Hypermiling habits to blame for DBW throttle weirdness? - Fuelly Forums

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Old 07-25-2008, 05:28 PM   #1
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Exclamation Hypermiling habits to blame for DBW throttle weirdness?

A few times now, my engine has stopped making accelerationworthy power. It is almost like cruise control. No matter how much I step on it it or how the road grade changes, it would not go any higher than a certain speed, which has been somewhere between 50 and 70mph each time. Usually I can let go of the gas pedal for a couple seconds and then resume and it works fine.

Today it happened at 70mph, and I didn't notice it (because I wasn't trying to go any faster) until I tried to set the cruise, which was not working. I had to shut off the car and turn it back on (EOC and bump starting made this unobtrusive) before everything worked right again.

I am pretty sure that it's related to Drive By Wire throttle. There's no check engine light, noise, smoke, bucking, vibration, or other symptoms, it simply acts like I'm not stepping on it harder. Could this be some computer confusion caused by shifting low (I rev higher on the highway than I ever hit while accelerating or at city speeds), P&G, or EOC?
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Old 07-26-2008, 02:33 PM   #2
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Quote:
Originally Posted by theholycow View Post
A few times now, my engine has stopped making accelerationworthy power. It is almost like cruise control. No matter how much I step on it it or how the road grade changes, it would not go any higher than a certain speed, which has been somewhere between 50 and 70mph each time. Usually I can let go of the gas pedal for a couple seconds and then resume and it works fine.
....
Is your timimg belt due for a change. That happened to my old old Corolla way way back and a few days later my timing belt went. Car died on highway, rolled to a stop and parked at next exit (so so lucky). On the previous days my car would just lose throttle. Thought I had a feul pump that was on its last legs (made lots of sense) but it turned out to be the timing belt. Im no mechanic and Im not saying thats what this is, but you might want to at least check it out. Hope you get your issue resolved.
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Old 07-26-2008, 02:49 PM   #3
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Thanks for the response. That's definitely not the issue, it's only got 8,000 miles on it.

It doesn't stall or slow, it just fails to accelerate.
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Old 07-26-2008, 02:49 PM   #4
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i had a saturn do this back before i knew about hypermileing. it turned out to be the fuel pressure regulator.

it could be the computer. just reset it and let it "relearn".
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Old 07-26-2008, 11:37 PM   #5
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Yeh it probably needs maintenance
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Old 11-14-2008, 05:46 AM   #6
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Update: A month or two ago I realized what this was, and it's not a problem. As I got better at bump starting I was quicker and more accurate in when I got on the gas pedal. In doing so, it got to where the ECU thinks that I'm not doing it on purpose, and is protecting me from over-revving immediately on startup, to protect against idiot drivers and/or malfunction.

I don't have to reboot the car to fix it, I just have to clutch in, let the RPM drop by ~500, then resume driving normally. That tells it that the startup routine is over.

The practical upshot of this is that I can floor it during the bump start, when on the highway, thereby reducing pumping loss during the start, making it easier to start (so less wear on the clutch) and eats up less of my speed. I floor it, bump start, and it goes up to 3000rpm and waits there...the perfect RPM to engage the clutch at 70mph.
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Old 11-14-2008, 07:30 AM   #7
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You don't reduce pumping losses if the motor controlled throttle is not opening. Sure sounds like an interesting ECU bug however.

(Thread jack...) My friend has a 95 Saab Turbo that had its starter replaced a few months ago and he can't get it inspected because the engine scan with the inspection computer and the ScangaugeI says Not Ready. They gave him a factory printout of driving instructions that took us 1.5 hours to run consisting of starting and stopping at different speeds and times to do each to teach the computer how he drives. It sort of worked but still comes up not ready when he goes from Gauges to Scan with the engine running. It comes up ready if he cold starts the engine again. Not sure if unplugging the ECU to make it relearn is the right thing to do - at least it wasn't in his case.
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Old 11-14-2008, 07:42 AM   #8
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The throttle does open when the engine is not running, according to OBDII accelerator position and throttle position readouts. It's not always obedient but it is in that case.

That ECU driver-learning issue is wacky.
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Old 11-14-2008, 11:24 AM   #9
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Are you sure the throttle plate actually opens or is it reading the pedal sensor? Guess if the engine it turning then you can't tell because it would be running then.
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Old 11-14-2008, 11:50 AM   #10
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The Throttle Position Sensor reading doesn't always follow the Accelerator Pedal Position Position Sensor. If it's not reading the throttle plate, I don't know what it is reading.
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