Blowing EFI fuse

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Joined
Mar 11, 2010
Threads
12
Messages
92
Location
Kokomo, IN
I have done a lot of research on here and I have found many threads and many suggestions and many different reasons for this to happen. Well it's happening to me and I can not fix it. I have taken it to 2 local shops now and they haven't been able to diagnos the problem. This has been going on since May 5th. I am sick of carpooling with my wife every day, ( not that I mind her company or driving her Hundy but I miss the independance of my own LC). The current mechanic working on it now thinks computer. Here is what we have done so far. I replaced the fusible link. It was cheap and worth a shot. The 2 mechanics traced wires from the fuel pump to the fuse box and every where between. I told them to specifically look behind the egr valve. One guy decided since he couldnt find anything he would eliminate the the whole wire from the fuse box to the firewall by just replacing the whole thing, (new wire from box to firewall). It now has a new relay, ( the one in the fuse box). I am desperate at this point and very frustrated. Who can help me?:bang:
 
have you checked over by the glove box behind the dash where the harness goes into the plugs into the ECM?

They can rub thru on the metal bracket that's part of the dash
 
[STRIKE]A couple more ideas

1. Have you got an aftermarket security system installed? The security system in mine + faulty electric door lock actuators = door opened while security armed. This resulted in a blown EFI fuse. Took me several fuses to figure it out but I removed all the power to the security system and that worked![/STRIKE]

I read your other posts...

2. wiring harness along firewall
3. What he said ^^

Good luck
 
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have you checked over by the glove box behind the dash where the harness goes into the plugs into the ECM?

They can rub thru on the metal bracket that's part of the dash
Thanks I'll mention this to the mechanic.

If this isn't it does anyone have a known good used or new computer I could borrow or buy?

I also forgot to mention the fuse does not blow right away. I can replace it, start up the LC and drive. I might make it 100ft or I might make it a couple days of driving it to work and back before it blows the next fuse. The Mechanic told me he thought he had it fixed but when I just blew another fuse last night he had a 30A fuse in it. No wonder I was able to drive around a couple days. Also it seams to blow the fuse when you let off the gas pedal. The last few fuses have blown in his or my driveway after driving it home.:bang:
 
he needs to 1st stop putting in 30 amp fuses or you wont need to worry about finding the problem as it will burn down to the ground . 2nd he needs to start looking at the harnesses that are rubbing somewhere like the trans/ o2 sensor wiring harness and the other wires that maybe pinched/rubbing with the motor rocking back and forth .
Id say hes just tossing parts at it now . 99% most likely its a wiring issue and not a component problem .
 
Update: I tried to drive it back to the shop this morning. No luck. With the proper 15A fuse I got about 3/4 a mile from the house. I put a new fuse in an it blew as soon as I put it in drive. This was new. I fiddled around with the wireing harness, I removed the glovebox and looked around the harness where it attatches to the computer. Then when I put another fuse in now it is blowing them as soon as I turn the key on.????
 
From my experience, automotive mechanics generally suck at fault finding electrical issues. The best net is to take it to a qualified auto electrician - for a start he wont be stupid enough to putting over rated fused in to cover up the problem.

You clearly have a major electrical fault that needs fixing asap. Your auto electrician, if he is smart enough, will systematically work through the whole fuse box and wiring harness back to the ECU as the most accurate way of eliminating all possibilities and to leave you with a solution.

If you wanted to have a look yourself, my bet would be to looken the fuse/relay box from the inner guard and remove all the wrapping around the cables. I'm pretty sure you'll find some shotrted out wires there with a fair bit of melting going on.

Do keep us posted on how you go. I can only imagine your level of frustration at this point.
 
I can virtually guarantee you it's not the ECU, so put that thought out of your head. I also second guzzla's suggestion: if you can't do the trouble shooting yourself, find a qualified auto electrician; the one you've got sounds like he doesn't know what he's doing.
 
More thoughts...

Have you read this thread https://forum.ih8mud.com/80-series-tech/84774-efi-fuse-blowing-ideas.html?

If the fuse is blowing as soon as you turn the key on, it will be easier to troubleshoot the problem. Start by removing the fuel pump relay. Now turn the key on. If the fuse doesn't blow, the problem is in the fuel pump wiring. If the fuse still blows, it's not the fuel pump wiring and you need to look elsewhere.

I know you "told your mechanic" to look at the wiring behind the EGR valve. But you didn't say if he actually did it. It's not an easy place to get to and you have to carefully remove the cable sheathing, tease apart the wiring bundle, and individually inspect the wires for burned insulation. Takes a while to do properly and a problem could easily be missed by a mechanic who didn't know exactly what to look for.
 
It's probably one of two things...

I was gonna suggest you check the link that TrickyT just posted...that one resulted in quite the adventure for me and my daughter...but that's another story...

Or as I mentioned in that same link, Morgan Fletcher had a link that he posted for a pre-95 LC that might be exactly what you're experiencing.
I had a 94 80 series that was blowing the EFI too and his instructions helped my mechanic trace the source of the short. It was the shorting out along the firewall. I'd check there first, since it's much easier than digging into the fuel tank to remove the fuel pump and examine the bracket...

