TBS and fuel economy

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McRuss

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Last Sunday was my 26,000 mile maintenance day: valves, TBS, air cleaner. All was good except TBS and I had to tweak 2,3, and 4 (#1 is base). But now my mileage seems to have dropped off significantly; from an average of 47 to a tank at 43 and another about the same. I keep a complete log of fill ups so I can see any abnormalities.

Only thing I can think of is the TBS tweaks changed something (other than running smoother than it has since new!)

Thoughts?

I'm getting ready to leave on a trip to Colorado in two weeks and don't want any problems. I'm not as concerned about the drop in fuel economy as I am about having perhaps altered something.

 

SkooterG

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#3 is supposed to be your base. I don't see how a TBS had anything to do with your fuel mileage. It's just an air bypass circuit used at idle. If anything, getting them closer should have helped. Most folk around seem to think a TBS cures all ills. :blink:

 

HotRodZilla

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Is it possible you need new plugs? I have seen poor fuel mileage magically improve with new plugs.

Also, Skooter is right. The base TB is #3, not #1. I'm betting either plugs or a bad vacuum cap. Or, just the tank of gas you got.

 

Fred W

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It doesn't really matter which one you use as your "base", because all you are trying to do is balance the throttles to each other, so it is all relative. And this is at idle so it has very little effect on the engine during operation (unless you do the unauthorized adjustment)

But the above posts were also spot on: You can not lose mpgs by doing a TBS, so it must be something else.

The other maintenances are all suspect. I suspect either the vacuum leak (previously suggested) or something else that you have failed to mention. Or else serendipity (placebo effect).

 

RadioHowie

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Did you go with OEM on the air filter?

Some folks have tried one of these after-market jobs and created ALL kinds of drive-ability and mileage issues:

3101-00-L.JPG


 

fastpappy

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How is the performance in general? Others have said with improved performance some have a tendency to twist the right hand a bit more which could affect millage. ;)

 

McRuss

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Well, after a couple more tanks, I guess it was just an anomaly, mileage is back up to 47+. I agree with all assesments too, except for #3 being base. I've heard that #1 is suspect because it runs hotter or something, but as Fred W pointed out, it is just a relative thing. Point of TBS is to get all TBs in sync (duh) and that I did. Maybe #3 is a little harder to get at with the screw driver to sync so that is the reason for using it. Plugs are a big factor, I put Iridiums in a while back and saw a mileage jump. Running stock air cleaner too. Overall performance has not changed after TBS, still runs great, just lost alot of vibration.

Interesting argument about TBS only valid at idle. A BMW tuner genius used to say set TBS just off idle. And my WeeStom service calls for setting the idle up to 1300 before doing TBS. I maintain that if the TBs are not in sync at idle, they won't be in sync at higher rpms either and the result will be felt. Thus the fact that setting my TBs reduced my 3k rpm vibes. That is my story and I'm sticking to it. :glare:

 

Fred W

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Well, after a couple more tanks, I guess it was just an anomaly, mileage is back up to 47+. I agree with all assesments too, except for #3 being base. I've heard that #1 is suspect because it runs hotter or something, but as Fred W pointed out, it is just a relative thing. Point of TBS is to get all TBs in sync (duh) and that I did. Maybe #3 is a little harder to get at with the screw driver to sync so that is the reason for using it.
I believe the reason they chose #3 as the base is because that is the throttle that is directly opened by the throttle cable. The other 3 are joined to it via adjustable linkages. You know, the ones that you are really, definitely, completely not authorized to ever adjust. ;)

Plugs are a big factor, I put Iridiums in a while back and saw a mileage jump. Running stock air cleaner too. Overall performance has not changed after TBS, still runs great, just lost alot of vibration.
Plugs should only influence mileage if/when they are failing. Most times, at the intervals we typically replace them at, they never get to that point, so you never see a difference. It is a very binary situation. They are either working (well) or not.

Interesting argument about TBS only valid at idle. A BMW tuner genius used to say set TBS just off idle. And my WeeStom service calls for setting the idle up to 1300 before doing TBS. I maintain that if the TBs are not in sync at idle, they won't be in sync at higher rpms either and the result will be felt. Thus the fact that setting my TBs reduced my 3k rpm vibes. That is my story and I'm sticking to it. :glare:
It depends on what you mean by TBS. The "official" dealer version of TBS is to just adjust the air screws. That will primarily affect the balance at idle. The reason for this is that the size of the adjustable orifice in the air screw path bypassing the throttle plate is relatively small. As you begin to open the throttle butterflies that small amount of added air becomes increasingly less significant. What will be of more significance is how well aligned the throttle plates are to each other, which is why the RDCUA TBS works.

The "Official" concept was that the manufacturer adjusted the throttle plates exactly during assembly and so you should never need to touch them. If that was actually true I don't think the RDCUA procedure would work so well.

 

SkooterG

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I maintain that if the TBs are not in sync at idle, they won't be in sync at higher rpms either and the result will be felt.
You can maintain till the cows come home, but that won't change the facts. :p

Adjusting the 'air screws' has ZERO effect on the throttlebody plates. And at idle, the TB plates are almost fully closed. Adjusting those screws affects the 'air bypass circuit' which is an idle only circuit completely separate of the TB plates.

 
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