Performance on my 08 Bullitt is off

[bujeezus]

Once a week, I drive an hour 1 way to race radio control cars. Last week after filling up, I got there no issues but when coming back home, something was up. Merging onto the interstate wasn't sluggish, but it wasn't quick like it usually is. My reported mpg (I know they're not exactly accurate) went from 20 to almost 25mpg and has stayed there. I also discovered that my cruise control will not come on anymore. I don't have a cel but I haven't checked for codes either. Probbably unrelated but my reverse lights are staying on too (wore out switch on the trans most likely). What are y'all's thoughts? Junction box corrosion was the 1st thing that popped into my head.

 

[86GT351]

Even with no CEL scan it for codes. When the PCM has a stored Engine replated code, the creature comforts like cruise control are designed not to turn on.

 

[StockishS197]

Could be a lot of things.

Check the CAI to ensure the filter is clean, MAF is clean, etc. check spark plugs as well.

If you can log, check fuel trims to see any changes there.

 

[bujeezus]

But why would those issues cause my reverse lights to stay on, and the cruise control not to come? It's intermittent also and only happens between full key cycles. Sometimes I crank it up and it's back to normal. Never occurs as I'm driving. Shut it down to gas up etc, and all the symptoms pop back up. There's a distinct click from the sjb when keying on. Is that normal? I don't recall it doing that before.

 

[Forty61]

But why would those issues cause my reverse lights to stay on, and the cruise control not to come? It's intermittent also and only happens between full key cycles. Sometimes I crank it up and it's back to normal. Never occurs as I'm driving. Shut it down to gas up etc, and all the symptoms pop back up. There's a distinct click from the sjb when keying on. Is that normal? I don't recall it doing that before.

I'm just throwing shit out there..

Are the reverse lights on, literally, all the time? Could that circuit being in that state stop the cruise control from activating?

If you're hearing a new noise I would at least investigate it, is it like a relay clicking?

These things may not all be related, surely the reverse lights would be the easiest to diagnose and fix, see if that rectifies the cruise issue. The SJB you'll just need to get eyes on, maybe cycle the key a few times with the covers removed so you can pinpoint the noise.

 

[bujeezus]

Yes, they do not shut off and that's probably why the cruise isn't turning on because the puter thinks the car is in reverse. So that tells me the switch is either faulty or something is up with the sjb. I did leave the cover off of it after cleaning the connections so maybe that's why I notice the sound now (it does sound like a relay).

 

[StockishS197]

But why would those issues cause my reverse lights to stay on, and the cruise control not to come? It's intermittent also and only happens between full key cycles. Sometimes I crank it up and it's back to normal. Never occurs as I'm driving. Shut it down to gas up etc, and all the symptoms pop back up. There's a distinct click from the sjb when keying on. Is that normal? I don't recall it doing that before.

Thinking about this more, if your reverse lights are on due to the reverse trigger switch being bad, it could actually be triggering the PCM to think it’s in reverse and limit throttle (and maybe prevent cruise control?).

I had a similar but different issue (reverse lights weren’t coming on) where in specific scenarios (turning, free revs/clutch in) the car would trigger transmission torque truncation throttle mode and limit TB opening. We had to zero out the torque truncation limits for reverse in the tune to regain full throttle in those scenarios. See my thread here: https://www.s197forum.com/threads/2007-gt-throttle-issue.148701/

Could be worth logging your pedal input vs throttle opening to see if the PCM is limiting anything. If it is like mine was, you can log torque source to see which throttle mode the PCM is commanding and go from there.

 

[Forty61]

Thinking about this more, if your reverse lights are on due to the reverse trigger switch being bad, it could actually be triggering the PCM to think it’s in reverse and limit throttle (and maybe prevent cruise control?).

I had a similar but different issue (reverse lights weren’t coming on) where in specific scenarios (turning, free revs/clutch in) the car would trigger transmission torque truncation throttle mode and limit TB opening. We had to zero out the torque truncation limits for reverse in the tune to regain full throttle in those scenarios. See my thread here: https://www.s197forum.com/threads/2007-gt-throttle-issue.148701/

Could be worth logging your pedal input vs throttle opening to see if the PCM is limiting anything. If it is like mine was, you can log torque source to see which throttle mode the PCM is commanding and go from there.

That was my line of thinking too.. and in that scenario it probably wouldn't throw a CEL because it thinks, technically, everything is working correctly.

 

[bujeezus]

I'm 99% sure it's the switch. If I unplug it and jump the connection on the harness side the reverse lights turn on and off so at least there's no short in the wiring. It only happens when the reverse lights fail to turn off. Going to replace the switch ASAP.

 

[StockishS197]

I'm 99% sure it's the switch. If I unplug it and jump the connection on the harness side the reverse lights turn on and off so at least there's no short in the wiring. It only happens when the reverse lights fail to turn off. Going to replace the switch ASAP.

Reply back here, as I never replaced my switch and I only tuned around it. Might have to look at mine too if it solves your performance issues.

 

[bujeezus]

Update:
Decided to be a cheapass and just unplug and wrap up the switch plug. A week of driving and the issue has not reared it's ugly head. Worse case, it could be whatever is inside the transmission that activates the actual switch but I'd bet it's just the switch itself.