Having intermittent misfires, but no check engine light. Mileage performance is down about 10% but everything else is fine. In fact, car passed annual emissions test. My mechanic said if it passed emissions test, an engine analysis wouldn’t explain the misfiring. (Had Throttle Body and spark plugs replaced about a year ago. Car has 106,000+ miles.)
Your mechanic is wrong, and it sounds like he’s jumping to conclusions
I’m a professional mechanic, who does smog inspections in California, arguably the most stringent state, as far as emissions testing is concerned
And I can tell you for a fact that a rather poorly running engine will often pass a tailpipe test with pretty good results
A professional level scan tool, with a competent guy to interpret the results, can provide much useful information
Is the compression good on all cylinders?
did you use the spark plugs called for in the owner’s manual, as opposed to whatever the guy at the auto parts store was trying to sell you . . . ?
By the way, misfires have to exceed a certain threshold before there is a code and a check engine light. Just because there is no code and/or check engine light . . . does not mean there’s no problem. It also doesn’t mean that hooking up a scanner is useless.
Is there a certain speed at which this misfire occurs?
That’s a pretty unusual report here. The engine computer is usually pretty good at noticing misfires. It may be you have a misfire code pending, so good idea to check for that. Sometimes the CEL wont turn on until misfires are detected on subsequent drive cycles. What makes you think you have a misfire? Does the engine seem to sputter, slowing down or losing power briefly? Or do you hear something different? If so, under what conditions? If during acceleration or going up steep hills, what you may be noticing is pinging rather than misfiring. Another problem that can cause misfire like symptom is a failing fuel pump. A shop can rig up a fuel pressure gauge so it can be read as you drive. If it falls rapidly at the same time you notice the symptom, you know there’s a fuel pressure problem and likely a failing fuel pump.
No, it isn’t
I can think of countless instances where the engine was misfiring, and it was noticeable to the driver, but not enough to generate a code or a check engine light
OP can type “misfire” into the forum search feature and check what other posters have said. Click that thing that looks like a magnifying glass, upper right.
There is no pattern to the misfires. They’ve happened at idle, under load, when the engine is heat soaked or at start-up, in damp or dry weather. They started happening when 95° plus weather became routine this summer. The misses are very noticeable (my wife can also pick them up). Ford literature I ran across said misfires not traceable to a particular cylinder won’t trigger a CEL unless they exceed a certain percentage of cycles.
For a cylinder to “fire” correctly it has to have these things
- spark, robust, and at the right time
- fuel and air pulled into the cylinder, at the right amounts & % mixture
- compressed properly
- & all exhaust gasses pushed out the exhaust pipe
So if you have misfires happening, one or more of those isn’t occurring. Without any other clues, no CEL, and no diagnostic codes, your shop will just have to make a guess where to start your testing.