Valve adjustment affect mileage??

[mhughett]

I got my Inspection I a couple of weeks ago and 15 of the 24 valves required different shims. While I take comfort knowing that the dealership appears to have been thorough in their work, I can't help noticing that my mileage since then seems to have gone down by 10%-15%. It's possible that it could be partially due to the cold weather which causes cars to run a little richer until they warm up. But could the changes made from the valve adjustment have anything to do with it? It sounds and runs great but the mileage drop is hard not to notice. Anyone have a similar experience? Thanks.

[thekurgan]

She shims are designed to wear (more than the valve stretching or seat recession, causing tight valves), so most likely the valves were on the loose side and a larger shim was introduced to bring the lash into spec, allowing more fuel/air mixture, as well as increase exhaust breathing. My mileage has always gone down after an adjustment, but after a few days, it levels again. Did you get the spec sheet as to the before and after tolerances?

[mhughett]

Originally Posted by thekurgan

Did you get the spec sheet as to the before and after tolerances?

Yes. But the way the tech filled it out, he never put the clearance measures on the sheet. He starts out simply citing whether the valve was within spec or outside spec. Then, if it was outside spec, he included the size of the existing shim, the one that he substituted for it and that the final gap was within spec. In all cases, the shim used was one size larger than the one it replaced.

[thekurgan]

Originally Posted by mhughett

Yes. But the way the tech filled it out, he never put the clearance measures on the sheet. He starts out simply citing whether the valve was within spec or outside spec. Then, if it was outside spec, he included the size of the existing shim, the one that he substituted for it and that the final gap was within spec. In all cases, the shim used was one size larger than the one it replaced.

If a larger shim was utilized, then the valves were on the loose side; he most likely had some scratch data where he calculated his measurements and micrometer values, he just didn't place that on the final sheet for you. In any case, the car may need some time to adapt to the new clearances, not that these cars vary much with respect to fuel mileage, I always have a little dip after adjusting them, but I generally set mine on the looser side of the spec.

[G80indy]

Originally Posted by thekurgan

.... I generally set mine on the looser side of the spec.

why?

[thekurgan]

Originally Posted by 330indy

why?

Cooler engine, safer for the valves, especially on the exhaust side.