So I've replaced my fuel pump, fuel filter, fuel pressure regulator and even got new BOSCH III injectors for my C4 just to clarify. I even replaced my ECT sensor and the ignition control module. New plugs, plug wires, starter, and battery. It runs great but I find that after driving for a while, shutting it off, and then turning back on within a short time, it struggles to start. Usually takes about 2-3 attempts of cranking to start, but it always eventually does. If I give it about 15 mins, it starts up normally even though it's still very warm. When it's cold, it starts up BEAUTIFULLY.

Anyone know any other areas I can check? I'm at a loss here as I think I've serviced every area that people have mentioned so far in the struggle to chase this down. Could it be something with the injectors still receiving a current and leaking into the cylinders? I know the injectors themselves are great because when the car is running it just hums but it could be after shutting off that they continue to dispense fuel? Any ideas would be greatly appreciated.

 

Let's try and diagnose this rather than guess or firing the part$ cannon any more times.

After a hot shutdown, pull the air filter off, and spritz the intake with carb cleaner or starting fluid (ether). If it starts up immediately, you've got a fueling problem. If the problem remains, it's an ignition problem.

If it's #1 drive it with the gas cap removed and see if the problem recurs (clogged tank vent). The pump pulling a vacuum the gas tank oil cans, but.... generally the problem goes away in 5-15 minutes. (my best guess)

It'll set a code / CEL for missing gas cap / large evap leak, clear it with scan tool or a 30 second battery disconnect.

Losing fuel pressure in the rail is GENERALLY caused by a drippy injector, but... it wouldn't matter 5 minutes or 30 minutes, it would ALWAYS long crank, hot OR cold.

If it's an ignition problem, oh boy, where to start. I'd want to hook up a scan tool and check coolant temp reading, a bunch of stuff looking for something out of range.

Perhaps put a meter or scope on one of the COP wires and confirm it's firing th eplug?

Any alarm or aftermarket remote start or stereo gear?

How long's this problem been going on. did this car ALWAYS do this?

Any misfires? Catalytic converter is intact, not removed or messed with.

 

I'll definitely go through troubleshooting with those methods, thanks a ton. The car has always been doing this as far as I know. But with the problem itself not occurring every time, it's difficult to gauge just how long it's been going on. Could've been longer than since I've replaced the aforementioned parts.

No alarm or remote start but there is an aftermarket stereo. Nothing too fancy, just a Pioneer receiver and some JBL speakers. No misfires and the whole exhaust system is new so the cat is fine.

You did mention: "If it's #1 drive it with the gas cap removed and see if the problem recurs (clogged tank vent). The pump pulling a vacuum the gas tank oil cans, but.... generally the problem goes away in 5-15 minutes. (my best guess)"

I was curious about what you meant with "The pump pulling a vacuum the gas tank oil cans"? I understand that the fuel system itself is pressurized I'm just confused.

Thanks again for all the help. I'll go down the list and get to trouble shooting.

 

The fuel system is pressurized but the tank is not.
In order to pump fuel OUT of the tank air has to flow in to replace it. Hence, the tank vent line, which bugs like to nest in.
Especially on seldom-driven cars.
If air coming in cannot replace the fuel going out fast enough problems result. It can even "oil can" the tank. (See video below)
A partially clogged tank vent line...give it 5, 10 minutes to "rest" and enough air bypasses the mud or spiders nests, or backflows into the
tank from the charcoal canister, etc, so the pump can work again.

You COULD put a fuel pressure gauge on there and see what's happening, but... taking the gas cap off is pretty quick and easy way to
rule tank venting as the cause of your problems in or out. GL.

Draining liquid out of a tanker car and not opening the vent: https://imgur.com/r/interestingasfuck/AbV0vIH

 

This is all such valuable information. I can't thank you enough. I'll take a look in the shop manual, but do you know where the vent line is located specifically? I'd like to take the car up on a lift and see if I can't fish some stuff out of there if that turns out to be the issue.

