Help please - intermittent stall - now solved (I think!)

[Cees Klumper]

Well I spoke too soon. As I wrote right above, on Dec 24, I picked up the truck from the shop who said they fixed the stalling by addressing a vacuum leak and fitting the new distributor, I took it for several test drives over the past few days and, when in over 5 trips totaling some 50 miles it never stalled or even faltered, I took the plunge today and drove it from LA to our new home in Fallbrook, some 120 highway miles. All was going 100% perfect until, after about 60 miles, the engine stalled again! Doing 65 mph in the middle of 5 lanes of heavy traffic, in the rain. I managed to coast off to the nearest exit, while putting it in neutral and starting it back up - it did start right back up but as I was taking the exit, stalled again. The exact same problem decided to rear its ugly head out of the blue, after more than 100 miles of trouble-free cruising where, just before the shop took it in, it would not drive for more than a mile and it stalled out 5 times on the 3 mile drive to the shop.
Oh boy, what can this be?!
I had no tools with me but I was carrying some kitchen stuff including a roll of tin foil. I figured what the heck and proceeded (in the rain, in a strip mall parking space) to wrap the TFI module and spout connector wires in some tin foil. Since the code I was getting could indicate some interference with the ignition signal from the TFI back to the ECU, e.g. if too close to spark plug wires. Started it up and went a couple hundred feet and it stalled again.
My last resort, before calling AAA, was to once again turn my attention to the inertia switch (inside the warm and dry cabin, that was nice). As I posted before in this thread, I had disconnected the plug and jumpered it, to try and rule out the inertia switch as a possible cause of my troubles early on, however that did not seem to make any difference back then. Then, when I went to plug the connector back into the switch, I noticed my jumper had backed out of one of the two connections, thus no longer providing the circuit that normally the inertia switch does. That was very strange and someone commented that it was indeed strange.
I decided to jumper the connector again, taking the inertia switch out of the picture.

But before I made the new jumper with the one piece of electric wire I happened to have in the truck, I decided to just unplug the connector and see if it would start. It started right up! Without the inertia switch connected, and without a jumper - just like before. Weird, still.

So I made a new jumper, inserted it into the connector and started off once more, this time avoiding the highway and sticking to local roads to continue my journey. I fully expected it would just stall out again within a mile, as I had not let the engine cool off (it all seems heat-related) but to my surprise the engine didn't stall or misfire even once, during none of the remaining 58 miles.

So, to recap: 6 or so trips totaling about 120 miles and not even a hint of the problem, then all of sudden after an hour and a half of highway trucking, the engine just stops cold several times. The only things I was able to do today was"insulate" some wires under the hood (but that did not seem to have fixed anything as it stalled out almost immediately) and then re-jumper the inertia switch connector, thereby taking the switch out of the equation.

But even without the jumper the engine was starting and running?

Yet it was no longer stalling now that I had a secure jumper in the connector. Here's my theory - I think it may be a combination of:

- a bad inertia switch (I can see many of these are sold all the time, they do go bad), AND
- one of the wires to the inertia switch is grounding out somewhere, usually closing the circuit that is normally provided by the switch (or a jumper)
- I did not notice the switch was bad since the wire was grounding out anyway, so with or without the switch, OR a jumper, the truck would just run - most of the time
- explaining why earlier, and today, I was able to start the engine without the switch connected and without a jumper in the connector. Normally that should not be possible
- HOWEVER, sometimes, when the engine gets good and hot, or it shakes a bit, or whatever, that ground that the defective wire 'normally' gets, is interrupted. If the bad inertia switch is connected, the ECU gets the message that the inertia switch has been 'tripped' and shuts off power to the fuel pump, killing the engine
- now because I today inserted a solid jumper, whether the wire grounds out or not, the signal to the ECU is "all is good" and no more stalling

Well that is the only logical explanation I could come up with for what happened today (and earlier in this saga).
Time will tell. Ordering a new inertia switch and going on more test drives, not too far from home this time ...

