Started resurrecting a project '81 last year. Rebuilt the fuel system, with fresh rebuilt carb, brakes, exhaust, and odds and ends. Got the car running but it would often die. ECM and Prom needed to be replaced and after doing so it ran much better. However, after reaching full operating temperature it dies completely as if it is running way too rich but will generally restart after sitting. The longer it sits the easier it is to restart and the longer it will keep running.

The old ECM would not flash hence the replacement. It has an OEM Prom that currently flashes 44 which indicates vacuum issue and I have confirmed the base of the carburetor is leaking badly. I will replace the carb base gasket in the next couple of days.

I am curious if the ECM is making the carb run too rich because of the vacuum issue and if any of you have had similar experiences?

It's difficult to test drive the car as my wife is always with me (I'm her caregiver) and most of our local roads have very few places to pull over if your car dies.

Gordon

 

Gordon,

To answer your question ...YES. A vacuum leak can throw off readings in the O2 sensor and make the Mixture Control Solenoid come into play.

I have MANY QUESTIONS:

1.) When the engine dies....are you coming to a stop QUICKLY...or rolling up to a stop slowly????

2.) Have you put a scanner on the computer system and monitored the data when running????

3.) Does your 'CHECK ENGINE' light come on when driving it????

4.) Can you tell if you have ANY exhaust leaks where the exhaust manifolds attach to the cylinder heads...or where your exhaust pipe attaches to the exhaust manifolds????

5.) Have you verified that the Throttle Position Sensor (TPS) is set/adjusted correctly???

6.) Have you visually looked to see if your carb is dripping fuel when running in the front blows CAREFULLY???? And when it dies....if fuel is then dripping also.

You are aware...that it is possible that the other vacuum control areas of the car can be leaking....such as the vacuum to your headlights. SO...when I am working on a car with a problem. I take the vacuum to the headlights out of the scenario by removing the hose from the intake fitting and capping it off. Then...once I get it working...I attach the hose and see what happens. I have had many Corvettes that the bad leaks in the headlights system caused for hiccups in how the engine runs. And I do not stop there...I verify that all vacuum hoses are in good condition.

By your comment on how the engine dies....I have to go by that ( because I am not there to see it die) and not get into possible ignition issues. Because if the engine seems to chug-a-lug and then die.....then that would make me feel it is a fuel issue....versus...just die like you turned off the power to it. It could possibly be in the ignition system...such as spark plug wires all zip tied together in a bundle and laying on a hot exhaust manifold ( for example)...

DUB

 

[QUOTE=DUB;1590622875]Gordon,

To answer your question ...YES. A vacuum leak can throw off readings in the O2 sensor and make the Mixture Control Solenoid come into play.

I have MANY QUESTIONS:

1.) When the engine dies....are you coming to a stop QUICKLY...or rolling up to a stop slowly????

A) Engine dies while moving, usually at any cruising speed. Feels like fuel starvation or way too much fuel flooding the carburetor.

2.) Have you put a scanner on the computer system and monitored the data when running????

A) No, I do not have a scanner... Yet. What kind of scanner would you recommend?

3.) Does your 'CHECK ENGINE' light come on when driving it????

A) The CEL is off most of the time. Flashed just long enough for me to realize it stored a code. Originally the light was off then I realized the bulb was bad. After replacing the bulb it stayed on continuously. Following the service manual diagnosis chart told me the ECM and/or prom were bad. The new ECM didn't change things so then hunted down an OEM prom. After installing both I finally got CEL feedback and the car ran better than ever before, at least since I have had it!

4.) Can you tell if you have ANY exhaust leaks where the exhaust manifolds attach to the cylinder heads...or where your exhaust pipe attaches to the exhaust manifolds????

A) I think I do have an exhaust leak but have not yet determined where. It is loud and distinct enough but I haven't located it yet. I cannot feel the exhaust leak anywhere that I can safely put my hand.

5.) Have you verified that the Throttle Position Sensor (TPS) is set/adjusted correctly???

A) No. The carburetor is a total fresh rebuild by a local carb shop. It came ready to bolt on and go.

6.) Have you visually looked to see if your carb is dripping fuel when running in the front blows CAREFULLY???? And when it dies....if fuel is then dripping also.

A) I will check the front bowls. All of my experiences with the engine dining have been when we are driving after I think I solved another issue.

