The teams of Gibbs' No. 18 and No. 20 Toyotas deliberately attempted to mask the true horsepower in their engines after Saturday's race at Michigan International Speedway.
Inspectors found magnets under the gas pedals of both cars when they were sent to the chassis dyno for examination.
NASCAR traditionally hands out penalties from the previous week's races on Tuesday, but NASCAR spokesman Kerry Tharp said the decision was put off until Wednesday.
NASCAR vice president of competition Robin Pemberton said Sunday the two teams put magnets a quarter-inch thick under the gas pedals on their cars in an attempt to hide how much horsepower the Gibbs motors still have even after recent rule change.
The two cars have combined to win 14 of Toyota's 15 victories in 25 Nationwide races this season. To temper that domination, NASCAR last month ordered all Toyota teams to cut about 15 horsepower in their motors.
Tony Stewart finished third on Saturday in the No. 20 in his final Nationwide race for JGR, and Joey Logano was seventh in the No. 18.
Joe Gibbs Racing president J.D. Gibbs said he and his father, owner Joe Gibbs, had no prior knowledge of their crew members intent. J.D. Gibbs apologized to NASCAR and Toyota and said it was "a really poor, foolish decision on the part of our key guys."
Both father and son have promised an in-house investigation and penalties against those responsible beyond whatever NASCAR levies. J.D. Gibbs said Sunday the team probe could lead to some employees being fired.
The Gibbs team also fields cars for two-time champion Stewart, current points leader Kyle Busch and Denny Hamlin in the Sprint Cup series.
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