Big Fans Killing Engine at Idle
 

Hello, I have a 1990 with a VVT engine and FM turbo kit. I recently installed the FM Stage 2 fans which basically cool the car from 190 to 170 in a matter of seconds but there's an issue.

If the car happens to be at idle when they kick on, the car will die. The RPM raises a bit as commanded and the IAC opens a bit, etc but it just can't handle the increased load and I am not sure what to do.

If the car is moving or even a small amount of pedal is in play, the car won't die. I also have an RX7 alternator, big yellow top, 0 gauge ground cables, etc.

The car is running awesome except for this one little annoyance.....Can I ramp in voltage to the fans more slowly? Can I run each fan separately to the MS to turn them on at separate times?

Thanks.

Do you do your own electrical work?

Yes, I have plugged in multiple TV's and computers.

Great.

- Get 2 heavy duty relays, about 12 feet of heavy gauge wire, associated crimpers, terminal ends and shrink tubing. Wire should be at least 5 mm.
- Wire heavy duty relays so they are triggered by the OE fan relays.
- Run power to the relays straight from the battery using the heavy gauge wire.
- Use leftover heavy wire to connect the relays to the fans.
- Modify your ECU setup to compensate for voltage drop.

Wash hands, remove mask, open beer and enjoy.

Sources on best ways to compensate?

Not all Megasquirts have voltage compensation. I'm pretty sure fan idle up delay can remedy most of the drop.

Is the + battery cable not heavy enough?

Is the stage 2 PWM controlled? Why are you running the temps at 170-190? What temp thermostat are you using? And what is your idle rpm?

I have maxed out the time for the fan idle up delay, it does rise but even so, when it hits the engine dies immediately. I'm running an MS3PNP.

Voltage drop compensation though......This might be where it's at for this, I have it set to compensate but maybe not enough, will measure, adjust, and report.

Idle is set to 900 and set for a 200 RPM rise with the fan on. It sits right at 1100 with the fan on all day long, it's just when I'm idling AND the fan turns on at the same time that the engine dies. If I am driving and it turns on then it will come back down to idle and stay there no problem.

They both come on at once, which is really the issue, I probably need to wire one to a second stage. Thermostat is a 180 in a coolant reroute. Once it hits 190, the fans turn on and then it just sort of sits on the thermostat.

I bet I could just unplug one fan and it would stay cool enough....this setup really moves some air.

I had the same problem with idle droop. I'm in the process of switching from 2 12" spal fans to a 13" single spal with a lower overall amp draw and a PWM ford fusion/c6 corvette fan controller which has a soft start.

I'm not familiar with the FM 2 fan. If it has its own controller that has min adjustments you should be able to control it directly with the MS3. There are two methods in the MS3, a table and curve. In the table you can request it to turn on at a lower % at idle rpm which would reduce the droop.

I have a reroute with a 180 stant super stat. I tested it in a pot of water and it starts to open at 180 but is not fully open until 187. There is no need to drive the temp down below the stats full open temp. Increasing your idle a little will help as well. Mine is set at 950rpm. I rum my + lead to the battery cable on the starter and the neg to the PPF which is just an extension of the batteries neg cable. I was able to make major improvements by logging when the fan would turn on. Zoom in on the graphs and watch the IAC and timing. There are several idle compensations to adjust as well.

You should also have aggressive idle advance on. I usually have 10 degrees of added advance by 100rpms below target. This will start compensating for the rpm drop as soon as it begins. Also when they're on and off, is your car returning to it's 900/1100 idle with zero droop? And if you target 600-900 rpm with the fans on or off, is it idling correctly with good afr and stable rpms?

you know, I was using spark idle compensation and turned it off while I was testing something else and left it off, I will put that back on also.

car is returning and holding steady idle at 900/1100 off/on, seems to like 13.5 AFR the most at idle and for the most part sits there.

the issue just seems to be when the fans turn on it hits the system so hard it just turns off, like switching off a light, it doesn't slowly die or stumble then die. it's just lights out in a split second.

if I am driving and they turn on (which is the case 95% of the time) then there's no issue (and they never turn off how they are set up right now, off at 160 - the 180 thermostat prevents getting back there). so once they are on, everything's fine.

Try the wiring method I explained in post #4.
I learned that method from @Joe Perez and it works.

If the fan is already on and the engine returns to idle does it stall or does it only stall when the fan initially turns on at idle? If you are running the fans all of the time you may need a larger alternator. I'm running a NA8 alternator and at idle the output is relatively low. Do a log in its current state and then try it after charging the battery.

Running an 8ga wire back to the battery would have more resistance then running a 10ga wire to the battery cable on the starter.

THIS.

I can idle at 700rpm, fans kick on and get a slight "dip" in rpm to maybe 500 and it just shoots right back up. Settling in around 1000rpm.

Idle AFR is another story. Mine is all over the place, but, it doesn't sit around idling long enough to even be an issue.

190 seems like a very low threshold for kicking the fans on, especially with a 180 thermostat. I'd go like 205/on 195/off personally.

Depends on your thermostat, if it’s a 180, then on at 190 is actually a little late. 205 for a 190, is technically over heating already.

Thanks guys, I will try some of the easier tune related options first and if that doesn't work I will look at wiring also. Unfortunately theres a lot to do other than Miata stuff so no instant gratification for this thread but I will update when there is one.

Leo, I do have an RX7 100 amp alternator installed and it made a big difference with voltage.

Also, I do not use a normal on/off temp because I don't want the fans turning on and off constantly. They cool the car down so hard and fast that it would literally take 10 seconds to go from 205 to 190 so I prefer to just set the off temp to something it won't get to (160 in this case) with a 180 thermostat. What that tells me is that I should probably just disconnect one fan and let it slowly cool down.

I am used to tuning cars that allow you to set fan speed vs coolant temp so the fan runs at 20% or whatever a lot to maintain a target temp. I don't see a table for that in the MS. It's all or nothing, which is annoying.

I thought they were pwm controlled fans, I now see that fm offers a stage 2 with brush and brushless fans. I will make a more comprehensive post about my cooling upgrades in a few weeks. I battled this issue to a lesser degree but only because I had lower draw fans. I also had cooling issues and could not increase the fans current draw/cfm because of the idle droop which was at my tolerance limit. Hopefully the pwm controller will solve the problem.