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I have a 97 toyota tacoma 2.7l, 5 speed, 4x4.
It has a p0401 and ltft always at +19.53.
The stft does funny things. At idle stft goes to +19.53. Tapping the throttle will immediately bring it to zero. When driving, at 1500rpm stft is ar around +10%, at 2500rpm its bounces +- 5% at zero.
I tested the exhaust for leaks and found one at the manifold around the first o2 sensor and have since had thr manifold welded there.
Is this scenario a guaranteed vacuum/exhaust leak and I should keep searching for more leaks or something else.
so if I am understanding you correctly, at 1500 rpm you have a combined fuel trim of 29.53%. At 2500 rpm you say your combined fuel trim numbers are getting better, ie going lower . Try checking stft and ltft at 1500, 2500, and 3500 rpm. see what your values are at these rpms.The way I am reading what you have said, your combined lt and st trims are getting better when you increase rpm, is this correct?
Last edit: 2 months 3 weeks ago by bills4065. Reason: clarifying what I said.
Fuel filter hasn't been changed in about 5yrs. The maf was changed about a month ago when this first happened and didn't have any effect. I had a spate maf and that was thr first thing i did.
personally, my next step would be change the fuel filter and then check the long term and short term fuel trims at 1500, 2500, and 3500 rpm. I would address the lean running condition before worrying about the P0 401. The P0401 is probably a different issue.
I'm thinking a vacuum leak can cause: 1) a lean condition; 2) EGR not able to to function correctly which could cause a P0401; 3) STFT fuel to react correctly when throttle is opened.
I might focus on insuring I had no vacuum leaks as a first step.
"An open EGR valve is a vacuum leak and will cause positive trim correction. Possibly related..."
Noah, my first thought was also a possible stuck open EGR valve. But then I thought that the P0401 "Insufficent Flow" would indicate that the code is a result of a vacuum leak not opening the EGR valve, so the code would be more of a result of vacuum leak not the cause of the leak. Would my thought be incorrect? Would "Insufficent Flow" be misleading in that it can mean both restricted flow and/or too much flow? Thanks.
After reading the code setting criteria, I agree. Insufficient flow is detected when the EGR temp sensor does not increase above ambient temperature, so most likely not that the valve hung open.
A problem with the vacuum switching valve or vacuum hoses could explain both codes as you pointed out.