You are aware...that it is possible that the other vacuum control areas of the car can be leaking....such as the vacuum to your headlights. SO...when I am working on a car with a problem. I take the vacuum to the headlights out of the scenario by removing the hose from the intake fitting and capping it off. Then...once I get it working...I attach the hose and see what happens. I have had many Corvettes that the bad leaks in the headlights system caused for hiccups in how the engine runs. And I do not stop there...I verify that all vacuum hoses are in good condition.

A) Yes, this car is a vacuum nightmare to me but I'm determined to get it running correctly. Most of the vacuum hoses are original and I've been trying to solve one or two problems at a time. I bought the car in 2003 and it sat until last year before I began working on it. The previous owner hacked up a lot of things, some of which I have repaired and some are waiting for their turn. The car only has 46k miles on the odometer.

By your comment on how the engine dies....I have to go by that ( because I am not there to see it die) and not get into possible ignition issues. Because if the engine seems to chug-a-lug and then die.....then that would make me feel it is a fuel issue....versus...just die like you turned off the power to it. It could possibly be in the ignition system...such as spark plug wires all zip tied together in a bundle and laying on a hot exhaust manifold ( for example)...

A) I have had electrical issues in other cars and I am confident this is fuel related rather than electrical. When restarting I have to feather the throttle to keep it running but it usually dies again very shortly.

I really appreciate all the questions and input. I've been tempted to gut all the emission stuff and ECM but I also kind of like the challenge of doing it right!

Gordon

 

OK.

Type of scanner. That is up to you. This computer system is so slow compared to todays systems...you do not need a really expensive scanner. And...there are many out on the market. Possibly OTC. I have an old Mastertech...and that is so I can patch into the connector under the ash tray and the lead will fit through the ash tray opening...but my old Snap-On MT 2500 will not do that. Do a bit of research and talk with the tech dept of the company that you plan on buying. They are not that expensive.

So...I am ASSUMING after you reply...NO CHECK ENGINE light???? Correct????

Do you have a Factory Service Manual...a GM one..and NOT a Chilton, Haynes or
Mitchell?

On the 'possible' exhaust leak issue. You really need to find it...ESPECIALLY if it seems at the cylinder head. I use a rubber hose to my ear and the other end like a stethoscope and listen CAREFULLY ....and I am careful that I do not blow out my ear drum if I get the hose right at the blow-out spot....SO...PLEASE use common sense and care.

Depending on IF the idle adjustment was turned on the carb at ANY TIME....this WILL effect your TPS setting. and THAT IS ASSUMING that it was perfectly set from the carb company. SO....knowing that to adjust the TPS takes a VERY SPECIAL socket....once a scanner is connected and you can read the TPS...this more than likely will need to be set correctly to FACTORY SPECS. YES..I am aware that there is a +/- value for the TPS setting....but this still need to be verified.

Dripping fuel in the carb will overload the system (as you know)...and make it die due to being too rich...so this was just a thought and easy to verify.

DUB

 

I have located an OTC 2000 scanner that I will use on this quest.

The CEL performs properly now. Stays off but came on when during my test drive. Flashes code 12 and 44 at this time.

I like the idea of using a hose to locate the exhaust leak. I do have a couple stethoscopes I use for more mechanical noises but they don't help in locating an exhaust leak. I'm sure I have one and it shouldn't be too difficult to locate using a hose. It is loud enough and sounds like it is coming from the front area.

I do have the factory service manual and have been doing a lot of studying since this car is so different from my other cars. I hope the service manual and OTC 2000 will help me make better progress. I really feel it is a too much fuel issue.

Next steps are solving the vacuum and exhaust leaks.

Thanks again. I will keep this thread updated as I go along

 

I was delighted when my old 81 convertible (it was not one of those turbo Duntov ones) finally spun a bearing and I was able to yank it out and rebuild it into something more powerful.
I had the heads magnafluxed and found that they were both cracked. Replaced them with some aftermarket ones and than used Edelbrock intake, cam and carb along with a DUI distributor.
I removed the computer items and ran it just like my old 63 Impala.
If I were you I would budget for a performance rebuild or better yet just use a crate engine.
If the budget won't allow it then just pull the heads and have them checked.
Hope this helps.

 

That's the path I was leaning toward but I'm going to wrestle with this as a personal challenge for awhile yet. This '81 project has so much stuff that doesn't work it's unbelievable. It's a great looking body style and I bought it really cheap, $1,000, when I bought my project '64.

Appreciate the input.

 

whats the ECM say the temperature of the engine is?

