EFI fuse missing, CEL on. (1 Viewer)

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Well I've got a problem and hope someone here has experience in this area. My 1991 80 series LC needed a new exhaust, I had been patching it up just forward of the CATs for a few years now. My local Muffler shop did the work, replacing the pipes from the header to the crossover pipe, which they welded in a few spots. This included adding new CATS but they used the same O2 sensors. When I picked it up, after hours, the CEL was on and the truck ran rough, especially at stoplights and signs. I called them the next day and they thought the light was on when I brought it in, it wasn't. Anyways, I took it back and they were unable to get the CEL off or smooth out the roughness. I have it now and the first thing I though to do was take out the EFI fuse for 15 minutes to reset everything, hoping that would do the trick. When I went into the fuse box the EFI fuse is missing! I have to admit, in the 7 years I've owned the truck I've never noticed whether it was there or not, the place where it goes is a little gnarled up and dusty so I don't think the shop removed it. I assume it has been missing since I bought the truck. Anyways, can anyone tell me first of all, why is the fuse missing? Is it a practice amongst some owners to disable the EFI? Why would my truck run so well (before this) if the fuse was missing? What should I do now? Should I replace the fuse? Can I jumper the fuse leads (I don't know why)? How do I get the CEL to clear now? The muffler shop used an OBD1 scanner (it showed codes for both O2 sensors, nothing else) and couldn't clear the CEL, is there a way to clear it without the scanner and with no EFI fuse? Does anyone have an idea why the codes even came on, would just taking out the O2 sensors and reinserting them cause this?
Sorry for all the newbie questions, not new to my LC but new to these electrical issues and there are no experts in any dealerships or shops anymore that have knowledge about these things anymore, I guess 1991 was too long ago and all the techs from that era have retired. Thanks in advance, Bob Nancarrow in Michigan.
 
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The truck shouldn't even run without the EFI fuse. I believe the EFI relay sends power to the circuit open relay (which powers the fuel pump). I'd check the EFI relay inputs/outputs with a multimeter to see if they are outputting 12v on key on. In order to solve the roughness you really need to read the code. I'm guessing its related to the CAT performance.

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EDIT: This wiring diagram is from a 96, so it may actually be wrong....
 
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I've included a picture of inside my fuse box, like I said the socket looks damaged and not recent. Anyways, I did drive it home, just ran more roughly than usual. If it's been missing awhile I can say the truck has run smoothly for years now up until this exhaust pipe exchange.
Are you referring to the 15 amp EFI fuse that is in the fusebox on the left fender under the hood directly below the 7.5 amp CHARGE fuse?

View attachment 2343040
Yes, that's the one, trying to take a picture of the blank spot now but having trouble loading it into the computer. Bob.
 
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I've included a picture of inside my fuse box, like I said the socket looks damaged and not recent. Anyways, I did drive it home, just ran more roughly than usual. If it's been missing awhile I can say the truck has run smoothly for years now up until this exhaust pipe exchange.

Yes, that's the one, trying to take a picture of the blank spot now but having trouble loading it into the computer. Bob.
Well then, a previous owner molested that circuit, no doubt trying to figure out why the truck wouldn't start for him/her.
No that is not common. Obviously, the EFI system is getting fed with +12 from another source, for without that there would be no B+ to the ECU and the majority of sensors under the hood. There would be no spark, no fuel, etc.
 
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Hmm, the previous own was the original owner and 80 years old when I bought the truck, a rich businessman so I know he didn't tamper with it, might have been a shop he took it to though. I will look more closely at the wiring to see if anything is amiss but there's no obvious evidence of tampering (except for the missing fuse). Thanks so far, Bob.
 
