QOTD: What Was Your Best Automotive Repair Diagnosis?

Some here at CC are accomplished mechanics, whether as professionals or as hobbyists.  Others of us, while mechanically inclined souls, are no threat to those who keep our cars running for a living.  I place myself in the second category.  From a clueless teenager who was shocked to find a clip on the back side of a screw which attached a piece of trim to the body of my ’67 Ford I have progressed to the place where I consider myself a reasonably competent twister of wrenches on basic repairs.  I suspect many readers here inhabit this space along with me, or perhaps will with more practice.

We had a recent discussion here about an ’89 Thunderbird.  This car resonated with me because I owned another car with a similar powertrain – a Fox body 1986 Mercury Marquis station wagon.  Beyond the fact that I liked the car a lot, it reminds me of one of my great triumphs as a diagnostician of automotive ills.

I purchased the car at a tent sale which east side Indianapolis auto dealers had set up in the parking lot of one of the city’s malls.  I had been looking for an inexpensive second car to replace my 83 Colt which had just been wiped out in a collision.  One of my rules of used cars is that the best buys are cars that are unpopular not because they are bad, but because they are out of style.  I knew that following the debut of the Taurus and Sable, those ugly duckling versions of the Fox body had become a great choice as a decent but cheap used car and I went looking for one.  The one I bought was not terribly old (five years) but had fairly high miles for the time at about 106K.  It was one of my more enjoyable cheap cars and was quite luxuriously equipped for what I paid with very few option boxes left unchecked by the original buyer.  Put wire wheelcovers and a chocolate brown “Brougham Decor Group” interior with the picture above and this was my car.

Photo source: Wikimedia commons


One morning made me start worrying, however.  I started the car and pulled away from the curb as I always did.  After a two block stretch on my street I stopped at a stop sign, preparing to make a right turn onto a busy thoroughfare.  I stepped on the gas and . . . not much.  The engine revved but the transmission seemed quite uninterested until it decided to bang into first gear off we went.  “Damn” I thought, “this is all I need – a transmission rebuild that may double my investment in this car”.  I knew how that would play with the finance committed at home.

I knew to check the fluid first and found it to be very, very low.  A bottle or two of Mercon took care of the problem but I knew that there was a bigger issue because that fluid had to go somewhere, a fact driven home to me two or three weeks later when the issue repeated itself.  The car did not seem to be leaking fluid, so just where the hell can transmission fluid go?  The radiator was an obvious possibility, but there seemed to be no contamination of the coolant.

I knew that oil goes away by either leaking or burning, but could transmission fluid be burned?  I was inspired to disconnect the biggest vacuum line I could find where it went into the intake and I’ll be dipped if it was not coated inside with slippery red Mercon fluid.  I knew that vacuum was involved in the operation of Ford automatics and quickly figured out that I had a bad vacuum modulator.  A little research revealed that my transmission was a C5 (instead of the C4 that I had assumed it to be) and a trip to the parts counter and a session on my back in the driveway had my transmission back in top shape, apparently none the worse for wear from being starved for fluid.

Looking back I remembered noticing a little puff of smoke upon startup for the previous couple of months.  That had not been (as I had wrongly assumed) my high mile 3.8 getting a little tired but had instead been my engine slurping transmission fluid through a bad diaphragm in the modulator.  Who needs Marvel Mystery Oil for lubricating those upper cylinders?

Boy was I ever proud of myself.  I had just diagnosed and inexpensively fixed an automatic transmission, something that I imagined few others could (or would) do.  That experience bolstered my faith in my mechanical abilities by no little bit and taught me that most of this stuff is not rocket science.  Instead it is reasonably simple for someone with some patience, some tools and willingness to do a little thinking.  I will also admit that this sort of thing is a lot easier with the internet, which offered me no help in 1991.  I still take my daily drivers in to the mechanic for most real repairs but do my best to diagnose a problem before I go there.  I am correct in the cause of a problem more often than not.

So how about you.  Is there an automotive repair diagnosis that you have made and remain especially proud of?  And for those real mechanics who may be tempted to chuckle and pat me on the head after reading this, hey, we all have to start somewhere, and you guys will probably have the most fascinating stories of all.