Are your manifolds leaking ? If they're not, then you wouldn't see a difference in mileage. His are, so the O2 sensors are telling the computer it's running lean and dumping fuel to compensate.

 

 

So you're arguing here why ? You had leaks then and leaks now and your mileage didn't change ? Well DUH !

 

I'd hate to see how those leaks have eroded the head flanges too after that length of time and mileage.

 

 

 

He said the exhaust leak wasn't loud or really noticable so I'd be surprised if it was able to suck in enough air to effect fuel trims to kill his mpg by 1/4. I quick tailpipe sniffer would answer if its running rich. I just hope he figures it out before handing over 1k+. if it isn't needed.

 

Every case is different. However, to the OP - I would do the "easy" stuff first, clean the MAF, check fuel filter, etc. Even without the O2 sensors being given bad info (and they very well might be seeing excess oxygen from the exhaust leak), a lean condition WILL cause you to use more fuel even though that's counter-intuitive. Lean mix means less power, less power at highway speeds means you have to give it more go-pedal, using more fuel overall.

Check the fuel trims before and after doing the maintenance work.

 

Nope, I couldn't figure why you were doubting everyone else. Every case can be different, different driving styles give different outcomes. I would have it repaired for no other reason than to prevent erosion to the aluminum heads from the leaks. Exhaust gases will eat the **** out of aluminum over time. I've seen the result on older Ford aluminum intakes.
Not to mention the irritation from the noise created. I had a Freightliner pop a half dozen exhaust studs once going across the PA Turnpike one night. Had to drive it all the way back to Louisiana cause the company was too cheap to have them replaced on the road. I had to put up with the noise and smell for 1500+ miles and that was NOT fun.

 

Great info submitted here! I appreciated everybody's input on this issue.

I had purchased the UltraGuage Blue scanner last year but had not taken the time to get the necessary gauges loaded on to the android app. After ford390gashog suggested to check the fuel trims, I had some Googling to do to find out what it meant.
I loaded the trim gauges in to the app and just finished a 200 mile round trip up to Malibu, CA. It's mostly flat, 80 degree air, and plenty of fast and slow freeway, with a healthy blend of stop and go. Typical Southern California driving.

After driving about 30 miles I checked the readings on the app.
Watching the trim levels as I drove the the short term levels varied about -6 to +13 and the number was not close to being steady at any time.
I read the Long term levels were a better indicator during driving. The long term average was +9 on one bank and +12 on the other.

From what I have read here and elsewhere online, this indicates it may be running to rich from bad O2 sensor readings. Makes sense to me.
I use the 89 octane (mid-grade) Arco fuel most of the time. It is the most convenient station near me. I have read the ethanol content was 10%.
I have driven Ford Vans for the past 50 years and this is my second v10 van. I do not drive it hard or aggressively and I do a lot of coasting. My driving style hasn't changed. There are 2 pedals on the floor. Everytime I push on one of them it costs me money.
10 MPG was normal on both V10 engine vans that I have owned. On this last tankfull, again I got 7 MPG.
I have noticed I need to push the gas pedal further to maintain speed on an uphill grade in the past several weeks. The 5 speed trans has been downshifting earlier as the uphill grade was increasing. I do not notice any difference on level roads.
No changes in the tire sizes or gearing have been made.

The mechanic service company I have been talking with is in Fountain Valley, CA. The company reconditions and resells used fleet vans, and mostly Fords. They have been around a long time and they have done work for me before.
They would be familiar with the e350 V10 know the best way to do the repairs.
The previous service manager had said the engine had to come out to access the studs. The new manager did not indicate removing the engine first. I looked over, under the engine compartment with the doghouse removed and could not see any easy access to the studs.
The broken studs I have are at the rear of the engine and somewhat accessible but the others further forward are buried. Welding a nut onto any other studs that may be damaged upon removing the remaining nuts appears impossible.
Tomorrow morning I will replace the fuel filter and check/clean the MAF sensor and double check on the tire pressures. Also, I will be stopping by another repair shop for an estimate.

 

Yep. Either bad O2 sensor, a vacuum leak, or an exhaust leak. Any of the above or any combination thereof will dump fuel and kill your gas mileage.

 

Yup, those long-term fuel trims are way up there. Now, whether or not that indicates it's really running rich is another thing. Like I said before, a lean condition will cause more fuel to be used because overall you need more throttle to do the same job.

It could really be way lean, and the computer is adjusting the fuel trims up to try to compensate, and not achieving it's goal.

Please let us know the outcome.

 

Does your app tell you vacuum on the engine? How's it reading? Idle? Pulling a hill? You may be able to check spark plugs and see if one is running lean or a bank is running lean. Of course pulling plugs on the v10's is no easy task either.

 

 

And before we continue the back-and-forth, everyone has a right to their opinions. Let's see how this plays out.