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390 messages, Last post on Oct 31, 2009 at 11:59 AM

You are in the Maintenance & Repair Forum. Your Host is mr_shiftright

What is this discussion about? Brakes, Electrical, Engine, Coupe, Convertible, Hatchback, Truck, Sedan, Wagon, SUV, Van

This topic will hopefully allow the more knowledgable of our community members share what they know with those seeking a good explanation for "how things work" in a car; or, this is a good place for equally knowledgable people to discuss the finer points of a technology.

Outline

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#246 of 390 zero fuel consumption while coasting by steve_ HOST Aug 24, 2006 (10:09 am)
There's a thread over in the Yukon group about fuel consumption while coasting, with one side saying no way can an engine operate without fuel, and the other side referring to ecu mod sites that say that fuel is cut off completely during coasting situations, like long downhills. Hits like engine overrun and (gas) engine vacuum pop up in searches too. Here's the thread start: elias, "2007 Chevrolet Tahoe/GMC Yukon" #1670, 21 Aug 2006 10:47 am Steve, Host SUVs and Speed Shop Replies to this message: jerrywimer (Aug 24, 2006 11:40 am) wwest (Aug 25, 2006 9:30 am) echota (Nov 18, 2006 8:27 am)
#247 of 390 Re: zero fuel consumption while coasting [steve_] by jerrywimer Aug 24, 2006 (11:40 am)
Replying to: steve_ (Aug 24, 2006 10:09 am) One of the last messages posted in the other topic claims that it's impossible for an automatic-equipped vehicle to use fuel shut off during coasting because there's nothing directly connecting the rotating wheels and the crankshaft to allow the vehicle's inertia to keep the crankshaft rotating, unlike in a manual transmission vehicle that is in gear. I had to do a bit of research, but I believe that this post is entirely incorrect- at least for vehicles with lockup torque converters when the converters are locked. I also believe this to be the case with the GM 4L60E transmission used in the 1/2 ton trucks and SUVs. This must be the case because otherwise my 2004 Silverado would have provided no engine braking when coasting down hills, and RPMs would have fallen off to idle levels from those obtained in normal throttle-on cruising at the same speeds (or, assuming GM is or was using fuel shut off during coast in the 04 Silverados, the engine would have died completely, as asserted by the other member posting that message). Am I wrong? Replies to this message: wwest (Aug 25, 2006 9:30 am) echota (Nov 18, 2006 8:27 am)
#248 of 390 Re: zero fuel consumption while coasting [jerrywimer] by wwest Aug 25, 2006 (9:30 am)
Replying to: jerrywimer (Aug 24, 2006 11:40 am) The torque converter does NOT have to be locked up in order to turn the crankshaft during coastdown periods. Some torque converters even have a one-way clutch incorporated for just this purpose. as we all know, or should know, torque converters are not very efficient for "conveying" engine compression braking, but they certainly do. The whole discussion gets a little confusing because automatics cannot be push started. But that's because there is no way, absent a rear pump driven directly by the drive shaft, to engage any of the necessary clutches without hydraulic pressure. And I rather doubt, strongly, that the "light" duty lock-up clutch would, could, ever be used to convey engine compression braking, keep the engine turning over during coast downs. But it is a simple matter to test, just downshift and then turn the ignition off during a coast down time and then back on a little later and see if the engine resumes "running". Replies to this message: echota (Nov 18, 2006 8:27 am)
#249 of 390 zero fuel while decel/coasting-downhill by elias Aug 27, 2006 (4:50 pm)
below are snippets about PCM from a university web page - note the repeated references to the zero-fuel decel condition. also, the point re torque converters are well taken. there's nothing much different here in the case of auto trans vs manual trans. the only reason i granted that might be a factor in the new tahoe v4/v8 was that there are certainly lots of techno things i don't know about that vehicle. it's funny that someone with a mech eng background cannot admit that an engine can run indefinitely downhill with zero fuel when even a lowly electrical-eng computer geek can figure it out! the condition is theoretically possible and empirically demonstrable with a diag laptop hooked up to the ECM - it will show fuel pump at "zero" in such condition. for example, VW vag-com can show the condition on most any VW, i bet . 5. Deceleration – when a deceleration condition is detected the fuel delivery is decreased or completely shut off. OPEN LOOP - the A/F ratio is controlled to provide the best driveability and minimal emissions under the following conditions: A. Start-up - richer mixture B. Cold engine operation - richer mixture C. WOT - richer mixture D. Deceleration - leaner mixture or complete fuel cut-off F. Idle > some engines G. Heavy engine load - determined from TPS & MAP or MAF
#250 of 390 Re: torque of lugnuts [Mr_Shiftright] by toronado455 Sep 12, 2006 (1:58 pm)
Replying to: Mr_Shiftright (May 01, 2006 9:44 am) You should ask that they be hand tightened. I've tried to ask mechanics to do this in the past and I always get mean looks. Why do they hate doing this so much? Also, I have an underslung spare with a nut-driven winder that tightens a steel cable that holds the spare up under the car (minivan). The owners manual says never to use an impact wrench on this and yet I cannot seem to get any mechanics to obey this rule. I even asked a mechanic who was using an air wrench to please stop and the owner of the shop was standing right behind him and told the mechanic to just continue using the air wrench. Replies to this message: Mr_Shiftright (Sep 12, 2006 2:58 pm) 0patience (Oct 06, 2006 3:51 pm)
#251 of 390 Re: torque of lugnuts [toronado455] by Mr_Shiftright HOST Sep 12, 2006 (2:58 pm)
Replying to: toronado455 (Sep 12, 2006 1:58 pm) I guess I'm just old-fashioned...I have this (call it crazy) notion that if you are paying someone $75 an hour they should make a feeble attempt to please you now and then.
#252 of 390 That so many technicians by marsha7 Sep 14, 2006 (9:23 am)
simply do not understand why they should hand tork lug nuts is disgraceful...I have been to 3 different Goodyear shops and they all hand tork lug nuts as SOP... I also get very upset at techs who will use an oil filter wrench to tighten and install and oil filter...where did they miss the concept that you HAND tighten the filter )after puting a thin film of oil on the gasket) but use the wrench to take it OFF???
#253 of 390 Re: torque of lugnuts [toronado455] by 0patience Oct 06, 2006 (3:51 pm)
Replying to: toronado455 (Sep 12, 2006 1:58 pm) If they do not torque the lug nuts on today's vehicles, then I would look for another shop. That is either incompetence or ignorance on their part. Lug nuts that are run on with air wrenches can cause problems with the wheels and rotors. Any REPUTABLE tire shop will torque ALL lug nuts.
#254 of 390 But, I think the point by marsha7 Oct 10, 2006 (6:55 pm)
is that if they can't do something correctly as simple as tightening lug nuts or install and oil filter, probably the first thing one does when working on their own car as a teen, then what does that say for the remainder of their work???...and these are, I hesitate to say, pros... Replies to this message: 0patience (Oct 21, 2006 2:12 pm)
#255 of 390 Re: But, I think the point [marsha7] by 0patience Oct 21, 2006 (2:12 pm)
Replying to: marsha7 (Oct 10, 2006 6:55 pm) That is the problem with a lot of the newer mechanics coming into the field. A lot of them start out in some lube place that emphasizes being fast. Get it in, dump the oil and get out. Then they go into some shop paying flat rate and they get paid for what the book says, so if they can skip a step and shave .2 hour off the repair, they make money. Now, don't misunderstand me, this happens less than folks would like to admit. But it still happens.