I just purchased an 06 Navi with 70k miles.

Since purchasing, I notice that when the vehicle is in gear with the brakes applied at a stop, the engine runs slightly rough. This sends a mild vibration through the vehicle. This happens only when the vehicle is warm and the engine is at low rpm, around 500-600.

I just purchased a Scangauge. It does not report any codes. I'm no mechanic, but I done my fair share of repairs over the years.

I checked the vacuum elbow on the passenger side of the throttle body and it is intact.

I suspicion was that there was a bad plug or coil, but without any codes, I'm not so sure anymore. Any ideas on how to diagnose this problem?

Thanks for your help in advance!

 

How bout if you slip it up into neutral - with the brakes applied - still rough?

How bout neutral with the brakes released - smooth out?

Only a SWAG, but you might have a vacuum leak in the power brake diaphram.

 

Thanks for the reply,

As soon as I slip it into N, the RPMs increase to 650-700 and the vehicle runs smooth. It only seems to happen when the Navi is in gear with the breaks applied and the engine is revving at 550-600.

 

SO, if its in neutral with the brakes applied OR with the brakes released it idles fine,

AND

If its in gear with the brakes released it idles fine,

I'd verify that the MAF is clean and look for a very small vacuum leak between the MAF and the throttle body. Also check the EGR valve gasket and the IAC valve gasket ( does the '06 have an IAC?)

Note: these are only my guesses, hoping someone will come along and know more about the '06s than I do.

 

Thanks, you described the behavior exactly.

I removed the MAF sensor and it looks clean. The box is still shiny and the small little thermo-resistors or whatever they are look very clean.

Vacuum hoses look new, with no obvious cracks that I can see, at least at the joints.

From what I've read so far, the '06s don't have EGR valves or IACs.

 

With the engine under stress you will feel a bad COP more easily. You will have to get a stress test on the electrical system to see which bad COP it is. That is my guess. Pretty common. Could also be just a fouled plug. Coils are about $90 each!

 

Thanks for the reply, Skoiv

Is it also common for bad COP or fouled plug to NOT through a code?

 

Yes... I just had a bad cop in my 2000 expy that did not throw a code.

 

Any idea of maintenance history? Since you just bought it and it has 70K, I would change the plugs, fuel filter, and air filter before I started replacing COP's. Read in here about your type of plugs, they can be tough. But it's minor stuff that may help you if you don't know the history from the PO.

 

Shortly after receiving advice on this post, I took my Navi to my local dealer to have the spark plugs changes, fuel system flush, etc. and asked to have try to diagnose the rough idle.

The called me with the good news that the spark plugs and fuel system flush had already been performed on this vehicle by them. Just lucky that I took it to the same dealer as the former owner. Bad news is that they had look at the idle and that 5.4 triton engines "just ran that way".

Now I'm not too sure they experience what I have experienced, as the severity is of the rough idle changes. I almost want to say that its worse in wet weather, but sometimes it is bad in dry weather as well.

Since then, I have had my fuel pump control module go south and it has been replaced. They tested the fuel pump at the same time and said it checked out good and did not need replacing (saving me ~$1000 in parts and labour). They also changed the fuel filter at this time.

I still do not receive and misfire codes. Just for fun, I removed one wire going to a COP. The engine did through a code and ran rough. I have to say that it was about twice as bad as what I experience normally.

Still tempted to buy 8 coil packs on ebay and replace just to see if it will help.

Edit: I also replaced air filter.

 

Sometimes the low RPMs at idle cause the problem you are describing. Some dealers have gone so far as to adjust the idle RPMs up a little to smooth the idle out. If you had a bad COP on an '06 it would throw a P0300 or P030x code. Prior to replacing all the COPs I suggest taking the air filter housing off and cleaning the throttle body. Then run some seafoam through the brake booster line.

By the way, warm idle speed should be between 600-650 rpm.

 

When the rubber boots on the COPs start hardening from heat and age, they tend to allow water down into the spark plug wells and cause misfiring. If it's worth it to you go ahead and get the 8 COPs and try it. For the $80 it cost for them, it's not worth losing too much sleep over.

 

I just had my 05 navi at the dealer to diagnose a drop in rpms at stops and it felt like it was almost going to die out. The dealer scanned it for codes and it threw out PO303 and 316 so they replaced all plugs, 7 boots, and eventually one coil pack for cylinder 3.

So I paid the 740 bill and drove it home about a week ago. Now I notice at idle it doesn't drop in rpms but the engine vibration and noise is worse than before the trip to the dealer. At idle the whole driver seat shakes??

Any ideas?

 

The P0316 code means that the PCM has detected a serious misfire in the first 1,000 engine revolutions. This code can indicate a problem with a crankshaft position sensor, camshaft position sensor (cam phasers), or a wiring issue. Since this code indicates a start up misfire, pay attention to items which may expand or contract when cold.

 

Thanks. I think I might just take it back to the dealer and have it checked out again. The CEL hasn't come on but the rough idle is annoying.