I have an '87 F150 4.9L fuel-injected engine with 117,000 miles it. The truck has two gas tanks. Each tank has a low-pressure lift pump that feeds the gas from the tank to the input of the high-pressure pump that supplies the final pressure to the injectors on the fuel rail.

I'm having an intermittent problem when running on the (absolutely new) rear tank. The engine sputters and dies as if it is out of gas, but the tank is at least half full. If I switch to the front tank, the engine fires right up and I can then immediately switch over again to the rear tank and the truck will run fine for a while. The problem appears to be random in happening.

I understand these trucks have an inertia switch that cuts power to the fuel pump. My questions are:

1. How many inertia switches are there? Is there one for each tank so as to cut off the low-pressure lift pump in the tank that's "running", or just one switch that cuts power to the high-pressure EFI pump feeding the injector rail?

2. Where is/are the switch(es) located?

3. What method is used to measure the inertia? From other posts on this forum, I understand the switch has a reset button on it. If there is a switch for each tank, is it possible the one for the rear tank has reached a "hair trigger" condition and is cutting in and out based on vibrations at its mounted location?

Thanks in advance for any help.

 

1. There is only one switch. It's purpose is to cut off the power two both fuel pumps in case of an accident so that fuel is not pumped onto an accident scene.

2. Not sure where it is located at on your truck, but it's location will be in your Owners Manual. There is a red reset button on it. It's either on or off and would not operate intermitantly or reset itself. Must be manually reset.

3. It is a basic accelerometer that is set to trip when the vehicle see's a certain number of G's. It is similiar to curcuit breaker on a house when it see's too many amps it cuts the power off. You have to hit or be hit pretty hard for them to go off. It would probably trip if you hit a curb hard enough to blow a tire out.

Your problem is probably a bad pump or a bad connection at the pump.

 

This may or maynot solve your problem but it worked for me once. Try running on the problem tank with the gas cap off. I had a tank that had a plugged vent and it would create a vacuum in it and the pump could draw fuel. Good luck.

 

Inertia switch on my 94 F250 is behind the side trim on the passenger side, under the dash. My trim has a notch in the top to reset the switch without removing the trim panel. My '97 Vic has it in the trunk.

The last inertia switch I reset, the girl has been run off the road into a ditch. When it trips, your won't get ANY fuel. I used it to do my compression test.


When you say "absolutey new rear tank" I assume you replaced the tank. Why?
Did you replace the rear fuel pump as well?
What about the front one?

I just bought a hi-mileage (200k+) F250 w/ dual tanks. Been trying to fix a rough idle. Did a lot of reading on the net, and apparently the fuel pumps on these are linked kinda like siamese twins...if one fails the other is too. So they have to be replaced in pairs to resolve the problem.

Check out www.2carpros.com. They have a lot of detail on this. Also good info on my 4.9.

The fuel selector switch can also fail & I believe there's a relay in there as well.

Good luck. Intermittents are a pain!

 

Problem solved. It was a badly gunked up dash-mounted selector switch. Surprisingly, it not only solved the rear tank cutout problem but also corrected an unknown but developing problem with delivery from the front tank also. Prior to cleaning the switch, the truck lacked pickup power in all gears and had a tendency to feel "jerky" in the lower gears under hard acceleration. At idle in neutral or the clutch in, the gearshift **** would occasionally shake quite a bit. I've been trying to solve that problem for a long time. Now, the gearshift **** is rock-solid at idle, and acceleration is very, very smooth and lots of power. At 50 mph in 4th, you stomp the gas to the floor and it just about throws you through the back window. In lower gears, you'd better be hanging onto something if you stomp it!

Thanks to everyone for their advice.

To answer your specific question, ladyfire3374, I replaced the rear tank within the last two weeks with a new one right out of the box from the after-market manufacturer. No, I did not replace the pump ... I reused used the old one after checking over the filter screen and running it submerged in a covered pan of gas (very carefully, I might add). I saw no reason to replace it.

