These MN12 cars haven"t been built since 1997. So that means that all MN12 Thunderbird & Cougars are 20+ years old. One of the things that goes bad over time, and not necessarily distance is the rubber hoses on the emissions system. Small rubber vacuum / emission hoses can only last so long under the continual under-hood heat and road conditions.

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If the emission hoses under the hood of your car are original, it"s time to replace them now, rather than wait. If you wait, it can lead to air leaks and cause idle or drivability problems, and most likely some diagnostic trouble codes like a P0171 or P0174. Your car may not idle right. The point is it"s worth the $25 or so needed to replace the hoses to ensure reliability of your car.

Now I must admit that 99% of the time I buy OEM parts. I"ll go to the dealer or shop online to get the quality of a genuine Ford part. I"ve heard that Ford/Motorcraft parts are designed to last 160,000 miles so sometimes they cost more, OK usually. In this case where we"re simply replacing small lengths of emission hose, it"s likely not critical to get OEM parts. For example, I inquired at the dealer about replacing one section of hose and I would have had to buy a whole kit with two rubber end pieces and the plastic part in the middle and just that part was $30. Well the plastic part of my setup was fine, so I just went to the local auto parts store and bought emission hose.

Note that these instructions apply to many more Ford 4.6L & 5.4L V8 engines besides just the T-bird. This same process applies to Mustang, F-150, Crown Vic, etc. although your hoses may be routed slightly differently.

Another thing to be aware of is that you cannot use regular vacuum hose to replace emission hoses. You must use emissions hose (or fuel line hose) on those hoses. Regular vacuum hoses just aren"t strong enough to work. The auto parts store may try to sell you fuel injection hose, that type of hose is rated to handle a lot higher PSI (pressure) and is not necessary. As far as I know it will work, but again it"s not necessary. I see there are silicone hoses on the market, I have no experience with them so I can"t say if they would work.

According to the factory Ford service manual, some (or all?) vacuum hoses are color coded. The manual mentions red and green striped hoses but I couldn"t tell if my lines had stripes or not, they were too old. For our purposes we don"t need the color stripes anyway. There should be a small sticker under the hood or on the front radiator support area labeled Vacuum Hose Routing, refer to that on your car to see where all your lines run.

Required Parts & Tools

In my case, I bought the following amounts of these hoses for my 4.6L V8 equipped car:

So I went to the local auto parts store for the stuff, but the total for those parts was a bit less (they sell hose by the foot even though you won"t use it all just buy what the amounts I mentioned).

Generally, this procedure should apply to all 1994, 1995, 1996, and 1997 T-Birds & Cougars with the 4.6L V8 engine. The process will be similar for 3.8L V6 and 5.0L V8 equipped cars, so refer to your underhood vacuum hose routing label for more details. There is a minor difference for the 1994/1995 4.6L cars, if your car is 94 or 95, see the note at the bottom of this article!

As for tools, really all you need is a sharp utility knife or maybe scissors to cut the hoses. You want nice clean cuts, preferably.

What I"m not including below is how I determined what hose sizes I needed. I used a digital micrometer and measured the internal diameter of the existing hoses and the outside diameter of the things they attach to. I figured out what is the closest size and that"s how I arrived at the three sizes: 1/4", 3/8", and 5/8". Technically the hoses could be a very slightly different size but all of the hoses fit OK. If you get a hose diameter that is too large you risk having a vacuum leak.

First off, to put this in perspective, this is all my old hoses laid out after I replaced them with new pieces. As you can see, they are crumbly and no doubt had a few minor leaks in them. When you touch the hoses, they leave black on your hands which means something is not normal. Vacuum leaks are not good, so let"s get started on replacing them.


One more thing before we start, here is a diagram with approximate locations of the hoses under the hood. The green marks in the photo apply to 1994-1995 cars:


I started by replacing the short section of 5/8" hose coming off the drivers" side valve cover. Simply twist and remove the short section of hose, lay it beside your 1ft length of 5/8" hose, and cut so the length is approximately the same. It doesn"t have to be exactly the same length but keep it close to the same length to make it easier. Ideally you should use emission hose here, however using the heater hose isn"t a big deal. You definitely don"t want to use heater hose on anything that will have vacuum, as the hose will collapse. The auto parts store I was at only had heater hose in 5/8" diameter and that"s OK for this specific application. If they had 5/8" emission hose I would have used that. Due to the oil/fuel vapor that may come in contact with the hose, the hose may break down sooner than the OEM hose, so it will have to be replaced sooner than the other hoses/lines. Again, this is a cheap fix so it"s OK here :-) Here"s a photo of the drivers side rotten hose:


To make things easier, I removed the air duct going from the MAF (mass airflow) sensor to the throttle body, which also has a smaller duct running back to the front of the throttle body where the IAC (idle air controller) is.

Next, I used a small section of the 3/8" fuel hose to replace the crumbling hose connecting from the passenger side PCV (positive crankcase ventilation) valve to the plastic tube that runs up to the intake.

