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 20 (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 20 (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 20 (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 20 (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 20 (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 20 (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 20 (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 20 (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 20 (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 20 (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 20 (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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post #12 of 20 (permalink) Old July 18th, 2019, 11:24
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According to the wiring diagram, the injectors are powered through the 40amp AM2 fuse to the ignition main relay (see page 57: http://www.mr2.com/files/mr2/techinf...%20Diagram.pdf)

They shouldn't be able to draw power from the EFI fuse.

However, that's for the stock wiring with the stock motor. I would think your Gen5 would be wired the same, but you never know.


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post #13 of 20 (permalink) Old July 19th, 2019, 04:29
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I had that problem once on my Gen2 3s, IIRC it was an engine ground that came loose.
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post #14 of 20 (permalink) Old July 19th, 2019, 07:42
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You could use a multi meter with amp function to find out what your amp draw on that circuit is currently. You might be borderline right now and then it only takes another 1 amp and pop. If tour amp draw is high all the time, it would be easier to find than a random surge.
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post #15 of 20 (permalink) Old July 19th, 2019, 15:39
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Yep, I'd see what the current draw in that circuit is key-on, then with the engine running with a multimeter in series (in place of the fuse).

If the draw is high all the time even with key-on, start unplugging stuff on the circuit and see when it drops down a bunch to isolate which component is drawing a lot.
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post #16 of 20 (permalink) Old July 21st, 2019, 00:05
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Can't add much than the guys have given some good ideas.

Here's the fuel pump wiring mod most guys have done.

https://midshiprunabout.org/mk2/supr...ump-power-mod/
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post #17 of 20 (permalink) Old July 22nd, 2019, 08:51 Thread Starter
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Clifton and Defsport, thanks for the feedbackóthatís a good idea. The car is in the shop having an axle boot replaced, but as soon as it is back iíll check out the amp draw on that circuit. It has only blown when the car was on track (runs fine on the street), so weíll see what it looks like in the garage with little load. But that might be enough give us some insight.
And jdm3sgte, thanks for the tip about the ground straps, I will check them as well. Iím not sure how that might be the issue as it only seems to happen (at least so far) when the car is on track and under extreme load, but it is definitely worth checking.
mr2_mike, thanks for the modded fuel pump wiring diagram. Hmm, wiring the fuel pp this way would give it a full 20A to use, as opposed to what it stock. I wonder how the Walbro pump was wired on my car? Iíll checkóthanks!
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post #18 of 20 (permalink) Old July 22nd, 2019, 16:34
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Does the walbro get noticeably louder as the car warms up? I just had to replace one on one of my mr2's that was going out. Took me like 5 hours to do, where are you at in ohio?
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post #19 of 20 (permalink) Old July 23rd, 2019, 18:06 Thread Starter
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The Walbro doesnít seem to get any louder as the car runs on track. The car as a whole gives no notice that it is going to blow the fuse when running laps. It just dies. I have a video that I will post shortlyóIíll let you know once iíve uploaded it (I just got an error message when trying to upload it just now).
And EFIpp98, Iím outside Chicago, but travel around the Midwest (Mid-Ohio) and east with the car visiting tracks.
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post #20 of 20 (permalink) Old August 4th, 2019, 12:42 Thread Starter
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Was on a vacation for a week, but Iíve now posted the video of the fuse blowing on track. The fuse blew during three different sessions, one after another, and at different parts of the track (so itís not, for instance, left turns that are causing the issue). The car seemed to behave fine up to the fuse going out.
Hereís the link on YouTube:
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