Car will lose acceleration while driving

I know this is a massive wall of text but I’m trying to not leave any details out because I am well aware how much of a pain diagnosing a car can be, so please bear with me.

I drive a '01 Honda Civic lx automatic with 170k miles.

About a week ago my car was having a difficult time starting, and I had to hold the key longer and longer to get it to turn over. Initially I thought it was my alternator/battery so I went and had them both tested. At first it said my alternator was fine but with some prompting I got the guy to test just the battery and it said the battery was bad, which he said was weird because if the battery was bad then his device should have said the alternator was bad too and the only way to really tell was to go get a new battery and then come back and test it. So I went and replaced the battery and it seemed fine at first so I didn’t rush back to test my alternator. The next morning it started to do it again but pretty sporadically. Sometimes it would take a couple seconds to turn over and other times it will start up immediately with no problems or what felt like it stuttering.

Unfortunately, today while driving, my car lost acceleration for maybe a second and then regained acceleration. Then about 30 seconds later it lost it again but regained it a second or two later. It did this 3 times in the span of maybe 3 minutes. Luckily, I was really close to home so I was able to safely park it in the driveway and not get stranded somewhere.

My car never revved up during any of these failures, like it did when my tranny failed (I got that replaced last year) and the rpm needle would start to drop. I also didn’t notice my radio dim or lose power (the radio was on at the time). However, this morning my cigarette lighter did stop working.

My check engine light did go on, but I got it coded and it says it’s my gas cap. I initially thought that the light was on because maybe I forgot to turn the cap all the way when I last filled up because it on about a quarter of tank in but with my car acting the way it is, I’m not so sure now. I know you’re probably thinking ‘well duh, you’re check engine light is on dummy!’ but this has been happening way before the light came on and in the past I did forget to fully twist my cap and that caused the same code and when I retightened it the light went off on its own about half a tank later, so I’m not sure if that’s really linked to the problem.

I have had my distributor go out on me in my '96 honda civic and these ‘stutters’ kind of felt like that except with my last honda there was no warning like this and it never regained acceleration.

Any help would be appreciated, I’m not a mechanic and I just don’t know what to focus on troubleshooting first.

There’s a main engine relay in Hondas that seems to be problematic according to posts we get here, so that’s one idea if you want to try the replace this, replace that approach. some caution is in order w/that b/c it is easy to run out of money before running out of guesses. Usually better to get the diagnosis spot on, then fix what’s broken.

First step is to check the voltage at the battery. Should be appx 12.6 volts before starting the engine , first start of the day, then 13.5-15.5 when the engine starts and is idling. Anything out of that range needsd to be addressed before considering other possibilities.

After that, checking the diagnostic codes. A problem with the gas cap could cause this, b/c the gas cap is supposed to allow air into the gas tank to replace the gas as it is used. If that failed, it could prevent fuel from moving from the tank to the engine. You could try (experiment only, unsafe to drive that way) loosening the gas cap and see if that affects the symptom one way or the other. Other problems can occur in the evap system that could cause this too, which might appear to be a gas cap code, but really an evap purge valve for example. There’s a bunch of hoses and valves involved in that system, any of which can fail. You’d need to post all the codes here for some more ideas on what to do.

A clogged up catalytic converter could cause this. Ask you shop to test for that w/a cat back-pressure test. A new cat will prove expensive, so cross your fingers it is not that.

Fuel pump problems can cause some of these symptoms too. A fuel pressure test would say yes or no to that idea.

Do you mean when the key is turned to Start, there is a pause before you hear the starter motor turn over the engine? [By “turn over the engine” we really mean “rotate the crankshaft inside the engine so the pistons are going up and down and the valves are opening and closing.” Or does the starter motor immediately start turning over the engine, but the engine doesn’t always start and run within a second or so?

If there’s a delay before the starter motor turns over the engine, and also brief intermittent loss of power, I would suspect first the battery and its connections, and second the ignition switch.

But first tell us more clearly what happens when the engine doesn’t start up and run within a second or so of turning the key to Start. A delay before you hear the engine turning? Or the engine turning but not starting quickly and reliably?