FP/INJ -- fuse blowing

Boxcars

Banned
Car drives fine, then suddenly shuts off. Found FP/INJ fuse blown. Replaced with another fuse and as soon as I turn the key ON, it blows. I have come to realize that it happens only after I make full throttle blasts. Make a WOT run and then, next thing you know, the car shuts off. The kicker is that after the car sits for roughly an hour, or sufficient cool down, install a new fuse and iit will start and run normally again. During my latest break down, yesterday, I was looking around for a possibly shorted fuel injection harness irregularity, or other faulty wiring, I noticed wet looking, probably oozing, ignition module tan/brown potting material around the base of the ignition module plug. So, my theory is that my ignition module is on it's way out, becomes overheated under WOT, and causes the FP/inj fuse to blow, and once it cools, will again function. I have other known good ignition modules and will install one today and will post further. Any other thoughts, or past experiences or fixes, concerning this problem is appreciated, and great information for those who will have this problem in the future.
 

TurboBuRick

Moderator
Staff member
Do you still have the factory oil pressure sender hooked up? That shares the same circuit. If its been deleted trace the old plug out and see if it's laying on the header or something.
 

Boxcars

Banned
Do you still have the factory oil pressure sender hooked up? That shares the same circuit. If its been deleted trace the old plug out and see if it's laying on the header or something.

I did look for that, but I think I cut it off years ago probably where it comes out near behind the alternator. I'm about to test drive the replacement ignition module. The ignition module I removed was all gooey with half of the potting material cooked away, so, definitely going bad, and of going to WOT would make more heat and I hope, somehow cause that fuse to blow.
 

Boxcars

Banned
OK, that wasn't it. It shut off after I went to WOT. I let it cool for a half hour and it started right up and I drove home. While it was blowing fuses, immediately after break down, I unplugged my injector harness, but when turning the key on, the fuse still blew, so it's not that. I'll have to unplug stuff and check everything more closely. What confuses me is, if I have a wire shorting and blowing a fuse, why does waiting a half hour un-short the short? Any ideas?
 

Boxcars

Banned
I've tried different ECMs with the same result. My fuel pump does go up into the 65 psi range, so seems fine. Today it shut down after I got out of it. I thought it was OK, then there was just no power, and had to find someplace to pull over. At least it still runs OK out of WOT until I find the culprit. :)
 

mikestertwo

Well-Known Member
That fuse only feeds a few things....Injector harness, oil pressure switch, fuel pump relay, fuel pump priming connector located behind the alternator, and the fuel pump. Does your car have a fuel pump hot wire setup on it? If so who's is it? Try unplugging the fuel pump at the back of the car after the problem comes up and see if it blows the fuse again.
 

Boxcars

Banned
Hey thanks for the very informative info. Hot wire is probably my own diy set up. I'll be sorting it out tomorrow. What is confusing is that the short "unshorts". Any theories?
 
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GNRick

Retired member
Could it be the fuel pump relay? Not sure if this is a similar case but sometimes when I turn off the radio, the antenna stays up. It usually will retract within the next couple hours. Does that sound like the relay?
 

Boxcars

Banned
Thanks all for your very helpful suggestions. I will be using all of your suggestions in my search for this intermittent fuse blower and will keep you posted.
 

Boxcars

Banned
Could it be the fuel pump relay? Not sure if this is a similar case but sometimes when I turn off the radio, the antenna stays up. It usually will retract within the next couple hours. Does that sound like the relay?

I sure hope it is the fuel pump relay. I changed it this morning along with the hot wire set up relay. I can't test it yet because traffic is brutal, besides, I'm not looking forward to being disappointed if this doesn't fix it. Yeah, for sure, you'd think that if it "unshorts" itself, more than likely has something to do with a sticky relay.

But, we'll see. :eek:
 

Jerryl

Tall Unvaccinated Chinese Guy
I sure hope it is the fuel pump relay. I changed it this morning along with the hot wire set up relay. I can't test it yet because traffic is brutal, besides, I'm not looking forward to being disappointed if this doesn't fix it. Yeah, for sure, you'd think that if it "unshorts" itself, more than likely has something to do with a sticky relay.

But, we'll see. :eek:
A defective relay very rarely creates a short. It can happen I guess, but hard to see how.
If the short is in the wires to the pump, in the tank, you already know what you are potentially dealing with. If the short was in the HW, would it not blow the HW fuse?
 

Boxcars

Banned
A defective relay very rarely creates a short. It can happen I guess, but hard to see how.
If the short is in the wires to the pump, in the tank, you already know what you are potentially dealing with. If the short was in the HW, would it not blow the HW fuse?

By the same token, how could the shorted circuit fix itself and not be shorted just because the car sits for 15 minutes? Could be the fuel pump cooling down I suppose. Oh, and thanks for posting that little gadget.
 

Jerryl

Tall Unvaccinated Chinese Guy
By the same token, how could the shorted circuit fix itself and not be shorted just because the car sits for 15 minutes? Could be the fuel pump cooling down I suppose. Oh, and thanks for posting that little gadget.
Great question. Not sure as I am electronically challenged. Lol

That gadget saved my arse though. Lol
Let us know what you find.
 

Boxcars

Banned
Great question. Not sure as I am electronically challenged. Lol

That gadget saved my arse though. Lol
Let us know what you find.

Electrically challenged? Yeah, you and me both! If the fuse blows again, I'll disconnect the fuel pump at the tank, insert a new fuse, and see what happens. If it doesn't blow, that might indicate it's the pump itself.
 

ScottHeidinger

Active Member
I had a similar issue with the Orange wire that goes to my ECM in my T-Type. When I got on the brakes hard the fuse would blow. I would put a new one In and it was fine. One day on my way to work I had to slam on the brakes and after that I was on the side of the road and fuses just kept popping so I cut my cord to the radar detector and wired it to the ECM. Later I just ran a new wire to the battery and forgot about it. A year later I pulled the engine out and found where the Orange wire had cooked on the passenger side head where I couldn't see it. You have a different issue but I think sooner or later you will find one of these wires is shorting out on something. The insulation might be melted off a wire or it may be touching on something as the engine twists during WOT. It's easier to find in the driveway or garage than on the side of the road. I would keep looking when your at home and stay out of WOT until you find it.


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No disintegrations

Well-Known Member
Maybe you could add compressed air to the fuel pressure regulator in the driveway. Since wot the fuel pump demand will be very high due to the one to one rise with boost. If it blows fuse then I'd say pump is bad.
 

GNX026

GM World Class Tech
look at where the injector harness passes by the egr valve. they are known to short there. and if you are having thermal shorts that would make sense. becaue you wont have heat there until you made a few wot passes. hope this helps. Fred
 
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