L98 Won't Start
09-03-2011, 07:25 PM
#1
Instructor
Thread Starter
L98 Won't Start
After replacing head gasket and reassembling upper motor, I am unable to ge my motor to start. It appears that I am not getting fuel to the intake. Timing seems to be OK, I sprayed starting fluid into the throttle body and it tried to start. I pulled the plugs and they were dry. What could the problem be. I have fuel going to the fuel rail so I guess the injectors are not spraying. Any suggestions?
09-04-2011, 04:34 AM
#2
After replacing head gasket and reassembling upper motor, I am unable to ge my motor to start. It appears that I am not getting fuel to the intake. Timing seems to be OK, I sprayed starting fluid into the throttle body and it tried to start. I pulled the plugs and they were dry. What could the problem be. I have fuel going to the fuel rail so I guess the injectors are not spraying. Any suggestions?
Last edited by Joe C; 09-04-2011 at 04:58 AM.
09-04-2011, 07:37 AM
#3
Instructor
Thread Starter
Yes it tried to start when I sprayed the starting fluid. I will check to see if the injectors are getting voltage. If I am getting voltage and it's pulsing, what else can I check?
09-04-2011, 08:14 AM
#5
Instructor
Thread Starter
Yes, my engine is '86 so it has the cold start injector, and it is connected. Is there any switches, sensors, etc that will prevent the injectors from spraying.
09-04-2011, 08:28 AM
#7
what is your fuel pressure? initally, when you first turn the ignition to the on position, it should jump to about 46psi, then after a couple seconds drop to about 38psi, and hold fairly steady. having fuel at the rail doesn't mean you have fuel pressure.
Last edited by Joe C; 09-04-2011 at 08:32 AM.
09-04-2011, 08:31 AM
#8
Drifting
Member Since: Nov 2009
Location: Perth Western Australia
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St. Jude Donor '10
L-98 Engine Start Sequence
When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.
Fuel Rail Pressurization:
When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails.
There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure.
The reading will go to 38-40 pounds nominal once the engine is running.test by attaching a fuel pressure gauge to the fuel rail at the shrader valve, on TPI and LT1 engines its located on the pass side fuel rail.
Initial Crank Action:
If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.
Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run.
(Note that you should have a black oil pressure switch/sender.units
It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad GM parts that stayed in the pipeline for some time).
The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter.
These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.
ECM Reaction:
(In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).
If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. If a cold start injector is installed then that will be activated also for a few seconds on startup.
The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to
start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).
The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54
volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts
during start and at idle from the TPS is very important to both starting and run performance.)
Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.
Engine "Catches":
When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how
much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms
nominally so as to attain a proper air/fuel mixture to ensure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).
The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.
If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be misadjusted. Alternatively, there may be a leak in the intake manifold or another
vacuum leak may be present. Listen for hissing sounds---there should be none.
ECM Mode:
The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.
Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an
intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning
the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly
strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.
Note that prolonged idling can force the computer back into open loop mode.
Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold
Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air
Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the
desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.
Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that
MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the
calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into
the sequence the same place that the MAF does.
Summary:
If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.
When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.
Fuel Rail Pressurization:
When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails.
There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure.
The reading will go to 38-40 pounds nominal once the engine is running.test by attaching a fuel pressure gauge to the fuel rail at the shrader valve, on TPI and LT1 engines its located on the pass side fuel rail.
Initial Crank Action:
If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.
Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run.
(Note that you should have a black oil pressure switch/sender.units
It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad GM parts that stayed in the pipeline for some time).
The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter.
These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.
ECM Reaction:
(In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).
If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. If a cold start injector is installed then that will be activated also for a few seconds on startup.
The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to
start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).
The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54
volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts
during start and at idle from the TPS is very important to both starting and run performance.)
Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.
Engine "Catches":
When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how
much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms
nominally so as to attain a proper air/fuel mixture to ensure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).
The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.
If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be misadjusted. Alternatively, there may be a leak in the intake manifold or another
vacuum leak may be present. Listen for hissing sounds---there should be none.
ECM Mode:
The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.
Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an
intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning
the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly
strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.
Note that prolonged idling can force the computer back into open loop mode.
Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold
Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air
Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the
desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.
Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that
MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the
calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into
the sequence the same place that the MAF does.
Summary:
If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.
Last edited by Lemme; 09-04-2011 at 09:10 AM.
09-04-2011, 08:34 AM
#9
good info Lemme - 
09-04-2011, 08:35 AM
#12
09-04-2011, 08:43 AM
#13
09-04-2011, 09:25 AM
#15
Instructor
Thread Starter
I just checked itand it seems to be connected if I'm looking at the correct switch. It is behnd the distibutor to the right(driver side). I did see another device that is not connected, it is a round black device with white wires that sits right at the back of the vavle cover on the driver side. What is this and does it need to be connected? I see one connector that it seems to connect to down by the oil pressure switch but I don't see where the other side should be connected.
09-04-2011, 09:26 AM
#16
Instructor
Thread Starter
I'm at the car now so anything you can suggest I will do it right away and let you know or if someone would like to call me I can provide a phone number. Thanks.
09-04-2011, 09:53 AM
#18
Burning Brakes
I just checked itand it seems to be connected if I'm looking at the correct switch. It is behnd the distibutor to the right(driver side). I did see another device that is not connected, it is a round black device with white wires that sits right at the back of the vavle cover on the driver side. What is this and does it need to be connected? I see one connector that it seems to connect to down by the oil pressure switch but I don't see where the other side should be connected.
not sure about early L98's but on mine the oil switch is BACKUP only in case the FP relay fails. I read the info about this switch but it is not correct, has been mentioned a number of times. ECM does not require 4 psi oil pressure to fire injectors. early models, it may have delayed for a couple seconds so that there was oil.
all components should be connected, yes.
09-04-2011, 10:03 AM
#19
Burning Brakes
if the tack works and shows RPM, tach wire should be working. if I read the fsm correctly.
09-04-2011, 10:07 AM
#20
Drifting
Member Since: Nov 2009
Location: Perth Western Australia
Posts: 1,340
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Received 2 Likes
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St. Jude Donor '10
I just checked itand it seems to be connected if I'm looking at the correct switch. It is behnd the distibutor to the right(driver side). I did see another device that is not connected, it is a round black device with white wires that sits right at the back of the vavle cover on the driver side. What is this and does it need to be connected? I see one connector that it seems to connect to down by the oil pressure switch but I don't see where the other side should be connected.