Fordnatics List Archive

This read-only message was archived from a public mail list.
Mail From: (email redacted) (JOE SABEH (X3762, Q257N))


[Forwarded from the -request address. Send list submissions to:
(email redacted) -- Chuck]

Hi Ford Experts,
While I was on a trip from San Diego to El Centro this week-end my
temp gauge kept going in to the cold area below the normal line. Specifics
are:

Car: Thunderbird
Year: 1995
Miles: 3125
Motor: 4.6
Trans.: AOD
Speed: 75 MPH
Outside temp range: 80 - 90 F
Terrain: Mountainous
aircon: ON

When I called the Ford Dealer he indicated when the thermostat
opened the motor temperature would drop that much. I questioned this
because dropping below normal would cause the brain to run the motor rich,
wouldn't it?
I would appreciate your thoughts on if this cold condition is
really a normal occurrence.

SABU

This read-only message was archived from a public mail list.
Mail From: chucko (Chuck Fry)

Date: Wed, 22 Mar 1995 10:05:43 -0700
From: (email redacted) (JOE SABEH (X3762, Q257N))

While I was on a trip from San Diego to El Centro this week-end my
temp gauge kept going in to the cold area below the normal line. Specifics
are:

Car: Thunderbird
Year: 1995
Outside temp range: 80 - 90 F
Terrain: Mountainous
aircon: ON

Under what conditions did the temp gauge drop? On downhills or uphills?

I frequently see the temp gauge on my Mustang drop when coasting down a
long grade in gear. This is to be expected, as the car is driving the
engine, and not much waste heat is being generated while air and water
flow through the radiator are both high, chilling things out rather
nicely. It's even possible the engine is not receiving fuel at all
under these conditions, in which case NO heat is being transferred to
the coolant!

If you see the gauge drop under other conditions, chances are (1)
there's a bubble in the cooling system directly under the temperature
sender or (2) the sender, gauge, or wiring is intermittently broken.
Cause (1) may be a symptom of a coolant leak or head gasket failure.
-- Chuck

This read-only message was archived from a public mail list.
Mail From: (email redacted) (Sander Pool)


Chuck wrote:

=> I frequently see the temp gauge on my Mustang drop when coasting down a
=> long grade in gear. This is to be expected, as the car is driving the
=> engine, and not much waste heat is being generated while air and water
=> flow through the radiator are both high, chilling things out rather
=> nicely. It's even possible the engine is not receiving fuel at all
=> under these conditions, in which case NO heat is being transferred to
=> the coolant!

I've often wondered about this. If you have a manual tranny and you
use the engine to control speeds when going downhill, shouldn't
the engine absorb the energy it takes to slow down the car?
It is my understanding that the engine slows down the car because
of it's compression of the intake charge, right? That will
generate heat. Of course this amount of heat is lower then
when the car is driven normally but there will definately
be heat transfer to the coolant, or am I missing something here?

Sander

This read-only message was archived from a public mail list.
Mail From: chucko (Chuck Fry)

[IMHO warning. Take the following with a large grain of salt.
Corrections welcomed.]

Date: Thu, 23 Mar 95 09:46 PST
From: (email redacted) (Sander Pool)

Chuck wrote:

=> I frequently see the temp gauge on my Mustang drop when coasting down a
=> long grade in gear. This is to be expected, as the car is driving the
=> engine, and not much waste heat is being generated while air and water
=> flow through the radiator are both high, chilling things out rather
=> nicely. It's even possible the engine is not receiving fuel at all
=> under these conditions, in which case NO heat is being transferred to
=> the coolant!

I've often wondered about this. If you have a manual tranny and you
use the engine to control speeds when going downhill, shouldn't
the engine absorb the energy it takes to slow down the car?

It should, and it does, but not for the reasons most people believe.

It is my understanding that the engine slows down the car because
of it's compression of the intake charge, right?

Not true! Everyone calls it "compression braking", but it's really
VACUUM braking. The engine is pulling a high manifold vacuum, trying to
suck air past a closed throttle plate. That energy is lost in pulling
the air past the throttle, and pulling the pistons down against the
pressure differential between manifold vacuum and crankcase pressure
(which is, or should be, near atmospheric). The energy used to compress
that thin air once inside the engine is substantially lower than during
normal operation.

You still have the same mechanical losses as during normal operation, on
the order of 10-20 HP or more lost to mechanical friction (depending on
various factors: engine size, ring type, oil viscosity, accessory loads,
etc.). I would wager this is where most of the energy goes in
"compression" braking.

When I said no heat was being transferred to the coolant, I was
exaggerating. Friction will continue to heat the engine. But since
gasoline engines dump a large fraction (I believe 2/3) of the heat of
combustion into the cooling system, and many EFI systems stop injecting
fuel under closed-throttle deceleration, the heat generated is
negligible compared to normal operation.

I believe the only engines that do true compression braking are Diesels.
-- Chuck

This read-only message was archived from a public mail list.
Mail From: guziec philip robert <(email redacted)>


When coasting down hill in a manual transmission equipped car with
the transmission in gear, you are breaking the car with the pressure drop
across the throttle plate. The intake manifold is at below atmospheric
pressure and we can assume that the exhaust is at atmospheric pressure.
Your engine is therefore pumping air from low pressure to higher pressure
and absorbing work.

The fuel injection computer is injecting fuel like always, it is just
that the power of the combustion of a stoichiometric mixture of air at
25% of atmospheric pressure is probably less than the friction of the
engine and accessories, like at idle.

Philip Guziec

This read-only message was archived from a public mail list.
Mail From: James A Larosa <(email redacted)>



> I believe the only engines that do true compression braking are Diesels.
> -- Chuck

Is it called a Jake Brake? That loud pap pap pap sound they sometimes
make when they are slowing down? Just curious...

James

This read-only message was archived from a public mail list.
Mail From: chucko (Chuck Fry)

Date: Thu, 23 Mar 1995 19:03:48 -0700 (MST)
From: James A Larosa <(email redacted)>

> I believe the only engines that do true compression braking are Diesels.
> -- Chuck

Is it called a Jake Brake? That loud pap pap pap sound they sometimes
make when they are slowing down? Just curious...

The "Jake Brake", if I understand correctly, is an exhaust blockoff
gadget. Anyone know for sure?
-- Chuck

This read-only message was archived from a public mail list.
Mail From: chucko (Chuck Fry)

Date: Thu, 23 Mar 1995 19:03:48 -0700 (MST)
From: James A Larosa <(email redacted)>

> I believe the only engines that do true compression braking are Diesels.
> -- Chuck

Is it called a Jake Brake? That loud pap pap pap sound they sometimes
make when they are slowing down? Just curious...

The "Jake Brake", if I understand correctly, is an exhaust blockoff
gadget. Anyone know for sure?
-- Chuck

- ------------------------------

"Jake brakes" operate on a diesel engine by opening the exhaust valve during the power stroke. The designs I have seen operate via oil pressure being used to actuate the valve opening when activated. The compression slows down the vehicle, and the open exhaust during the power cycle prevents full power from being generated, thereby causing a casual braking effect.

(email redacted)
Ford team technical editor