Strange oily film on inside of windshield


Dirt-Old 20+Year Member
Jan 29, 2003
Hey folks,
I discovered a strange film that is slightly oily on the inside of my windshield, above the passenger side heater vent. Anyone have any idea of what this might be from?
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Buy a U hose. Inside your engine bay on the passenger side, you'll see two hoses going into your firewall, thats where you bypass it. Take the hoses off and then slap on a u hose on the engine side. You'll know what I'm talking about once you have a look.
I'll add another agree to the heater core idea -- but just in case, I'd probably clean it off real good first and then watch to see if it comes back.

You can also check around the carpet there and see if it's wet, that is another sign. Sweet smells inside the car as well.
Your A/C comes on when in Defrost. The oily residue is on the windshield. A/C systems do contain oil. I think you would smell the coolant, but who knows. I think it is either a Heater Core or an evaporator. You wouldn't smell the A/C system as much as the heater core.
yup, gotta sweet smell, and it does come back!

Looks like heater core time... only 55k miles on this car, but nearly 20 years of age..

No wet on the carpet, I put down some garbage bags and towels just in case.. I can't get to it for about a week unfortunately.

How much time, realistically, should I allow to do this?

My skill level is so so. I have taken apart the cluster on this car before, and have removed seats etc., but never this dash specifically. It looks like the hoses under the hood are tough to get to also, and they are original, so I will change those too.. I would like to take my time and make sure it doesn't rattle when back together also!

Oh, any other small $5 parts I should change when in there? The blower motor isnt expensive, should I pop a new one in? :shrug:

From what I hear changing the heater core is a right of passage on this board, wish me luck!
It would be nice if some of the pics in his write up actually worked. My car is starting to get the fog on the passenger side and when you turn on the heat it lets out the sweet, sweet smoke screen. Luckily my passengers dont have a green foot bath... yet.
Ive done heater cores in newer F150s and some E series vans (probably the easiest heater core you can ever do) but Ive never tried one on an older car....
I've done a lot of mechanical work, and the heater core is probably one item I would almost pay someone else to do. Realistically, first time out, it's a 6 hour job, but doable. If you bypass the hoses, make sure you run your U-hose on the hard heater lines, dont loop the dead core together instead (how it sounded to me). You could just not use the heater/defront until you swap it, it doesn't circulate through there unless it's on.

The film you speak of happens on brand new cars too. Not to discount what many have said about the heater core. It is a PITA to change and I am not looking forward to changing mine, but at some point I will have too, I'm sure. And it may well be the core going bad. However it's isn't neccisarily what's happening.

My 98 GT gets this film inside the windshield too. My Cougar got it, my Taurus got it, my Tempo's all got it and my F150 got it too. It's pretty normal. If you clean you winshield on the inside regularly it doesn't usually get the chance to build up to the point where you notice it.

Most regular glass cleaners I've tried don't even touch it. They just smear it around. What I found that works bet is ammonia and water mixed 1 part ammonia to 2 parts water. Now this mixture is a little on the strong side and I recommend you use scented ammonia cause it's harsh smelling. I don't know if it will harm the dash, but I wipe it up off the dash after sprayingthe windshield then I wipe the windshield down. It's one maybe two wipes and the winshield is litterally squeaky clean.
omg yall are mustang fanatics an you cant take out the dash??
1. remove the cluster
2. remove the glove box
3. remove the center console
4. remove the trim around the radio ( be careful it WILL BREAK)
5.remove all the black 10mm and 12mm bolts underneath the dash
6. slightly pull up, then towards the back seat then pull up again( two ppl its easy) also remember there are plugs. should be one on the air bag and I think two on the driver side. ill post up some pics