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autominus

Pontiac LeMans 1.6 TBI - fuel pressure problems?

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Hi all,
I have a Pontiac LeMans, 1989 year of manufacture, engine 1.6 with Rochester TBI 700.

I think that I have a problem with a fuel pressure. Engine idling is bad. Engine idle has 800 rpm with fluctuation +/- 120 rpm and it uses a lot of gasoline.
Spark plugs are black with feathery carbon deposit.

 

I tested fuel delivery pressure with manometer.

Engine not running:
When the ignition is placed in the run position, the fuel pump works for two seconds.

  1. The fuel pump works two seconds. I'm reading pressure: 0.8 bar (11.6 psi).

  2. The fuel pump stopped. I'm reading pressure: 0.7 bar (10.2 psi).

Engine running:

  3. The throttle is closed: 0.9 bar (13 psi) with minimal fluctuation +/- 0.01 bar (0.14 psi).

  4. The throttle is wide open: 0.9 bar (13 psi) with minimal fluctuation +/- 0.01 bar (0.14 psi) - the pressure is not falling off.

 

In addition, I tested the fuel pressure regulator:

I started the engine for a few minutes. But at runtime, I'm reading pressure: 0.9 bar (13 psi). When I stopped the engine, pressure's down to 0.7 bar (10.2 psi).
After a few hours I'm reading pressure 0.7 bar (10.2 psi). This pressure holds where the engine not running.



Has the fuel pressure shouldn't be constantly at all times? - both when the engine is running and turned off.


Thank you for your help in advance.

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Hello autominus. Welcome to FP.

From what you are describing, your stock fuel pump/pressure regulator appear to be working properly. The stock GM TBI systems required around 9-13 psi to the injectors. Everything you described appears to be correct.

You mention that your spark plugs are black and have carbon build up and the vehicle has bad fuel economy. This points to a rich fuel condition to me. Since this is a 1989 car I will assume it has a lot of miles on it (over 100,000). This leads me to believe that you have dirty or clogged fuel injectors that need to be replaced or removed and cleaned. Once dirt gets into an injector, it will disrupt the spray pattern and more fuel is often allowed to flow than it should, leading to the rich fuel condition. Assuming this is a high mileage vehicle you might be better off replacing the injectors. I would also recommend putting a bottle of fuel injector cleaner in your fuel tank for the next couple of fill ups to help clean the entire fuel system. I would also take fuel injector/carb cleaner to the rest of the TBI unit to remove any gas varnish build up in and around the fuel injectors. Of course, you need to replace your spark plugs too.

Edited by Frosty

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Welcome. Sounds likes frosty is right, a dirty injector problem. 

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21 hours ago, autominus said:

Hi all,
I have a Pontiac LeMans, 1989 year of manufacture, engine 1.6 with Rochester TBI 700.

I think that I have a problem with a fuel pressure. Engine idling is bad. Engine idle has 800 rpm with fluctuation +/- 120 rpm and it uses a lot of gasoline.
Spark plugs are black with feathery carbon deposit.

 

I tested fuel delivery pressure with manometer.

Engine not running:
When the ignition is placed in the run position, the fuel pump works for two seconds.

  1. The fuel pump works two seconds. I'm reading pressure: 0.8 bar (11.6 psi).

  2. The fuel pump stopped. I'm reading pressure: 0.7 bar (10.2 psi).

Engine running:

  3. The throttle is closed: 0.9 bar (13 psi) with minimal fluctuation +/- 0.01 bar (0.14 psi).

  4. The throttle is wide open: 0.9 bar (13 psi) with minimal fluctuation +/- 0.01 bar (0.14 psi) - the pressure is not falling off.

 

In addition, I tested the fuel pressure regulator:

I started the engine for a few minutes. But at runtime, I'm reading pressure: 0.9 bar (13 psi). When I stopped the engine, pressure's down to 0.7 bar (10.2 psi).
After a few hours I'm reading pressure 0.7 bar (10.2 psi). This pressure holds where the engine not running.



Has the fuel pressure shouldn't be constantly at all times? - both when the engine is running and turned off.


Thank you for your help in advance.

 

1 hour ago, Frosty said:

Hello autominus. Welcome to FP.

From what you are describing, your stock fuel pump/pressure regulator appear to be working properly. The stock GM TBI systems required around 9-13 psi to the injectors. Everything you described appears to be correct.

You mention that your spark plugs are black and have carbon build up and the vehicle has bad fuel economy. This points to a rich fuel condition to me. Since this is a 1989 car I will assume it has a lot of miles on it (over 100,000). This leads me to believe that you have dirty or clogged fuel injectors that need to be replaced or removed and cleaned. Once dirt gets into an injector, it will disrupt the spray pattern and more fuel is often allowed to flow than it should, leading to the rich fuel condition. Assuming this is a high mileage vehicle you might be better off replacing the injectors. I would also recommend putting a bottle of fuel injector cleaner in your fuel tank for the next couple of fill ups to help clean the entire fuel system. I would also take fuel injector/carb cleaner to the rest of the TBI unit to remove any gas varnish build up in and around the fuel injectors. Of course, you need to replace your spark plugs too.

Autominus, there’s a good chance Frosty is on the right path, but I would recommend you also check a few other things that more often than not are the actual culprit for what you are describing.

First the MAP sensor can cause this without setting a code. Also, even more often than the MAP is the engine temperature control sensor, down toward the lower part of the block on the passenger side. A false signal/bad will tell the engine it’s cold when it’s not.

Third is the fuel regulator that sits in the TBI, this doesn’t happen often, but it can. Also make sure your fuel filter is good. If there are over 50,000 miles on the engine and you’ve never changed the filter, it could cause problems. 

Likewise make sure your plug wires, distributer cap and rotor are good. These if not up for the task can create a rich condition.

Edited by Last Indian

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