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Hmm, the previous own was the original owner and 80 years old when I bought the truck, a rich businessman so I know he didn't tamper with it, might have been a shop he took it to though. I will look more closely at the wiring to see if anything is amiss but there's no obvious evidence of tampering (except for the missing fuse). Thanks so far, Bob.
Bob. You're not missing the EFI fuse.
You're missing the EFI relay.
When I asked you if the fuse that was missing was directly below the 7.5 amp charge fuse, you said yes. But I'm staring at that fuse in your picture.
 
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And just to come full circle on this, removing the EFI relay does NOT clear codes and reset the ECU. You need to remove the 15 amp EFI fuse. That's the blue 15 amp fuse that is upside down in your picture. The one directly below the 7.5 amp CHARGE fuse.
 
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OK, well, somebody did Something! There's no wiring going to the EFI fuse location and there's this funny looking gizmo with wires from under the fuse box going to it. Looks like somebody rewired it for some reason and it DID run nicely for many years this way. There's no way I could ever figure this thing out. Does anyone have a suggestion of how to clear the CEL (which has not been on for at least 7 years now) and get this thing running smoothly now with the new cats?

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And just to come full circle on this, removing the EFI relay does NOT clear codes and reset the ECU. You need to remove the 15 amp EFI fuse. That's the blue 15 amp fuse that is upside down in your picture. The one directly below the 7.5 amp CHARGE fuse.
Well, thanks John, good advice, I will try it now and let you know the results. Bob.
 
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OK, well, somebody did Something! There's no wiring going to the EFI fuse location
Bob, I don't know how I can possibly make this any clearer for you.
There is an EFI FUSE and an EFI RELAY. They are 2 different items that live in this fusebox. If you look at the cover of the fusebox, it has the locations of the items printed on it. I also gave you a picture from the FSM in post #3.
You are NOT MISSING the EFI FUSE.
Your EFI RELAY has been relocated and can be seen in the last picture wrapped in electrical tape with white tie wraps around it.
 
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OK, so far, disconnected the battery for a bit, no change. Took out the
Bob, I don't know how I can possibly make this any clearer for you.
There is an EFI FUSE and an EFI RELAY. They are 2 different items that live in this fusebox. If you look at the cover of the fusebox, it has the locations of the items printed on it. I also gave you a picture from the FSM in post #3.
You are NOT MISSING the EFI FUSE.
Your EFI RELAY has been relocated and can be seen in the last picture wrapped in electrical tape with white tie wraps around it.
I understand John, the last post I mde was before I read what you wrote, thanks, Bob.
 
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OK, took out the EFI fuse for 15 minutes and replaced it. No change in the rough idle OR the CEL (still on). I think my next move is to get under the truck to check the O2 sensors and wiring and make sure they didn't bugger anything up. I won't be back here for awhile but if anyone has any other suggestions, I'm all ears (or eyes).
Should the truck run rough for awhile after the EFI fuse is replaced? Does the truck learn things after this and eventually smoothen out?
 
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OK, took out the EFI fuse for 15 minutes and replaced it. No change in the rough idle OR the CEL (still on). I think my next move is to get under the truck to check the O2 sensors and wiring and make sure they didn't bugger anything up. I won't be back here for awhile but if anyone has any other suggestions, I'm all ears (or eyes).
Should the truck run rough for awhile after the EFI fuse is replaced? Does the truck learn things after this and eventually smoothen out?
Typically, the idle will be higher than normal until the ECU relearns the sensors. If the ECU is giving you O2 codes and it's running poorly, then there's your answer.
You also stated that as soon as the exhaust shop was done, it threw codes and ran poorly.
 
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Typically, the idle will be higher than normal until the ECU relearns the sensors. If the ECU is giving you O2 codes and it's running poorly, then there's your answer.
You also stated that as soon as the exhaust shop was done, it threw codes and ran poorly.
Yes, that's when the codes started and the rough idle. Is that an unusual occurrence or normal? Do you think that just driving it now after doing the EFI fuse trick will make it better? The Muffler shop was no help, they just didn't have the knowledge about old trucks to figure out how to get rid of the CEL. Bob.
 

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