Ok guys, here goes. My 89 XT6 gets consistently poor fuel mileage. There are a few other symptoms that may or may not be related. I will list the symptoms, and the stuff I've already tried. If anyone has any ideas on where to go next, I'm wide open to suggestions.

Cold start high idle lasts a very long time, over 3 minutes, unless I shift to Neutral, and will sometimes surge and hunt for idle for a few seconds until it settles to about 1800 RPM.

Exhaust smells very rich.

Occasional surge/hesitation until warmed up.

So far I have replaced both coolant temp sensors, with connectors, oxygen sensor, all injectors, fuel pump and filter, cap, rotor, plugs, and wires, tried a used but good fuel pressure regulator, cleaned the MAF, checked ignition timing, and tried a different but unknown idle control solenoid. The car runs great after warmed up for a few minutes, and not really that bad when cold, just different. I am getting around 15-16 mpg and it doesn't seem to matter how I drive, it's always the same. I'm hoping one of you might know something I'm not thinking of. Thanks in advance!!!

Did you make sure you don't have any precat exhaust leaks?

 

Pull the plugs again and see if they all look the same color/wear.

1. You’re probably driving around town mostly?

2. What check engine codes exist?

3. Have you read the ECU codes in memory? Post all the codes here  

4. Swap another ECU. They fail with age or overcharging alternators and nearly every one isn’t on the original alternator by now. The smell of fuel and poor mileage with most addressable fueling parts replaced is suggestive of ECU issues.

5. Since you smell it this probably isn’t the case but Check caliper slide pins and front ebrake cable to ensure it’s not sticking  - clean and grease with high quality grease so they’re effectively stay off the rotors at all times.

Are you ever cruising 60mph for 30 minutes or hours straight? That’s the best comparison. 

If you’re DDing around a town you’re going to get bad mileage particularly in a 6 cylinder.  This has been a very common misperception amongst people for the last 10 years as more people drifted to urban settings. 

I see people all the time who think they aren’t city driving because they work in a town but make escapades here and there to visit family or go to parks. But by and large their commute daily or buying food multiple times a week involves what amounts to city driving.  Even in rural areas this holds - people moving to small towns rather than the family farm. Etc.