Fluctuating gas mileage - horrible to exceptional - Fixed

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#11

11-28-2007, 11:56 AM

classic_cruiser

Join Date: Jul 2007

Posts: 43

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RE: Fluctuating gas mileage - horrible to exceptional

I usually check if the torque converter is locked by hitting the gas, not real hard, and observing if the RPM's immediately go up. If the RPMs jump, the converter is not locked, if they follow the vehicle speed then it's locked.

Reason I brought that point up is that I've found driving mine around 40-50, that if you do slow acceleration, it sometimes waits a bit longer to lock it. Accelerating fast and then getting off of it makes the TCC lock sooner, in a big heavy SUV the slushbox automatic burns a fair bit more gas when the TCC is not locked.

Here's something else to try: stick the trans in '3' and drive to work and back home a couple times; since you're only going about 50 the RPMs shouldn't get too high. See if the gas mileage results are still different between fast acceleration and slow acceleration in this case. If they're about the same as what you got before, then it's not the TCC.

#12

12-02-2007, 09:05 PM

mdkuni

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RE: Fluctuating gas mileage - horrible to exceptional

Quote:

ORIGINAL: classic_cruiser

I usually check if the torque converter is locked by hitting the gas, not real hard, and observing if the RPM's immediately go up. If the RPMs jump, the converter is not locked, if they follow the vehicle speed then it's locked.

Reason I brought that point up is that I've found driving mine around 40-50, that if you do slow acceleration, it sometimes waits a bit longer to lock it. Accelerating fast and then getting off of it makes the TCC lock sooner, in a big heavy SUV the slushbox automatic burns a fair bit more gas when the TCC is not locked.

Here's something else to try: stick the trans in '3' and drive to work and back home a couple times; since you're only going about 50 the RPMs shouldn't get too high. See if the gas mileage results are still different between fast acceleration and slow acceleration in this case. If they're about the same as what you got before, then it's not the TCC.

I am pretty sure you have hit the nail on the head here. I was doing some testing this weekend and found an issue with the converter. I was towing a trailer with a snowmobile and was accelerating slowly. I was doing 60MPH and the RPM's were about 2,300. I thought this was high, but let it for for awhile. Still the same RPM's after a couple miles. So I accelerated with my crusie on up a couple miles per hour and let off the gas. The rpm's dropped to about 1,600-1,700 and stayed there.

So accelerating slowly probably was just the converter not locking up?

Would running a bottle of Lucas Oil Transmission Fix help the converter lock up easier?

#13

12-03-2007, 12:02 PM

classic_cruiser

Join Date: Jul 2007

Posts: 43

RE: Fluctuating gas mileage - horrible to exceptional

My suspicion is that it's not a hardware failure (like sticking of the TCC valve) because it does lock, so using conditioners in the trans fluid probably won't help it (although it wouldn't hurt to try, as it might be related to transmission fluid pressure). When I've seen this stuff on older TCC vehicles, most often it's been electronics or total failure of the TCC valve. I don't have enough experience with my own blazer to say for sure, but any of the inputs that the computer uses to determine when to apply the TCC could be causing the issue; my first thought would be that the engine vacuum is slightly lower than where the computer thinks it should be for a given vehicle speed, so you could start by checking the vacuum just to see if it's in the ballpark.

There are so many potential contributing factors that it might take a while here to narrow it down. On mine, the problem is so infrequent that my short-term solution is to just speed up a bit and then let off the gas a bit to get it to lock. Seems to work for now!

Terry

#14

12-17-2007, 12:27 PM

mdkuni

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RE: Fluctuating gas mileage - horrible to exceptional

Well I am more confused then ever. I took my truck to the repair shop and they tested the o2 sensors at idle and while driving. They found no problems.

I drove two days to work and back and got 13mpg. I drove 3 more days same exact route and filled up, this time it was 19mpg. I have been watching the converter and it has been locking up correctly during normal driving when I am not towing.

Does anyone have any ideas?

#15

12-17-2007, 05:15 PM

RegSo

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RE: Fluctuating gas mileage - horrible to exceptional

Quote:

ORIGINAL: mdkuni

I own a 2000 ZR2 Blazer with about 76,000 miles on it. I have only had it for a couple months.

Thanks to this form I was able to replace my fuel filter and found my thermostat was broke as I was only hitting 154 degrees. I have replaced my thermostat with a Stat SuperStat 195. Now my temperature is normal.

I also rand two tanks of gas with Seafoam and one treatment through the oil as well. The oil was changed after about 100 miles.

My problem is I get great horrible gas mileage at times as bad as 11-14mpg yet other times I have got as good as 22.5mpg which I never expected to get with the Blazer. I just wish I would get that all of the time.

The drivng that seems to be killing me is the driving I do to work and back. This drive is 50mph and is ten miles long with only about 4 stops along the way. I have been accelerating very slow (under 2,000rpm). What I have found now is the faster I have been accelerating the better my mileage has been. I can not explain it? The engine is very very slow to warm when driving to work slower. I do not even get to full temp by the time I get to work. Yet driving hard I am up to temp about half way there. Could a sensor be stuck casuing this issue.

I have tried drving slow versus driving faster on a 80 mile trip this past week as well. This posted the same results. 22.5mpg driving faster. 13.89 driving slower.

I am no mechanic when is comes to cars but I can use a wrench fairly good!

where did you get the new thermostat, and was it easy to install

#16

12-17-2007, 05:39 PM

mdkuni

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RE: Fluctuating gas mileage - horrible to exceptional

Quote:

ORIGINAL: RegSo

where did you get the new thermostat, and was it easy to install

Advanced Auto. Not to bad considering I have never done one before. I did not bother with my coolant though. Once I replaced it I took the Blazer to Penzoil and had my coolant flushed since I figured it was about time anyways. I got all of the information to change the stat from this site.

#17

04-10-2008, 07:41 AM

mdkuni

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RE: Fluctuating gas mileage - horrible to exceptional - fixed

My Blazer is finally getting consistent mileage about 4mpg better then before. I checked the rear brakes and found 3 out of 4 caliper bracket slide pines were rusted solid! Also the left side pads were rusted pretty bad in the bracket and required a chisel and hammer to remove them. They were worn completely down. I am sure every time the brakes were pressed the pads would not release until they were worn.

I am now getting close to 17mpg for my work commute compared to the 13mpg I go all winter. This site is a great resource, I have been able to accomplish a lot thanks to this site. Thanks guys.