1997 Ford Ranger with 190K miles has check engine light on with P302 computer code, meaning misfire on cylinder 2. Engine runs rough at idle only once it warms up, which only takes a minute. It is usually fine while driving, but it’s a lot easier to feel a misfire at lower RPMs than at higher ones. Added fuel injector cleaner to gas for a couple tanks, no change. My mechanic has checked spark plugs, fuel injector, o2 sensor, and everything is ok. Dry compression test shows good compression. He thinks it may be a valve sticking, which means a valve job: expensive. Comments? Is his assessment right? Is there anything else that can be done? Why is there no misfire when it is first started?
A “Misfire” code means the ECM’s measurement of the crankshaft velocity isn’t showing it accelerate like it should after number 2 is programmed to fire. Anything that would prevent a firing to occur, or to confuse the ECM’s timing could cause that.
There’s probably at least a dozen reasons a person could think of, but if I had this problem I’d eliminate a spark problem as the cause as first order of business. Your mechanic could do something as simple as pulling the spark plug and checking for a consistent visual good, hot spark. Moving things from cylinder to cylinder to see if the problem follows is another common debug technique. Spark plugs, spark plug wires, coils. If you have a conventional distributor, check under the cap for problems, like corroded contact points, or carbon tracking. A visual check of the engine compartment at night can show a spark going somewhere it shouldn’t, like to the engine block, that couldn’t be seen in daylight.
Fill us in with the reason your mechanic thinks it might be a sticking valve problem. Have you had prior valve problems or valve work? Has the oil & filter consistently been changed on schedule and used a name brand oil and correct viscosity as recommended in the owner’s manual? The reason I ask is b/c if a valve is sticking, you’d think that would show up in a compression test. One simple thing that might spot a valve problem is a intake manifold vacuum test. All that requires is a simple vacuum gauge. Has that been done?
How does he check spark plugs?
What are the dry compression readings and does this mean the compression was checked on that cylinder only or all of them?
If the compression is allegedly good then I have to wonder why he suspects a sticky valve.
Thank you both for your help! My mechanic has checked all details of the spark situation. The oil was more than a quart low when I took it in, which is my negligence. That was not a factor for him, though, he just can’t find any other cause. I’m no expert in these things, but if a seal or the rings were worn then it would show in the compression test, but if a valve is sticking it would not, right? Please explain if this is not the case. I will talk to him about an intake manifold vacuum test, but that won’t rule out a sticking exhaust valve, right? I will also ask him what the compression readings were and if he compared to other cylinders or compared to the expected compression for the make and model.
The engine is now missing continuously, not just at idle. It still does not miss when first started, only for about the first minute.
My mechanic said he checked the injector by feeling the regular pulsing (my words, not his) but he has not taken it out and inspected it because that would cost about $120, but that’s the next step.
Any advice or further thoughts are appreciated.
@scrap Per your last post - a sticking valve will definitely show up during a compression test. The problem is you don’t have a misfire when you first start it, which would most likely be the condition during a compression test.
Have your mechanic swap the injector with another cylinder and see if the misfire code switches to the other cylinder. That’s the best way to check an injector.
I agree with @jesmed1. Swap the injector.
You do have a spark issue or a compression issue. The computer is monitoring the voltage required to make a spark. If the voltage varies too much from the other cylinders, it means that there is either something wrong with the coil, spark plug, wire etc, or the tip of the plug is not under the proper compression with the correct fuel air mix. This is what sets the code.
I doubt that you have a sticky valve. In 50 years of amateur messing with engines, I have never seen a sticky valve. I don’t know anyone who have ever actually seen one either. I have seen burned valves that were diagnosed as stuck valves. Back in the old pre-smog, leaded gas days, carbon and lead deposits used to build up badly on valve stems and it is possible that the deposits eventually kept the valve from fully closing, which lead to burning off the face of the valve, but all I ever saw was the burned valve. I could not determine if it was ever stuck.
You don’t see these deposits today so I don’t think you would have a stuck valve, but it could still be a burned valve. I have seen some pretty badly burned valves that did not show up on a compression test, but a leakdown test will reveal them. You mechanic should do that check. Also, if your valves are adjustable, then the valve lash should be checked, and that should be done ASAP as a tight valve will soon become a burned valve.
A very quick and simple thing to do would be to use a spark checker at #2 spark plug. That would eliminate the ignition system except possibly spark plug # 2 itself.
I would have to agree with @keith that a sticky valve would be an extreme rarity.
My first thing if I owned it would be new plug wires.