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Hi I’m new to the forum and I’ve been having this issue for almost a year now so I’m hoping this may help me somehow.
So I have a 2003 Lincoln Town Car with 86k on the odometer and it has one very irritating issue. Drives just fine, tranny is all good, just passed NYS inspection with ease, but when I’m at a red light (for example) and I’m sitting with my foot on the brake in drive for more than about 10-20 seconds, a rough idle kicks in and my whole car shakes like a cylinder or two has just shit the bed. There’s a loss of power for about 20 more seconds when you finally give it gas and drive, then it’s all good again. Never has it actually stalled out though.
Another thing to mention is that when it does this, engine light comes on and it’s a code for coils. I’ve went through coil packs a few times because of this. So now I completely avoid the idling by putting it in park or neutral when stopped, and it doesn’t have any problem.
I’ve talked to many people, even mechanics and no one seems to know about this. My guesses are a possible vacuum leak, a bad fuel and air mixture, TPS, or just too low of an idle. Each of these makes sense to me but I can’t quite figure it out. I know Fords from 90’s to early 2000’s are kind of notorious for idling troubles of many types but I’d like to get this solved.Any help on diagnosing would be greatly appreciated or some suggestions would also be great help. Thanks – Tyler
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I know this is an off shot, so im only throwing it out there based on how well detailed your description is that you know enough to check most of obvious places. But have you checked for electrical arcing around the coil packs wires etc.. ? Just an outside the box thought. I would check everything else you think it could be first.
these cars are no strangers to coil issues. when you say you went through the coils i assume you mean you replaced them. have you looked at the plugs at all? theyre more than likely not the direct cause of the rough idle but bad plugs can cause the new coil packs to fail prematurely, especially since theyre known for coil issues to begin with. also im not sure of the brand of coils you installed but cheap coil sets usually dont agree in these cars. i would recommend installing high quality aftermarket coils if you haven’t already. these cars also dont use plug wires, they use a coil over plug design and arcing is not as common with these as they are with regular plug wires. however if they do fail and you happen to find arcing (most likely from the lower part of the boot) you would wind up replacing the coils anyway since in most case they are a part of the coil.
One last thing to mention is using dielectric grease inside the coil boots. moisture likes to build up in the plug valleys and can also screw with the coils. using the grease keeps moisture away from the connection and keeps it from corroding.
Yeah it could be something more electrically based like you guys both said. But I think it’s just that bad idle that causes a cylinder to go out of whack and fry the coils. I personally think it’s a little chain of events haha, like something causes that idle, then goes the coils. I got the car a little over a year ago and right before I bought it the previous owner did have all new spark plugs put in right before so never really thought about that. The car runs just fine until I’m stopped in drive for some time so maybe the plugs have trouble with a certain idle rate? Too little fuel or too high of air in the mix I’m thinking. I appreciate the response I’ll look into those plugs
the rough idle shouldnt do damage to the coils. im thinking your problem is within the ignition system. the plugs may have been replaced before but sometimes youll see people throw in the cheapest plugs they can find just to be able to say they put new plugs in, and on a few awesome occasions its the wrong plug completely so id definitely look into those since theyre easy to get to. as for the air fuel ratio, the only way to tell is with a scan tool. if you have access to one youd have to look at the short term and long term fuel trim while the problem is occuring. a good value should be slight fluctuation between + and – numbers very close to 0. the closer the percentage is to 0 the better but anything within 10% +/- is pretty acceptable. it would also be a good time to see if there are any codes stored in the pcm.
Well thanks I appreciate the insight, I’ll definately look into the plugs and coils. Hopefully it is something cheap and easy like that. One can hope so, thanks again
What exactly do you mean by check it though? And even if it is bad would it cause these ignition coils to fail on me?
When you are not braking, there is a pressure being applied to the brake booster diaphragm from engine vacuum. There is a check valve in this line so that when the engine is accelerating (engine has no vacuum) it maintains this pressure. The 10-20 seconds you describe seems like the length of time it would take for this pressure to be relieved while your foot is on the brake if the diaphragm or valve is leaking. Once the pressure has fallen, now the engine vacuum at idle (while you are braking) is trying to restore this pressure, and basically looks like a large vacuum leak to the engine. This would explain the rough/low idle. I suspect you may have a problem with either the brake booster diaphragm or check valve.
Check that the hose from the iacv isnt blocked or dirt inside of it.
It would have nothing to do with your coil problem may have something to do with your idle problem.
With your coil problem I would make sure you have good connections on your battery and check that you have good earth too. If you have changed the coil packs in the past it would be a weak ignition system somewhere and first would be to check that you have a good battery and good connections.Wow I never would of thought about that brake/vacuum leak but it makes sense too. It’s one of those problems where it can about a dozen things haha. The only thing to keep in mind is the coils that fail with the idle. The brake diaphragm sounds like a total possibility but how would that idle/misfire effect coils?
Codes do not tell you that a part has failed. Codes only tell you that the computer did not receive the expected data or that the data was out of range for a condition. A failed part can certainly cause unexpected or out of range data, but there are other reasons for codes besides a failed part. Vacuum leaks introduce unmetered air into the combustion chamber. This dilutes the air/fuel mixture. This causes misfires. Misfires cause codes. Many times these codes are simply reference codes rather than failure codes. Reference, in this case, means that the code refers to a symptom, not a cause. The computer has no idea what a coil is, much less a spark plug, fuel injector, battery, or even a vacuum leak. All it knows is that it is experiencing a fault condition and responds with a flag according to what the engineers/programmers instructed it to do when a particular fault occurs.
if it was a coil code like a P0351, it trips when the computer sees a fault in the circuit for the coil such as a driver circuit short to ground, a completely open circuit, bad connection or a bad coil. it shouldnt come on from a vacuum leak. usually with unmetered air causing a miss youll see a misfire code P0300 sometimes paired with a mass air flow code and maybe some others depending on the situation. its possible that you have a vacuum problem causing a rough idle but i dont think it would be related. the best way to tell is too look at the fuel trims while the problem is occurring (if you have the ability). if you have a high positive value, you may have a vacuum leak but if theyre within a reasonable tolerance, id keep looking into the ignition system. otherwise without a scan tool, youre pretty much guessing.
Yeah it was a cylinder misfire code but the failed coils caused that. I’m now leaning more towards the electrical/ignition systems because I just had it inspected in NYS with no problem and I’m sure they check for air leaks or bad compression within the motor right? This sucks because I’m terrible with wiring and such but hey maybe I can learn something, thanks again.
if the code that shows up is just a misfire code and not specifically a coil code, then you could be dealing with many different things. i was thinking that the code that was coming up was an ign coil fault code like a p0351. if you dont have access to a scanner. just start with the basics. visually inspect plugs, connections, hoses, clean the mass air flow sensor with MAF cleaner, make sure all the intake duct work is tight and sealed up, inspect and clean idle air control valve and its path through the throttle body, and clean the throttle plates. if you have access to a scanner, get the latest codes (if there are any) and check out some of the data like fuel trim, o2 sensors, and idle air control counts. it can sometimes be pretty tricky to diagnose a misfire without a scanner and some experience especially when it doesnt occur all the time.
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