Miata Fuel Pump Fiasco – Stranded

My Miata left me stranded while driving into work. The first portion of my drive was normal. Everything sounded and felt right as I pulled up to a red light. The light turned green, I started in first gear, and during the shift to second gear the engine shut off. I pushed the clutch in and attempted to restart. The starter cranked away but It was clear that it wasn’t going to run, so I pulled into the hatched divider area between the lanes. I checked the obvious stuff with the limited tools I had in the car. The battery was cranking strong but the engine wouldn’t run.

Stranded Miata

I could not fix it on the side of the road and had to organize a rescue mission with my Jeep Cherokee XJ. I connected a kinetic rope between the vehicles and put Rebecka in the driver seat of the Miata. I chose a kinetic rope instead of a tow strap to avoid aggressive jerking during turns. The mission was a success and I was able to diagnose the issue in the comfort of my garage.

Recovery

Once home, I could begin diagnostics with all of my tools available. After some quick googling, my symptoms aligned with a common camshaft position sensor issue. I ran to the parts store and got a new one. Got home and plugged it in…. And nothing.

Next I needed to actually start problem solving instead of throwing parts at it. Every engine needs air, fuel, and spark. I had already cleaned the mass air flow sensor and replaced the air filter. I also recently changed the spark plugs, coil packs, and spark plug wires so I was confident the problem was fuel.

I attempted to crank the car again and listened carefully for the fuel pump. I never heard it turn on. I focused my attention on that circuit and checked fuses and relays which were all good. Next I went to the pump directly and it was getting power at the connector but not doing anything.

Investigation

I removed the fuel pump assembly from the tank and immediately discovered the problem. The strainer/filter inside of the tank disintegrated and became the debris that it was supposed to block. The fuel pump had clearly been ingesting this debris and caused it to fail.

I bench tested the old pump directly with a 12v power source and it did nothing. Problem confirmed. Dead fuel pump.

I still had to suck the remaining debris from the tank or any replacement pumps would suffer the same fate. I used an electronic transfer pump to suck the debris out into a Tupperware container. I was surprised at how well it worked. This prevented me from dropping the tank or draining it completely. I thought that the debris would also destroy my transfer pump but I got lucky and it survived the job.

I ordered a new OEM Denso pump with a new strainer/filter from Amazon. While I was there, I also ordered a new fuel filter. Chances are that some debris made it through and got trapped in the in line fuel filter. This proved to be a smart choice because the old filter was completely full of dirt and debris.

New Fuel Pump

Fuel pump isolator modifications

The fuel pump I received was slightly different than mine and required modification of the rubber isolator that holds it onto the assembly. I used an Xacto knife to remove enough material so that the isolator sat flush in position. I cleaned up the remaining parts to avoid introducing anymore debris into the system. Everything went back together easily with no problems.

I primed the new fuel pump and filter using the diagnostic port box in the Miata engine bay. From here I was able to directly send 12v to the fuel pump by bridging the fuel pump pin to ground. This energizes the coil side of the fuel pump relay and allows the fuel pump to run without the engine being on. This priming got any air out of the lines to avoid an extended period of time cranking the engine which would put unnecessary strain on the starter.

After everything was buttoned back up, I turned the key and the Miata fired up immediately. I went on a test drive and noticeably felt a power difference. It’s crazy how proper fuel flow can make an engine perform better.

The Miata is back on the road and driving better than ever. This also seems to have fixed my intermittent o2 sensor codes. My theory is that the engine was running lean due to fuel starvation. This lean condition caused the o2 sensors to have fluctuating readings and throw a check engine light

Thank you for reading. Hopefully you were able to learn something about problem solving. This spontaneous unassigned project helped me learn more about my Miata’s fuel system and some of the electronic circuits involved. Learning how to systematically solve your problems like this will save you tons of money over your lifetime. The total cost of this repair was $125 without including the cost of my tools. My tools have paid for themselves time and time again with repairs like these.

Let us consider the scenario where I was unable to diagnose and repair my vehicle. I would have to call a tow truck to bring it to a local repair shop. The tow would cost at least $300. The repair shop would charge AT LEAST $700 for diagnosis, parts, and labor. I’m sure they would find something else that they insist needs to be repaired/replace immediately. Even if anything additional is declined, the total cost to get back on the road is $1000! Almost 10x what it cost me.

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