Vapor-lock in a Ford truck? (Off-topic)

[montanaman]

Hey: I got an '89 Ford F250 with the 5.8L (351) V-8 with 282,000 miles last winter for $1000. I'm hoping my XJ brothers can help out with a problem:

It's showing the classic symptoms of vapor-lock. When I start the truck cold, it runs great, and will keep running great for hours, no matter how hot it is outside. BUT ... if I let the motor warm up, then stop it and let it sit for 5 or 10 minutes or so, it has problems. It will start up again with no problem, then cough and die after about a minute. This happens a lot when I take it to run errands.

It's definitely a fuel starvation issue. When it starts to cough like that, I can take my foot off the gas, and it will "catch up" and run again -- and keep running as long as I'm real light on the pedal. But if I floor it, it will surge and then die for sure. It will do this several times after it's been sitting, then return to normal operation.

When it's running properly, it's runs really fantastic -- silky smooth. So I don't think it's an issue with the injectors or the ignition.

Here's some more info:

- Previous owner said it's done this ever since he got it 2.5 years ago.
- Gas comes from a high-turnover gas station. I use the same gas in my XJ with no problems.
- Dual gas tanks ... does the problem with both tanks.
- Fuel pumps in each gas tank, plus another fuel pump on the fuel rail (3 total).
- Fuel pump on the rail has been replaced.
- Fuel pump relays have been replaced.
- Ignition module has been replaced.

So whaddaya think? What does this sound like to you? Any suggestions for a solution?

Thanks!!

 

[kloker]

Oh, yeah, this brings back bad memories. I had the same problem with the same year model and engine truck, years ago. It was the ignition module mounted on the left fender well inside engine compartment. It was shot and was getting hot. Replaced it, problem solved. Get the best quality module you can, I had to do this twice, the first one was a cheap crummy one and did the same thing after only two weeks. Then I got a good Motorcraft one and the problem went away forever. If the module is getting hot enough to burn your hand, then you know it's no good.

 

[montanaman]

Oh, yeah, this brings back bad memories. I had the same problem with the same year model and engine truck, years ago. It was the ignition module mounted on the left fender well inside engine compartment. It was shot and was getting hot. Replaced it, problem solved. Get the best quality module you can, I had to do this twice, the first one was a cheap crummy one and did the same thing after only two weeks. Then I got a good Motorcraft one and the problem went away forever. If the module is getting hot enough to burn your hand, then you know it's no good.

Ok ... but what I don't understand ... my motor doesn't do this when it gets hot, it only does it when it's hot, and THEN SITS for awhile. It seems that if its a heat issue with the ignition module, it would do that even while you are driving, not only after it sits for awhile. Make sense? Did yours do it only after it had sat awhile?

 

[Matthew Currie]

Ok ... but what I don't understand ... my motor doesn't do this when it gets hot, it only does it when it's hot, and THEN SITS for awhile. It seems that if its a heat issue with the ignition module, it would do that even while you are driving, not only after it sits for awhile. Make sense? Did yours do it only after it had sat awhile?

When you park it it gets a good bit hotter briefly. I had a similar problem with an XJ with a bad injector harness. It would usually run fine until it was parked hot. After that, a resistive splice in the harness would get bad enough to stop one injector from firing.

I'd look for wiring and connector faults. A test of the injectors and harness sounds like a good place to start.

However, a weak ignition might also cause the same apparent starvation, so don't count on it being fuel.

You might also consider running a vacuum gauge on it to see if you get any odd readings there. It's possible that an ignition problem will show up on the gauge, but when I was going through this, I found that a dead injector is invisible to a vacuum gauge. That makes a possible test for which problem you're having.

Another thing you might try, just for testing, is to take a nice big old wet towel along, and throw it over the ignition module when you park. If your problem goes away, then you can pretty much bet on its being a hot module.

 

[montanaman]

Another thing you might try, just for testing, is to take a nice big old wet towel along, and throw it over the ignition module when you park. If your problem goes away, then you can pretty much bet on its being a hot module.

When I was a teenager, I had an old air-cooled VW that ran hot and got vapor-lock in the summer. I used to carry a water bottle and squirt the fuel pump with water. That fixed the problem every time. I'll try the idea of the wet towel on the ignition module and the on-rail fuel pump to see if either one of those is the problem.

Also ... this one has the ignition module mounted directly onto the distributor. Ford moved the module to the fender in later years. Does anybody know if the later, fender-mounted module will work on the '89?

Thanks!! is really awesome.

 

[montanaman]

Ok ... I'm pretty sure it's fixed.

I touched the ignition module to see if it was running hot. Motor cut off immediately. Turns out the wiring harness was not plugged in.

The guy who sold it to me said he replaced the ignition module, so he must've not plugged in the wires properly. I just pushed the connector onto the module, and the problem hasn't repeated since.

Thanks for the help!!