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Old 05-08-2004, 10:41 AM   #1
David R
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Default "Fuel Trim" values

Yes, I'm still trying trying to solve my fuel economy problems. If I've done this right a graph showing short and long term fuel trim should be visible. I adjusted the fuel pressure down which has helped a bit, but the car is still decidely below spec. I've continued to adjust the fuel pressure to put the Long Term spread either side of zero.

A question comes to mind, should the Long Term values be jumping around as much as they are? The solid lines are two point moving averages, which connects the gaps, but smooth the plots as well.

I've searched the web on this topic, the implied behavour is a slowly adjusting value. Is that slow for people time or computer time?

Any comments?

David
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Old 05-08-2004, 08:03 PM   #2
wolfman
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The PCM shoots for an "average" (always looking for stoic) fuel mix and looks to the O2 sensor data to see how it's doing. It makes small adjustments accordingly but is always shooting for that average rather than a precise mix. It will also try to compensate for things like reduced fuel pressure by altering injector pulse duration times, so in other words by monkeying around with the fuel pressure you are actually working against yourself as the PCM is fighting you all the way. You will eventually get to the point beyond which the PCM can compensate for software wise, at which time it will either got nearly always open loop, set an SES light (unlikely) or continue to try and compensate for things resulting in a fuel map that is all over the place.

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Old 05-11-2004, 09:04 AM   #3
David R
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Wolfman, I appreciate what you're saying. That's actually why I adjusted the fuel pressure, because the PCM never (over a two week period) seemed to reach equilibrium. At the end of that time the spread of highs and lows centered at -14% Long Term and still falling. The fuel economy would continue getting worse until I got annoyed and reset the PCM. Having adjusted the fuel pressure down (was ~38psi at idle), the PCM is now swinging around zero.

My question was actually about how rapidly the Long Term Fuel Trim is adjusted. The time frame on the graph should be x10 (stupid Excel). It just seems odd that something called "long term" would be jumping around by 3 to 10% per second.

David

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Old 05-11-2004, 11:44 AM   #4
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David,

I don't know what you have done so far to diagnose your problem so you might have already tried this.

Have you checked/replaced your injectors? If one (or more) is leaking, your PCM would continue to try to lean out the mix. Maybe you have one or more injectors delivering the wrong amount of fuel. Make sure your injectors are operating to spec and go from there.

Good luck.

Brian

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Old 05-11-2004, 12:01 PM   #5
David R
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I believe I did have them checked last fall, cylinder balance, I think they called it.

I did checking into replacing the injectors, but they were very expensive, to do on speculation.

I have however been thinking of getting a complete fuel rail, with injectors from a wrecker and just swapping to see if makes any difference.



David

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Old 05-11-2004, 07:02 PM   #6
eljefino
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What are your spark plugs like? All equally black?

The used injectors and rail sound like a decent idea to try.

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Old 05-11-2004, 07:45 PM   #7
Kevin94SL
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I had a used fuel rail, Throttle body and injectors put into my car last fall. Made a bit of a difference. Not sure how much exactly, but, it's cheaper then using all new parts if you don't know exactly what the problem is. Give it a shot and let us know how it goes

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Old 05-11-2004, 08:23 PM   #8
TedL
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You can get four injectors cleaned, tested and returned with new screens and seals, all for about $50 from cruzinperformance.com. I got a used set for $20, sent them for cleaning/testing, then put them in. Something to consider, so you know that what you're putting in is better than what you're taking out.

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Old 05-11-2004, 09:15 PM   #9
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Sparkplugs are a chocolate red-brown hinting at a tan colour (red/green colour blindness doesn't help) all more or less the same.

I'm going to make a few phone calls to get an idea on prices. We'll see, surprisingly few wreckers in my area are interested in dealing with the public.

David

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Old 05-13-2004, 11:21 AM   #10
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Some follow-up information, did the numbers on the last two tanks of gas, pure city 17.4mpg. Second tank pure highway, except to get on and off 36.4mpg, which is right on the money for this car (Environment Canada ratings). Interesting thing is, I ignored the 63mph speed limit and drove almost the whole trip between 78mph and 81mph. My usual style is around 68mph and its a struggle to get 30mpg on the highway.

Inspite of what I've been told it kind of suggests a leaking Injector(s).

Might also explain why the Long Term seems to move around as much as the Short Term.

David

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Old 05-13-2004, 02:00 PM   #11
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Keep in mind that the O2 sensor is reading the output of ALL cylinders combined, not individually. Even ONE leaking or stuck injector will result in the PCM "seeing" a lean or rich condition on ALL cylinders and trying to adjust accordingly. The possible cause for your readings...

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Old 05-13-2004, 03:19 PM   #12
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You might have someone check your ect sensor before you spend money on injectors.We see ect sensor problems daily, and they can be off by 50 degrees and not trip the engine lite. The sensor has been updated to a good brass unit now, was plastic,and they don't weep coolant into the wiring harness and corrode the connector . Basically, the temp. the sensor is sending to the computer should be about 10-15 degrees of what the temp. in your radiator resovoir is showing. I use a cheap digital thermometer to check this in conjunction with a tech-2 scanner.

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Old 05-14-2004, 11:10 AM   #13
David R
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Found a wrecker with some parts online up here, but I need to know some compatibility information.

I have a 97 DOHC, they have fuel distribution parts (rail, injectors etc.) for:

1996 DOHC $150cdn
1997 SOHC $125cdn
1999 DOHC $150cdn
2000 SOHC $125cdn

If I remember correctly 96 is a direct drop-in, the 99 uses a different pressure regulator. I'm not sure about the SOHC parts wouldn't the injector be the same parts?

David

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Old 05-14-2004, 11:17 AM   #14
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Saturntech-6

I've changed out the temp sensors several times now, but it doesn't have any effect. I 've also done the thermostat several times as well with some success, but that's a different issue.

David

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