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Old 03-16-2009, 09:28 PM   #1 (permalink)
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Actual mpg vs % that mpguino is off

I have been wondering about this, how to get the mpguino more accurate, so I did some charts with my fuel log data. I charted miles driven, gallons, mpg, % guino is high (from calculated fuel flow value), and % of trip downstate (basically nonstop). Something that really stood out was when plotting the actual mpg against the % the guino was high that time. To make the charts better readable I subtracted 30 from one car's mpg and 25 from the other, to keep the same data trend but to place them together.

The other things did not have near as much correlation.

So essentially the less pulse width my injectors are getting, the more the guino is off. Since the injector delay is constant, a shorter pulse width means a higher percentage of each pulse is made up of injector delay, and a lower percentage of actual fuel flow. The guino thinks I have more fuel flow at these shorter pulse widths and therefore fuel use reads high. So it sounds like I need to bump up the injector delay so it will consider less of each pulse as fuel flow and more of each pulse as injector delay. Then readjust the usec/gal because a little change in injector delay will probably make a big change in usec/gal needed. But once the injector delay is correct, that should make the rate of change correct, then the usec/gal can be set and it should be pretty linear.

I'll have to think about it for awhile, but it seems there should be a way to figure out what to set my injector delay to, in order to get things properly on track. Maybe a system of equations or something. It's been a long time since I have done that kind of thing.

Any math whizzes wanna play around with it?

Edit: The last value on the Celeb chart should be ruled out as an outlier. The tank ran out considerably sooner than it should have. I found no fuel leaks and no guino problems, so I think some of my gas got stolen.

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Old 03-16-2009, 09:31 PM   #2 (permalink)
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It may be nothing, but can you unplug an injector and measure its ohmage? Also, what sorts of precautions do you use when refueling?
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Old 04-14-2009, 10:18 PM   #3 (permalink)
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Hi dcb, I finally got a chance to check some things out.

When refueling, with both cars I always try to pump the gas at the same speed. At about .1 gallons per second, or 6 gallons per minute. I just imagine I am watching a clock. I go to the first click of the handle and call it good. Last year I went to 2 clicks but that would overfill the wagon so since then I have been going to the first click for all cars.

Then I go in and usually my daughter takes a potty break and looks around for a few minutes to decide on something to nibble on for the road, then I go pay and come back to the car. I always look (especially lately since I'm suspicious) and I never see any drips on the ground. I also check after a hilly stop and go drive like going to work, and never see anything on the ground.

This season I have done all my fillups at the same station, that never gave me any problems with the other car so I would assume I am not getting ripped off. Last season the problems started when using other stations anyway. Before that it was consistently 1-8% high and no more. The problems started with one tank being 1.3% low, then was ok for a bit, then bam, 10% low. It still continues to be about 10% low.

Now I did crawl under the car at lunchtime yesterday and nosed around at the tank and lines. The tank is starting to get rusty, but no sign of leaks and no gas smell. The hoses from the tank are dry, but when I grab them and pull them around, they are still dry but my fingers smell like gas afterwards. So they are passing vapor, which means it is time for replacement. I ordered a new tank off ebay, since I have to drop the tank to change the lines anyway, might as well just do it once and be done with it. Every one of my a-bodies has needed a tank replacement anyway. I doubt I am losing over a gallon per tank just as dry vapor, but I will get at these things once the weather shapes up, they can't hurt.

One thing I was curious about was the pressure regulator. If it was leaking, it could be drawing gas into the intake, therefore burning the gas and not needing as much through the injectors to get the same engine power. But I disconnected it and not even a smell of gas, so I am good there.

Tonight I got a chance to test the fuel injectors. To get to them I had to pull the plenum which requires removal of a bunch of other things. I am not totally sure if they are all to spec or not. 4 of them are 12.6 to 12.9 ohms. The middle one on each bank is 11.6 for one, and 10.0 for another. I have a service manual that says the bank as a whole (3 in parallel) should be at least 4 ohms, but in the same sentence says that each injector should be at least 8 ohms, which doesn't jell. Various websites say that they should be no less than 12 ohms which makes more sense. I do have 2 spare banks of injectors, one is in similar condition with some low ones, the other is all consistent at over 12 ohms.

I may get at swapping the injectors this week just to see, not sure. Friday I get my wisdom teeth pulled, so I will be out of commission for awhile.
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Old 04-14-2009, 10:36 PM   #4 (permalink)
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I was trying to tell if you has peak and hold injectors with the ohmage readings, but it sounds inconclusive.

If it is consistently reading low, then it may be that some injectors were partially clogged and have cleared up? I might run a bottle of injector cleaner through it and recalibrate.
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Old 04-15-2009, 12:23 PM   #5 (permalink)
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I am finding more pages that say the proper ohms is 11.8-12.6. I think my GM factory service manual has a misprint.

The injectors are GM Multec, and the injection type is MPFI. Not TBI. I highly doubt they are peak and hold. GM TBI injectors it shows as between 1 and 2 ohms.

That's possible they could have been clogged. I'll try some cleaner at the next fillup.
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Old 04-17-2009, 07:41 PM   #6 (permalink)
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Wednesday night it was nice out so I swapped the injectors after work. I also tested the old ones both hot and cold and WTF now they all test fine? Must be an intermittent short in them, they got the old readings both hot and cold as well but a day earlier. The new-used injectors seemed a bit sticky at first from sitting, finally after about 20 miles into my 25 mile work trip, they fully cleared up and it has ran well since. Guino said 28 mpg to work, which makes sense, it ran horrible at first. I also filled the tank at lunchtime. Forgot fuel injector cleaner this time, can always do later. Trip home guino said 38 mpg, best reading ever, but of course it may not be correct. One thing I noticed is the gutsy low end seems even a tad stronger. It also started well before, but now it starts absolutely instantly, like in 1 turn of the crank. Oh and fuel pressure is very strong, proper range, tested that too. Got my gas tank here today too. And I am feeling pretty normal after getting the teeth pulled this morning, so hope to do the gas tank and lines in the near future.

Now I'm really curious what the next few tanks will bring for guino readings, then I'll have an idea where to go with recalibration. Only thing is with getting good mileage it takes a long time to use up a tank, which is a good thing.

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