Timing Adjustment needed if switching to 91 Octane gas?

[batook]

My understanding is that the timing advance will change slightly if the timing was set for 87 octane gas and 91 is used. Will the Previa's ECU/EFI system alter the timing advance on its own as it detects the difference in combustion from the higher octane gas, (possibily through input from the knock sensor??), or must the distributor be manually adjusted to correct the timing on the Previa? Or, will the timing not be affected that much from the switch? This question specifically applies to naturally aspirated Previas in my case...not sure if the S/C would do things differently or not.

I must admit that I've never set the timing on the Previa before...just never had the occasion or need to do so (never removed the distributor). I do have a timing light, and my understanding is I would just hook it up to the appropriate terminals on the check connector, shine it at the crankshaft pulley, and see where the timing markings on the pulley are falling as compared to the little needle indicator as the light strobes. The pivot bolt on the distributor could then be loosened so as to turn the distributor up or down to adjust the timing. Is this correct? I imagine it's best to do this once the engine is warmed up and idling normally. Thanks, Eric.

[timsrv]

Set the timing per the manual. Be sure to put the van in self diagnostic mode before checking/adjusting so the electronic advance won't interfere. Once you have it set, pull the jumper from the diagnostic terminals and use the timing light again to verify the timing jumped (this means the electronic advance is working). Don't worry about the octane of the gas. Just put the cheap stuff in there. The Previa has a knock sensor and will self adjust the advance if predetonation is detected. If you're towing or climbing hills with heavy loads you might benefit from 91 octane, but unless you're doing that I'd stick with 87 (nothing to be gained with the 91). Tim

[batook]

Well the reason I'm thinking of using the 91 is there's a station near my house that is selling ethanol-free gas, but only in the 91 grade. In Colorado, the low grade is actually 85, because the higher altitude supposedly allows for it. I've been putting the ethanol-free 91 in my wife's new car, and it increased the mpg by 3-4 almost instantaneously over the 91 with 10% ethanol. So I wanted to see if my Previas would exhibit a similar performance and fuel economy increase by using pure gas instead of burning 10% corn, but I wanted to make sure the timing wouldn't be off after the switch to 91, since that's the only way I can get it without ethanol. If the experiment proves to not make much of a difference, I'll go back to the cheap stuff, but I also like the idea of supporting this station and their efforts to supply the pure gas, since it's one of the only stations in the denver-metro area that even sells it (and per CO law they can only sell it from March - November). Eric

[timsrv]

It's true that higher quality fuel will give you better MPG (more btu per gal). Will the increased MPG off-set the increased price per gal??? That will depend on difference in price vs increase in MPG. I'd run a few tanks of each then do the math. It might actually be cheaper to run the good stuff. Tim

[batook]

Yeah that's the plan. I've been tracking my mpg using an app on my phone, so I'll be able to recognize any increase after switching to 91. Only variable that might influence the results is the transition from winter to summer gas, which might be happening soon. Either way, I'm curious to see how it does. Eric

[mountainhick]

Once you have it set, pull the jumper from the diagnostic terminals and use the timing light again to verify the timing jumped (this means the electronic advance is working). Tim


Service manual shows using an SST to connect TE1 and E1 on the DLC1 diagnostic connector will retard to 5 BTC. Is this just a simple jumper that shorts the terminals or something else? I don't want to short it if that ain't what it's fer.

[timsrv]

On the Previa I use a metal paperclip. I like the bigger ones as they fit better. The SST is just a little wire with the proper ends to match the terminals in the box.

[mountainhick]

Thanks Tim