i have an 86f150 with an 88 fuel injected 4.9l. my truck recently developed a high idle (around 1450 rpm). but sometimes it idles perficlty. as it warms up it seems to get slightly higher. At times it also has a dead spot as soon as i touch the throttle. i have repaced the iac and the tps sensors already in hopes of fixing it. yesterday i scanned it and i got code 21 coolent temp sensor out of range. but my temp gauge works just fine. cold this be the problem?