 

Even with just replacing parts you have an hard time hitting it on the head first guess. I totally agree with wadenelson about going through the simple process of "trouble shooting" Throwing parts gets expensive fast but it looks like you know that now. Early money should be spent getting the Factory Service Manual for your year Corvette, this is critical for long term ownership and Long term Happiness! The information in the FSM is priceless when you have to trace down a problem as they have GM's trouble shooting guides. Hard Copy's are available for about $85 and DVD's with the manuals on them for about $25 on Ebay and at RockAuto.com.

I would attach a Fuel Pressure Gauge to the fuel lines and see what is happening while the car shuts down and re-starts. The fuel pressure is critical to a "Properly Running" L98 engine and if it dips below you would want to know. You can borrow one from some auto parts stores. They cost about $35 at a Autozone and are handy to have with the dawn of EFI and other High pressure fuel systems.

You mentioned replacing the "ECT sensor". Where was the sensor located on your Corvette? How many wires were in the harness attached to the sensor?

The Coolant Temperature Sensor is used by the ECM as well as manifold air temp which are different and in different locations on the L98. I would read that sensors Resistance to get the approximate temperature it corresponds to. That number has to be fairly accurate or problems will and do occur. If the CTS thinks it is cooler than it is it could add fuel to starting it when warm, this could be a problem.

What is the voltage your battery is sitting at while the car sits? How old is the battery and are the lead posts and battery connectors nice and clean? Measure the voltage of your Battery using the battery ground first and then use the engine as a ground, the numbers should be the same.

Get one of the Infra Red Thermometers and read the output of each cylinder at approximately the same position on each cylinder at the exhaust manifold. You are looking to see if any of your cylinders is not functioning right and you would see a lower temperature on the weak cylinder. This is always a good test.

Have you set or adjusted either the TPS or the IAC? They control your idle and can be checked easily. This is where a scanner is the best tool you could have when you are trying to see what the engine is doing to find what is wrong at the same time.

How old is the oxygen sensor in your Corvette? I see you replaced a lot of parts but that should have been one of them. The Factory style, UN-heated Oxygen Sensors were designed to be used for 24 months or 48-50k Miles. The New OBD2 systems go 4 years or 100,000 miles before the oxygen sensors need to be replaced. This ONE sensor makes such a big difference if the old one was bad. Old O2's are going to slow down the signal and send a weaker signal to the ECM who makes thousands of computations per second to keep your Corvette running it's best. The voltage output of a Oxygen sensor will be between 0.1 and 0.9 going from rich to lean very, very quickly. It is hard for some multi-meters but not the more expensive ones. The average should be 0.45 Vdc or so. My personal feeling are "replace it" when it is older than 24 months, others have sensors that have worked for years and years.


When your engine sits for a few minutes and then restarts it starts in "Open Loop" as your O2 has to warm up to about 600* before it starts to produce a signal. In the "open loop" mode the engine looks at the Coolant Temperature Sensor and Manifold Air temperature and uses them to calculate the fuel ratio. Once the O2 is Warm the engine will switch to "Closed Loop" and then it is using the temperatures along with the A/F ratio from the Oxygen sensor. When you come to a stop light and the engine idles for three or four minutes it will go out of "Closed Loop" after the Oxygen sensor cools down and the engine goes to "Open Loop" seamlessly. When you stand on the gas pedal and hold it down you are in "Open Loop" while doing so. The engine would return to "Closed Loop" after getting the Oxygen sensor signal after it warms up again and you let off the gas pedal. Most people rarely can tell when their engine switches to and from "Closed Loop" or "Open Loop". Today's cars use a Heated Oxygen Sensor along with one before and one after the catalytic converter in many applications.

Hope that some of this helps!

 

I had the same problem many years ago with my 85. I did all of the same troubleshooting and part replacement. For me it was two things. Leaky injectors and a ruptured fuel pulsator which caused the pressure to bleed off quickly after shut down. Both are very common on an 85. If memory is correct, this little box is after the fuel strainer sock and before the fuel pump. Easy fix is to replace this thing with a short section of submersible fuel line. Cheap easy fix and I was back to holding pressure and much improved starts.

 

Seriously GREAT information! I cannot thank you enough. I appreciate this immensely and am going to working towards troubleshooting this very soon.