Long Start Or Crank Time On Early C4


The Grumpy Grease Monkey mechanical engineer.
Staff member
My L98 became increasingly hard to start in the morning. It acted like it was starting in a lean condition. Once I got it going it would run fine for the rest of the day. Putting a fuel pressure gauge on it revealed that it only pumped up to 30 pounds when the key was turned to ignition. It started, but with great difficulty. After getting it running and switching it off, the pressure would bleed down relatively quickly and I thought it may be a check valve in fuel pump or fuel pressure regulator, but this was not the problem. I elected to install a new fuel sending unit, which includes the pump, pulsator, fuel tubing, and fuel level sending parts. Cost me 280 from Amazon for a Delphi unit. This has solved the cold start problem. I did a 8 minute video of the removal and install. On youtube if you want to take a look. Not the best quality, but it may be helpful to someone. The task was really not too difficult. Keeping trash from falling into the tank was the hardest part.


"quote"since I deleted my 9th injector":rolleyes:
  • thats a bit like saying I pounded a 6" spike through my big toe with a sledge hammer and now it hurts when I walk
gas as a liquid fuel ,does not burn well, GAS vapor does ignite easily, the hotter the engine is the more rapidly the liquid gas vaporizes thus making starting the engine more easily, the 9th injector is used to richen the F/A mix, to make starting far easier, later engine versions use a software change that lengthens the injector pulse duration to significantly richen the F/A mix its this longer duration in a cold engine that makes starting easier, so you either require the 9th injector or the more updated software that uses the engine temp software to make starting the cold engine easier

as always it helps to have a shop manual for your year and model car, then I'd suggest, you get out your multi meter and verify the alternator is putting out about 14 volts while the engine runs,pull the trouble codes with a code reader, then check the electrical grounds and fuses as a first step, in isolating the problems source

reading these links will be helpful
yes I know it will take some time and effort to isolate and test

but its the only 100% sure route to finding and fixing your problem,
don,t get over whelmed,
simply break the problem down to testing each basic sub system,
test each related sensor and electrical component and electrical sensor and connection.

some reading on the threads posted below, a bit of logic and deductive reasoning, and a multi meter and a shop manual will go a long way toward finding and fixing the problem.

Measured Value
Engine Coolant Temperature Sensor. 185 Ohms @ 210F, 3400 Ohms @ 68F, 7,500 Ohms @ 39 F.
Engine Oil Temperature Sensor. 185 Ohms @ 210 F, 3400 Ohms @ 68 F, 7,500 Ohms @39 F.
Oil Pressure Sender/Switch. 1 Ohms @ 0 PSI, 43 Ohms @ 30 PSI, 86 Ohms @ 60 PSI.
Fuel Quantity Sender. 0 Ohms @ Empty, 45 Ohms @ 1/2 Full, 90 Ohms @ Full.
MAT (Manifold Absolute Temperature Sensor). 185 Ohms @ 210 F, 3400 Ohms @ 70 F, 15,000 Ohms @ 40 F.
Outside Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
In Car Temp Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
MAF (Mass Air Flow) Sensor. .4 Volts @ idle, 5 Volts @ Full Throttle.
Oxygen (O2) Sensor. .1 Volt Lean Mixture, .9 Volt Rich Mixture.
TPS (Throttle Position Sensor). .54 Volts Idle, ~ 5 Volts Full Throttle.

Sensor Locations


Engine Coolant Temperature Sensor. Front of engine, below Throttle Body.
Engine Oil Temperature Sensor. Left rear of engine, just above the oil filter.
Oil Pressure Sender/Switch. Top, left hand rear of engine.
Fuel Quantity Sender. Top of fuel tank, beneath filler pipe escutcheon panel.
MAT (Manifold Absolute Temperature Sensor). Underside of manifold air plenum at rear.
Outside Temperature Sensor. Right side of engine, top right corner of radiator.
In Car Temp Temperature Sensor. Coupe: above left seat near interior courtesy light, Convertible: center of cargo compartment lid.
MAF (Mass Air Flow) Sensor. Front of engine ahead of throttle body.
Oxygen (O2) Sensor. Left side of engine, in exhaust pipe.
TPS (Throttle Position Sensor). Right side of throttle body at the front.



you always need a base line to start from, on a corvette.
a logical step by step approach and keeping accurate notes helps.

youll NEED a multi meter, a shop manual
and a timing light and fuel pressure gauge at a minimum,

set and verify your ignition timing, pull trouble codes,set your tps and iac,, then check for vacuum leaks on the lines and intake,then get out your multi meter and verify all the sensors, chances are good a logical step by step approach will lead you to the problem, youll be amazed at what youll learn reading links. use of a shop manual and multi meter can be very helpful

helpful links











if your early l98 corvette engine is set up to use that 9th injector to enrich the start up procedure,

and you removed it:oops:
without upgrading to the more recent EFI controller,
designed to run without that 9th injector...
you will have cold start issues unless the other injector flow rates compensate by flowing a bit richer mix.

both the MAF and plenum temp sensors could be partly responsible and Id certainly check the 02 sensors
having a shop manual and basic tools like a fuel pressure gauge and multi-meter will help

ID certainly verify your fuel pressure at start-up,(when you turn on the key but before you start the car is running in the 38-42 psi range and your fuel pressure regulator, and fuel pump are in good condition)

86 pacer....have you verified for closed loop operation? If you don't have a multi-meter, you can use the car's built in field test mode using a paper clip.

The same as pulling codes but you start the car this time when jumping the A&B terminals of the ALDL plug. Two flashes per second on the engine light means the car is in open loop. If the flashes drop to 1 time per second, the car is entering closed loop.

Vacuum leaks can introduce unmetered air entering the motor bypassing the MAF sensor, which throws off the mixture as the ECM tries to compensate.
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L98/ TPI 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.

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.


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.
youll find a hour or so reading thru the links and sub links, on this site in the threads, will provide a great wealth of related info and incite into related factors, or the function or testing of sensors, that you may not currently be thinking about, or things that you might not think that are related to your issue that PROBABLY ARE

without testing your simply guessing READ THE LINKS AND SUB LINKS AND THINK LOGICALLY... AND ASK













links to info that may help







https://www.hotrodhandbooks.com.au/eBooks/TPI/TPI On Line-03-1.html
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take the time to do the math,
I can't begin to tell you how many times I talk to guys at car shows or corvette clubs etc.
where the guy has swapped to an aftermarket cam, and better heads and now thinks his engines making 500 plus horsepower,
:rolleyes:o_O yet he has never upgraded the size of the injectors and has no clue what his current fuel pressure is, no clue what his fuel pump volume is,
and has no idea of the exhaust back pressure or exhaust or intake flow rates,
and has never checked his ignition advance or effective compression,
or even ever done a compression test.:rolleyes:
if your still running a TPI with stock injectors you can be damn sure your engines making far less than 500 hp no mater the heads or cam you used.

if you ignore reading the linked and sub linked info your skipping most of the related useful info












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removing the 1985 corvettes cold start 9th injector | Grumpys Performance Garage


if you don't have fuel pressure in the fuel rail check the fuel pressure regulator

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