LTFT high at idle

[blackyote_317]

My 99 GT auto with 80k miles seems to have high long term fuel trims at idle. Seems to be about 17-19%. Short terms look pretty good and usually hover between -3% and 0. It runs and drives well but occasionally is slow to start when cold.

I'm planning to check for vacuum leaks tonight but I'm leaning towards this being a fueling issue due to slow starts. Since I bought this car I've been refreshing things as it is my daily. I'm thinking I'll tackle the fuel filter, injectors, and plugs/coils soon. Hoping that normalizes things. Anything I'm missing or I should check/replace?

 

[wmburns]

If a vacuum leak is suspected this will usually cause LTFT that decrease towards zero as the RPM's go up. IE as there's less intake vacuum there's less pressure differential to cause un-metered air to bypass the MAF.

If this were my car I would double check that the fuel rail pressure sensor (FRPS) intake vacuum reference line is connected and leak free. Also inspect the line for raw gas that will indicate the FRPS is bad.

If a fuel injector problem is suspected I have had good luck with an injector cleaning and flow test service such as InjectorRX.com.

 

[blackyote_317]

I did both propane and soapy water tests. Could not find a vacuum leak. Good tip on the frps. I'll check that today and then the fuel filter. If that doesn't solve things, I'll start working my way through the system.