RE: MB problems (4500rpm cutoff)

Orin writes
> It seems that it turns into a MAP system once the air flow meter
> tops out.  Given MAP (manifold pressure), RPM, intake air temperature
> and a table of the engine's Volumetric Efficiency (VE), you can
> calculate the fuel requirement of the engine.  There are some
> equations somewhere on the the diy_efi site.

I think the above is key to your answer Phil et. al.,  For 4500rpm cutout ck
the following:

MAP sensor is working correctly.  It is rated to 2.0 bar.  If you have modded
your car, and you are/have whacked that sensor at it's maximum too many times,
it will start to fail (pressure measured is lower than actual > wrong tables

Charge Air Temp Sensor operating correctly.  Historically a problem with the
Mac-o2 computers, but the symptoms are the same for the MAC-11.  I don't have
the Bentley handy, but you should have a specified ohm value on the computer
plug at room temp.  (my suspicion, double ck this).  This is read to the
computer at 1.1> bar, which means that you should be able to exceed 4500rpm
without boost.  ((For older Urq owners the MAC-02 spec is 12-33 ohms at room
temp (at the plug))

Someone else mentioned an MC swap into an urq.  Remember, you must change the
CATS to the MC part, the temp resistances are different (right Dave L?:)

A few lingering "might as wells"...   Check the ground and ECU plug on the
coil, clean all the coil connections.  Bypass the fuel pump relay, drive and
test, if it solves the problem, ck the idle switch for a short.  Also, ck the
readings of the coolant temp sensor.  3 ohms resistance at the WOT switch is
significant to confuse the computer.  A lower WG feed hose leak will overspin
the turbo, creating serious charge air heat at 4500rpm up.  Check fuel pump
output and system and control pressrue.  I discount the knock sensor, since
that codes immediately.

Phil, the mods you described to the car shouldn't affect the operation of fuel
in terms of quantity, sounds like the computer is doing it's job, just has
garbage coming in, you just need to clean that up.  A 2.0 bar car with larger
valves, cam, exhaust and big turbo, should be able to run on the stock fuel
system (BTDT).  You can mess with the control pressure if you find yours is
running lean.  

Bottom line, 4500rpm stumbling isn't a fuel quantity problem.  If it is, the
result would be knock, and subsequent overheating of the engine (read blown
head gasket).  Mild tuning to the MC motors really doesn't create stumbling
due to fuel starvation.  Given the similarities to the MB, I would say all of
the above applies.


Scott Justusson
'87 5ktqwRS2
'84 Urq