I have a 1999 v10 it has power changes all the time i can pull a hill as fast as I want to in od and then two hours later come over the same hill and have to shift down all the cyl are firing and not check engine light does any one have an idea

 

A few things to check: fuel pressure when it's doing this. It could be the fuel pressure regulator not going to full pressure under load. When was the last time you changed the fuel filter?

Clean the MAF.

 

You'll need some way to monitor whats going on when this is happening. A good start is definitely fuel pressure, less than spec fuel pressure will cause a lack of power issue but I would think a lean code would be set if this were going on for an extended period of time.
I would also check EGR flow at the time of low power. It's possible the EGR valve is sticking open too far allowing more flow than necessary.

The MAF is another good candidate for intermittent run problems. I really think this is where your problem lies. However I do not reccomend cleaning it. The way a MAF operates is it keeps a bare wire inside the sensor at 200 degrees C and the air flowing in past it cools that wire, then the voltage necessary to keep the wire at the set temp is how the PCM calculates the incoming air flow. Cleaning these only further damages the sensors ability to correctly estimate the amount of incoming air as if whatever you use to clean it with will no doubt leave a finite residue on the wire and cause the sensors readings to be skewed. I have been a tech now (graduated from FORD ASSET program) for 10yrs and it has been my experience that cleaning a MAF never leads to a good result. You only contaminate a possibly still good sensor or waste time cleaning a bad one.

 

Not to argue off-topic, but cleaning the MAF has had good results for many, and never EVER has it been reported that it's made anything worse. The MAF on the modulars is a glass-coated filament that can be cleaned quite easily.

However, even though it's rather sturdy, it's still possible to break it or otherwise damage it, and if not completely cleaned could possibly make things worse.

I've been doing it for years on all my modulars and others' vehicles including GM and it's always led to better throttle response and crisper shifting.

 

I won't argue with that. I was just using the hot wire MAF as an easy way to explain MAF operation. I guess being a tech I just never want to put a part back on a car that might not fix it. A new MAF is guaranteed (provided you get a good one out of the box) to fix a concern caused by a MAF malfunction so why chance the problem coming back?

I'm sure cleaning it may help and if you have had good results, it may be something for the OP to try out. If it was mine and cleaning the MAF had an improved result, I still think I would replace it just for peace of mind in and reliability stand point. I don't wanna be towing 1000mi from home and have the thing malfunction again and cause my trip or vacation undue stress.

 

I have cleaned the MAF and it has a new feul filter the MAF is after market and bigger which was recomended to me after installing a banks system. I have had to have the fuel system cleaned after the fuel in Oregon and other states started useing ethanol fuel. The computer has not gave me a dtc code for any lack of fuel pressure. These are the things I have added to the the pick up maybe this will help any ideas. Has a banks ottomind chip, bigger MAF, throttle body spacer, cold air intake.

 

my 2 cents... I have had good results with cleaning MAF sensor with no issues. Its one of the first things on my check list when dyno tuning a vehicle or buying a used vehicle. Last year had a 2005 F150 5.4L with 95k miles on dyno... i asked owner if he had ever cleaned Maf...he said no with that puzzling look .... did a wot run and cleaned maf did another wot run; truck picked up about 10 rwhp.. I consider the maf the most important sensor on the vehilce so as mentioned be careful handling it and what you use to clean it with or you can damaged.

 

Um, did you get a custom tune for the larger MAF? Otherwise, it's going to be way off...

 

If the larger maf is not calibrated for your truck delivered fueling and timing will be off even though the pcm has some adaptive measures to help correct but has limits... maybe try installing the stock maf back in truck and seeing if it helps with issues.

 

Sounds like a great place to start. I bet you'll have some results with that.