So I need some help. When I got my C4 it had some very minor black marks over the exhaust pipes and ran super rough. I changed out the injectors and everything immediately became smoother, but whenever I floored it, it would let out a huge cloud of black smoke. I eventually figured the injectors I got were too large (42lbs) and just recently swapped them out for these: https://www.ebay.com/itm/184152672935

Now the issue is that my vette shakes a bit when idling. It almost feels like it’s misfiring, but this goes away when I accelerate and it smoothens out again. Additionally, it’s letting out white smoke when at idle that smells slightly of fuel. This never happened with the older, larger injectors.

I’m lost, did I do something wrong? Could it even be the injectors at this point?

 

Check all the basics, check for vacuum leaks,(at injectors also )egr valve that everyone forgets. Ignition from plugs to distributor. Keep it simple . If you can scan see what block learn is, and read other sensors to make sure they are within parameters .

 

First I would like to Welcome you to the Corvette Forum rspownz! This is the place to be IF you have questions regarding a Corvette!

I agree with Highwayman9011 that you should go through the "standard stuff" before attacking the more complicated stuff. Start at the simple stuff and work your way up to the more complex parts. There is so much experience here that you should get your answers quickly and easily. Then YOU and only YOU can decide what you are going to do next.
We love to hear how the fix went so we can all learn a bit more, please follow up with what fixed your problems on your C4. To reduce the number of times you get asked the same questions we ask that you fill out your Profile on this site.

Second, do you have a set of the Factory Service Manuals for your particular year Corvette? You will NEED One to own a C4 and keep it on the road. They are available in print for about $85 from Helm Publishing and roughly $35 from RockAuto for a DVD copy of the manuals. They really are important to have when working on a C4.

I would start by checking the Fuel Pressure Regulator as it must have been adjusted or modified to use the 42 lb injectors. Pull the vacuum hose connected to it and see if there is the smell of raw gasoline which would indicate a failed diaphragm inside the FPR. I would check the vacuum like suggested to be sure that it is is still holding a vacuum and the reading is nice and smooth. The EGR uses vacuum to control the EGR Valve and it is controlled by a small Vacuum Solenoid attached to the thermostat housing. If you pull a vacuum on the hose leading to the EGR Valve the idle will change a bit indicating that it is moving and working. Your Corvette might have an aftermarket Adjustable Fuel pressure Regulator and that would need to be re-adjusted to get the right fuel pressure at the fuel rails on the Engine. If it is an adjustable one then you might get away just -re-adjusting it to a set number close to what the factory FPR used. Get a fuel pressure tester with a long hose and attach it to the fuel rail and watch the pressures it delivers at the idle and while driving.

If the Oxygen sensor is older than 24 months or 50 k miles then I would consider replacing it. Older OBD1 systems use an unheated oxygen sensor which cost more than the early style. The early O2's were expected to last 2 years on the Corvettes like they were on other new OBD1 Cars. They are cheap and make a big difference.

The Coolant Temperature Sensor is another biggie in the system. If the CTS fails and tells the car it is below freezing the engine will start up with the Cold Start Enrichment activated and you will have black smoke all the time. This mode would make a warm start tough. When it fails thinking it is too hot then it will be hard to start when cold and it will run the engine a bit leaner due to the high engine temperature it thinks it sees. Either way the CTS is important to be accurate all the time. The CTS uses a resistance value to represent a temperature range. It is not hard to test but you will need the page from the FSM manual that shows the resistance values and their respective temperatures.

Adjust the throttle body Throttle Position Sensor and be sure the Idle Air Controller is working like it should. The TPS needs to be set to a precise number for the engine to act normally. You might want to check your running engine at night-time when it is dark and see if any of the spark plug wires are arcing to ground under the hood. You won't see the arcing during the day. Depending on how old your Distributor cap and Rotor check them carefully. They can develop Carbon Tracking inside the Distributor cap which causes a regular miss. It never hurts to put a new distributor cap and rotor on the Car every so often.

I am not so sure that buying Fuel Injectors on Amazon or EBAY is a good idea. I buy mine from a place called South Bay Injector and I get good working "matched sets" of injectors when I buy them from South Bay. Just because they are "New to You" doesn't mean that they work unfortunately.

If your engine was running that rich it might have plugged the catalytic converter. White smoke is okay at startup as it is water condensation but if the white smoke continues past startup then you might have a coolant issue getting past the head gaskets likely.

I have written this not knowing what year Corvette you have. My experience is with the 1988 model C4 as I have had one for almost 30 years. If yours is a newer C4 with the LT1 engine then you need someone with LT1 Experience specifically as they are a unique bird. Most of what I have covered will apply to the 86-91 C4's specifically.

Good Luck and let is know how it goes!

 

Those injectors seem a bit sketchy to me, unbranded and low priced can lead to unequal performance between each injector causing issues. Since you can do the work yourself always go with a name brand IMO.