lt1 dies after a few seconds

[homerneedspeed]

i have a problem with a lt1, it will start and die after a few seconds of running. we drove it to the gas station and it ran fine but died and will not start after filling up, the car is full of gas, the fuel pump is running, all electrical connectors are connected, fuses are good and it has electricity, it just won't run for more than a few seconds. it's raining and dark so i'm looking for common problems or advice from people that have had similar situations.

[ngrimm]

Fuel filter clogged? Perhaps unhook the hose and see if gas sprays into a can when the pump is running. Also have you checked for spark at the coil and then at the plugs? Good luck. Norm

[that240guy]

i would start looking at the optispark distributor if it failed while driving in the rain. The module usually goes bad at the same time as the opti. you can tell if the module is bad by looking for hot or melted spots on it. It is also possible that your VATS module or VATS eliminator has gone bad unless you have eliminated it through tuning. get a scanner on there and start looking for clues. Optisparks are notorious problems in the wet though

[LS1 240Z]

press the scrader valve on the fuel rail to see if it has pressure at the rails.. (little screw off cap on the fuel rail)

[jbc3]

I second the VATS diagnosis.

 

Jody

[Pop N Wood]

I know the time the air snorkle leading to my MAF became disconnected the engine did exactly that. The computer typically ignores a lot of the sensors during start up.

 

If the car use to run and suddenly doesn't , then I would not suspect the VATS.

[grumpyvette]

you may want to read this (its the same for the L98/LT1) AND THE INFO IS FROM MY CORVETTE MANUALS and SEVERAL CORVETTE SITES, WHILE IT MAY OR MAY NOT BE YOUR PROBLEM IT APPEARS TO BE SUSPECT

 

 

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.

 

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. 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 units that stayed in the GM parts 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:

 

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. (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).

 

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 insure 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.

 

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.

 

 

Most of the C4 Corvettes used a MAF (Mass Air Flow) sensor to determine how much air is being pulled into the intake manifold. The exceptions are the 1984 Corvette that used a speed density system--a sort of predictive method of measurement---and the 1990 through 1993 C4 models which were also speed density based. In 1994, Corvette went back to the MAF based system but used the speed density approach as a back up. (1989 Bosch MAF installation shown at right).

 

A Mass Air Flow sensor has an extremely fine wire inside its bore. The 1985 through 1989 C4 engines used a Bosch MAF sensor that heated the wire to 100 C. The 1994 and later C4 models used a AC/Delco MAF that heated the wire to 200 C. The amount of current required to reach the temperature is measured in each case. (Note: the LT-5 engine used in the ZR-1 used a speed density system and continued to use that system in 1994 and 1995 since the engines had already been made prior to the last two years of production. The ZR-1 therefore has no MAF even after Corvette went back to the MAF based system).

 

Theory of Operation

 

As the air travels past the heated wire enroute to the intake manifold, it will cool the wire and additional current is added to again heat the wire to the design temperature. Since the amount of air moving past the sensor is directly related to the amount of cooling experienced by the heated wire, a feedback condition is established whereby the exact amount of moving air is directly related to the amount of current passing through the wire and the intake air is therefore precisely measured.

 

Once the amount of air is known, the computer controlling the engine can add or subtract fuel as required to maintain the magic 14.7:1 air-fuel mixture resulting in the cleanest burn possible from an emissions (pollution) standpoint.

 

It does this by varying the "on time" of the fuel injectors. The injectors are pulsed on and off and the width of the pulse is lengthened or shortened as required. When you first start a typical engine, the pulse width is around 4 milliseconds but as soon as the engine "catches" the pulse width is shortened to about 2.2 milliseconds for idle. During operation, the measured air flow through the MAF will cause the computer to increase or decrease the pulse width as explained above.

 

MAF Operating Conditions

 

The Bosch MAF is more complex than the AC/Delco version. Both measure the air flow but the Bosch MAF has a circuit called the 'burn-off circuit' that cycles on for about 2 seconds when you shut the engine down. This circuit heats the wire to a high enough temperature to burn off any residue that may have collected on the wire during operation. If you are in a quiet area, you can hear the relays click on and then off on a 1985-1989 C4 as the burn-off cycle occurs.

 

There are two relays involved with the Bosch MAF: A power relay that passes current to the MAF wire during normal operation and the burn-off relay that provides the current for the cleaning cycle. Both are located on the firewall in the engine compartment, just behind the battery on the drivers side. Bad MAF power and burn-off relays can cause hard starting problems and should be changed periodically as preventative measure and any time you experience hard starting conditions.

 

The AC/Delco MAF has a power relay but no burn-off relay. For this reason, you should pay even closer attention to the condition of your air filter on a later model C4 than normal since a contaminated wire in a AC/Delco MAF is going to stay contaminated for the most part and cause false signals to be passed to the computer.

 

Also, the Bosch MAF outputs its information as a analog signal to the computer but the AC/Delco sends its signal as a digital component of varying frequency. For this reason, you cannot measure it's operation directly.

