EFI fuse blows after 3 laps on very hot days on Gen 5 swap - MR2 Owners Club Message Board
 
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post #1 of 11 (permalink) Old July 6th, 2019, 18:16 Thread Starter
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EFI fuse blows after 3 laps on very hot days on Gen 5 swap

This is a strange one: my 92 turbo track car has a gen 5 swap from about a year ago which has performed very well on trackóup to this past weekend. We had very hot, humid weather (100 degrees, 70% humidity). The car ran fine for the first two morning sessions, then in the third 25-minutes session (running 12psi boost), the car just died. Got towed back to the pits where I found the EFI fuse had blown. Replaced the fuse and the car started and ran fine. Went out next session and the fuse blew again on the third lap. Same routineógot towed back to the pits and tried again the next morning on track where the fuse blew again on the third lap (on another hot/humid day). The fuse blows always under acceleration, and with the car hot. No missing, hesitation or any other signs of distress before it goes, just everything is fine to completely dead in an instant. The new motor swap ran one other fairly warm event (low 80s) last summer, and ran fine.
I have my next event coming in two weeks and would like to have this diagnosed ASAP. Any ideas what is causing the fuse to blow? Is this due to the heat? Humidity? Something wearing out?
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post #2 of 11 (permalink) Old July 6th, 2019, 20:48
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As a best guess Iíd say itís an injector going bad which causes excessive current draw. Could check the injector resistance but might not tell truth if it only fails during hot temps. Could always throw in another set which is not a bad idea especially if they have been cleaned and tested. Iíd be concerned running car with a fault like this as could run a cylinder lean.
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post #3 of 11 (permalink) Old July 7th, 2019, 13:23 Thread Starter
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Thanks, benckj, for the quick response! Iíve sent on your idea to the engine builder/installer as it seems a good idea (I also tried running the car with the O2 sensor unplugged, but it still died about 3 laps in), and one that would be easy to fix. I donít know what injectors are installed right now, but am assuming they are readily obtainable so that this can be done before my next event.
Thanks again for being a resource, and I will keep you and the forum apprised as to how this all plays out.
And, of course, any other ideas you think could be causing this issue would be welcomed!
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post #4 of 11 (permalink) Old July 10th, 2019, 09:22
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Doesn't the "engine room cooling fan" aka intercooler fan run off the EFI fuse?
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post #5 of 11 (permalink) Old July 10th, 2019, 15:39
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No quite. It has power sources from 4 fuses.

7.5A AM2
20A VENT
7.5A GAUGE
7.5A ECU-IG

The fan itself gets power from the 20A VENT fuse.
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post #6 of 11 (permalink) Old July 10th, 2019, 19:39
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Could also be a fuel pump issue. The pump is probably the largest single draw on the EFI fuse. Might be worth while to run a new circuit / fuse to the pump with a relay that is triggered by the current pump circuit. This would also isolate the pump such that if it blows the EFI fuse again it's probably not the pump, but if it blows the new pump fuse you know it is.


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post #7 of 11 (permalink) Old July 14th, 2019, 07:54 Thread Starter
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Thanks for your ideas. The engine builder/installer has sent me a new ECU to try at Mid-Ohio this coming weekend (idea #1), along with two other ideas to try at the track while Iím there. Idea #2 is to disconnect the idle air motor (which has a slight draw on that circuit), and idea #3 is to try a slightly bigger fuse in the EFI circuit (20A vs the standard 15A). The wiring guy says the wiring in the circuit can handle the 20A fuse). Other than that, the problem is probably the fuel pump, as you pointed out, Alex. That is a bigger job, so weíll leave that for either after the event, or at the event as I will have a garage.
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post #8 of 11 (permalink) Old July 14th, 2019, 15:24
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Make sure that the fuse holder is clean and has sufficient spring tension. Bad contact in a fuse holder could cause a thermal runaway.

Another historic cause for sudden EFI blow is O2 heater circuit shorting to exhaust or axle. You said that you unplugged the sensor, but I am guessing that you didn't disconnect the entire sensor harness.
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post #9 of 11 (permalink) Old July 16th, 2019, 10:06 Thread Starter
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Thanks for the tips, rmeller, I appreciate your input! I did check for the possibility of a short with the O2 sensor wires, but found none. And the fact that the fuse only blows after about 3 laps (and at different parts of the trackónot in a hard left turn, for example), makes me think itís a component that is getting hot and then drawing more amperage. The fuel pump is an upgraded Walbro unit (255lph, I believe), and new as of the date of the engine swap (11/17). It has worked flawlessly up until this last event a couple of weeks ago.
We had a bit of a miscommunication with the new ECU being sent out, so I will have to sit out Mid-Ohio, but Iíll put in a new ECU in another week or so when it arrives. The fuel pump is the last thing we will deal with as it is such a pain to get to.
Other than that, any other ideas?
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post #10 of 11 (permalink) Old July 16th, 2019, 15:46
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My money is still on the injectors. Hard problem to diagnois but like spark plugs can cause issues under stress. They would be subjected to high heat and duty cycle so a definite possibility they have could have insulation issues leading to a short circuit.
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post #11 of 11 (permalink) Old July 18th, 2019, 09:16 Thread Starter
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Benckj, iíve been told that the injectors are on a different circuit (different fuse, at least), than the EFI fuse in the engine bay fuse box. Assuming that is true, could there still be a connection between the injectors and the EFI fuse?
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efi fuse, gen 5, swap, track car

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