 

[RobbieD]

- a bad inertia switch (I can see many of these are sold all the time, they do go bad), AND
- one of the wires to the inertia switch is grounding out somewhere, usually closing the circuit that is normally provided by the switch (or a jumper)
- I did not notice the switch was bad since the wire was grounding out anyway, so with or without the switch, OR a jumper, the truck would just run - most of the time
- explaining why earlier, and today, I was able to start the engine without the switch connected and without a jumper in the connector. Normally that should not be possible
- HOWEVER, sometimes, when the engine gets good and hot, or it shakes a bit, or whatever, that ground that the defective wire 'normally' gets, is interrupted. If the bad inertia switch is connected, the ECU gets the message that the inertia switch has been 'tripped' and shuts off power to the fuel pump, killing the engine
- now because I today inserted a solid jumper, whether the wire grounds out or not, the signal to the ECU is "all is good" and no more stalling

The circuit through the inertia switch to the fuel pump is its power; (+) 12 volts. If that circuit touches ground while the pump is running, it'd be a direct short and would blow a fuse.

Since it's the pump power running through the inertia switch, if the switch is unplugged then the pump could not run.

With what you've described, it sounds like at some point in the past somebody's "hot wired" the circuit, likely to bypass the inertia switch. It honestly sounds like you've got some past bypass wiring that's getting flaky.

1990 fuel pump wiring; 2.9 and 4.0 fuel pump wiring are identical.

 

[Cees Klumper]

Thanks RobbieD. What's interesting to note, though, is that, a few years ago, I hit someone's trailer hitch which triggered the inertia switch, killed the engine as designed and I could not start the engine until I pressed the re-set button on top of the switch. So at least at that point all inertia switch wiring was as intended, no bypass. Aside from that one shop that resealed the intake manifold and replaced the distributor two weeks ago, nobody has touched the truck except me and I have not messed with any wiring. So between that fender bender back two/three years ago and more recently, something in the wiring has changed to where it will tend to run without switch or jumper in place. Sounds like it can only be the ECU getting an "all ok with the inertia switch" signal when it's not supposed to, somehow.
I tried finding an auto electrician somewhat close to me a couple of weeks ago but no luck in the time I had before moving house. Now we are more settled, I can look again around here, if the truck acts up again. Thanks for your help and the diagram, I can try to see if I can troubleshot some more.

 

[RobbieD]

Sure is a head scratcher, with the history you just related. If you need anything else out the EVTM (wiring book), hit me up.

 

[bobbywalter]

while unlikely... what can happen... is that in the factory inertia switch connector the wiring is bridged from resistance sometimes ...generally heat related due to the failing inertia switch...

since you are in rustfree cali....it is likely if the vehicle is equipped with ac .....or the drain vents in the cowl are plugged from tree litter and rabid squirrels....water will drop right on that connection under certain conditions....

its often one of the initial rust failures...at least screw hole wise here in the rust belt...


follow the power right from the pump back to the source.

it can not operate with it unplugged from the inertia switch for damn sure. so you have to prove that circuit or overlay a new one.



i suspect the relay wiring is inverted.

or the static weld on the split point is getting janky.


regardless ....the battery positive power to the pump is not incorporating the inertia switch. that is a known. proving the circuit is key.

 

[Cees Klumper]

