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Old 10-12-2008, 04:23 PM   #1
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Default Fuel pump diagnostics for 96 F-150 4.9L

Hello,

I have a 96 F-150 4.9L 2wd, which started to smell of gas, and finally visibly leaked it from (or perhaps just near) the vapor canister. I'm told that no liquid fuel should be anywhere near it, so while the hoses look like they want to be replaced, something else must be wrong.

My "mentors" were unanimous in suspecting the pressure regulator. In getting to know my way around the fuel system (so far, I have done shocks and lots of cooling system work on it - otherwise I'm still fairly green at this), I noted that it appears to have cross-filled into the rear tank. If I understand what I have read, that probably points to a stuck open check valve in the rear pump??? How would I confirm that?

So far, I have found that the engine off pressure is low - to be honest, I'm not sure which tank was selected, because I was just getting started with the sequence and trying to read ahead for special tools I would need, etc.

Around that time, I began to notice that the procedures referred to DLC and STI connectors that my truck appears not to have. Should I keep looking, or are they missing on the 96? How does one test a fuel pump on the 96? Is there a way to get the selected pump to run continuously? Doing that, I could see if it continues to cross-fill, and whether it goes one way or both, stuff like that. Hopefully there are detailed procedures for this, but I have yet to find them.

If it turns out to be one or both fuel pumps, how ugly is it to replace one? Dropping the tank sounds like the worst of it, but how bad is that? One friend says to use air to dry the tank before working on it, and saw the warning to use a brass punch to loosen the ring holding the pump. But to get to the tanks, would you remove the bed instead? Of course, if I do that, I have to put it back. Is that just eye-balling the alignment and torquing some bolts, or is there more to it?

What do you think of alldata.com? I am thinking of subscribing. I have the Ford shop manual (two volumes), but there is a separate diagnostic set too. Would you get that, subscribe to alldata, or both?

Any pointers will be appreciated.

Bill
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Old 10-12-2008, 04:50 PM   #2
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Does the fuel seem to cross between either tank, or is it just one direction? (only goes from front to rear tank when running on the front one)

Will it run on either tank, or is one tank not working?

A quick check on the pressure regulator is to have the truck running, and pull the vacuum line off the regulator. The idle will go up a bit, and if it's shot, fuel will squirt out the vacuum fitting.

I don't think the FPR is your issue, though... Each tank has it's own fuel pump, and each pump has a check valve to keep fuel from returning. Some mechanics will try to convince you that the two tanks are completely separate systems, but the lines do connect at some point.

The vapor canister should only be getting 'vapor' as the tank builds pressure inside. If it's getting fuel, it might be something as simple as the filler cap.
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Old 10-12-2008, 05:52 PM   #3
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Having found gas pouring out of it, I have grounded it. It appeared to run on either tank, but (IIRC) filled from front to rear, so I assume the rear tank has a bad pump. I am fairly certain that is the pump giving a low engine-off pressure too, but I need to get clear procedures and go through it all carefully. For now, I am trying to match the procedures I am reading to what I see on the truck, and they do not agree.

I removed the vacuum line from the regulator and pressurized the system (verified by gage on the rail) - no gas. What if anything else does having the engine running do? Applying a vacuum is obvious, but you are telling me to remove it; I'm not challenging you, just trying to understand. I would be willing to remove the other end of line from the vacuum system (far lesser risk of screwing up and being eaten) with the engine running; would that work? Or is it enough to not see a leak into the vac line while under pressure?

I suspect the rear tank is overfilled from the cross-filling, so one of the next things I want to do is move some gas from it to the front tank. Before cranking it, (speak up if you disagree) I would prefer to start with the pumps hotwired to run to see if I can reproduce the cross-filling, and also answer your question about whether it goes both directions, or only front to rear.

So far, the only instructions I have found for making the pumps run continuously mentions a connector that my truck does not appear to have, I assume because of OBD-II. Any ideas?

