Originally Posted by semcha
Other sites said it may indicate a vacuum leak, so I inspected all my vacuum connections. I found a hose clamp that had become loose at some point. It was on my crankcase breather hose. The hose was loose and had some oil around the hose. I replaced the hose clamp, disconnected the battery for 45 minutes, reconnected, and ran the datastream test again. The values dramatically improved, but I still need to do a test drive. I also tested for additional leaks using carb cleaner. I replaced a couple fittings that looked like they might have an issue in the future.
While it;s good to eliminate vacuum leaks, this is NOT the cause of your problem.
You were given this suggestion by some well meaning individual who assumed that your fuel injection system used the conventional (by today's standards) MAF, or Mass Air Flow sensor. If a MAF system has a vacuum leak, then the leaking air gets into the intake without passing through the airflow sensor, and so can be labelled as "unmetered air". The PCM injects the correct amount of fuel for only the metered air, but the engine gets both, so runs too lean.
You have a MAP or Manifold Absolute Pressure system, in which the pressure in the intake is measured, and a calculation is made from this to determine the mass flow rate of air entering the engine. If you have an air leak in a MAP system, then the leaking air raises the pressure in the intake, and that pressure is passed to the PCM which computes the correct amount of air and the correct amount of fuel.
Unmetered air is a real concern in MAF systems, but not at all in MAP systems.
If the physics doesn't convince you, you can create a large air leak by pulling the PCV hose of the back of the throttle body. That will give you a 1/4" hole directly into the intake. the engine will run just fine, make good power and do everything its supposed to EXCEPT that the idle speed will not go below about 1100rpm.
Originally Posted by semcha
I didn't find anything else. Hoping for the best!
I'm afraid that hoping for the best is all you're doing here. You have done an inadequate fuel pressure test, and you have admitted that you have not measure fuel pressure when your problem happens.
Like many others including our wise and gentlemanly sage, Subford, I believe your problem will turn out to be the fuel pump. he is too much of a gentlemen to say you are in denial about this; I am not.
You have chosen to believe that because the previous owner replaced the fuel pump, it must be good. However you can search this forum and find posts from people who had to do 2 fuel pump changes before they got one to work for more than week or so. And i often find myself reminding people that anyone selling a used anything has likely not used it much for a while, has probably skimped on the maintenance, and has only fitted the minimum cheapest parts to make it appear fit to be sold.
In addition, you are probably in denial because changing a fuel pump is a horrible job, and there is risk of damage, and apparently no guarantee of success. I understand all this; I had to change my own fuel pump in 28 degrees after i removed 55 gallons from the tank.
Be all that as it may, the fact remains that you seem to be fortunate enough to be able to reproduce this power loss, and so what you should do is the simple, cheap fuel pressure test while driving, only you need to do it all the time.
You say this can't be done, but I don't see why not. Contrast the time and money spent on rigging up a gauge where you can see it, with all the cost and aggravation of hanging out perfectly good parts.
1 You can drill a hole in your doghouse cover for the pressure hose to allow the gauge to be placed where you can see it while driving. After all is fixed, you can plug the hole, or repair the hole in the fiberglass doghouse cover.
2 You can get a longer hose and lead it forwards, over the radiator, out through the grille and support the gauge on the outside of the windshield.
3 You can get a
AUTO METER 2663 Z-Series Electric Fuel Pressure Gauge
$220 from amazon. how much have you spent already on diagnostic tools and parts you didn't really need?
And btw, did you know that if you pivot your dashboard up, then to the right of the ford instrument panel, Winnebago has provided two holes for this size of gauge. All you have to do is cut out the plastic part of the dashboard over the home in the metal. Fit it and leave it there. And put a transmission temp gauge in the other hole while you're at it
So, yes, changing fuel pumps is no fun. But the test is easy. And although the problem is intermittent now, it will eventually fail, and that will probably mean a towing bill.
So don't be in denial.