2006 Grand Marquis, Intermittent Rough Idle followed by Overheat

Asked by Feb 13, 2017 at 04:44 AM about the 2006 Mercury Grand Marquis LS Premium

Question type: Maintenance & Repair

I have a 2006 Grand Marquis, Normal 4.6L V8.

Every couple days, my car will randomly begin
idling seriously rough, with the rpms dropping to
300-500, then jumping back to normal every few

After a few minutes of this, the engine temperature
suddenly climbs and enters overheat range within
one mibute of the temperature beginning to

The engine is returning 2 codes:
Misfire (Cylinder 5)
Bank 1 running too lean

I have carefully cleaned the MAF sensor with MAF
cleaner, and I cannot detect any vacuum leaks. The
Ignition coil on Cylinder 5 appears normal, but the
spark plug on 5 does have a white appearance on
the diode, and feels rough.

Also noteworthy, the engine doesn't change in any
noticable way when unplugging the EGR valve's

This is causing quite a significant problem as this
is used as a security patrol vehicle, and it likes to
do it during my shift. Any ideas?

18 Answers


The white on the spark plug tells me it is running lean in that #5 cylinder. Nothing wrong with the plug, it is just telling the tail. The fuel injector could be the fault, or there is a vacuum leak with the intake manifold. What I do for checking for vacuum leaks along the intake manifold is spray carburetor cleaner along the gasket area when the engine is idling. Best to do this when the engine is not too hot. Just warmed up. The rpm's of the engine will pick up if there is a vacuum leak.


To diagnose the #5 miss-fire, move the #5 spark plug and coil to another cylinder to see if the miss moves with the parts. If the miss moves either the coil or the plug is bad. You cannot tell by looking if a coil or spark plug is bad. If the miss is still on #5 when you move the parts a faulty fuel injector or injector wiring may be the problem. How many miles on this engine? Have the spark plugs ever been changed? Have the coils ever been changed?

The engine has around 145,000 miles. I bought the car at 136k, so I do not actually know if it has ever had the coil packs or spark plugs changed. The plugs were a near-future plan. I will try moving the coil-pack next. The "Bank 1 lean mixture" and the overheat are my priority issues though. It does not appear to be a vacuum leak. I spent a few minutes hitting the vacuum lines with WD40 since I had no carb cleaner available. The engine didn't react at all. It was worth a try though. Testing the sensors is going to be difficult without a live- scan computer.


I would try to get the #5 miss fire fixed and get it running on all 8 cylinders and proceed from there. Is it using any coolant? Bank 1 lean could be caused by the problem that is causing #5 to miss fire.


And maybe the radiator & cooling system needs a flush, it wouldn't hurt to change out the thermostat. The coil pack you can take into a auto parts store to have it tested, and it just sounds like a good tune-up is in order, this vehicle must do a lot of idling.

The vehicle does indeed do a lot of idling.---It does not appear to be using any coolant. The overflow is still at the fill line and is the appropriate green color. I suppose a flush couldn't hurt though.---There is a key detail that seems important. The engine only overheats after it starts running rough for a few minutes. It doesn't start running rough unless its been running for a while and is already up to temperature. The overheat and misfire appear connected.---I'll see if I can solve the #5 misfire, though it seems to be a symptom, not a cause.


I think the plugs, a tune-up is needed. Cylinder #5 spark plug has run its course, in with the new out with the old. I think the overheating or engine getting warmer than it should is a cooling problem in itself. I personally would do a tune-up, but I would check the compression of the cylinders to make sure there is not a problem with #5 cylinder, compare it to the rest of the cylinders compression, all of the cylinder compression's should not very more than 15% of each other. And I would have the cooling system flushed, but I would change the water pump. It is possible the impeller is eroded and not circulating the coolant like it should. Probably a lot of miles on it anyway. Also change out the thermostat. Then you know you are in good shape since you depend on this vehicle.


An over lean condition can cause the engine to overheat. Please keep us posted on what you find.

Will do. I suspect I may have an O2 sensor on one side going out. My ignition coils and spark plugs are apparantly fine. This is even throwing mechanics for a loop because its a randomly occurring problem that occurs after the engine has be running a while and the thermostat is already open. I'll let you know when I find anything new. Thanks

Got a more comprehensive scan. I have ruled out the spark plugs and am waiting on the engine code to return after trading the ignition coils for 1 and 5. Also going to replace the thermostat and ECT Sensor and see what happens.


Replacing the Engine Coolant Temperature Sending Unit Is going to help out a lot by the looks of these codes. One thing I notice is to make sure that battery and charging system is good also. A low battery throws codes. The wheel speed sensors are of another problem going on at the wheels, bad sensor, or bad connectors.


Forgot to mention, clear the ECU codes after you do the work so you know the next time if any trouble codes show up, you know what to address. The ABS codes will just come back until they are fixed.

So, I just realized that this entire problem may have been caused by my cooling system after all. But an extremely rare problem. My radiator, safely hidden behind my condenser coil, looks like your average lint trap after washing several loads of towels. So either the water pump is not pumping after all, so the water next to the temperature sensor is not getting hot until the car is overheating, or the sensor itself is just bad, and the car is hot before it tells me it is.


If the radiator has a lot of d-bree on the front of it, then it should be cleaned out.

This problem is proving to be extremely annoying. I suppose I should replace my water pump next.--------- ---Radiator is now clean, thermostat is new, coolant has been verified to be clean and free of oil, there does not appear to be a coolant leake anywhere.------ ------The kicker is that the engine seems to be getting hot before the guage registers the increase in temperature. The car may very well be overheating without showing on the dash, causing it to run rough. This makes it APPEAR that the car is running rough before it overheats, when in reality it is the other way around.------------This is a serious pain.


Just have to eliminate one problem at a time.

Both you guys have been a tremendous help, but the advice has gotten a bit discombobulated. Lol. Here's what All I have done so far.------Cleaned and tested MAF--Checked Spark Plugs--Checked ignition coils--Tested battery Voltage 14.4v--Tested Fan Controller INPUT at 14.4v--Tested fan controller OUTPUT at 14.4v--Tested fan via test leads-- Checked head gaskets.----------Just now confirmed the fan IS failing sometimes. It started to overheat as I was typing, and THIS TIME the fan wasn't coming on.


Well there's a problem to be fixed.

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