Lower performance when the engine is at operation temp.

[Laserbait]

I have an 06 Merc Mountaineer, with the 4.6l 3v V8. When I start it up in the morning, the performance is good, responsive. But after the engine gets up to temp, it feels sluggish. It has 128K miles on it, and it's been this way since I got it at 95K miles. I'm not sure if it's the engine, or the transmission, that is at fault.

I replaced the plugs 5K miles ago, and there was no change in behavior. There are no check engine lights, knocking, or decrease in MPG.

The only thing that sticks in my head was that I really stated to notice this phenomenon right after I changed my tires from a nearly bald set of crappy Bridgestones in the 235/65-18 size to new Kelly Springfields in 265/60-18.

Initially, I chalked it up to being the bigger tires throwing off the overall drive ratio, but that wouldn't explain why it feels responsive when the engine is below operating temp.


Any ideas?

 

[Explorer75]

Have you plugged and ODBII monitor so you can actually know at what temps this actually happens? The dash temp gauge won´t tell you much. It stays right in the middle for a wide range of temps. Also, how are your MPGs before and after you noticed this? (after the tires?)

 

[Laserbait]

Have you plugged and ODBII monitor so you can actually know at what temps this actually happens? The dash temp gauge won´t tell you much. It stays right in the middle for a wide range of temps. Also, how are your MPGs before and after you noticed this? (after the tires?)

No - I have to get an ODBII monitor. I was thinking Torque for the Android.


Mileage stayed about the same after the tires (once I ran the math). The old tires were about 29" in diameter, and the new ones were 30.5".

 

[pzy3339]

Can you better define "sluggish" ?

Have you ever cleaned the MAF?

Oxygen sensors don't work till they reach temp. It is possible they, or one of them, is giving an inaccurate reading but is within parameters so it has not thrown a code yet.

Not sure how these engines judge spark timings. If it uses vacuum to adjust timing maybe there is a tini hole or crack in something between the manifold and the sensor.

Don't know much about this phone app you refer to, but I do know a dealership can do a very detailed diagnostic and get every detail about all the sensor readings. They also have detailed knowledge and experience with these vehicles so they can make pretty accurate judgments on where this issue is most likely to be found. Last time I had this done it was a 1 hour job, and they typically charge $90-100/hr.

 

[Laserbait]

Can you better define "sluggish" ?

Have you ever cleaned the MAF?

Oxygen sensors don't work till they reach temp. It is possible they, or one of them, is giving an inaccurate reading but is within parameters so it has not thrown a code yet.

Not sure how these engines judge spark timings. If it uses vacuum to adjust timing maybe there is a tini hole or crack in something between the manifold and the sensor.

Don't know much about this phone app you refer to, but I do know a dealership can do a very detailed diagnostic and get every detail about all the sensor readings. They also have detailed knowledge and experience with these vehicles so they can make pretty accurate judgments on where this issue is most likely to be found. Last time I had this done it was a 1 hour job, and they typically charge $90-100/hr.

By sluggish, I mean it just doesn't feel eager to accelerate like it does when it's cold. I feel like I have to drive it like I'm mad at it to get it going. Unfortunately, I don't have a better metric to go by.

I have not cleaned the MAF yet - but that is a good idea. I'll try that!

I remember testing for cracked manifolds/bad gaskets back in the day with an unlit propane torch. Is there a better method now?

 

[Flag Gibby]

I am thinking plugged fuel filter or the fuel pump is worn out.

 

[colintrax]

^ I'm thinking the opposite. Engine runs richer when cold. I'm betting
Go it's a small vacuum leak

 

[Flag Gibby]

Colintrax, With the knock sensors on this generation the PCM does a good job of retarding the timing etc. on the vehicle in order to keep it running so it is very hard to tell when you have fuel delivery issues other than what can sometimes be felt as a loss of power on a warm/hot engine. To me a vacuum leak would cause a consistent acceleration problem hot or cold.

 

[colintrax]

Colintrax, With the knock sensors on this generation the PCM does a good job of retarding the timing etc. on the vehicle in order to keep it running so it is very hard to tell when you have fuel delivery issues other than what can sometimes be felt as a loss of power on a warm/hot engine. To me a vacuum leak would cause a consistent acceleration problem hot or cold.

A pinhole though could be masked by the rich condition when cold. Of course the engine is in open loop when cold and should cprrect for the leak when warm

 

[Flag Gibby]

True, I guess that we'll just have to wait and see what it ends up being.

 

[Laserbait]

Would a ScangaugeII be a help in diagnosing this?

 

[Bobmbx]

What kind of air filter do you have? Do you have an intercooler?

 

[Laserbait]

What kind of air filter do you have? Do you have an intercooler?

Just a stock paper air filter, replaced about 5K miles ago. No turbo or supercharger (yet ) so no intercooler.

 

[boominXplorer]

Would a ScangaugeII be a help in diagnosing this?

Yes as it will aid in the "self diagnosing" process. I would start by looking at the first O2 PIDs and compare both sides. Then you can narrow it down to 1 side if they read different. If they read the same you have a common issue between all cylinders and you can start checking fuel pressure, vacuum leak and so on.

 

[Bobmbx]

I'm going to go with "better performance with a cold engine". Engines will feel a bit "crisper" when they're cold....because they are. There's less friction, the air that gets to the cylinder is still cold (therefore denser and packing more O2...more power), the fuel is cold (energy density per volume is higher). Once things warm up your HP drops a bit. Every engine will exhibit a different cold vs. hot performance curve.

If there are no symptoms other than "feels sluggish", I wouldn't put a dime into trying to figure it out.