PCV valve emission hose


While I"m at it, shake the PCV valve when you have it out, see if it rattles. Generally if it rattles, it"s still good, if it doesn"t, it"s time to replace it. That"s an easy DIY maintenance task to do.

Next up I replaced a length of hose that runs across the passenger side valve cover. This uses the 1/4" fuel line hose, approximately 5 inches long. Again, take the old hose off, hold it up to the new hose and cut off approximately the same length of hose.


Next up I replaced the L shaped hose that comes directly out the back of the throttle body and goes off toward the passenger side of the car. The original hose may have been molded with the bend, however I used a straight piece of 1/4" fuel line hose and it conformed well enough for my purposes. This hose was really rotten and cracked on my car.


There is a similar hose, same size L-shaped coming off the front of the throttle body, replace that one too. I don"t have a picture handy but it has the same bend off toward the passenger side of the engine.

Next I discovered a small short piece of hose down past the air intake box. So, I removed the air box to make it easier to access, I suggest you do the same. Basically the emission line that runs over the passenger side valve cover (remember you just replaced a section of that hose earlier), well, it leads to a plastic piece of line, then this small rubber connector goes between that and another rigid plastic line. You can see that in the very center of this next photo:


That"s it for what I replaced "under the hood". I traced that line that I just replaced the small connector line and noticed that it goes down underneath. the inner fender. So... I took off the right front wheel and removed the plastic fender shield for easier access. Plus it made it a lot easier to take pictures :-)

So, what you"ll see down there is the charcoal canister which is part of the EVAP (evaporative) emissions system. It has some rigid plastic lines, wiring, connectors, etc. There is a U-shaped piece of tubing/hose and that has a few small rubber pieces that need replacement too. Here is a photo of the area with the charcoal canister in place before I did anything. The other photo shows the U shaped piece that I removed as a unit (you need to disconnect one wiring connector, you"ll see which one), and that allowed me to easily replace the small pieces and put it all back together.



The previous two photos apply to 1996 & 1997 Thunderbird & Cougars. 1994 & 1995 model year vehicles have a very slight difference at the canister. Instead of one line going in, there are two, as you can see in the following photo courtesy of TBIRD9491. So there are 3 small pieces of 1/4" line that need replacement:

1994 / 1995 Models:


In addition to the hoses shown above, TBIRD9491 mentioned some other hoses in the fuel system that may need replacement. There is one hose in front of the fuel filter (you may need to loosen the fuel filter bracket and pull it down slightly to make it easier to access), there is one hose behind the fuel filter, and one near the fuel tank. FYI the fuel filter is located basically underneath the front passenger"s feet, under the car. The two rubber lines in front and behind the filter may be easiest to access by cutting/removing both and removing the plastic piece in between them (it"s on the same line), then putting on new rubber pieces and reinstalling:

In front of the fuel filter


Behind the fuel filter


Here is what TBIRD9491 has to say regarding the hose near the fuel tank, along with a photo:

Follow it some more and it goes up into a space along the fuel tank. This is a pain to get off and on, So it takes some patience. I used the flat head to get the metal line side off while pulling up. then to get it off i used the square pliers and pulled down . To install i grabbed the plastic line with needle nose"s held it in place while pushing the hose on ; then for the metal line grab the hose with needle nose and push down while pushing metal line up. (Note - only 1in or 2in space so take your time)


Well that covers the hoses that I replaced. I got all the rotted rubber hoses replaced that I could see, let me know in the forum if I have missed anything, or if you have any feedback or comments on this article.

UPDATE: Another hidden EVAP hose found!

Since writing this article, nall_one discovered an extra hose and notified me. Thanks for pointing out a hidden hose! You have to remove the passenger side inner fender (plastic) liner to access it. Mine was rotten, check it out:


Extra Note for 1994 1995 4.6L Cars

NOTE: If you have a 1994 or 1995 4.6L car, there are two additional 3/8" hoses to be aware of.

First is a Tee in the vacuum routing system. It leaks sometimes, so here is a photo of the T, it"s kind of hidden behind the throttle body and plenum on the passenger side. It is circled in the following photo (sorry I don"t know the source of this pic). A tip is to follow the plastic vacuum line off the PCV and it will lead back to this T. If you need to replace it, look for a Ford part number F7LZ-6B899-AA or a Dorman HELP! part number 47072:


Secondly, the tube that goes off from the Tee toward the drivers side then goes into a plastic tube, and at the other end of the plastic tube is another short rubber piece. This is awkward to get to, that line runs under the plenum / throttle body along the fuel line into the intake manifold.

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TBIRD9491 contributed this snippet regarding the T:

I disconnected the other rubber piece (other side) off of the intake maniold, by squeezing some needle nose pliers and working it off. It"s a very tight squeeze, have to be careful not to pinch or break fuel line. Then I pulled towards me by grabbing the rubber hose I just pulled off. Which came out with the plastic tube that goes across and connects to the T.