QuestionI've got a 1990 Toyota pickup (V6, 4x4, SR5, 5-speed, 227,000 miles). I haven't had any prior engine trouble, and hadn't had any recent maintenance work done. Recently the engine began running rough. There are certain RPM ranges where it does run smooth, but it runs rough throughout most RPM ranges (especially when accelerating). I have tried to correct the problem by replacing the spark plugs, distributor cap, rotor, plug wires, air filter, and fuel filter. None of this has helped. The "check engine" light has not come on, but I decided to put a jumper across the leads on the diagnostic port anyway just to see if the light would blink out an abnormal code. The "check engine" light did indicate a problem with the air flow meter. I then removed the air flow meter and checked the resistance across its terminals as described in the shop manual. They all checked out within the specified ranges. Could the air flow meter still be bad even though the the resistance across its terminals check out ok? Any ideas on why the "check engine" light didn't illuminate, but then blinked out a code indicating a problem? If its not the air flow meter, any guesses on what the problem could be? Oh yeah, the last time I replaced the timing belt was about 40,000 miles ago. Thank you!
AnswerYou have almost as many miles on it as my '90 4-Runner(254K) and still going.
What was the trouble code you had retreived?
The a/f meter normally doesn't set a code and can malfunction without doing so but I don't think it's the problem based on the description.
The reason the check engine light did not come on is because the computer only saw the malfunction once, it needs to see it two times(two trip detection logic) to make the check engine light come on and stay on.
Since all of those parts were already replaced we can rule out an electrical misfire, this leaves an engine mechanical problem as a possible cause, check the compression and check for a possible head gasket problem.
