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Motor Runs Rough- 1991 Honda Accord EX

My Honda has ran well over the years but is now experiencing some problems. The car has 200K miles on it and due to the age of the car it does not have an advanced computer system but does have a basic fuel injection system.

The motor runs pretty good when cold. But when it is fully warmed up, it has a noticable roughness when driving at a contant highway speed. You can feel the significant roughness as soon as it gets to “normal” driving temperature.




Have you used a paper clip to jump the two-pin electrical connector behind the passenger’s right kick panel to get any trouble codes to flash the check engine light (symbol)? And…?
Have you done spark plugs and wires, distributor cap and rotor replacements this year? Replaced the fuel filter? Checked fuel pressure?

Good questions.
This problem started a couple of months ago. Then 2 weeks ago, the ignition system died. I replaced the distributor (consists of ignition coil, ignition module, and another electronic piece). Also replaced the distributor cap, rotor, and ignition wires due to the mileage on the car. The car runs again but back to the same symptoms as prior the ignition system going out.
Check engine light is not on.
I am not familiar with the paper clip jump you are mentioning. Tell me more about that and what it can tell me.
I am thinking that it may be related to a fuel mixture issue, because it runs fairly well until the motor gets to normal temp. As soon as that happens, it gets significantly rougher. Kind of like the “automatic choke” turning off on an older car with a carburetor.
When pushing the gas peddle to the floor, the engine is smooth and accelerates OK. No roughness…
What do I need to fix now???

Since there is no lit engine symbol (check engine light), there won’t be a DTC stored in the engine computer. So, there won’t be a need to jumper the blue test connector under the edge of the glove box.
The oxygen sensor is a good guess. If it had a problem bad enough, it would turn on the check engine light and set DTC 1 or 43. It must not be that bad. You can get a replacement oxygen sensor for about $60 (more, if you wish). With 200,000 miles on the original oxygen sensor…

Does the engine idle smoothly when hot?
I think the ignition system failure you mention may be connected to the poor engine operation. Could your engine be running in “limp mode”?
Before replacing any parts, scan the computer trouble codes. There may be stored codes even if the CEL light is off.

The engine seems to run smooth at idle.
I will check the computer codes even though the “check engine” light is off.
Could you explain “limp mode” and what I need to do?
Thanks. I appreciate all the info.

OK… Here is the latest update.
I have now replaced the fuel pressure regulator. The motor still runs rough when driving at a constant speed. 45 mph is rougher than 55 mph. I think the rpms are around 1700 at 45 mph. Still runs smooth on hard acceleration.
More advice…?

I too have a 1991 honda accord with identical symptoms. I have replaced everything i could possibly think of to remedy this with no luck. i however do get an occasional check engine light but have not been able to ground out pins to determine code and its not frequent enough to determine how i can get it to come on. if you have had any luck please let me know.