Loss of power and loose engine note when hot
 

Ok guys, I`m hoping someone here has some input for me. I`ve been dealing with this problem for years now and the dealer has been unable to resolve it. It seems like if it`s not the computer spitting out a code, they are not much help.

Now that summer is coming and the heat has been turned up, a problem I`ve been trying to resolve for years comes with it:

When my 8 gets warm (and the air is warm or its been sitting in the sun) - I loose a good chunk of power, and much more noticeably, my engine sounds like crap. The exhaust note goes from a nice tight wrrruuppp - to a ghhhhhhhaaaa.

It most noticeable over about 5000rpm - that's when you really hear the engine start to struggle - the higher you rev, the worst the sound gets and the less power you make.

I have a 2007 with 100,000 KM. Immaculately serviced, oiled and I;m the original owner. I had a MS CAI that`s been removed to make sure it wasn't`causing the issue.

In the winter, I had all my coils, plugs and wires replaced - to which I was treated (much) better fuel economy and a nice increase of power. The only thing not stock is the MS LWFW.

No misfires, and no codes according the the dealer over my many trips there regarding this problem.

Any thoughts on what to do to troubleshoot this?

Should I be taking it to a tuner shop, i'm not really sure where to go from here. I'm relatively mechanically inclined, and work on my motorcycles - if I were to relate this issue - it's as if one of the spark plugs was unplugged on a inline-4 motorcycle engine - that's what it sounds like. However the car runs absolutely perfectly in cold weather, or when you first start driving in hot weather provided the car is parked in the shade.

Have you gotten a compression test? Just to rule that out.

No I haven't - its honestly been running the best it's ever has this past winter - would compression be an issue only in the summer? I've no experience with it, but the engine runs so damn good when its not hot out (the engine can be hot in the winter with no problems).

No problems starting when hot, even in summer.

I don't think it's a compression problem, just asking because it's worth ruling out at some point.

I suspect most that it's going to be related to intake air temp, and what that's doing for your AFRs. Cars that pick up noticeably engine response and power when it's cold out tend to have slightly richer AFRs and the cold dense air leans it out just a bit. Cars that pick up similar when it's hot out tend to have slightly leaner AFRs, so the hot thin air richens it up a bit. The reverse is true for each as well. Yes, the MAF is supposed to be able to correct for the temp and air density, but it is never a 100% accurate correction, so things can still move a bit.

If your front O2 sensor is old, you may consider replacing it and resetting your fuel trims, and see if it continues happening.

Or, another possibility, is that you have a very very tiny vacuum leak somewhere, which is causing added fuel trims over the winter and the transition to hotter weather is resulting in too rich AFRs.

Just some theories for you :)

Wow thank you so much! That is a huge help, and gives me something to start into - a direction. My O2 sensor is over 7 years old now. Your comment made me think of something from summer of last year. I threw a code: P2096

The tech comments says:
Code: P2096. POST CAT FUEL TRIM SYSTEM LEAN. CHECK PCM CAL LEVEL AT LATEST LEVEL. SEARCH MS3 FOR TSBS NONE FOUND. CHECK MODE SIX MISSFIRE COUNTS IN NORMAL RANGE. DALALOGGER PIDS FOR 02S AND ALL FUELS TRIMS ALL IN NORMAL RANGE. CLEAR CMDTCS FROM PCM. NEED TO DRIVE WITH STOCK AIR BOX AND SEE IF CODE RETURNS.

This was never resolved, as they cleared it and never returned. Code happened on a road trip in hot summer and car was performing lousy. Should I be replacing this sensor? Is it a rear o2 sensor?

Thanks again!!

Well, that specific code is the rear O2. And it is possible that the rear O2 is the one causing the problem, as the ECU uses the rear O2 as a check and balance against the front O2. When one of our members attempted to force the engine (through fueling maps) to run the engine very lean at low loads, the rear O2 sensor was being used to pull the fuel trims back into safe ranges. And since O2 sensors tend to fail by reading lean, it's possible that the sensor is hovering on the edge of failure for a few year (unlikely, but possible), and keeps pulling the fuel trims slightly richer.

OR, it could be the front O2 sensor that is failing on the lean side and pulling the trims richer. Since you had a code for the rear O2 sensor, and it's the cheaper and easier one to replace, it's probably the better one to start with.

You could also disconnect the rear O2 and reset the fuel trims and see if that changes anything. It's not going to harm anything (except popping a CEL for it). If there isn't a change, it's more likely a front O2 problem, if it improves noticeably, then it's likely a rear O2 problem.

Thank you! I'm going to start into diagnosing these sensors - I'm inclined to just change both out at this point :)

I'm going to look at the vacuum leak as well - I know my next research topic! It does make me wonder as today was super hot compared to the previous cooler days we have just had - and the car seems to run worse then ever. Its a good theory!

Hi i have same problem with my car now. After resealing engine the power in low rpm is very good but when i rev it to 5500 it does as you said My O2 sensors are 10 years old after warm up i am getting low circuit voltage and high circuit voltage pid for bank 1 sensor.

As i understand it its failing primary o2?
So if i am going to change primary o2 sensor should i change secondary too?