Bill
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Old 10-12-2008, 05:57 PM   #4
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Your logic and knowledge of the truck appears to be sound. Familiarizing yourself with the problem before jumping into the middle of it is always a smart thing to do. I have only changed these pumps from underneath utilizing a hoist. On jack stands, a floor jack can be used, a smarter jack would be a low transmission jack. Others here have done it by lifting the bed and propping it up on one side. A brass punch to remove the locking ring is for safety reasons the only way to go. The low fuel pressure engine off does point to the check valve being kaput. It doesn't matter which tank is selected. The tanks are in parallel to each other. The selector switch controls which pump is running, the check valves prevent the cross-filling. Give it a shot and let us know the results.
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Old 10-12-2008, 06:11 PM   #5
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Your typing is faster than mine. What Alliens was saying about the vaccuum line at the regulator is a test to see if the regulator is faulty. The presence of fuel out of the regulator is bad. No fuel should be there, no smoking please. Run the engine and pull off the line. When the engine runs, the vaccuum maintains the pressure to 30-40 PSI depending on engine load. Engine off, and regulator is bad (fuel out of fitting) fuel pressure will drop as your does. So is it a faulty regulator or a bad check valve, or a leaking injector. Do the engine running test, regulator is probably good, leaving the check valves and injectors suspect to low pressure at engine off, with cross filling happening the rear pump is at fault. That is the tank that is cross filling?
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Old 10-12-2008, 06:44 PM   #6
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if i understand the problem right, the problem is in tank switching valves. the rear fuel back line valve is stuck ot not closing completely. i think it also may affect the rear pump pressure if it's stuck. the only way of repairing it that i know so far is replacing a pump (it must be supplied with that plastic case). my '93 seems to have stuck valves for a while, and i'm still looking for a way to fix it without replacing pumps, no success so far.
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Old 10-12-2008, 06:55 PM   #7
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Hi Ham, haven't spoken in a while, I'm sure either you or Sean will come up with a way to fix those check valves. I'm past the time of attempting to find a lesser expensive way of doing things. Not that I'm wealthy, at my age, having fun is the priority. 45 years of twisting wrenches and busting knuckles is not high on my list of things to do.
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Old 10-12-2008, 07:33 PM   #8
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I'm not as familiar with OBDII systems, but there is a way to run the fuel pumps to test them. I don't know how to do it, my manual doesn't say. But it isn't required any way, as you have all the information you need.

It doesn't matter where you pull the vacuum from, as long as the pressure regulator doesn't get vacuum any more. Then watch for the leaking fuel from the line and for the increased idle. The regulator is probably fine.

There is no further testing required for the pumps. If the front pump is selected and the rear tank is filling then the rear valve is bad and vice versa. The valve comes with the fuel pump. The low static fuel pressure is also indicative of a stuck valve. If the tank is overly full then fuel can be forced through the vent line and come out at the vapor canister.

I don't think you should remove the bed to change the pump even though some people do it that way. It just seems like a more difficult way to do it. Bolts can be rusty and hard to remove or can break off or strip, and then good luck getting the bed back on. But who knows, I've never done it that way. It might be easier.
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Old 10-12-2008, 10:21 PM   #9
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Sean has just added the last piece of the puzzle. I was not sure whether the bad check valve could explain the fuel at the canister. If you guys are buying that, then the rear pump is probably the culprit.

Because things happened quickly, I want to see it cross-fill again, only safely this time. Any objections to siphoning gas from back to front, cranking, looking carefully for leaks, and then seeing if gas moves, and only one way? If it is all the same to you, I won't repeat the overfull tank and gushing canister

Any swags on how long it should take to move 3-5 gallons? I'll check the specs in Haynes, but that will no doubt be if all is well, which it isn't.

Since the regulator is not off the hook yet, an underpressure test would be a good idea (from Haynes). They say to pinch off the return line downstream of the regulator. Am I right to be a little nervous about doing that to the line? I would remove the tether and pull back the sheath, but still... The point of the test is that if the pressure rises sharply, the regulator is bad. Right or wrong, I think of it as restricting flow to maintain pressure provided the pump works. If the return line is blocked, I'm not seeing what the regulator would do to keep pressure down. What am I missing?

I also did a static leakage test. It holds reasonably well. It held better than specified max 5 psi loss over one minute. Does that reduce your suspicion of the pump check valves?

Haynes also defines a regulator check valve test that might help, but I'm only beginning to read that one so I won't bug you with it.

Looking ahead, fair enough about dropping the tank instead of removing the bed. Just how bad is it to drop a tank? There will be some hassle with things that are not visible until the tank comes down some, and no doubt will break if still connected when it is down a long way. I would plan to drain the tank (anything better than a siphon?) as much as possible to reduce the weight, and might see if I can find a cheap tranny jack that would work. I just like to know what I am getting into before I start backing off bolts.

Thanks!

Bill
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Old 10-13-2008, 12:08 AM   #10
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This is excellent info! Since I've had bigger fish to fry on my truck, I haven't dealt with my fuel tank issue yet. My rear tank returns into the front, but instead of coming out the canister, it comes through the filler cap.

Mine was fairly easy to figure out, though - the front pump hasn't worked for quite some time, so the check valve was a sure bet. I climbed around under there forever, trying to figure out a short-term fix until I was ready to rebuild the front tank, but couldn't find any solution that wouldn't make repairing it right a much bigger hassle than it already is.
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Old 10-13-2008, 12:08 AM
 
 
 
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