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Old 08-09-2020, 09:09 AM   #1
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Default negative fuel trims

This is the '98 DOHC that I recently replaced the head gasket on. I've been driving it some but has a lack of power; especially at low rpms in high gear. Like at 45-50mph, it can hardly maintain speed without downshifting.

Finally put my OBD2 scanner on it Friday evening. Fuel trims were way negative. During slow idle, ST at -2% and LT at -14%. Raised rpms to 2500 and ST went to -28% and LT at-10%.

Unplugged front oxygen sensor and battery. Let car sit overnight.

Took short drive this morning. Since oxygen sensor was unplugged, it stayed in open loop mode and the fuel trims both stayed at zero. Power was restored and seemed normal.

I do have a P0440 Evap code but I believe this is because the nipple on the purge valve where the fuel line connects is broken off. I discovered this during the head gasket reinstall but didn't fix it because I wanted to make sure the car would run first. I did order a new valve from Rock Auto yesterday but doubtful this will fix it. I think if the purge valve started sucking unmetered ambient air it would actually cause the fuel trims to go positive, not negative.

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Old 08-09-2020, 09:32 AM   #2
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Join Date: Apr 2006
Posts: 4,134

1998 SC2
1998 SW2
Default Re: negative fuel trims

Purge Valve:
When the purge valve stuck open on my ‘98 DOHC, the warmed-up engine wouldn’t start without flooring the gas pedal for total fuel cut-off. After shutting off the engine, there was a strong smell of gasoline vapors from under the hood. Fuel vapors were wafting out of the air filter box.
I do not recall ANY codes or warning lights being set when the purge valve stuck open.
Competition Clutch 10Lb. Flywheel. 14 stalls to date (and loving it).
Oil use is 1 ounce per 100 miles with ~230,000 miles on the engine

Last edited by bumpdraft; 08-09-2020 at 09:38 AM. Reason: No MIL for the purge valve stuck open
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