Hey guys,

I'm chasing down a rich condition in my car. Its an '87 350 TPI. I am only getting 23 psi of fuel pressure while idling. When I turn off the motor the pressure goes to zero immediately. I have run a series of tests and determined my injectors including cold start, are not leaking. The fuel pressure regulator is also fine. So I know that either the fuel pump or the rubber hose in the tank is busted. When I pulled the fuel rails to test for injector leaking, I noticed my runners and intake manifold are covered in fuel. I also have black rich exhaust, the rich condition is confirmed by datalogging. I have also determined my sensors appear to be reading correctly. So how is the engine getting so rich?

My theory. Since the fuel pump or hose is leaking fuel pressure back into the tank, the tank must be getting pressurized, right? Now, the EVAP Charcoal Canister system is designed to remove fuel vapor and pressure from the top of the gas tank. With the pump not working correctly and pressurizing the tank, the EVAP system now has to work much harder. Maybe the charcoal canister is dumping the excess pressure into the engine through the bottom of the throttle body. It is dumping much more fuel/fuel vapor than normal, causing the excessively rich condition.

Sound fair?

Anything else for me to check out?

If my theory is correct, I should be able to loosen my gas cap to relieve the pressure. In this setup the rich condition should go away, right?

Thanks for taking a look.

John

 

Hmmm, fuel pump outlet hose leaking in the tank pressurizing the tank. The only way that could be is if the source for the pump is outside the tank.

Your theory doesn't wash. If the fuel is going from liquid to vapor inside the tank, that would raise the pressure. But, not just from a leaking hose (put a vacuum cleaner inside your room, close/seal the doors and windows, start the vacuum cleaner - does the pressure go up inside the room?).

Low fuel pressure keeps the injectors from spraying properly. The ECM cycles them longer to make up for it. Result is unatomized fuel = rich.

Did you check fuel pressure with the regulator vacuum unhooked?

Get the fuel pressure right, your problem should go away (assuming the O2 sensor is doing its job properly).

 

I have verified my fuel pressure with with the vacuum line disconnected. I have also tested the fuel pressure at pre-startup prime. It goes up to 12psi then right down to zero, probably why I'm having starting problems. The engine BLMs are basically pegged at 108, the O2 sensor reeds 750-800mV the whole time, which makes sense with how rich the exhaust looks/smells. I'm getting 1-2% injector duty cycle at idle. I was wondering if my pressure was low enough to cause atomization issues.

I'm trying to identify any other issues I should look at when I do the pump replacement. I want to make sure I have all the ducks aligned. Being 300 miles from the car makes diagnosing problems challenging and slow.

So if fuel isn't atomizing correctly it won't get combusted correctly in the chamber and some raw fuel makes it to the exhuast. So would it be correct to assume the real air/fuel mixture that is being combusted could be ok, or even lean? On the other hand, the ECM clearly thinks the engine is rich because of the BLMs and O2 readings. So why isn't it adjusting injector pulse width to make the O2 sensor happy?

 

wow you're really over thinking this.. fix the known problem.

 

Yes I know. But I don't like driving 300 miles everyweekend when something new breaks on this car. So if someone has a idea of a thing to check when I'm with the car next week, I will certaintly check it out, thus saving me another 600 miles of driving the following weekend. Of course I'm going to fix the fuel pump.

While I understand low fuel pressure with cause poor injector spray performance, I still haven't ratioinalized why the ECM isn't pulling fuel to make the O2 sensor happy. And why is my whole intake covered in fuel? Seems like it may be coming from further upstream, hence thinking about the EVAP system. I've never had wet gas in my intake when the car is running properly.

Thanks

 

five7kid,

I was thinking about what you said. You mentioned the ECM adding fuel to compensate for the low fuel pressure and unanatomized fuel. However wouldn't it only do this until the O2 sensor read good? I'm getting rich O2's across the board.

Thanks