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1978 Cadilac Seville won't run

Started by D.J. Hall, July 10, 2010, 03:16:49 PM

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D.J. Hall

Hello to all, I am new to this so I will give it my best. I have a 1978 Seville with a tad less than 41,000 miles on it. I have owned it for a little over 2 years now and it has been a wonderful car. Last week I took it for a little ride and it let me down. I was running down the road and it started to surge a little and then just quit. After getting it off the road I hit the key and it fired right up but only ran about 5 seconds. When it is running it sounds great but only for about 5 seconds. Long story short, towed home, both fuel pumps replaced, still the same. Crank it, fires right up then dies in about 5 seconds. Anybody out there that can give me any ideas, I am about to give up. Thanks in advance

D.J. Hall

Dave Shepherd

#1
Got good fuel pressure at the rail, clean/new fuel filter, will it keep running if you spray carb cleaner into the throttle body?, some diagnostics must be done to pin this down, there is no quick answer here. Do both pumps run?

bcroe

My guess is the ECU is failing.  Could be the internal MAP sensor.  Check that the fuel pumps/pressure is maintained until the engine stops.  The speed sensor could also have failed, that's the 2 switches low down on the HEI shaft.  Your car also has a spark advance control, an ESS module on the firewall connected to the HEI; make sure you have spark.  Here is my check list. 

   
76-79 Cadillac Fuel Injection Repair

Here is some general stuff.  If you think only half
the injectors are firing, check that the black wires
on the injectors have a good ground connection
on the engine.  There are usually 2 bolts grounding
the lugs, one for each bank.
_____________________

There is always the possibility of bad fuel
injectors; it is usually possible to get them cleaned.
Replacing ALL the injector gaskets and high pressure
fuel hoses might save you a fire; be sure to check for
leaks after repairs.
_____________________

Regarding the 76-79 350 and 76 500 engines, they
use different distributors, different fuel injectors, in
fact about the only parts on the engine that are the
same are the temperature sensors.  The computers
are calibrated/adjusted differently.  The 77-79 425
parts are very similar to the 500, but use the same
injectors as the 350.  The throttle position switch
seems to be the same for all models.  The 78 and
later 350 engines use a small bore throttle body on
an aluminum intake manifold, others use a large
bore on an iron intake manifold.  The aluminum
intake has corrosion problems around the coolant
passages and should be checked to avoid leaks.  I
would recommend milling about .030" off and putting
in some of those Mondello fiber gaskets over the
regular valley pan.
_______________________

If you switched computers from one car to another,
a car should at least start and run a bit.  So you can
test a computer in any running car.   Also the SPEED
SENSOR in the distributor is the same.  If you are
taking parts, save the computer.  The computer
parts can be used to fix other computers, and I can
convert a 350 computer to run a 500

You need to know if your car won't run because of
fuel or spark.  Make sure there is plenty of spark at
the plugs. 

Try turning on the ignition but not starting the engine.
You should hear the fuel pump run for about 8 seconds
and then shut off.  There should be 39 psi (3.6 Bar) of
fuel pressure at the injectors.  If no pressure, check
pump power from computer.  There is a fuel pump
fuse in the wiring connecting to the computer.

If pressure is very low, try squeezing closed the hose
on the return pipe from the fuel pressure regulator.  If
this raises pressure its the regulator; if not its the
pumps.  There are 2 pumps, a low pressure pump
in the tank and a high pressure pump mounted in the
frame. 
_______________________

Try this test on the computer.  Take off the air cleaner. 
Turn on the ignition but don't start the engine.  Slowly
pull the throttle all the way open, and you should hear
the injectors click 21 times.  If this works a lot of your
injection system is working.  The electrical unit on the
the throttle body shaft is a multi contact switch which
generates acceleration enrichment pulses plus
detects full open or closed throttle.  It is very reliable,
but sometimes must be adjusted (turned) to close
the closed throttle contact.  Throttle body shaft
bearings can be worn like a carb, and repaired the
same way.
_______________________

The MAP sensor is located in the ECU, fed by a vacuum
hose in the wiring harness.  If there is a leak anywhere,
the engine will run rich.  A short hose inside the ECU
can leak or come off.  Use a hand vacuum tester and
apply a vacuum where the hose attaches to the throttle
body.  Find and repair any leaks.
_______________________

The 78-79 distributor uses an Electronic Spark Selector
module (ESS) mounted in the engine compartment and
connected to a special module in the HEI distributor.  It
provides spark advance (high vacuum) or retard (for
starting) but is essentially independant of the EFI.

The 76-77 distributor is essentially standard except
for a modified curve and the added Speed Sensors.
________________________

Do your periodic maintainance.  Worn out distributor
bearings or weight pivots, bad ign wires, or bad vacuum
advance will severly affect operation.  All of these cars
are so old the timing chain and sprockets should be
replaced regardless of mileage.  The plastic teeth will
be falling off the cam sprocket if not already worn away.
________________________

The SPEED SENSOR is a pair of reed switches operated
by magnets, located in the lower part of the distributor.
There is a 3 wire connector to it.  If it is working, the
switches will alternate closing for a short time, one
switch each engine revolution.  With a meter on the
wires, the white wire will have about 9 volts.  The red
and the black wires  will each briefly rise to about 7 volts
in 2 engine revolutions (one distributor revolution).  If
this fails, the engine may start and barely run by pumping
the pedal hard.

If only one of the SPEED SENSOR contacts is not closing,
none of the injectors (either bank) will operate.  If only one
contact is shorted, one of the injector banks will operate.
If the SPEED SENSOR is unpluged, an ohm meter may
be used to check the contacts.  As the distributor rotates
slowly, there should be a brief contact closing (zero ohms)
alternating between the following pairs of wires
   white to red
   white to black
_______________________

The EGR passages on these systems often get plugged
up.  This will cause the engine to run lean.  I recommend
taking off the EGR valve and throttle body, and cleaning
them out.  If the extra diaphram unit fails, I set them up
with the simpler system used on a 79 Olds 350/403 EGR.
_______________________

The idle air valve is in the throttle body.  The assembly
may be removed by removing the electrical plug and
air cleaner stud, and turning the plastic block 1/4 turn.

The idle speed may fail to shut all the way down if the
idle air valve does not close all the way.  Usually this
can be fixed by use of an extra spacer (like a dime) of
some .040" to .080" thick in the center of the moving
part of the valve.  Power is supplied from the fuel pump
circuit, which will shut down after 10 seconds if the
engine is not running.  If power does not flow through
the circuit, idle speed will be very high.
________________________

The 2 thermistors measure air and coolant temp, and
are interchangable.  These should measure ballpark
1000 ohms if good; usually the failure mode is open
circuit.  An open will cause the engine to go to the hot
mixture level, no cold enrichment.  The coolant sensor
has much more effect on opperation.  So if you only
have one good one, put it in the coolant position to
get things running.
_______________________

I can test your ECU for postage.  Repairs a little more. 

Bruce Roe
5719 East Skinner,
Stillman Valley, IL    61084-9215
USA


D.J. Hall

Thanks Bruce, I needed some simple horse sense advice and will look at the car again this afternoon
D.J. Hall