Also, on your advice, I read all entries listed at the 2carpros.com website regarding the F150. Thanks for sending me that link. Yes, they do say it is necessary (in a number of their answers) to replace BOTH pumps if one goes bad or you suspect a problem (and, of course, the cost of $75.00 per pump plus labor to remove & re-install both gas tanks is of no concern with them!). That solution almost sounds like their stock answer to any fuel problem.

Speaking as Mechanical Engineer with 30+ years of experience, I can see absolutely no logical reason whatsoever why the operation of either in-tank pump would be dependent on a balanced performance with the other pump ... not with the way the system is designed by Ford. I'm certainly open for further discussion on this matter and I'd love to have someone convince me I'm wrong. But if someone wants to give me their best shot, I want them to do it in detailed engineering terms of pump mechanics, hydraulics, and electrical control systems used in this particular fuel system design ... and be totally logical every step of the way while doing it. Until that happens, I can only conclude that the alleged requirement to replace both pumps at the same time was the result of someone in the past mis-diagnosing a fuel delivery problem and erroneously concluding that this two-pump replacement solution was necessary to correct it. And now it is perpetuating itself as the definitive answer to any fuel pump related problem.

I like to be wrong and have someone convince me I am ... that's how I learn. Does anyone want to try with this alleged solution of needing to replace both pumps at the same time? If so, bring it on! I'm ready for ya!

 

Traderjoe,

Glad your problem was solved simply and cheaply. I've resolved a number of issues just by cleaning. I appreciated the detail of your reply.

I'm a dedicated DIYer with no formal mechanical training. From experience I've developed my own repair methodology - I select the most likely, and then start from the simplest, cheapest point. For anything electrical it's power, followed by contacts, then wiring. For fuel issues, the fuel filter, and so on. You always have to evaluate the quality of the information on the Net. I'm always looking for some one who has more knowledge or experience (preferably BOTH). My favorite source is an experienced mechanic. If I'm doing something a little tricky for the first time, I prefer an amateur who can report on pitfalls that a pro would assume are known. I looked at a lot of the 2carpros material, and compared their solutions with ones I have known to work. That's why I recommended them.


I'd like to offer a few thoughts-

First, a notion as to how & why 2carpros makes their recommendation:
As an engineer, you might appreciate this example:
**The example assumes that the mechanic is honest and isn't angling for a bonus for selling more work.

If as a mechanic, I fix a problem on the first try, my customers are generally happy and reassured that I am competent. If the price is high, knowing that the problem isn't likely to recur will soften the blow.

If, OTOH, I try to fix the immediate problem for the lowest possible price, and the customer has more problems of a similar nature, they are inconvenienced, annoyed and dissatisfied. They also doubt my competence. If the problem is a difficult one, I may wind up doing work for free, and still get cussed out.

Experience allows me to test each hypothesis, and validate (or refute) my assumptions. Given the criteria above, I want the answer that produces the most positive results, and that answer is the former.

Second, why sometimes the right answers produce the wrong results:
Engineering doesn't exist in a vacuum. As an example, the rotors on my '97 Crown Vic are perpetually warped. I don't drive like a maniac. This was a known issue soon after they were adopted in 1992. It wasn't resolved until 1998, when the Vic went to 16" tires to accomodate larger rotors. I know that engineers can calculate the HP input, the weight of the vehicle, brake pad area & material, and a host of other relevant variables and figure exactly how much heat those rotors will have to dissipate. It seems like a reasonably straightforward ME problem. So why did it take so many years to fix? Why is my only option for solving the problem to replace my brake hardware, tires & wheel with the newer design? Did the engineers screw it up that badly??? I suspect not. I think compromises were made that rendered the design inadequate for the intended task. I'm just speculating here. A clearer example is the O-ring on Challenger - an engineering decision was hijacked by company brass and agency politics. With predictable results.

I know mechanical mythology runs rampant, supported by few, if any, facts. I'd also like to see this proven, mechanically or electrically. I worked in technical support, where I had to deal with many of the same pressures faced by mechanics. I'll take an efficient, practical answer that works most of the time, over a documented "correct" answer that results in a lot of re-calls.

It's always refreshing to hear from an engineer - most of us don't think with such clarity.