Issues with Super Highway Mode

Hi all--

I recently (as of about ten days ago) bought a 2007 Prius from Steve's place, Autobeyours. It took me a while to work out the kinks, but I've been able to extract some pretty good mileage out of it once I got the tires pressed up properly and read up on how to stretch the fuel just a bit further, especially with SHM and similar techniques.

However, I've noticed an issue with SHM - I know that in theory one holds 14IGN/18TPS on the scangauge at (insert your low 50's MPH speed here) to hold the fuel consumption meter at roughly 1.5x speed or better, which I have seen plenty of times.

However sometimes I've found it completely impossible to hold 18TPS for more than a second or two - it'll suddenly and inexplicably jump to 21TPS without the IGN changing or my foot moving at all. And when I try to back off the accelerator, it instead drops into warp stealth, with 5IGN and 14 TPS . Given the largely flat areas I drive on and already lower-end speeds I drive, it means I have to re-accelerate in a hurry and it costs me a bunch of mileage. Here's a couple shots of the fuel consumption display - on one day, the car behaved properly, and on the other, 18TPS was impossible to hold. Mind you, this was over the same stretch of highway:

This is with the Prius behaving properly:



And when 18TPS was impossible to hold:



One thought: I considered the possibility that driving at night would increase the electrical load (headlights, etc.) and potentially put further load on the hybrid battery's SoC, but figured the warmer temperature might mean the fuel economy was a wash?

Does anyone have any thoughts/known causes of the Prius being unwilling to hold 18TPS and instead jump to a higher number (and burn much more fuel)?

 

Hi Cory151--

I'm using a ScanGauge II, version 3.15. I bought it in early 2008.

 

Hi GreenJuice--

The issue isn't being able to hold speed on gradients - I generally only use it on flat or gently downhill slopes.

What I'm saying is I back off to 18 TPS/14IGN, and then after about 2-3 seconds, TPS jumps to 21, IGN is still at 14, and instantaneous MPG has dropped from 90 to 65, with no appreciable difference in performance. If I try to back off the pedal to compensate and get it back to 18 TPS, it just drops into warp stealth instead at 14 TPS.

 

Hi GreenJuice--

I've noticed this, especially when I take the Prius out on the highway. And you're correct inasfar as that the Prius simply refuses to drop to 14/18 when SoC is less than ~59. It'll drop into warp stealth, and if I nudge it back out, it will hold 13/18 or 14/18 for about one second, with the IFCD at ~90 MPG like it should, but then with no difference in performance or acceleration it'll jump to 21 TPS and iFCD drops to ~65. If I try to back off TPS below 21 by easing off the gas pedal, it just drops back into warp stealth.

To minimize this, I try to hypermile as much as possible when around town, keeping SoC between 60 and 65, although it does drop to 57-58 from time to time.

(Side note: I've also noticed that the only way to avoid killing your battery down to two bars when stuck in the S1 warmup phase is to hit the throttle wide open, and to hell with the emissions. Either that or limp around accelerating more slowly than most public transit, getting < 15 MPG the entire time.)

Problem is, I've noticed it happens about 10-15% of the time even when SoC is conisderably higher (I could get on the highway with a SoC of 65 and still have the same 14 IGN/21 TPS problem, pulling less than 70 MPG even when trying to coast down in speed.

 

Hi Jaquimo--

This may be the cause of the lack of willingness to go into SHM. Most often when I get on the highway, the entrance ramp is about four blocks way from the house. Oftentimes I'm barely out of S1 when I start accelerating on the entrance ramp. Granted, it dumps me onto a construction zone ill-suited for SHM (heavy traffic, etc.) but even after 5-10 minutes on the highway it still won't go into SHM. Perhaps it's because I'm stuck in S3 purgatory because I never truly have a chance to stop the car again? I'll have to check into this more.