Changed lower intake gaskets because of coolant leak. Also changed valve cover gaskets at same time. After the job was finished it stumbles when first started for the first 15 to 60 seconds. Sometimes so bad that it will die when the peddle is depressed. The MIL light will aluminate after a few trips in the car but there will not be any active codes but there will be codes P0171 and P0174 in the history. (Bank 1 and Bank 2 too lean)





Fuel pressure is 55psi with engine running and when the throttle is snapped it will jump to 60psi so I feel the fuel pump is fine.



I pulled the upper intake off and reattached the fuel lines to the spider and pulled the injector tubes out of there holes and pressurized the fuel pump and looked for any leakage anywhere from the fuel spider. No drips or fuel noted.

After watching the live engine data on my snap-on MT2500 the only thing I notice is a higher than normal MAF reading 14 grams/sec until the stumbling stops then at that time the MAF reading is normal at about 5.3 grams/sec. The mass air flow was dirty but even after I cleaned it the readings are the same. If I unplug the MAF it still stumbles the same.



I have checked the timing readings and they are 0 degrees cam retard and 20 degrees advanced ignition timing.

It is just like a vacuum leak but it goes away after a while so I don't think it can be that. I did spray carb cleaner around the intake and could find no change in RPM so I don't think it is a vacuum leak.





I need some more ideas.

 

I know guys its got me stumped too.

 

Have you done fuel pressure leak down , your #'s are barely there.

 

I have done a leak down and it fails. The problem is it has failed the leak down for over a year but it has always ran fine. I did replace the fuel regulator when I had everything apart thinking that would fix the leak down problem but it did not make a difference so I guess the check valve at the fuel pump is bad. Even with that failing my fuel pressure is good when the engine is running and when I step on the gas the pressure instantly jumps.


What #'s are you saying are barley there.

 

There are no "engine running" fuel pressure specs for this system. All pressure and leakdown tests are done with the engine off.

Key ON, engine OFF, fuel pump running: pressure must be 60psi to 66psi, (regulated fuel pressure). Pressure must remain above 55psi for at least 10 minutes after the pump shuts off. If it fails one or both parts of the test, pressure and leakdown must be checked at the fuel filter to determine where the problem is. All fuel flow and pressure must end at the fuel pressure tester, (no fuel allowed to the engine). While the pump is running, pressure must be 73psi to 108psi and must remain above 55psi for at least 10 minutes after the pump shuts off. If you need a parts list to build the adapter, just ask.

 

Parts list would be great. Would love to check the actual pressure the pump is able to achieve. The only problem I have noticed about the leak down failing is it cranks a couple seconds longer before it starts. That is what led me to check the leak down 1 year ago to start with.

So do you think if the pressure is 55psi vs 60psi that could cause this stumble for only the first 15 to 60 seconds after starting. Once it gets past that it runs great with no problems.

 

If regulated fuel pressure, (engine off, pump running) is below 60psi, you will have problems, guaranteed. The PCM is programmed to deliver a certain amount of fuel when the injector opens. If regulated pressure is low, the engine will run lean. If the engine is at operating temperature, and properly tuned, if the pressure drops much below 55psi it will not start, (on its own). At zero degrees F, and a cold engine, it won't start. When regulated pressure drops below ~43psi, the poppet valves, (if still equipped with them) will not squirt any fuel. If the excessive leakdown is at the pump, it will not build sufficient pressure.

"So do you think if the pressure is 55psi vs 60psi that could cause this stumble for only the first 15 to 60 seconds after starting?"

Without a doubt, yes.

Here is a link to the parts list for the adapter: http://i561.photobucket.com/albums/s...ps5c876ad4.jpg

 

I have a similar problem going on. My engine will start and run fine no problems smooth everything. But it has stalled three times now and will not restart until it sits overnight. I have done your checks and here is my results.
With the fuel pressure gage on the valve on the back of the engine, with the engine off I get 60 psi, in ten minutes it drops to 50 psi. When I hook it to the fuel filter and start the pump I get 70psi and in ten minutes it drops to 60 psi. does this sound like a fuel pump problem?

 

Ok, removed fuel filter and installed gauge and fuel pressure climbs to a little over 100psi. My gauge only goes to 100psi so I am not exactly sure what the pressure is but it is between 105 and 110 I would guess. It drops to 55 after about 2 min. After about 4 to 5 min it is at 30psi where it seems to hang for quite a while.


Even though this does not pass the leak down test I do not believe this would cause the stumbling problem I have for ONLY 15 to 60 seconds after starting and then run great after that.


I do believe this is what is causing the longer than normal crank times.


Remember the fuel pressure readings have been the same for at least a year and other than longer than normal crank times it NEVER stumbled or stalled.


Only after replacing the intake gaskets did the stumbling problem appear.

 

I replaced the fuel pump today. As I figured it did fix the longer than normal crank time as it does build the right amount of pressure and it holds the pressure when the fuel pump is turned on then off.


But as I also figured it did not fix the stumbling problem. It may be slightly better than it was but it is still the same.


Any other ideas would be helpful.