P1399 is a Acura specific diagnostic trouble code. It means that more than one of your RDX’s cylinders is misfiring (maybe just one, more on that below), and is most common fix is to adjust the valve lash or replace the bad ignition component.
P1399 Quick Facts
- Most commonly caused by valves that are out of adjustment or an ignition issue.
- P1399 is a drivability issue and needs fixed as soon as possible.
- Triggered by variance in the Crank Position Sensor speed (which would indicate a misfire).
Acura RDX P1399 Symptoms
Since the engine is almost always actually misfiring when P1399 is present, there are almost always noticeable symptoms. Here they are:
- Rough running engine
- Smell of fuel
- Decreased fuel mileage
- Check engine light
Acura RDX P1399 Diagnosis
Here are the common issues that commonly cause P1399. They are valves that are out of adjustment and ignition issues.
The good news (if you want to call it that) is that usually your RDX will behave differently depending on which one it is.
Valves Need Adjusting
If your Acura RDX’s valves are in need of adjustment, it’s usually way more noticeable when the engine is cold. Once the engine warms up, it’s not as pronounced.
Example: You’ll start the vehicle up, it may have a hard time finding the right idle speed and it’ll die when you put it in gear. Or it’ll die when it gets to the stop sign at the end of your neighborhood. But, by the time you get to the grocery store, it’s running like a top.
If the above example describes how your vehicle has been running, and you have code P1399 by itself, without looking at anything else it’s about a 7/10 chance that the valves need adjusting.
Now, it may be running skunky even when the engine is warm and still in need of a valve adjustment, but we’d want to look at the ignition system components before jumping straight to a valve adjustment.
Ignition System
There’s one thing we need to keep in mind. If the crank sensor is telling us that the misfire is “random”, is it? The crank sensor can only tell the speed of the crankshaft. So, P1399 can be caused by a misfire in one cylinder only.
You’re going to want to start by looking at the spark plugs. If they are looking rough, change them. This happens a lot on Acura’s for whatever reason.
If the spark plugs are looking ok, it may be that you have a bad coil pack (or not). If you are feeling adventurous, you can pull one plug at a time and see if that stops the misfiring sound. Here’s a video of a guy doing EXACTLY THAT. He was able to find P1399 being caused by a spark plug that looked just fine.
Using the above method, you should be able to tell which cylinder is misfiring(or if the misfire is truly random). At this point, you’d want to slap a new spark plug in there and see if that solves the problem. Here’s more on bad spark plugs.
If it does great! We’re done.
If not, you’d want to take a look at the coil packs and test them. Here’s how to test them.
Now if your RDX is old enough to have an engine with a distributor, it could be a bad spark plug wire, cap, or rotor that is causing the misfire. And, it really may be random. You’ll need to test these components. Or if they haven’t been replaced in a while, slap new ones on and see what happens.
If it appears the ignition system is fine, it may be time to look at valve lash. Before doing that, you may want to check the EGR passages.
EGR
Clogged EGR passages have been known to cause P1399, and they are super easy to check. It would be pretty unfortunate to end up doing a whole valve adjustment, only to find that the EGR system just needed cleaning.
Acura RDX P1399: Conclusion
The most common fix for this code would be a valve lash adjustment. But, don’t rush in and do that without checking the RDX’s easier to check potential issues first. Good luck!