Poor acceleration when hot

nissan
sentra

#1

I have an 08 Sentra Se-R spec V that I bought recently and it has really poor acceleration when the engine gets hot. It feels like it’s running rich.

So to give some background, the car needed some work when I first bought it. I had to replace the muffler with a Tanabe as it was completely rusted out (it was a Flow master), the manifold had a crack and I replaced that, and replaced the spark plugs.

I thought those repairs might have fixed the acceleration issue I’m having. Basically after driving for 15-20 minutes, the acceleration is extremely poor. I also notice some stuttering in the acceleration when I’m moving at a constant speed. I’ve pulled the O2 sensors, one at a time, and didn’t notice any improvement in performance. I’m thinking it could be one of the other sensors such as MAF but I’m not sure how to test that before I replace.

The issue seems to have gotten worse over the last month to the point where is very sluggish, especially after getting off the highway when the engine is very hot. It runs great when it’s cold though.

Any ideas?


#2

What mileage? Any diagnostic codes?


#3

Mileage is about 104k. The check engine light was on until I replaced the cracked manifold. It went off until I started pulling the O2 sensors and now it remains on even though I reconnected them.

I plan to take the car to an auto parts store to have the codes checked this evening. Will let you know what is found.


#4

Hot makes it sound like it is overheated. You mean to say it runs poor when it is at operating temp? Motor is same temp at 20 mins of running vs 2 hrs of running. Sounds like a bad cat to me.


#5

Heat related problems are often due to ignition system problems, particularly the crank position sensor, so that’s something to consider. Acceleration requires good intake flow and good exhaust flow, so the cat and engine air filter are other possibilities.


#6

What you mean started to pull the sensor? Did you remove it and put it back in without damaging the wiring/connectors? What is the code?


#7

To follow up on some of the comments: the title should be “poor acceleration when warm” instead of “…hot”

The check engine light came on when I unplugged each of the O2 sensors. I took the car to a car parts retailer to check the codes; the code indicated an issue with the O2 sensor. I disconnected the battery, check engine light went off and hasn’t come back on.

For clarity, prior to disconnecting the O2 sensors the check engine light was not on, but I had been experiencing the poor acceleration issue for several weeks.

The acceleration issue is most noticeable when accelerating from a stop after having driven at ~70 mph for 20 minutes. I take off from the stop and shift through the gears, and when initially shifting into 3rd it feels like I’ve hit the brakes. Performance is poor in all gears but is more conspicuous in the higher gears.

I also notice a bit of a front-to-back rocking feeling particularly on the highway (although I notice it at lower speed, most obvious when ascending a hill) as if I’m very subtly changing the amount of pressure I’m putting on the gas pedal. At low speed I notice the exhaust note change slightly corresponding with the rocking feeling.

When I first purchased the car I drove it about 200 miles. I didn’t notice the acceleration issue but I think that may have had something to do with how gingerly I pressed the gas pedal for fear of the horrible noise that would come from the huge hole in the side of the muffler. However, I have the impression that whatever the issue is, it’s gotten worse.

On this particular car the manifold has a catalytic converter and there’s an O2 sensor on the mk
manifold before and after the cat. The exhaust also has a cat inline which a mechanic told me could be removed and straight piped.

One last piece of information which could be completely unrelated, but for the sake of completeness… a previous owner replaced the stock 6 speed transmission with a 5 speed. Why this was done has baffled many but the mechanic who replaced the manifold assured me it looked like it was done professionally. Perhaps a side effect of the transmission swap is that my cruise control no longer works.