Months ago, a friend of a friend asked if he could use my as part of his wedding procession. “Sure, the thing runs great,” I replied. A few weeks ago, I let the groom drive the car; all systems were a-go. Then this past weekend I went to upgrade the cooling system. This was a major mistake.

There are few things that I, a former powertrain cooling engineer, hate more than marginal cooling systems. Keeping a vehicle from overheating isn’t rocket science: Make sure you have an effective and clean radiator; ensure you’ve got airflow through said radiator (make sure your fans/fan clutches are working properly, ensure there’s nothing hindering airflow to the heat exchangers); maximize that airflow with well-sealed fan shrouds; use a proper thermostat; have the right coolant mixture flowing through your system (this prevents corrosion, too, which is important); keep air out of that coolant; tune the engine properly and keep it within its designed operating points; run a good water pump; keep the cooling system sealed (i.e. pressure test it to make sure there’s no leak into, say, a combustion chamber due to a bad head gasket); and I could go on.

The point is, it’s not that hard, especially on a 1966 Ford Mustang, whose cooling system literally consists of a radiator, water pump, thermostat, heater core, mechanical fan, in-tank transmission cooler (this Mustang has the C4 three-speed automatic transmission), and some hoses. This is why, when I saw the vehicle’s temperature needle creep towards the top of the range a few weeks ago in traffic in Ann Arbor, I decided to take my Mustang under the knife. The car would probably have been fine for the wedding, but there was no way I was going to risk this thing overheating, because it turns out: The car that takes the groom to a traditional Indian wedding is a big deal, if I am to believe the website :

Grooms take great pride in his choice of transportation. We have seen everything from helicopters, yachts, exotic cars and of course elephants from time to time. The most common way to arrive is by horse. The horse will be adorned to match the groom. The groom wants to make a statement when he arrives to impress brides family and 

Holy crap that bolded part is making me feel some real pressure right now [pulls collar, steam billows out], because yesterday wasn’t a good day for the formerly-abandoned Candy Apple Red Ford Mustang that I rescued from Charlottesville, Virginia as a college student back in 2012.

The vehicle ran great until I upgraded my cooling system. I installed a six-blade fan and a fan shroud:

This replaces the four-blade fan that the Mustang came with from the factory:

I also threw a transmission oil cooler up front, as well as a that bypasses the cooler until the transmission oil has reached an appropriate operating temperature (roughly 180F). The TBV isn’t strictly necessary, I should mention, but I’m not about to give up cold-weather efficiency if I can help it.

Anyway, the installation procedure was no problem, and everything appeared to have been working well when I took the Mustang on a post-operation Woodward avenue jaunt. But then, the Mustang left me stranded:

The machine sputtered and died when I let off the throttle on my way back east towards Troy from Woodward Avenue. Now the trusty 289 cubic-inch V8 no longer idles in gear (it does idle in park or neutral).

What exactly happened? Well, it’s clear to me that the new fan is adding more load to the engine, but would this cause the motor to stall when sitting at a stop light? Not likely. Plus, I should be able to adjust for that by twisting the idle speed screw a half a rotation or so. Unfortunately, I found that I have to turn the idle to a ridiculous speed to prevent the vehicle from stalling in gear.

So what is it? Well, it’s possible the engine got a bit warmer than it had before, possibly due to air in the cooling system, but mostly due to the fact that it was 90 degrees out, and I was putting the Mustang to the test before the big day. The vehicle didn’t overheat, but especially when I shut the engine off, it was clearly hot in the engine bay.

What this short period of elevated temperatures could have done — and this is just my current line of thinking — is exacerbate an already-existing vacuum leak. One that, when small, I could accommodate for with carb tuning, but that has now grown thanks to thermal expansion.

Spritzing some starting fluid at the base of the carb confirms a vacuum leak, but whether it’s more significant than what it was before my cooling system upgrade, I don’t know.

I did add a significant restriction to the transmission cooling system with the bypass valve and cooler, and while that might jibe with the fact that the vehicle only stalls out in gear, I just don’t think that’s the issue. The vehicle’s engine is dying at idle under load (it otherwise runs great; the car still accelerates nicely), and I have reason to believe it’s because the motor is running lean.

I’ve had wrenching deadlines in the past. I’ve had , , and . But this time, the stakes are higher than ever, because it involves someone’s wedding day. And as you can see in the block quote above, this car is going to “set the tone for the Baraat.” This is no joking matter, which is why I’m a little anxious about something that, let’s be honest, shouldn’t be that hard to fix.

It’s time to spin some wrenches.