Ok, I've searched and searched; to no avail.

97 Prelude SH
CEL Codes- P304, P1399
Random surging idle also

I have the above listed codes, which only come on if the car is left idling. Under normal to heavy driving conditions, everything runs good, no check engine lights, healthy motor. When left idling you can hear a barely noticeable misfire at the exhaust and the motor shakes a VERY TINY bit, hardly noticeable, but enough for the ECU to catch it.

I changed the cap, rotor, spark plug wires(including coil wire), and spark plugs also. That helped the misfire a lot, as I was getting codes for misfires on all cylinders, and now its only for cylinder number 4.

To test it, I checked compression and it checked in at 220PSI, which sounds about right, or so I hope. Spark is also good, very bright blue spark.

What's iffy about the problem is that I unplugged the number 4 injector, and to my surprise there was hardly any change in how the motor ran, little to no change. My first thought was the injector was bad so I swapped number 3 injector with 4, to see if any changes happened. Nothing happened, the problem persists. Same exact symptoms on cylinder number 4.

Anyone please help? It's driving me crazy. Thanks in advance, and sorry for the LONG post.

 

Sounds like a vacuum leak of some kind, the surging idle is pretty well known in hondas. If its only one cyl, maybe its an intake gasket leak there? One way to check it is to spray something flamible (carb/brake cleaner, starting fluid, etc.) in the area of the suspected leak. If it clears up the misfire, thats where its leaking.

GL
Mike

 

Do you think the vacuum leak would cause the misfire situation? I've had idle issues on other cars before and it doesn't cause misfires. I could be wrong though. I will try the carb/brake cleaner today when I get a chance to look at it.

Also, why do you think that when I unplug the number 4 injector, it causes no change on how the motor runs? Shouldn't it make a big change? Because when I unplug the other injectors and a very noticeable change and the motor runs rough.

 

I think whats happening is that you are running lean on cyl 4 due to a vac. leak and when you unplug the injector, it just goes leaner. This is because a 5th gen uses a MAF instead of a MAP. The computer is not accounting for the extra air being introduced into the cyl, because it measures the flow before the throttle body, and the leak is in the intake runner to cyl. #4, which is after the TB. Again, the carb cleaner at each joint of the runner should pinpoint the leak. Be careful though, hot engine + flamible liquid = no eyebrows or worse if it catches fire. Again, GL.
Mike

 

Thank you so much for the help!

One question though, the car has a MAP sensor mounted on the throttle body, I don't seem to find a MAF....?

 

So I did the carb cleaner test, and low and behold... A leak at the intake manifold gasket! Thanks a lot for your help!

 

Prelude's don't have maf sensors, only map.