Hello everybody, in spite of my mechanics efforts my 1985 4 plus 3 is still running rich. When I bought the cam I ordered a PROM. I am wondering if the PROM can be the problem. Can the PROM make it run rich. Does anyone know a reliable person I can contact to program a new PROM. Thank you

 

Program or prom usually needs some tweaking after cam change. Tuner may want datalogs to revise tune. You should contact your tuner or PCM of NC is popular.

 

There are many parts on the Corvette that can cause fueling issues like the ones you are enjoying. Are you sure it is related to the Cam and Prom?
Yes, a re-programmed prom can make the engine run richer, whether your new prom is causing it or not, IS in question.

How did it run before? Was the Corvette performing okay before the work was done? Was it running a tad rich back then as well or is this problem new?

How old is the oxygen sensor? An old O2 can make the make the air/fuel mixture incorrect and make it run richer. They were designed to last 24 months or 50k miles and should be replaced on this schedule on OBD1 vehicles. If you have OBD2 then go to five years or 100k miles.
As a O2 sensor starts getting old it starts to lag behind the requirements of the ECM. The width of the signal coming from the O2 is between .1 and .9 and it oscillates very quickly in operation. Older O2's tend to start getting narrower signals like .2 to .8 and so on. If your O2 is older than 24 months replace it, the difference will affect your performance and the economy of the Corvette! Whenever the O2 acts up it hurts the performance and makes the mileage go down rather quickly.
There are so many people still running old O2's it is amazing, the problem is people assume they work until they stop working and that is more rare now days. The last time I had an O2 Fail while driving was in 1990. When it fails completely the Corvette will go into "Limp Mode" and that will ruin your day or trip when it happens. IF it is over 2 years old replace it...

Is the CTS indicating the proper temperatures? A faulty Coolant temperature sensor can make the car think it is colder needing extra fuel enrichment. It is easy to see with a scanner or you can read it's resistance and cross-reference the chart in the FSM.

Any other changes to the Corvette?

Best regards,
Chris

 

Hi and thank you for the reply. I do not know how the car ran before because I bought it as a project. And it is. Good thought on the CTS. I kniw my 84 actually got one mile per gallon when the CTS was no good. All of the sensors O2 , MAF, MAT , TPS , CTS, Ignition Control Module are new. Scanner? Does the car need to be running or just key on. Either way I will ask my mechanic if he can scan the car.

thank you again

 

Hello again jseremba!

Who put the new O2 on the car? Unless you did it or had it done assume that it could be older than you thought. Out of 4 I bought a few months ago only 3 were good and that was for the Bosch brand. Don't assume just because it is new that it works properly. I am also been lucky enough to buy a bad MAP sensor from a reputable auto store. Were all the sensors installed by you or did it come with them pre-installed? Was the camshaft already installed?

On your Corvette I might try using the old prom again and get the car running the way it was supposed to. Even if you have a non-stock camshaft installed. This would help you understand if it is an Engine problem or a programming problem.

You might want to pull the injectors and look for any leakage. How fast does the pressure drop after shutting the engine down? You can clamp off the return line to eliminate that from allowing the pressure to drop. On my C4 the pressure would drop very quickly and it turned out to be a very leaky injector that been flowing way too much fuel. I replaced my injectors and now the pressure stays up there for hours and hours after shutdown.

I replaced my FPR diaphragm while replacing my EGR and de-carbonizing my EGR passages. My EGR failed and totally plugged up my vacuum passages. It took more time to clean the damage up than it did to replace the parts. The EGR is another important part that can affect the engine if it is not functional. Without a functional EGR the engine will run hotter and have detonation issues requiring 93 octane gasoline in some of the worse situations. It appears that my EGR Vacuum solenoid had failed and then the EGR stopped working and plugged the engine up. Nasty mess to deal with.

Have you pulled the vacuum line off the Fuel Pressure regulator and smelled inside of it? If you smell gas that could be the problem, when the FPR's go bad they like to go rich. FPR's LOVE to fail so don't be surprised, the newer diaphragms are designed to do better with the Ethanol they put in the gas today. I replaced the diaphragm on my FPR with a stock replacement and it was not too hard to do. While replacing the EGR components I removed the top of the intake system to make it easier to work on. I also found that my Cold Start Injector (a.k.a.9th injector) was not functional so I replaced it as well. Removing the parts gave me the time to sand blast them and re-paint the intake system and replace a lot of parts in the process.

Depending on the Scanner you usually have the key in the "Run" position with the engine off. Others like the Snap On MT 2500 are fine with the engine running as it will show you a continuous flow of data.

If you are going to buy and fix older Corvettes you should get yourself a good scanner. The best options are using something like Tuner-pro and a Moates Connection system if you want to see "everything" working in live time. The lesser expensive options are the Snap On MT2500 and other older scanners made for OBD1. The Tuner-Pro/Moates combination will likely be my next investment. The Snap On MT 2500 is very handy for most stuff and I am still following the software module for the Early GM Bosch ABS equipment. This software was designed to assist in maintaining the ABS that we were blessed with. It also helps bleeding the ABS unit I have been told.

The CTS failures are more common than I would like. There are people who have starting problems due to bad CTS'. When they are reading colder that the actual temperature then it will add fuel, when Hot it will reduce the amount of fuel. I have seen them fail in both directions. Frequently with just "starting" or poor "mileage" issues.

Good Luck and let us know what you find.

Best regards,
Chris

Also inspect the wires supplying power to the injectors. The computer supplies a ground signal to energize (Pull them to ground") the injectors as the 12 volts DC is at the injector all the time while the engine is running or the key is set to Start or Run. I have seen dry brittle insulation on some old Corvettes and it would be possible for a Corvette to have a injector short to ground causing the injector to squirt fuel out of the ordinary pattern. This would cause a rich condition. It is work a peek!

 

First of all. This is a great post. Thank you

I purchased and installed all of the sensors and control devices.
i installed the cam shaft.
The old PROM was damaged. I need to get a new one. I found one on eBay. But, how do I know it is working. I do have the burner. So with the correct files I could just reset the chip.
Injectors are new from Bay Shore Fuel Injector, Rhode Island, for the 1985
The FPR is new and adjustable. The fuel line retain pressure for at least ten minutes when I shut the fuel pump
I have TunePro RT on y laptop. But, I forgot which files I need to run it. Besides I think it will only work with the car running. The other scanner is an idea. Will it tell me if the ECM is seeing the CTS.
I did the voltage check on the injector wires. Good idea

Thank you

 

This is an update the situation with the 1985 running rich. I checked the voltage on all the wires going into the ECM. My mechanic pulled the connector and found that one of the pins in the ECM connector was bent/broken. We also noticed that the re-manufactured ECM was missing a chip. I was already in contact with Scott Hansen. He graciously offered me an ECM with the proper chips installed, already in the ECM, for a reasonable price. Fingers crossed, please.

And as always that you all for your help.