LV_c4

86 c4 engine turns - won't start

tried to retrieve codes - no flash

dropped ecu -found wire harness for VATS but no module

engine idles with starter spray

dont want to waste money trying unnecessary replacement parts

go carb?

Any help.....ideas....thoughts? Thanks

-js-

You can find this info around the net using search engines.
Just search on "L98 start sequence"
Here is one below:

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 Schrader 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.

TWISTERUP

VATS does two things--starter enable (since the starter engages that portion of VATS is bypassed)-second thing is VATS module sends a reference signal to the ECM which in turn pulses the injectors. VATS can be programmed out of the ECM so the VATS signal is not needed.

I would plug a noid light into one of the injector harness connectors and see if it blinks while cranking. If the light does not blink the missing VATS module is likely your problem and reprogramming would be IMO your best option. If the light does blink VATS is not the problem and someone may have already had the ECM reprogrammed .

LV_c4

thanks for responding.....

noid doesn't light or flash

does anyone know good place to reprogram ecm to bypass vats module completely?

is it possible for the lack of a vats module causing diag failure for code retrivel?

Is it possible to jump the vats harness to allow ecm to pulse inj?

thanks agian for the replies

TWISTERUP

"Pcmforless" is one of many companies which can do the ECM reprogram. There are also online companies which make VATS bypass modules which generate the signal the ECM must see in order to pulse the injectors. I've also seen them on ebay, just be aware that you need the complete VATS module bypass-not just the "key pellet" bypass which would still use the OEM VATS module which you don't have. The signal is a 5 volt reference signal so --no you can't just "jump" the VATS harness. Use the search feature here there's a lot of info on VATS--there are vendors here also which do ECM/PCM reprogramming and likely some which sell the VATS module bypass you would need.

engle1147

LV_c4

Thank you all for your help and advice. The problem has been resolved.... I bought new performer manifold $60 A like new 600cfm $75 remanufactured hei w/vac $50 assorted gaskets & plumbing $50 and elect 5 psi pump old stock free .... total out of pocket $235.
Result - 8 hours labor and my trusty 86 came to life in a big way. Engine was smooth and strong. I know there are going to be some trade offs for the swap but my biggest gain is in knowing that I won't be at a loss for answers on how my engines doing. This vette turned out to be one sweet deal. Thanks again everyone