EDIT: just checked some older posts...the bad news is that the links to his actual posts are no longer active...the good news, sorta, is that I did see that it's related to the O2 sensor wires...so that might be a place to start looking...

https://forum.ih8mud.com/80-series-tech/17772-i-keep-blowing-efi-fuses.html
 
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At this point, this might be a long shot but if you haven't checked it, it's simple to do.

Look under the truck on the passenger side where the wire harness to the O2 sensor runs along the frame.
Mine had come loose and flopped on top of the front driveshaft and worn through over time and shorted out the power wire to the O2 sensor.

I could drive for sometimes miles/hours without it shorting out, other times it would blow right away.

I repaired the wire and re-routed the errant harness to it's original position and haven't had a minutes trouble since.

g'luck, Dan.
Before_repair_w-text.jpg
 
Id check the wiring under the black fuse box under the hood. Mike
 
Have you read this thread https://forum.ih8mud.com/80-series-tech/84774-efi-fuse-blowing-ideas.html?

If the fuse is blowing as soon as you turn the key on, it will be easier to troubleshoot the problem. Start by removing the fuel pump relay. Now turn the key on. If the fuse doesn't blow, the problem is in the fuel pump wiring. If the fuse still blows, it's not the fuel pump wiring and you need to look elsewhere.

I know you "told your mechanic" to look at the wiring behind the EGR valve. But you didn't say if he actually did it. It's not an easy place to get to and you have to carefully remove the cable sheathing, tease apart the wiring bundle, and individually inspect the wires for burned insulation. Takes a while to do properly and a problem could easily be missed by a mechanic who didn't know exactly what to look for.

That thread was very interesting. I can't believe I had not seen it before. Guess my search skills suck as bad as my diagnostic skills. I forwarded the important info on to the mechanic ( i will call him Mike from here on out, it's shorter).

It's probably one of two things...

I was gonna suggest you check the link that TrickyT just posted...that one resulted in quite the adventure for me and my daughter...but that's another story...

Or as I mentioned in that same link, Morgan Fletcher had a link that he posted for a pre-95 LC that might be exactly what you're experiencing.
I had a 94 80 series that was blowing the EFI too and his instructions helped my mechanic trace the source of the short. It was the shorting out along the firewall. I'd check there first, since it's much easier than digging into the fuel tank to remove the fuel pump and examine the bracket...

EDIT: just checked some older posts...the bad news is that the links to his actual posts are no longer active...the good news, sorta, is that I did see that it's related to the O2 sensor wires...so that might be a place to start looking...

https://forum.ih8mud.com/80-series-tech/17772-i-keep-blowing-efi-fuses.html

What an ordeal you guys went through. I read your whole thread. I hope your daughter is way over the ear infection.;) I have been trying to avoid the high price of the dealership but at this point I wish I took it there first. I am ready to be over this. Do you know, on the link you posted, when this guy says main EFI Relay is he talking about the one in the fuse box where the fuse is that keeps blowing? I have had the same idle issues he describes but we already replaced that relay and it didn't change the idle or the rough idle between blowing fuses (before it started blowing them imediately that is).

Id check the wiring under the black fuse box under the hood. Mike

Mike (the mechanic) took that box out and unravled most of the harness from the fire wall, behind the EGR and engine, and all the way to the fuse box. He found nothing but replaced the wire straight from firewall all the way to the fuse just incase it was shorted where he couldn't see it. Obviously now, that did nothing but being stumped gets you to try anything.

I will take all these suggestions in and look extra hard at o2 sensors and all wires from there up and if that doesn't do it I will tell him to pull the pump and check there. Thanks for all your help so far. I will keep you posted.
 
it's my understanding that it won't throw a code while this fuse is blown. No power to ECU.
 
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I wish I would have posted this thread a month ago instead of all the searching. I mentioned the o2 sensore and wiring to both mechanics (the 1st one I had literly gave up after 3 days) and they both told me they did't see anything in that area. I stressed it again this morning to Mike and he looked again at the sensors and also the fuel pump. Apparently It had the exact same thing fj80toyman had in his picture. It literly rubbed a clean spot on the drive shaft where it had came off the frame and been laying there. How could 2 professional mechanics miss this after being told to inspect those locations. I am Kicking myself for not looking there first before I took it somewhere. Being electrical it scared me enough to not even try. I imagine it would have been giving a check engine light but I have had one on for months and it is that EGR system code. All the searching for that led me to believe it was that VSV under the intake. I will **** if repairing this wire makes the check engine light go out for good! For now I think I am in the clear. Mike is going to reroute and tape up all the wiring he has opened up and clean everything back up tommorow and I better be back on the road. He took a picture of the offending wire and I'll try to post it up tommorow. Thank you all again for your help! This site is amazing!
 
Wow!


Glad I could be of some help. I felt your pain, I struggled with this one for some time too before finding the trouble.

Pay it forward.




Dan.
 
glad it worked out for you. :cheers:
 
I'm glad I found this thread. My 94 started quit today, I changed the fuse and it blew again when I turned the key on.

Got a ride home, found this thread, retrieved the vehicle and sure enough it had a shiny front drive shaft and wires worn through.

back on the road!
 
I had this same problem about 6 months ago, and took me 2 days of searching top to bottom, should've started bottom to top, and was the exact same thing o2 wire on front driveshaft. Crazy
 

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