I have a 1986 that starts, idles, runs, and accelerates fine. Problem is when i accelerate hard the motor bogs down until i let off of throttle. Light acceleration is fine. Sitting in driveway and rev it up hard it will bog down also. I changed the fuel filter. This started after it sat for about 4 months.
Any ideas on where to look?

 

Put a fuel pressure meter on it to see what happens you stomp on the throttle.

 

Also sounds like a knock sensor retarding the timing, but i would check out the fuel pump first.
could also be a bad coil or dist. Module, but module failure is rare unless there is no paste under that module.
paste is a temperature transmitter.
it helps wick away the heat to keep the module from overheating and turning off till it cools down a bit.
if it gets hot enough, it can fail.

 

Check fuel pressure, check for knock counts.

Possibly plugged up cats too.

 

Change plugs, wires, cap, rotor, new fuel.

 

Any change at different temperatures? Been having similar issues that appear to be temp-related.

 

Look for clogged intake or exhaust. Rodents ?

 

I let a Car sit for about the same amount of time and when I tried to start it up the engine would start fine and idle but would not rev past ~2500rpm. I was able to identify that the catalytic was the problem by removing the O2 sensor and letting it breath a bit more air. Even without the O2 it was still barely able to go 25 mph. Sitting unused for a few months was all it took for my catalytic converter to get sealed up internally.

Pull your Oxygen sensor and start it up, normally there should be ~5 psi of exhaust blowing out with a open exhaust. If the engine runs better without the O2 in place that would point to the exhaust or more likely the catalytic.

Get an infrared Thermometer and verify the temperatures are all about the same, a bad catalytic will show up hotter than normal. Go through and check each cylinder's exhaust port temperature while you are at it.

I would also spray down the MAF sensor's tiny wires with some MAF sensor cleaner to be sure that the MAF is not aggravating the situation. The MAF is responsible for measuring the volume of air being ingested by the engine, if it is off even a little it can adversely affect your drive-ability. This is why it is important to make sure that the Burn-off relay is still doing it's job. In the dark the little MAF Sensor wires glow yellow while they cleanse themselves. If the burn off function is un-able to do it's job then you might start to have MAF issues.

You should probably check your engine's fuel pressure as well to be sure that it is not a factor. With the Ethanol in the gasoline the rubber diaphragms are susceptible to breaking down. Attach a Fuel pressure Gauge and tape it to the windshield so you can see it while on the road. Drive the Corvette and document what happens as far as fuel pressure goes. We want to be sure that your Fuel pressure regulator is still working correctly.

The suggestion to check your Knock Counts is a very good one. My knock sensor went bad and my engine felt like a four cylinder under the hood. Easy sensor to test, with the engine running simply tap the cylinder head or block with a metal wrench or ratchet and the idle speed will stumble a bit.

Last but not least is the oxygen sensor. Do you have any way of testing it? They are supposed to oscillate between .1 and .9 Vdc very, very quickly and the average should be ~ .450 Vdc. As O2's start to fail their signal is not as wide and nor is it fast enough for the ECM and a Moving Corvette. If by any chance you have an old O2 this would be a good time to change it. They were designed to be replaced every 24 months or 50 k miles. This one part is critical to a well running Corvette. It will improve your economy and performance compared to the old O2 sensor.

 

Thanks for the suggestions. Long story short is I saw something under the intake and turned out to be a mouse nest, I cleaned it out and when my back allows i will look for eaten wires and or vacumn hoses in the foward part under the intake. Also just for info, i have straight pipes, no cats, and no o2 sensor that i know of.
Thanks again. One more question is that do you think vacumm leak could be my problem ?

 

Opps. I was wrong. o2 sensor was replaced last year.

 

Could absolutely be a vacuum leak.

 

My first guess is the fuel pressure. Check that under heavy load and it should read 42 PSI. If that is OK, start with the basic's as listed above.

 

Checked fuel pressure today. 42 key on 46 start motor 46- 48 rev up motor shut off motor goes back to 42

 

You need to check it under load not a free rev..

 

I went for ride today with fuel pressure gage where i could see it. key on 42 start 46 driving 45 pushed accel down 46 and motor bogging down, steady accel to 80 46 no bogging down No codes,
I have put in air filter, fuel filter, injector cleaner.
Motor starts, idles, and runs, fine until you push hard on acelerator like you want to pass someone. Then it will act like it is running out of fuel.
Any ideas?

 

The Mass Air Flow Sensor could possibly be causing you grief as they carefully measure "Very precisely" the volume of airflow going into the combustion chamber. ANY leakage will mess up the drive-ability.

I bought a Test harness that plugged in between the MAF and the wiring harness and it let me see the MAF working. The kit I bought was from Mid-America and has several setup or test harness' that allow you to set up the emissions system. They are very handy and can be built by yourself very easily with a few parts. The harness' all use a digital Volt Ohm meter to see what the setting is or what the sensor is doing. The kit included Harness' for testing the O2, MAF, CTS, IAC, IAT, Knock Sensor.and Cold Start Injector systems. When you have these it takes no time to get a Early C4 L98 Corvette back up and running properly.

If you have a Brand New MAF and OLD control relays then your troubles can come right back. The "Burn Off" Function is really important and has to happen. If it doesn't happen then the junk that builds up on the tiny wires does not get cleaned off in the "Burn Off' Function. If the "Burn Off" function is not happening the MAF is not going to be as accurate as it needs to be and can have a limited lifespan.

Have you verified the Throttle Position Sensor as you rev it up from idle is working correctly? It has a very specific setting and should be tested every so often. Sometimes dead spots appear in the rev range and the TPS needs to be reset or replaced.

When the bogging occurs is the throttle "wide open" or is at partial throttle? When you push the throttle "wide open" the ECM shifts back to "Open Loop" and that reduces the items being used to calculate the air/fuel ratio. It sounds like it is happening at lower the Wide open throttle conditions.

You might try checking the throttle shafts for vacuum leaks, someplace air must be getting in between the throttle body and the MAF. Why it would work and allow the car to accelerate moderately but not under Higher acceleration rates. Something is being overwhelmed when it gets asked for harder acceleration rates.

If it is not something to do with the air going in then I again suggest checking the Catalytic Converters for a partial plug. You initial description sounds more like a Cat issue if nothing is glaring on the intake side shows up. Catalytic Converter or a bad TPS could easily be culprits, you need to verify they are both working properly.

Be sure to let us know what you find as you get this Corvette back on the road!

 

Got it!!!!!!! Mass Air Flow Sensor. found a local corvette mechanic, he suggested same.
Unpluged maf sensor and started car and went for a test. Problem gone..
Waiting for new one to arrive..
Thanks to everyone for suggestions and support.
Al