Re: Repeatedly Running On A Low Tank?

Jun 11, 2008 2 Replies
C

----- Original Message ----- From: "Bill Putney" Newsgroups: alt.autos.ford,alt.autos.honda,alt.autos.nissan,alt.autos.toyota,alt.trucks.chevy Sent: Tuesday, June 10, 2008 5:42 PM Subject: Re: Repeatedly Running On A Low Tank?


I'm puzzled about one thing that i used to experience sort of
> related to this, so I'll pose it here: In the late 80's/early 90's,
> I lived in Colorado - periodic emissions inspections were required.
> I had an '86 MPFI turbocharged Subaru station wagon at the time. On
> two occasions, it failed the emissions isnpection - and both times
> the technician asked if the air filter had been changed recently.
> On those two occasions, the answer was 'no'. I changed the filter
> both times, and it passed the re-test with flying colors. After
> that, I made sure to replace the filter just before taking it for
> inspection if it had not been changed recently, and it always passed > with no problem.
>
> So my question is: If the computer can compensate for the slightly
> clogged filter for normal driving, why can't it do so for emissions
> measurements (idling situation)?

Your car was running in open loop mode during the test. If the car is not in closed loop mode, the PCM is running the engine on a stored lookup table (the amount of fuel injected is determined by the engine rpm, TPS reading, MAF reading). Older systems did not do a particularly good job of "learning" parameters at idle. Still, the air filter, unless really dirty, should not have kept the car from passing. At idle the air flow through the filter is relatively low, meaning the pressure drop across the filter was also low. I suppose the combination of the lower atmospheric pressure at high altitude and a dirty filter could have resulted in an air flow vs. sensor relationship that was not addressed in the PCM's look-up table, resulting in too much fuel for the air flow, causing you to fail the test. Still, if the car was warmed up, the converter should have been able to mask any excess fuel condition. Did it fail because of high hydrocarbons, or high carbon monoxide readings? Was the car originally sold with a high altitude calibration (back then cars sold in high altitude areas often got a different set of parameters loaded to the PCM).


Ed



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I did know to make sure it was good an warmed up before I got there. IIRC, they rev'ed the engine to something like 2000 or 2500 rpm for several seconds and held it while the testing computer sampled the exhaust. I believe also that a second part of the test was at idle. You're saying that at idle, it would have been open loop regardless?

I could not tell you what parameter caused it to fail.

I do not believe there was any high altitude issues with taht car, i.e., that there was any sort of kit or flash available for it. It certainly ran well, even in 16k feet mountains near Denver. I do recall, it being turbocharged and fuel injected, the amazing difference between how it and the carbureted Chevy Citation V-6 I had at the same time ran at the

10k plus heights.

There is one other possibility that I can think of: I bought eh Sube with 140k miles on it, and though I sold it years later with over 275k miles on it with the original turbo unit and engine running as well or better than it did when new, I suspect that the turbo shaft seals leaked a little. Perhaps, with the intake fan of the turbo unit between the air filter and the throttle body, the extra vacuum on that from a partially restricted filter would have pulled oil thru that seal and that that is what made it fail the test. If that were the case, I would guess that high hydrocarbons would have been the problem.

One other factor: Coincidentally, the particular inspection station that I used was like 3 blocks down the street from Burt Subaru - the largest volume Subaru dealer in the U.S. at the time (turbocharged and 4-wheel drive Subarus were very popular in Denver because they did well going up into the mountains near Denver). Perhaps the special knowledge that the tech had about replacing the filter to pass the test was specific to turbocharged vehicles, or turbo-charged Subarus in particular (perhaps they all leaked a little oil from the shaft seals at higher mileage). But that also is pure speculation on my part.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

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Ah- Turbocharged!! No high altitude calibration(because it "turbo-normalizes"-and quite possible it WOULD run open loop at idle.

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