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"Check Engine" mystery

My 95 Civic engine light keeps coming on at exactly the same spot in the highway every time I drive to my mother’s house about 81 miles away. It has done this for about 9 months. However, when I drive the same distance in a any other direction, the light does not come on at all. I took the car to a mechanic when the light came on the first time, who hooked it up to a computer and said it was the fuel system and that I should tighten the gas cap. It worked! Now every time I go to my mom’s I just plan to stop at the same spot, tighten the gas cap, and go home. The light always goes off and does not come on again until my next trip to her house. I guess my question is, why doesn’t this happen when I go anywhere else, and what can I do about it?

How many times within the past 9 months have you driven to your mother’s?

How many other similar trips have you made in some different direction? (“Similar” would mean things like distance, road conditions, road type, elevations, hills/valleys, etc.)

20-25…my mom is elderly so I drive down to care for her regularly. I do drive in a different direction approximately the same distance about once every six weeks to visit my grandson, but the light has never come on when I make that trip.

Do you always fill up the gas tank before you leave? At the same gas station? Maybe you, or the gas station attendant, is not tightening the cap enough. If it’s you, then why it would only happen when you fill up before this particular trip is a mystery, but if it’s a full service station and the attendant is consistently doing the same thing . . .

OK, that’s just one guess. Also, try tightening up your gas cap at the start of the trip. Most gas caps you can turn until they click a couple times. They won’t break. Perhaps that trip has some road bumpiness at just the right frequency to loosen your cap, but I’d think if you tightened it up at the start you would prevent that.

OH, don’t “top off” your tank when filling up. That can cause some problems in some cars.

The good news is that this is a symptom of a simple EVAP system problem rather than something destructive.

Your gas tank breaths in through the charcoal canister as the pump pumps gas from the tank. It is possible that the canister is restricted, possible from a gas-wetted charcoal bed, and on this long trip you’re developing a vacuum in the tank that’s interfering with the pump’s ability to pump gas. Perhaps it can still breath well enough to recover when you stop to tighten the cap. It would be nice to know exactly what the mechanic found, what codes he read.

I wonder if it makes a difference that my gas cap is not a Honda cap, but rather one I bought at Auto Zone. I have a bad habit of driving off and leaving the cap behind. My husband suggested that the problem might be alleviated if I just bought another Honda cap. I also had the thought about the road bumpiness loosening the cap. Yes, I always fill up before I leave, and most of the time (not 100%) I fill up at the same station. I always run the complete tank of gas to the bottom before I fill up, so that topping off isn’t a problem. I appreciate your input.

You misunderstood my topping off comment. What I meant was that when you fill your tank, don’t try to keep adding more after the pump first shuts off. It’s pretty common to have the pump click off, and then pump some more in slowly to round out the money or just to get as much as possible in the tank. This can cause problems for the evap system.

Your husband’s suggestion of getting the OEM cap might help, or might not. It won’t hurt.

To prevent leaving the fuel tank cap behind, buy one that has a strap attached and attach the strap to the fuel tank filler door.

You could also post a note to yourself and put it where you can easily see it telling you to screw the cap back on and turn it for at least 3-5 ‘clicks’ to ensure the cap has sealed.

Is your mom’s place at a very different altitude?

“the problem might be alleviated if I just bought another Honda cap”

That just might fix it.
Some gas caps have a check valve to let air into the tank as gas is drawn out.
Your aftermarket cap might not work at the “right” pressure difference.

Ha! I do that all the time…round up to the nearest dollar! I wonder if that has anything to do with it. Maybe there’s something in the manual about it. I"ll check. And I’ll STOP! Thanks for the clarification.

I think I"ll get another Honda cap WITH a strap on it if I can find one. Maybe that will take care of my problem. Thanks for all the suggestions.

peculiar! I’m wondering as well if altitude is a factor. does the route you take to your Mom’s by chance take you over a high elevation at exactly 81 miles from your house??? just a layman’s thought.

I live in Oklahoma…flat, not much elevation at all, so I don’t think that’s the problem. Thanks for commenting.

I’m of the opinion that the gas cap is coincidental and maybe more of a symptom than a cause.

Drop by a local Autozone, O’Reillys, Advance Auto, etc. and have them pull the codes. Post the results back here for discussion.

What you were told by a mechanic may have simply been one of those “try this, see ya” things.

Thanks…I’d need to go to a dealership, though, because, apparently there are two types of 95 Civic LXs…mine is one that was made outside the country and the computer can only be read by a dealer’s device. I tried Autozone and another store and was told the same thing, that their reader couldn’t provide codes for my car. I took it to a dealer and he was the one who told me the problem, according to the computer, was something to do with the gas line or cap. I’ll take it back by and see if he will give me the exact codes. Thanks again.

The trouble you are having is very unusual. If the light is turning on at the same spot on the road then I would have to think you may be chasing a ghost. If there was a problem with the car then the light should turn on at any place you go to. Since it only happens at a particular spot then I would have to think there is some electrical interference in that area that is getting into that circuit and causing the code to be generated. There could be some high voltage wires buried under the road or perhaps some sort of high level RF signal coming from somewhere. The car engineers do a lot of design testing to prevent this kind of trouble but I think that may be what is happening in this case. If this idea is correct then shielding the wires to the sensor may help stop the trouble.