 

A scan tool is generally the best way to troubleshoot engine problems and with the 1994 and later Corvette, it is virtually mandatory. (An oscilloscope will also work on the AC/Delco MAF but a regular test meter will not).

 

MAF Problems

 

Faulty MAF sensors will normally light the check engine light on the drivers information center if the problem is constant and store a trouble code. If intermittent, a trouble code will still be stored as long as the battery is not disconnected.

 

Normally, the problem is a poor connection at the sensor and wiggling the wires, unplugging and reinserting the connector will often cure the problem.

 

A faulty MAF will normally cause a no start or difficult start condition and although you can eventually get the car into the "limp-home" mode in most cases, you need to attend to the problem ASAP.

 

AC/Delco sensors can become intermittent or give false readings if the wires become contaminated as explained above.

 

The MAF is a critical part of the emission control system and as such will cause the computer to react to problems very quickly, setting trouble codes and reducing performance in ways that cannot be ignored for long.

 

MAF Mods

 

The Bosch MAF is often modified by removing the two screens that are present in the front and rear of the cylinder. Removing these screens significantly increases the air flow through them and this results in more horsepower. Removing the screens is an old trick from the Corvette Challenge days in 1988 and 1989. It does work but is illegal in many states so be advised not to do anything that will get you arrested for a pollution violation.

 

The AC/Delco MAF is not readily modified. It is what it is but since it is a larger diameter than the Bosch, it responds well to changing the air filter to a free flowing type such as the K&N filter.

Welcome to C4 vette codes it is very ....repeat very

important that if you are not savvy of working on your

vette ...you would be better off - taking your car to a

dealership for repairs on your trouble codes.

However if you feel that you want to dive right in ..than you

have come to the right place.First locate your car's alcl

this component is located just below the instrument panel and

to the left of the center console. Remove the plastic cover

the first two slots to your right are the A & B slots for a drawing of

the alcl module's picture is added below.

The A slot is the diagnostic slot and the B slot is the ground

slot. insert the computer key into these slots (with the engine

off) this is very important...now only put the ignition key

to on ( not start !!!) the check engine light will display a

code 12 which is one flash followed by two flashes.

this code will be flashed three times ..followed by the

trouble code stored in your car's computer.

what ever the code is it will be flashed three times.

have a paper and pencil ready and write down the

code .

 

code 13 =1 flash followed by 3 flashes =>oxygen sensor

code 14 =1 flash followed by 4 flashes =>coolant sensor

code 15 =1 flash followed by 5 flashes =>coolant sensor

code 21 = 2 flashes followed by 1 flash =>throttle position sensor

code 22 = 2 flashes followed by 2 flashes=> throttle position sensor

code 23 = 2 flashes followed by 3 flashes=> manifold air temp sensor

code 24 = 2 flashes followed by 4 flashes=> vehicle speed sensor

code 25 = 2 flashes followed by 5 flashes=> manifold air temp sensor

code 32 =>egr system

code 33 =>map sensor

code 34 =>maf sensor

code 35 => idle air control

code 41 => cylinder select error

code 42 => electronic spark control

code 43 => electronic spark control

code 44 => lean exhaust

code 45 => rich exhaust

code 51 => PROM

code 52 => fuel calpak

code 53 => system over voltage

code 54 => fuel pump circuit

code 55 => ecm

code 62 => oil temp

please remember that if you have the computer key installed

in the alcl and you start the engine ( you will ruin the engine's computer

)

only put the ignition to on (not to start)

If you should get a check engine soon display.. you can use

the above procedure and codes to buy the right part

or at the very least to keep from getting taken for a ride

and be made to pay hight prices for some inexpensive

module that you could have installed yourself.

You never ask a barber if you need a haircut ..

so you have to be on guard they will see you comming

a mile away.

If your engine displays a trouble code ... your engine will

go into limp mode ..it will still run but very poorly.

you might be able to reset the computer if it will not start

( just to get home ) by disconnecting both battery cables

and re-installing them ...this is not recommended ..but if

you are stranded it might help unitl you get your car home

or to a repair shop..good luck

[homerneedspeed]

ok, i've got an update. we replaced the maf and tps, neither helped. it is getting fuel pressure, allt eh ignition is new and functioning. the car is now rinning for a few seconds at 1500 rpm then stumbling to it's death.

[Tim240z]

I will bet it's a vacuum leak after the MAF. Unplug the MAF (leave it in place, just pull the plug) and start it.....it will default to speed density and take the vacuum leak into account and run fine. If that works, then find the leak, fix it and plug the MAF back in.

[homerneedspeed]

when i unplugged the maf it did not start at all.

[homerneedspeed]

no fuel pressure was the culprit. the previous owner installed the fuel pump pick up on the drain plug at the bottom of the tank, when we drove it the rust from the tank slowly clogged the filter before the pump rendering it useless.