Thanks bobbywalter. The area where the inertia switch is located in my truck is very dry, and the switch, connector and everything around it looks like new. But, when I was tinkering with it yesterday, right after the stalling occurred again, I did notice that the switch was very hot to the touch. I thought that was because I was running the heat to the floor, but now I am thinking it was due to the failing of the switch as you write.
What I now think, based on what you and RobbieD have written, is that quite possibly a previous owner did rig up constant 12 volts to the fuel pump. I've come across several hacks around the truck including a replaced TFI pigtail (that was quite professionally connected, credit where's it's due, other 'modifications' were not so proper. However, this 'permanent 12 volts to the pump' is wonky and although it works most of the time, it doesn't always, e.g. due to heat.
When the inertia switch was working properly, that would complete the (factory-intended) circuit in case the PO-rigged 'solution' was temporarily out of order. So, when I had my fender bender (actually it completely damaged my bumper, and I got a new aftermarket one through LMC truck, surprisingly cheap and good as well) the temp fix must have been 'out' and the bump tripped the switch, stalling out the truck. I was able to push the reset button and the truck ran again.
Over time, the switch has started to fail, leaving only the wonky 12 volt hack to ensure power to the pump. When that inevitably failed (due to heat, movement, poor wire/crimp connection or whatever) as happened on the highway yesterday, the proper switch circuit was no longer able to 'compensate'. Until I bridged the connector with a proper wire, hence explaining why the stalling stopped and I was able to complete the journey.
I guess I could test my theory by just removing the jumper again, and seeing how long it will be until the engine dies again. But firstly that's unsafe and secondly I am afraid of jinxing the situation - leave well enough alone!
New original Ford inertia switch on its way, surprisingly cheap ($15 plus shipping). Need to drive the truck today for errands, so we'll see how that goes.

 

[RobbieD]

I don't know if you've checked for this yet, but it might give you something more to go on:

1) Key "on", you should hear the fuel pump run for about 2 seconds then it turns off.
or
2) Key "on" you can hear the fuel pump run and it doesn't turn off.

If the truck does 1), then the computer is telling the relay to power the pump. This would be the original circuit as designed, and you just then need to determine why/how the pump power is getting past the inertia switch.

If the truck does 2), it would indicate either the pump circuit has been changed (rewired) to ignition power (more likely in this case), or the computer's control circuit is not operating as designed (less likely, plus there's the issue of the pump power getting around the inertia switch).

 

[Cees Klumper]

I just checked and when I turn the key on the fuel pump runs for about a second and a half, then stops (if I don't then start the engine, I imagine). This has been consistent since I bought the truck - so apparently it's 'as designed' except for its ability to run without inertia switch connected, which I will try to figure out when I have some more time. Thanks for all your suggestions and knowledge, I think we're getting somewhere, finally.

 

[RobbieD]

OK, good. You might try the "key on" test again, with the inertia switch unplugged. No pump run to prime, would indicate that the circuit is actually OK; if the pump still runs to prime then you do have a wiring issue which bypasses the inertia switch.

 

[Cees Klumper]

I tried that just now, and with the jumper removed from the inertia switch connector the pump does not prime. I haven't checked whether maybe the pump is energized when I start the engine (yesterday it did start and run for a bit while I was testing this same scenario) but maybe it's possible that it would run for a short while on the remaining fuel pressure? Will try starting and see if and how long the engine may run without the switch/jumper connected. Possibly another theory out the window, and back to square one on the possible cause here. Maybe it's a bad/loose connection from the TFI module - I did manipulate that a bit as I was putting the tin foil around the wires and although it still stalled after that, maybe it 'settled in' a bit later and restored whatever bad connection there may be in the wiring. And my jumpering the inertia switch connector had nothing to do with the fact that afterwards the truck kept running for another 58 miles without a hitch. Intermittent issues are the worst! (I call them Miss Intermittent).

 

[RobbieD]

it's possible that it would run for a short while on the remaining fuel pressure?

Yes, that's what I'm thinking. Generally, fuel delivery will be a stumbling engine stop, and if it just instantly stops running it's more likely ignition / electrical.

 

[Cees Klumper]

That's what I was thinking also. The stalling is instantaneous, no stumbling or struggling then dying. It does sometimes want to 'run on' but generally it's like flicking a switch. I replaced distributor, TFI, coil, ignition switch on the steering column, but of course there's lots more to the ignition wiring. I was getting code 18 as reported above but when the shop reported 'issue fixed' and it looked for 120 miles it was, I did not think too much about that possibility anymore. Now it looks like I will need to open that can of worms again, and I have almost 0 experience with electricals unfortunately. Anyone know of a good auto electrician down south towards San Diego / Temecula maybe?