 

I haven't received my scanner yet. The temp gage says right around 200 when the problem occurs.

 

Forgive me if I'm stating the 'obvious'; but sometimes it's good to ask about the basics.

Your '81 came with the first [crude] ECM system in a Corvette. It is a one-year-only system with a unique carb (has a mixture solenoid....doubt that the replacement is correct), distributor, O2 sensor, throttle position sensor, ECM and wiring harness. If all that original equipment is not there, the 'system' won't run properly.

Can you clarify what portions of the engine management system you still have in place?

 

7T1vette, I am confident all of the engine management system is still present. I highly suspect most issues will be resolved when I correct the vacuum leak and presumed exhaust leak. Then I can move on to luxury items like heat and AC, door locks, etc.

 

Glad to hear it's all there. Most carb rebuild shops don't want anything to do with those 'electronic' carbs.

 

Lars won't even touch them

 

YES...if a person has the Factory Service Manual...and reads about the carburetor. When you get to the section on replacing the Mixture Control Solenoid. IT IS VERY SPECIFIC and PRECISE. CODE 44...LEAN exhaust...meaning RICH FUEL.

I agree that all of the components need to be in place...but they do NOT have to still be original equipment parts.

And...it all depends on the carb shop. I have no problem in getting them restored when needed.

I will be curious to know what the TPS value is when the scanner is connected to it.

Also...what the COOLANT TEMP SENSOR reads.

AND...the voltage of the Manifold Air Pressure (MAP)....which is also very important.

THEN...when the scanner shows the computer is in CLOSED LOOP....which is controlled by the OXYGEN SENSOR getting hot enough.....I would like to know if the computer goes back and forth....from OPEN LOOP to CLOSED LOOP to OPEN LOOP. A RICH FUEL condition can cause this 'going back and forth' scenario....especially at IDLE. Which is why the exhaust leak is important to find and repair....along with ANY vacuum leaks like previously discussed.

DUB

 

DUB, you make this sound manageable, LOL. I'm not giving up on this as it will be a nice accomplishment when it is solved. I hope to have the scanner by this weekend so in the meantime I did pick up a carb gasket today and will change it out over the next couple of days.

I will report back accordingly.

 

It is manageable...but only if a person chooses to expand their knowledge and learn something new. Gish know I had to. AND if I can do it...anybody can do it.

DUB

 

Received my scanner and did a lot of reading and even a little on-car studying. I'm excited to use this tool on my '81. In the meantime I am making progress on my vacuum leaks. Found one small port with nothing connected to it and a couple of hoses in the wrong place so far. Improved the carb-to-intake leaks but not fully yet so I will keep working on it.

I can see light at the end of the tunnel!

 

Update: My vacuum leak was definitely coming from the carburetor mounting to the intake. A new gasket, proper torque, and correctly routing vacuum hoses seems to have solved that issue.

My exhaust leak is coming from the LH AIR Pump check valve. Since the pump no longer works I plan to plug the fitting going into the exhaust manifold. The engine sounds pretty darn good with no vacuum leak and a rag muffling the exhaust leak.

I'm excited about digging deeper into the engine management system with my recently acquired OTC 2000 analyzer. The more I read, the easier it is becoming to understand the dependencies and the potential impact. I think I will actually be able to solve this car's engine performance issues and it will run like an '81! ��

 

Update: After solving the vacuum leaks between the carb and intake and properly routing some vacuum hoses, I removed the LH AIR check valve and capped it with a pipe cap. The cap eliminated the exhaust leak and the engine actually sounds pretty good now.

The OTC 2000 scanner is probably the best tool I could have ever bought for this car. I have pretty good skills in the garage but trying to decipher the flow charts in the service manual without having a scanner was driving me nuts! The input from DUB and 71T1vette gave me hope and now I am happy to report everything is running smooth and the Check Engine Light has stayed off.

Recap: Replaced ECM and prom; Solved the vacuum leaks at the carburetor; rerouted some vacuum hoses; reconnected the MAP sensor that was disconnected; fixed the exhaust leak at the AIR pump check valve with a pipe cap; and finally, replaced the oxygen sensor.

I have plenty of other things to fix on this project but at least now I can drive it to the parts store.

 

Latest performance issue left me stranded with barely any engine power. Determined the Charcoal Vapor Cannister was spitting carbon bits into the air cleaner and carburetor, severely restricting fuel flow. Just installed a new Cannister and am anxious to get the carb installed and find time to hit the road.