Save Your Main Relay! FM Fuel Pump Rewire Kit (FM Live)

[@ThomasHinton-i4b]

I just found your video which is more helpful than all my previous searches, so thanks. I'm the original owner of a 2000 Miata LS 5-speed, 1.8 ltr. For the past year, I've encountered a problem, apparently linked to heat, where the car starts and runs perfectly. But if driven for some distance and then shut down, the car will not restart. However, if left for an hour or so, it restarts just fine, but the problem will recur if stopped again. My Miata mechanic has not been able to find the cause of this. He hasn't been able to replicate the problem so he cannot get any error codes to be generated, but I conclude that he doesn't run the car long enough. He's replaced several of the other normal causes of starting issues, but nothing has yet succeeded. As a result of your video, I replaced the main relay, but this did not eliminate the problem either. I am now assuming that the problem must be either the fuel pump relay or the fuel pump itself. Would you hazard a guess as to where is the culprit? I should add that the car remains in original condition and has never been mechanically modified. It has only a bit over 44,000 miles on it. Any help would be greatly appreciated!

[@FlyinMiataVideo]

Yeah, these intermittent issues can be a struggle sometimes. In no particular order, here are the things I'd throw money at, cam and crank sensors, coils, fuel pump relay, and fuel pump. You could rig up a separate light to troubleshoot the fuel pump from the fuel pump relay but since these cars have some age on them now, I don't think it's a bad idea to just replace them either.

[@chrisschneider7727]

This is always one of the biggest things to update if you're seeking +200hp. Old wiring and thin diameter wire is the main cause of high resistance.

[@FlyinMiataVideo]

Definitely a good preventative upgrade for higher horsepower cars!

[@frogers58]

Yep, know this problem very well. '05 MSM, 285hp. Over a 18mo period went through a dozen relays. At one point had a bag in the glove box with 5 or 6 that I would bend back in place so they made contact. My symptom was that the tach would swing all the way to 9K at key on then hear a relay clicking as I cranked it. Of course it wouldn't start, so I swap the relay. Tried the 40A and 80A versions. Eventually the socket in the relay box melted and warped. Pulled the relay box and cut off all the burned wires. The double crimp on the relay I separated into two circuits and added a second relay. Been a few years and the problem hasn't come back. I think the last time it happened was on a FM Summer Camp tour. I probably ended up with something similar to what Andrew came up with.

[@michaelh.9866]

Thanks, Keith!

[@merlyn2121]

Ooh. Wonder if Miata Mike or Brandon are going to do an install video. If I ever replace my fuel pump I'll be adding one of these.

[@FlyinMiataVideo]

We can add that suggestion to the list! Thanks!

[@merlyn2121]

@@FlyinMiataVideo I don't think people know how valuable your install videos really are.

[@scmiata99]

Hi guys Rich from Jersey , just wondering if that fuel relay kit is ok to run on a LS Miata? Thank you for your time and support.

[@FlyinMiataVideo]

It's definitely OK to run on the LS - but your GM harness may have the wiring and relays for the fuel pump and injectors in the fuse block already. Keith has removed the Mazda main relay from his LS car.

[@ricepony33]

I’ve had the main relay fail twice on my 94 in short succession. Both resulting in a tow… really made the car feel old.

[@FlyinMiataVideo]

It's a right of passage for owners of older Miatas.

[@mars-lv9xo]

Could this be the reason my 91 engine dies randomly? I’ve gotten towed 3 times now in the last month. Car will drive fine & suddenly the RPM needle will drop to zero. Car will roll to a stop & then wont start again. Really frustrating because I’m always wondering if I can make it to and from home.

[@FlyinMiataVideo]

Possibly. When the main relay fails, it cuts power to the ECU so the car stops running like someone turned off the key. Most other failures have the engine sputtering to a stop. They don't usually come back again, but it's possible letting the relay cool might bring it back. It's certainly worth investigating.

[@RobertMeany]

I was wondering if you guys have any experience with ND2 fuel pumps and related electrical? I recently put in a supercharger kit from Fab9, which included upgraded injectors and pump.. While tuning, we're having issues with fuel starvation at high RPMs and I'm wondering if perhaps the stock wiring/relay is not up to the task...

[@FlyinMiataVideo]

Sorry, we don’t. We used factory pumps in our V8 conversions and amazingly they were up to the job. Direct injection engines start acting weird when you get past 25% duty cycle on the injectors, which will happen at high rpm first.

[@calebburt9632]

My fuel pump relay went out a while back. I ended bypassing it with a little wire and it has been fine ever since

[@FlyinMiataVideo]

Not a great idea, as you want the fuel pump to cut out if the engine stops. Otherwise you've got a very high risk of fire in a collision or if a line breaks.

[@brandonlow27]

What wire gauge are you using for the main power wire?

[@FlyinMiataVideo]

The power wire in this kit is 14 awg.

[@starstar5657]

Don't put a spare in the gloves box, replace it and keep the old in the glove box if you must and forget about it. they generally last over 20 years. my current one is still going after 25 years.

[@FlyinMiataVideo]

Something is better than nothing!

[@MrSprintcat]

First on race day.

[@wesleydotcom]

Should I expect there a parasitic drain on the battery if I install this?

[@FlyinMiataVideo]

Not at all.