2005 Hyundai Accent

My (5-spd manual transmission) Accent has begun misfiring, but it noticeably worse when highway driving. Barely noticeable issues when city-driving, but after 30-60 mins on the highway, it starts to mildly shudder when changing speed/gas. When I started up again from a toll booth, it had very little power. Tried to coax it, but then just gave it a lot of gas, which got it going again. The engine light flashed a bit, but then went away. I was able to drive at highway speeds and get home (with the same mild shuddering). Haven’t driven it since then, except in/out of the driveway. I’ve had the sparks, wires and coils changed very recently, and the shop even changed the coils again under a warranty repair, just to make sure, but that doesn’t seem to be the problem.

How many miles on the engine? Is the check engine light on? Have you had the codes read to see if any are stored? What are those codes?

Why don’t you just drive it (or have it towed) back to the shop that did the coils and let them determine what is wrong?

The car has about 76K miles. I keep up with all repairs and regular maintenance and service.
The engine light is NOT on. As stated in original posting, the engine light flashed for a bit, but then went away… notice that I did not say that it stopped flashing (which might have implied that it stayed on), but rather, I stated that it went away, or went dark. There are no codes.
Yes, I have taken the car to a shop, but they are having trouble identifying the problem. And, of course, driving it on the highway for 30+ minutes is not a smart idea, if only for safety reasons.

Correction: It had a general mis-fire code, but not one that clearly identified a cause.

How do you know there are no codes now? Even if the light is NOT on, codes can still be stored.

What was the actual code number that you call a “general mis-fire” code? Was it a P0300?

When? When the coils were replaced for the second time? of after this;

So did you have it towed to a shop “since then” or have you had a traveling mechanic visit your home and try to diagnose the problem?

I am trying to help. If you want some person on the internet who hasn’t even seen your car to tell you some simple thing to magically get your car running again, you are going to have to give us more clear information. If that upsets you, tough.

Then the bet advice I can give you is have it towed to a different shop since the first one can’t diagnose the problem. Good Luck

Thanks for your help. I do appreciate it. It is impossible to ensure that all necessary information is included in a first e-mail, so follow-up questions are always anticipated. I do, however, endeavor to be precise, but succinct in my emails. Please don’t be offended by my brevity.

The last information I have from the shop is that it only had a general mis-fire code. I can not be more specific at this time, but will attempt to get that information.

Here is a more detailed history:
The coils were replaced based on codes (December?). A couple of weeks ago, the car misfired after highway driving and gave only a general code, nothing was found to be wrong, but shop replaced coils under warranty “just to be sure.” A few days later, the car misfired in an even more pronounced manner during highway driving. I had to take it different shop (another story), but so far no luck identifying the problem. This shop has given me the understanding that it is also a non-specific code.

I appreciate the help.

Given that the car doesn’t have a ton of miles - I’d expect 130 to 170K in 13 years - it may be a bad spark plug got installed o it may be a bad fuel injector. You need air, spark and fuel to make a car run, soooo

Given plugs, wires and coils were replaced, I’d check for error codes first. I’d check to make sure there are no tears or cracks in the intake downstream of the MAF sensor letting air sneak past.

I’d then do a check of the injectors to see if all are firing and then disable each injector to see the drop in rpm to make sure they are all spraying fuel and that the pressure is OK.

I’d then remove the spark plugs, run a compression test and look for issues with the rings or valve seals. I’d replace any plugs that don’t look like they were firing the plugs again and then check for mis-fires.It seems these cars are a bit notorious for this problem. Some solved it by using premium iridium spark plugs. Some by gapping the spark plugs at 0.035 inch rather than the recommended 0.044 inch.

Be prepared to pay a shop for this kind of diagnosis. It takes time and you don’t want someone “parts slapping” it because they didn’t look hard enough to find the problem.