Hey guys I have a question, I have a 1986 TPI, she idles perfectly fine, and when I hammer down on her, she drives great, but when I am cruising, she seems to be misfiring like crazy. Any ideas?

 

Hello there jt7691,

I would start by replacing the standard tune up parts that wear out like the Cap, rotor and Plugs and if needed the Spark Plug wires. If you have a MAF sensor I would also clean it as well. They make a special cleaner to spray on MAF sensors and avoid damaging them. Another critical item to have is the Factory Service manuals for your year Corvette, they are so important to have. I could not have a Corvette without one of these sets of the FSM's

When you are cruising down the highway at first is the Corvette hitting on all eight cylinders? Is it smooth until you stomp on the gas pedal? Do you have or know someone with a OBD 1 scanner? The real time data is what I like to look at as it tells you a lot more about the engine and what is happening when.

There are three major components that control the fueling at speeds. One will be the Oxygen sensor, the second is the Coolant Temperature Sensor and the last but not least is your MAF sensor. When cruising down the highway your Corvette is using data from the big three (as I like to call them). Which one will cause the misfiring? The MAF will cause drive-ability issues but all the time and would be very bad if lair is leaking in between the throttle body and the MAF sensor. It is critical that NO air leaks in that section. The last item is the coolant temperature sensor which is critical as it tells the ECM what temperature the coolant is.

First, The Oxygen sensor on these OBD1 Corvettes were supposed to be replaced every 24 months or 48 k miles. They can cause your problems very easily if the Oxygen sensor is old. The older the O2's get they start slowing down their signal and the signal gets narrower. Either is bad for your engine and fueling. If your O2 is over 24 months old or you don't really know then replace the Oxygen sensor first.

I have found a lot of misfires going down the road can be aggravated by an old distributor cap and or bad spark plug and wires.. If it is old you might want to replace it and the rotor.

Take a multi meter and verify the resistance values of your eight injectors. They should all be consistent if they are still good. After shutdown How fast does the pressure drop? How far does it go?

Your EGR valve should be in action while driving your Corvette. Take a look at it and see if it moves when you apply vacuum with a hand vacuum pump. If the EGR does not move it could be the problem you are experiencing. There is a Vacuum Solenoid that operates the EGR and it is mounted on one of the thermostat bolts. Make sure it has it's power connected as well as the vacuum lines.

I get serious flak for suggesting a Fuel injection cleaner but I suggest a can of BG 44K Fuel System Cleaner might help you a lot cleaning up and smoothing out the idle. If you have a serious problem it will not resolve it for you, but for routine cleaning it is awesome. Whenever the V8 doesn't idle smoothly I drop a can of BG 44K into the fuel tank and it will smooth right out.

What kind of pressure is your fuel pump is making in your system? You measure it at the Shrader Valve on the passengers side fuel rail near the firewall. I ask this because a low pressure might make your Corvette act strange on the highway if it is not getting enough fuel. Verify the fuel pressure and let us know what it is.

Also before testing anything start with a fully charged battery as a weak battery just makes it harder to solve sometimes.

This should get us started so let us know any other information that is connected to this action.

 

What does it feel like? Slight bucking?

 

Need to know exactly what happens when it does this, how long after startup, what speeds youre cruising when you jump on it, etc.

Otherwise plugs, wires, cap/rotor, fuel filter, report back