wschwab

Hello all,

Ok, I'm confused I have been stuck trying to get consistent data on my '96 4.9, and finally decided to subscribe to alldata. They already turned my low-pressure condition into a valid static pressure. One of my favorite corollaries to Murphy's Law is "A man with one watch knows the time, a man with two is never sure." Alldata is probably right; I'm pretty sure it is indeed a bad pump (check valve in particular). Still, I want to run through the whole procedure for "fun" and needed practice, not to mention possibly turning up something else that might be the real flaw.

There are scan tools, readers, etc. Do any of you have experience with free OBD-II software such as on scantool.net? Computers are easy to come by, and the cables are cheap. Speaking of cables, do any of you recommend a J1962M to DB9 appropriate for the 96 F-150? There are hints that some care is required in the selection, and I can believe that given some of the gizmos I interconnect at my day job.

One of things I am supposed to do, and want to do to confirm suspected cross-filling, is rig the fuel pumps to run continuously. Alldata says to hook up a scan tool and enter "output test mode." That sounds a lot like running the fuel pumps the way I need to do. It would explain why the connector Haynes tells me to find is not on the truck. The problem is that smart guys are telling me that scan tools do not do anything like that. Any ideas? I am thinking of renting the AutoZone scan tool just to see what it does (it's not safe to drive the truck to them yet). Before I shell out one to three hundred dollars, I want to get a $15 dollar cable and try try the various open source, or highly recommended commercial programs. Comments/suggestions would be appreciated.

One of the open source programs talks about tests "coming soon." Is that what I would need?

In a sentence, should I be looking for a way to hot-wire the pumps, or looking for a tool that tells the computer to do so? Sources I trust do not agree on this, hence my concern.

Bill

wschwab

I rented the AutoZone scan tool, and just like they said, nothing of relevance. It will go back soon, so no harm done. FWIW, the truck seems happy.

Back to my immediate problem, while I am fairly certain I know what is wrong, I should hotwire the pumps to repeat the cross-filling. Anybody done this on a '96? It will be interesting to see what AllData says about the scan tool and the output test mode.

Bill

Just call me Sean

iTrader: (1)

Here is the problem you are having. The Haynes manual only gives information on the OBDI system, and while the manual goes up to '96 the '96 is the first year of OBDII which is why you can't find the connector shown in the book. OBDII requires a scan tool to read codes. I'm not familiar with OBDII so I can't say for certain if the scan tool will or won't energize the fuel pumps on command, but it may. There isn't any reason to hotwire the fuel pumps to check for cross filling anyway. If you drive with one tank and the other fills up for no reason then you are getting cross filling. If you really want to hotwire the pump then find the relay with the yellow wire, the red wire, the light blue wire with an orange stripe and the dark green wire with a yellow stripe. Put battery positive to the dark green wire with the yellow stripe and the pump will run.

wschwab

Understood about the ODB-II vs. -I. However, it is a little surprising just how bad the published data is on this, transition year or not. I do want to run the pumps under controlled circumstances because I do not remember all of the details of the truck's most recent gas-spilling adventures; it would be nice to verify it.

Thanks for the info on the wires. That is probably the next step.

Bill

wschwab

One snag: I do not see any individual wires in the distribution box. I am fairly certain I know which relay sends power to the fuel pumps (presumably via the selector switch), and the relays are each etched with a schematic that explains how they work. Each relay has two outputs: one on by default, one off by default. It should be as simple as shunting switch in to switch out on the correct relay.

Anybody nervous about that?

Bill

Just call me Sean

iTrader: (1)

The selector switch comes after the relay. The relay hots the power wire to the fuel pumps then the switch selects which pump.

wschwab

No surprises, which is great. I was slowed down by a cold, but hope to actually get something done in the next couple of days.

Assuming it is the rear pump, the steps to replace are clear enough, but I suspect there are some tricks required to safely jack the tank down. Any recommendations for a jack or tips for "balancing" the tank on the jack? Somebody somewhere mentioned a low-profile tranny jack. I have a floor jack, but will consider buying something that looks like it would good to have around.

Bill

wschwab

Having convinced myself that the confusing stuff I was reading was in fact just plain wrong, and having identified the fuel pump relay and understood how it works, I crimped connectors on a piece of wire and was ready to run the pumps.

Before I could do that, I siphoned about 9 gallons of gas out of the rear tank. It was not just full, it was PAST full, at least if the sending unit works at all - I'm sceptical, but sorta believing it. So far, I have run both pumps intermittently, and the front one for about 15-20 minutes total over four or five sessions.

