Electrical Bizarre issue

Shawn_Mc

Member
Oct 1, 2021
40
12
18
Anahiem
I've had this car since day 1, off the lot. 1989 LX 5.0 5sp convertible. It's now got 190K miles on it and it runs as well as the day I bought it (with the expected maintenance) but recently, it started doing a weird thing.

I replaced the slosh module in the dash with a solid state one from NPD. That corrected several issues I was having with the gauges. Then it got a 2.5" set of tail pipes as the OEMs were rusted into a jig saw puzzle. I replaced the sender and the fuel pump.

The car gets 21 mpg at 70ish down the freeway.

The weird thing? If I top off the fuel tank, put 13.5 gallons in it, it'll occasionally die, like someone is killing the power to the entire car. It did it once on the freeway at 75mph and I noticed that the tach signal disappeared, then came back when it started back up like nothing had happened. If I'm going 15mph. The car will die completely, sometimes it'll restart, sometimes it wont.

But once it burns off that top couple gallons of gas, it runs perfectly again. I'm good with schematics. but I'm not connecting these dots. I'm missing something.

If I only put 10 gallons of gas in it. It's as reliable as the day is long.
 
  • Sponsors (?)


When you say the car dies does it just quit running but all the electronics are still on i.e. the dash, radio, etc.? Just making sure what "killing the power to the entire car" means. Any chance the fuel pump wiring is getting pinched?
 
It sounds to me like your fuel vent line and canister are filling up with fuel.

I do not know if this will cause a flooding condition or not as I've never run into it. Maybe @Mustang5L5 knows? I can't recall who it was that I saw digging into this system before... It's been a few.
 
  • Like
Reactions: 1 user
That is a strange issue. I agree that i'd take a look at the charcoal canister setup to make sure the solenoid is stuck open and it's just sucking raw fuel into the intake. I'd just disconnect the vac line to the manifold and cap it and see if the issue goes away.

Other than that, wiring is prob where i'd look. The tach signal dropping is strange, so i wonder if there is a short in the wiring? Was the fuel pump ever replaced recently? Is the wiring pinched? What confuses me is the fuel level affecting the condition. Fuel is a poor conductor, so that shouldn't have any role in it.

I'd troubleshoot the canister purge setup first, and if that checks out ok i'd drop the tank.
 
  • Like
Reactions: 1 user
Oddly enough, it started right after I had the tail pipes replaced. I got a set of stainless 2 1/2" tail pipes from I think Summit for $180, and had those installed.
When I say it dies, It's only happened when the tank is topped off and I've only done that during daylight hours so I dont really know if I lose ALL the power, just that when it happened that once on the freeway, I looked down and noticed the tach signal was saying 0 but I know it was still spinning 1800 at least because I was going 70+ in 5th gear still. Then it came back on and drove down the freeway for 30 more miles no issue.
The canister thing is a possibility, I guess. I understand the purge cycle there and when that activates. But I wouldn't think the tach signal would drop to zero from the thing just getting doused with fuel.
I have had the tank out a couple times. Once to change a dead sender, another to change a dead fuel pump, and another to change the filler neck seal when it failed the evap portion of the smog test. I also replaced the fuel pump relay. And the ignition module (both done in the parking lot of the Autozone because I had a no start condition).
 
Yeah, the tac falling on its face is electrical and the fuel line isn't going to make that happen. A short, however, can def kill an ignition system.

I would be thinking about dropping the tank about now and maybe even swapping a pump while I was in there looking at it all.
 
A dead tach (stall) is an ignition issue.
Shutting off while driving down the highway and restarting on its own is 98% likely the PIP sensor. When they start to fail you can experience intermittent issues like that.

Having a full tank is probably just coincidence.

When the EEC loses the pip signal, the engine will shutoff, and the CEL will illuminate. A code is not usually stored. When the pip signal comes back, the engine restarts.

The PIP can be tested using a multimeter. The problem is having a safe place to pull over next time so you can do the test.
 
With regards to the tach dropping out and it being an ignition issue I would agree that it could be the issue. Tach gets its signal from the ignition module or the negative side of the coil. It gets a key 12V+ and is grounded. So more specifically it could drop out and be a ground or 12V+ issue and the car could stay running.

OP needs to chime in and let us know if the car stays running or not when this happens.
 
I already changed the ignition switch. I did that when I changed the slosh module. When the engine dies, the tach signal is also gone, and when the engine comes back, so does the tach signal, which is why I know it's electrical and not a fuel deal.

And when I say it dies and looses the tach signal, there's no back fire, there's no stumble. It's literally like you killed the power to the coil and turned it back on.

The weirdest thing is, if I dont top off the gas tank, the car is dead reliable. I'm driving it back and forth to work, 60 miles a day at 80mph (and 20mpg!) down the freeway, 5 days a week.

The dots I cant connect are what is driving a full tank of gas to kill power to either the coil, or the ECU. Something with the fuel sender signal at full is causing an electrical issue. I what I dont have is enough information regarding the sender signal. Is a full signal running through the ECU? So, if that signal is compromised somehow mechanically to ground, is that killing my ignition circuit?