1999 Ford F150 6 cyl Four Wheel Drive Manual 78000 miles
My truck does not start when it is hot outside - especially if parked in the sun.
If it is cool out (60's) it start fine and runs great. If it's in the 80's and parked in the shade it will start fine - if moved into the sun it will not start. If its 100 out it won't start no matter what.
I found that when the ignition is turned to on (engine is not running) and its hot out there is no buzz from the fuel pump. If its cool there is a buzz.
So I check the voltage at the plug on the frame rail by the fuel tank. When it will start it shots up to 12 or 13 volts. When it won't it only goes up to 7 or 8 volts.
Next I thought maybe it was the fuel pump relay. In the fuse box under the hood there were several relays that looked the same. I took the one for air suspension, which I don't have, and switched it with the fuel pump relay. This did not change anything.
I have tried starter fluid when it will not start and it runs briefly and dies.
One other thing I have done is sprayed the truck with water when it won't start. If its parked in the sun and in the seventies this will cool it down enough to start. If its any warmer this will not work. As you can probably tell I have been trying to figure this out for a long time. What should I try next?
Thank You in advance!
First be sure you have 12 volts a the relay feeds, if you only have 8 in it may only give 8 out. If ok, test the continuity (ohms) of the wire from the fuel pump relay to the inertia switch(under right side of dash) then test the same out to thepump right at the tank unit. You have a bad connection somewhere that is losing contact when it expands due to heat/load
May, 11, 2009 AT 10:58 PM
Thanks Jack, I tested everything and confirmed there are 12 volts at the relay feed. When the ignition is on there's 6.5 volts at the fuel tank and 6.5 at the inertia switch whether it starts or not. When the truck does start, at those spots the volts jump to the 12-13 range when the ignition is *initially* turned on. On the other hand, when its hot out and won't start, the volts only jump to the 7-8 range.
So my question is, what makes the voltage jump up to 12-13 when the ignition is turned to on? It seems like this is where the problem is.
The continuity is good in the areas you talked about. I tested the green wire from the fuel relay to the inertia switch and it passed. I then tested from the green wire at the relay to the red wire at the tank and this passed. So the green wire from the relay to the inertia switch is good and the red wire from the inertia switch to the tank is good. There are 3 other wires at the relay and I'm not sure how to test these. I can see that 2 go to fuses but then I'm not sure were they go from there? Other wires I'm not sure about are the 3 at the tank plug (Black, small yellow and small blue/white) and the one small gray/orange wire at the inertia switch.
What do you think I should try next?
Thank very much for your time Jack!
May, 13, 2009 AT 7:45 AM
When itis only giving out 7-8 test the feed to be sure itis staying at 12 during this time. Also test relay power and ground and trigger to see what voltages are where and the resistance to ground at the relay when it is messing up
May, 13, 2009 AT 11:57 PM
Thanks again Jack,
Today it was around 100 degrees so all the tests I did tonight were with it not starting. At the feed there were 12 volts. At the relay power there are also 12 volts. At the trigger when the ignition is on there are 12 volts. And at the ground with the ignition on there are also 12 volts. I want to test the resistance to ground but I don't know how to do it -- can you describe how to test for this?
I had it parked in the shade and it started fine. Pulled out into the sun and it was 90 degrees out. Left it idling for 1.5 hrs and the temp went up to 100 during that time but it never stalled. I shut it off, waited a few seconds, and it wouldn't start. Not sure if these details would help diagnose.
May, 14, 2009 AT 7:39 AM
Those are with it not starting? So it has 12 in but how much out to the pump when it wont start? Find the ground wire and use a meter on " ohms" setting. One lead plugged into the ground the other to the chassis or other known good ground to measure the resistance in the circuit
May, 14, 2009 AT 9:29 PM
Thank You Jack,
I think we are on to something! First to answer your questions: Those readings were with it not starting. Going out to the pump its 6.7 volts.
I checked the ground wire tonight and nothing happens it does not zero out. Just to make sure I was doing it correctly I tried it on another relay and this one did zero out. Next I spliced into the ground wire just below the relay and then grounded that wire to the motor. Resistance at the relay ground plug was then good. I turned the ignition on and the fuel pump came on but did not shut off like it should?
Where Does the ground wire go, and how do I figure out where the problem is?
Thank You for your time!
May, 15, 2009 AT 2:08 AM
Well now that its cool enough, I went back out and the truck would start. So I re-tested everything and what I am assuming is the ground performs the same way when it starts. Now I'm not sure there is a problem with the ground.
The wire on the fuel relay that I think is the ground is kind of a blue color w/ an orange stripe.
On all the other relays the ground wire is black and when I check the resistance they zero out. The fuel relay ground doesn't do anything on the gauge when I test the resistance, whether it starts or not.
When the ignition is on - the power in on the fuel relay shows 12 volts, power out shows 0 volts, the trigger shows 12 volts and the forth wire which I think is the ground also shows 12 volts. This is whether it will start or not.
On the other relays the grounds show 0 volts.
I am now lost again!
May, 15, 2009 AT 7:38 AM
According to my wiring diagram there are 2 lt blue/orange wires. One of them comes from fuse #10 and should be hot all the time, the other is the trigger from the PCM to turn the pump on.
Ther red wire is fed from the batt junction box" fuse 18" and should also be 12 volts.
The dark green/ yellow wire is fuel pump monitor, also for the PCM so it see the pump on or off. The PCM power relay feeds thefuse block that supplies the 12 volts, check for 12 volts at fuse 18 and the othrs in the same fuse block. The wire you grounded sounds like the trigger from the PCM that is why it wouldnt shut the pump off
May, 16, 2009 AT 2:01 AM
Fuse 18 and the others in that block show 12 volts. With the ignition on the green/yellow power out wire at the fuel pump relay reads zero volts. Whether the truck starts or not.
I was telling you the power out reads 6 or 7 volts, but that is only when I break the circuit somewhere such as unplugging it at the fuel pump. When its all connected it reads zero. Is that normal?
When the truck is not starting, and I initially turn the ignition on I get 3, 4, 5, up to 6 volts at the green/yellow wire at the relay. It then falls back to zero.
When it will start the green/yellow wire jumps to 11 or 12 volts and then goes back to zero. With the truck running the power out reads 13 or 14 volts.
Thanks for the help
May, 16, 2009 AT 8:45 AM
Did the relay you tried have the same number of pins where the connector goes? According to my diagram that is the monitor wire(PCM) as well as the feed to the inertia switch(Fuel Pump), so if it isnt giving out a full 12 I would think the relay is the issue if all other feeds are in spec. Have you tried jumping terminal 30 (Power) to 87 (fuel pump) with a fused jumper wire when it is hot to see if it will start?This will basically by-pass the relay