The front pump makes a higher-pitched sound than does the rear. I suspect their pressures are both right around 55 psi. I do not know whose numbers to believe, but that is somewhere in the right range. I can try bleeding and pressurizing to compare the two pumps to see if there is a difference.

There might be some cross-filling, but if so, it is very slow. Part of why I have been so insistent about being able to test the pumps is that I have NO idea how quickly the transfer would occur. Any ideas? If the check valve is stuck open (it could be intermittent), would 15 minutes move ounces or gallons? I suspect not too much; sing out if you know otherwise. I think I am seeing a gentle decrease in the front tank level, but not quite as much of an increase in the rear. I also do not see any puddles, so I do not think gas is leaving the truck, though it could be backing up into places it would ordinarily not go.

The level changes are not dramatic, so it is difficult to be certain what is happening. I am also wondering how the battery is holding up. Running the pumps might tax it slightly, and the truck has been sitting for 3-4 weeks now. Statically, the voltage has been 12.2V and is now down to around 12.1V; take that for what you will. What effect might that have on the gas gage reading?

I have kept an eye out of fuel leaks, and don't see any. A couple of times I caught some vapour near the engine, but nothing I consider unusual. The leak that lead me to stop driving the truck was from/near the vapour canister.

With 55 psi on the rail and gas circulating, I pulled the vacuum hose from the regulator, but at the manifold end - it's easier to reach. Nothing. That's checking for diaphragm integrity, right? Disagree if you want, but it seems to me that it has to be ok with that pressure and no leakage or fumes from the end of that tube.

Any ideas? I am starting to think that the leak from the vapour system was due to the rear tank being over-full, and that draining it should prevent dumping more fuel. Put another way, I am thinking it would be appropriate to test drive it at this point. How far would you expect me to need to drive (laps in the neighborhood will do nicely<g>, in order to see some cross-filling?

Would you replace the rear pump on the current evidence? Any other tests you would run?

Bill

wschwab

The truck has now been back on the road for some test driving, and it could be going better. The first priority was to ensure it was not leaking gas, which seems to be the case. Then I noticed it running a little rough, then a lot rough, and then it quit. Thinking of air+fuel+spark, I wondered if I hadn't seated the relay properly. Then I wondered about the pump having quit. The things I messed with all appeared to be in their proper places, so I turned the key on and heard the selected pump run and then stop as expected. It cranked, but it ran. It was rough at times, but it got home just fine.

After a phone call, I tried some more and at speeds over 30 (45 max for a mile or so) to see how that went, and when I slowed to make a right into my subsidivision, it quit again. Sadly that delay was all it took to get a couple of road sloths in front of me - really obnoxious ones, especially since I wanted to get back home in case it was going to quit again

When it stalls, I suspect the exhaust smells unusual. I was reminded of burning charcoal briquettes or a lawnmower. Please note that both of those things were probably in use nearby, but I still suspect the truck's exhaust smelled funny, and not unlike those items. I notice that smell only when it stalls.

It was looking like the rear pump, but I will reconsider the pressure regulator. Anything else? In the end, all I did was kill power to the control module (forcing it to relearn its strategy - but this seems a bit much for that), bypass the pump relay a few times, connect a pressure gage to the fuel rail (I removed the gage and replaced the cap before cranking it), and siphoned off a bunch of fuel. I did not disconnect any fuel lines or remove any other components.

Between trying to grok conflicting data and losing a week to a fairly ugly cold, the truck sat for a month. I would expect it to complain some, but not this much. It has been a little cool at night lately, so there might have been some condensation in the tanks?? Temp and oil pressure look ok. Any thoughts on what might be wrong other than the regulator?

Bill

aliens8mycow

Does the truck run like this on either tank? Were you test driving on the front or rear tank?

My guess is that the check valve on the rear tank is shot - that's why it's letting itself over-fill. To stop the over-filling from the front tank to the rear, you'll need to replace the rear pump anyway.

My truck has the opposite problem - the front tank pump is bad, so I drive around on the rear tank. The front one over-fills, and if it's cold overnight, then warm and sunny in the afternoon, fuel will seep out around the filler cap due to the pressure rise just sitting in the sun. If I drain some of the gas, it can't seem to get the pressure right for awhile, and does about what you described. My truck has a complete new tank/sender/pump/wiring harness/you-name-it on the rear tank.