evolution x tech:

Potential Relay issue which may cause your car to run lean!

By: Chris Black

Recently I have seen an event happening which I felt would be worth mentioning to those of you that own an EVO X. I may have found an issue that would affect the fuel system of the car and make the car run very lean, and possibly cause engine damage. The issue is related to the stock fuel pump voltage switching relay. The Evolution 8, 9 and now the 10 all have a relay which switches the fuel pump from low voltage to high voltage when certain conditions are met (full throttle, or boosted conditions).

The purpose of the relay switching the voltage is so that the fuel pump will last longer due to the reduced load it will see about 9 volts. When the car goes full throttle full battery voltage (13.5-14.5V) is needed for enough fuel flow even at the stock tune’s levels.

The issue that I have been seeing is that the relay will be stuck in the LOW position. Obviously you can see how this would be a major issue. I had it happen to me once here at AMS, I swapped it out the with Daytime Running Light relay and everything seemed to be fine, and I thought nothing of it. Recently though, on a tuning trip to Texas I had the same exact thing happen to me 2 more times. I would hop in a customer’s car and do a baseline pull to see where the car was at before making any changes, and it would be the 13’s and 14’s for air fuel ratio. After messing with the first car I was tuning a bit trying to figure it out, I remembered the issue that I had on the car back at the shop and I swapped the relays. Immediately the car was super rich when in full boost because of all the extra fuel I had added in at the lower pump voltage.

I thought that it was odd that it had happened twice, but then again the next day when I was out tuning cars it happened AGAIN! Again I swapped the relay and everything was all good. Later on that day I received a call from one of my customers saying that his DRL’s were staying on and he was asking me I knew any possible reason why…

So, in summation I would like to make it known that there may be a possible issue with some cars and their fuel pump relays. I think this may be the cause of several of the motor failures that have been noted around EVOM and may currently be the cause of other supposed engine failures due to weak parts.

What you should do:

If you have an EVO X with a wideband A/F gauge in it, just check your A/F ratio.

If you have an EVOX without an A/F gauge in it, I would swap out the fuel pump relay with the DRL relay and see if there is an issue with your DRL’s staying on with the car off. I have had several cars where I swapped the relay out with the DRL’s and have had no further issue, however I believe that one relay was so far gone, that it actually kept the DRL’s on.

If you have an EVOX and are super paranoid about what may happen, I would get a volt meter and a buddy and pull up your rear seat and find the fuel pump positive wire. With the car running you should have about 9V, when the car is in boost you should have above 12V and there should be a clear point at which it switches.

If you are super duper paranoid and you don’t care about your warranty it may be a good idea to hot-wire your pump.

Here is the circuit:


This issue seems to be prevalent on cars with higher mileage on them.