Welp, my 97 F-150 4.6l won't start. We know its not getting gas, we sprayed some starting fluid in the throttle body and it ran (not good, lol) off of that, so its definatly getting spark. I just replaced the fuel pump and fuel filter..but still nothing. The shutoff switch isn't tripped, I checked that already. Any other ideas?

 

It can be numerous things. It can be anything from a throttle postion sensor that has gone bad to a MAF. I will explain all of these. PCM's have a thing called clear flood mode and when the PCM see's a full 5 volts from the TP sensor it goes to this mode. It is for when the engine is flooded and doesn't need anymore fuel that it kills the injectors. The only time it sees this is KOEO (key on, engine off) with the pedal to the floor, or if it fails and sends a full 5 volts back to the PCM. The MAF could be reporting an untrue amount of air entering the engine so it isn't delievering the right amount of fuel. There are just several things that could be wrong. My advice is start checking to see if you got fuel pressure on the rail at the injectors. Then check to see if the PCM is switching the injectors. It could be a bad fuel pressure regulator that isn't allowing the fuel system to get enough pressure in it. You need to look a little deeper into the problem. Also does your fuel pump run with KOEO (key on engine off). It should only run for about 2 seconds. Just some ideas

 

argh..its not getting any pressure at the rails, and the fuel pump doesn't turn on with the key on. Thanks for the help so far

 

Ok check all your fuses first and make sure your relay is working right. If they are all good then check to see if the pump has power coming to it. Also check ground connections because these can cause some crazy stuff. If you have ever been in a wreck before or if it has ever been in the shop for anything serious, you will want to check to see if they may have left a ground wire off or didn't tighten the bolt tight enough. If you have any other questions just be sure to ask we are more than willing to help.


Matt

 

how do I test the relay? And the relay you speak of is the one on the passenger kick panel near the computer right?

 

You can have someone turn the key and put your finger on the relay and you should feel it click. There is a more precise way of testing it but this is just a quick check you can do. Also don't forget the fuse. A fuse can look good but may be blown at the bottom so be sure about the fuse being good.

 

Thanks for all of your help so far. The relay under the hood did click, but we switched it with another one and tried it anyway, still no luck. We tested the power to the inertia switch, it is not getting a full 12v. The truck hasn't been in an accident and it hasn't had any major repairs involving the wiring (other than a new starter last November). The fuse under the hood is also in good shape, we looked at it pretty closely.

This was a pretty good truck up untill about 2 years ago, then one thing after another started happening. It probably doesn't help that it sits in our driveway alot, it only has 66k miles.

 

Ok you shouldn't have a 2v drop anywhere in the circuit. The only place that voltage gets dropped is across the motor and it will be 12V. So you need to find out what is wrong there. It can be high resistance in some wire (corrision, etc...) Check to see how much voltage you have at the pump. Also you can ohm out the pump. The resistance values for your fuel pump are 22.5 ohms to 145 ohms. The less amount of gasoline you have in the tank the lower the lower the ohms will be. That is why they say you burn up your fuel pumps by running a tank dry because there isn't enough resistance in it and the pump gets to much voltage for too long. Let me know what you find. I still have more ideas

 

In the fuse box under the hood you have an PCM power relay and a fuel pump relay.
If either of those doesn't work your fuel pump wont run.
At the inertia switch you will only have power for about 2 seconds when you turn the key from off to run. You can check that with a test light or voltmeter while you cycle the key from off to run. If you dont get 12V at the inertia switch for 2 seconds when you cycle the key then you know it's something ahead of that such as one of the relays not working.
If both relays are working you could even have a bad PCM. It might not be telling the fuel pump relay to turn on.

 

Do you have a wiring diagram for the circuit. If not you defiently need to get one. I can email you any wiring diagram you need for this truck. It is relatively simple to read since it is from the service manual at the dealership. Just let me know if you need anything. If you don't understand how to check something either let me know and i can provide it in the simplest terms so anyone can understand.

 

Ya I could use that diagram. I would post my email but for some reason the site won't let me do it..argh

 

If you email mmorton through the system here, by clicking on his name and then hitting the email link, he will have your email address and he can send it there.

 

I need this one post. It says I need to be a member for 15 days or have 15 posts, so haha. edit: Hmm he wishes not to recieve emails.

 

Sorry guys i was outta town for the weekend deer hunting. My email is mmorton@siu.edu or mmorton@mchsi.com I can be reached at either. I would be more than happy to get you a wiring diagram when i get your email address. Sorry for the delayed response.



Matt

 

ok my 97 is doing the exact same thing.......
right about 60,000 miles and sits alot
have changed the relays and the same thing
if we jump the terminals we can get the fuel pump to run and the truck will start but wont do anything above an idle
im totally at a loss about what to do now