Question about 1999 Lincoln Town Car

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1999 Lincoln Town Car: This car runs and sounds like a diesel at speed after it warms up and the gas mileage has significantly declined. Tune up didn't help and now I am being told that the catalytic converters may be clogged. I am also told that sometimes after market converters may cause the O2 senser to give bad readings. What is your experience with after market converters with this car.

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There should be no difference in using an OEM or aftermarket cat to the O2 sensors-as long as it fits with no leaks, it will do exactly what an OEM cat would do, with a significant savings. When the pipes are dropped behind the cats, inspect for 'clogging'-although it does happen, your cats behind your 4.6 are not as prone to breaking up as the earlier generations of catalytic converters, so look yourself for proof that they are damaged. If you do replace the cats, the rear sensors only monitor the cats thenselves they do NOT affect the way the engine runs at all. You are being sold a bill of goods in my opinion.

Posted on May 24, 2011

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  • tsudduth627 May 24, 2011

    the O2 sensor issue aside, can the catalytic converter cause the engine to misfire and reduce mpg?

  • mike mountain May 24, 2011

    if one or both cats are plugged, it can affect gas mileage - I doubt that the misfire is caused by a plugged cat, all that would do is call for a need for checking the secondary ignition system-a code would be set for a misfire and would tell you what cylinder is misfiring.

    it'd be nice of you to offer more than 2 thumbs since my advise is accurate and honest-it would be very unusual for a cat on a modern car such as yours to plug, but it is possible-and you can install an aftermarket cat, much less expensive than an OEM cat. The facility that is telling you an aftermarket cat will affect a O2 sensor is not being accurate-a O2 sensor after the cat is designed ONLY to read the efficency of the cat, it does NOT affect the engine computer's decisions on what it does to control your engine functions. A cat is a cat, it heats up ('Lights off') to about 600 degrees and burns any unburned hydrocarbons that may not have been burned during the combustion process. The Os sensor tests to see if it is, in fact, 'lighting off' and should read a fairly steady voltage reading around .5 to .7 volts-it does not switch rapidly like the front O2 sensors which range from about .1 to .9 volts.

  • tsudduth627 May 24, 2011

    The mechanic that looked at it said the engine is not throwing ANY codes at all. The muffler shop did a pressure check and said it produced about 3 lbs of back pressure when revving the engine. He also said that a new one produces about 1.5 lbs of back pressure. Do you know what the normal back pressure is for this car? The mechanic was leaning heavily on the fact that too much back pressure would cause the knocking noise it makes and the reduction in power it exhibits when it warms up. Without any codes to follow what is the next step. I would hate to spend a grand on 2 new cats if their not needed and don't fix the problem. As far as my rating goes, I am not trying to be **** and you have lessened my doubts about using after market cats, but I am still at square one and I do not have an unlimited supply of money to throw at this car. I need help.

  • mike mountain May 24, 2011

    Have the aftermarket cats installed, and keep the old cats (they are worth about 50-$75 a piece because of the platinum content:)-I realize you are doing this because you have been told that the performance issues you have are due to what someone has determined to be a clogged cat-there is no need to install an OEM cat, they are very expensive and it is a waste of your money to do this. Your rear O2 sensors are not throwing codes, so if you do not have a visual inspection of components, technically you don't even have to have cats-it will still pass an OBDII connection test, since there is no code thrown. No way should it cost $2000 to install 2 AFTERMARKET CATS-they can be found for less than $125 each and it would be less than $500 out the door, you need to shop around and get someone to do this for you (try advertising on Craigslist, really, it is not rocket science and there are all kinds of people out there who could do this in their garage, it really is that simple)-the blockage, assuming it is there, is 'loading up' the engine and will cause misfires due to weaknesses in the secondary ignition (high voltage)-so why it isn't throwing codes is a mystery to me, it should...if you are being told the truth about the cat(s) being blocked...I have a Mark VIII and am very familar with the 4.6 DOHC and 4.6 SOHC engines, I can't say I have EVER seen clogged cats on a 99 TC-and I would think if it happened, you'd get symptoms like a CEL for misfires and MAF readings that are very out of range. Have the cats removed and drive it without, see if it's cured (it'll be super loud, but you will know right away if that is the issue by how it runs).


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I am in a similar position. I have replaced the cat, both o2 sensors and various other sensor and such still haveing problems and it's throughing the code for a bad converter. Which has been checked and i'm told it is fine. Now my mechanic is in the guessing phase and i don't have money to be guessing. I've talked to numerous people and no one seems to know what it wrong. It knocks and rattles and idles rough. I was told the same thing about after market converters and the check engine light. My light has been on since the new cat was replaced and it comes back on within a day of resetting it. I have talked to other people that have had the same problem, if there is nothing wrong with my cat why the code for a bad one. doesn't make since to me. Hope you got/get your problem still at square one, very frustrating.

Posted on Oct 18, 2011


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