2000 7.4 liter bravo cold start issues

[fa635s1]

I am having issues with cold starting on my 7.4 mpi bravo engine. It starts up and revs up to 1200 rpms, then falls down fast and it can't catch it to keep running. I changed the IAC motor with the same results. I have tried a different engine coolant sensor and a different MAP sensor and the same thing. Once the engine warms up it will idle all day. It is a new engine (Long block) this summer. It has done this since the new engine was installed. If I use the shift lever to hold the RPM's up it won' die. Can anyone please help. I did have it back at the shop where the engine was installed and they said there were no codes. They can't, or don't want to figure it out. Do you think it could have anything to do with the timing? It runs great when warmed up and at high speeds!!!!

 

[Fun Times]

Re: 2000 7.4 liter bravo cold start issues

When it is warm over [154 degrees] What is the idle speed at, Exactly?

 

[fa635s1]

Re: 2000 7.4 liter bravo cold start issues

The idle speed when warm is 650 rpm's on the gauge. I would like to test all of my sensors but I don't know how many volts they should be at when cold. If anyone out there has a service manual for this motor I would appretiate it if you would share that info for the IAC motor, MAP sensor, Coolant temp sensor, Intake air temp sensor, and Throttle position sensor.

 

[Fun Times]

Re: 2000 7.4 liter bravo cold start issues

The idle speed when warm is 650 rpm's on the gauge. I would like to test all of my sensors but I don't know how many volts they should be at when cold. If anyone out there has a service manual for this motor I would appretiate it if you would share that info for the IAC motor, MAP sensor, Coolant temp sensor, Intake air temp sensor, and Throttle position sensor.

To be honest It sounds like to me you Might not have enough fuel pressure at start up or have air in the fuel system some place,[IMO] You might want to check fuel Pressure first, You should hook up a fuel pressure gauge to the fitting on the fuel rail area, And then turn the key on with the engine off one time and see what pressure you get, Then key it up a few more times and see what psi you get then try too start the engine cold and see if it will stay running on it's own. Then find and unhook the vaccum hose going to the fuel pressure regulator and see if the fuel Pressure goes up with the engine running like it should.

 

[fa635s1]

Re: 2000 7.4 liter bravo cold start issues

I checked the fuel pressure last week and it was around 40psi. I didn't cycle the key though. I don't think it is fuel related because if I start it up and unplug the IAC motor when the RPM's are high it will stay at the high RPM. I think something is telling the IAC motor or the Computer that the engine is warm. The only thinks that I am not sure of are the Throttle possition sensor and the Intake Air sensor.

 

[Fun Times]

Re: 2000 7.4 liter bravo cold start issues

I checked the fuel pressure last week and it was around 40psi. I didn't cycle the key though. I don't think it is fuel related because if I start it up and unplug the IAC motor when the RPM's are high it will stay at the high RPM. I think something is telling the IAC motor or the Computer that the engine is warm. The only thinks that I am not sure of are the Throttle possition sensor and the Intake Air sensor.

Well if you do end up unpluging the Intake Air sensor, What will Happen is the ECM will read -32 degrees And the ECM will think it is varry cold out side and the ECM will add more fuel to the engines fuel system by holding the fuel injectors open a little longer and making the engine run a little bit richer is all, And set a code. Tyr pulling some spark plugs to see if they look rich.

Try cylcling the key a few times just to see what happends.

When you hook up a scan tool and read the voltages between the sensors and the ECM, The volt range is anyware from .5v up to 5.0 volts. And if a sensor falls out of that volt range window by up or down then it will set a code in the ECM.

 

[fa635s1]

Re: 2000 7.4 liter bravo cold start issues

I will try cycling the key a few times before I try to start. I won't be able to get to it until the weekend........

Well if you do end up unpluging the Intake Air sensor, What will Happen is the ECM will read -32 degrees And the ECM will think it is varry cold out side and the ECM will add more fuel to the engines fuel system by holding the fuel injectors open a little longer and making the engine run a little bit richer is all, And set a code. Tyr pulling some spark plugs to see if they look rich.

Try cylcling the key a few times just to see what happends.

When you hook up a scan tool and read the voltages between the sensors and the ECM, The volt range is anyware from .5v up to 5.0 volts. And if a sensor falls out of that volt range window by up or down then it will set a code in the ECM.