How low is too low?

I am trying to resurrect a 1996 Mazda B-3000 (Ford Ranger in disguise). It seems to skip a little under power, and then the check engine light comes on. The code reader says "Misfire on cylinder #3". New spark plugs and plug wires didn't help. I checked the compression on #3, 100 psi, and #1, 120 psi. Is 100 psi low enough to cause a misfire?

We had that on our Ford Focus. It was because the EGR system was malfunctioning causing the misfire. I suppose it could be a lot of things.

Some of the older cars I had when unleaded gas came too being. Remember some of the crazy pre ignition stuff? Try NGK copper core plugs. As soon as you said under load that is a sign that you need copper plugs. Give it a shot and see if it works. Fixed all three VW cars I had, Remember,,,,,NGK plugs!

Bob N.Y.Check the distributor cap and rotor for carbon arcs.

Sorry, it has coil pack and no distributor

I got a 96 Ranger, 4.0 doing same thing. Done plugs, wires, coil pack, cam position sensor, and fuel injectors. Still got a stumble under load. Haven't done comp check yet, wretched job on that engine. I'm getting to end of line with this thing. Maybe this post will get an answer. George

I once babied a Ford Aerostar van all the way from Washington DC to Nebraska with a miss under a load. I didn't do anything along the way for fear of winding up worse off than I was.

Coupla weeks later, I was under the hood for something, and noticed one of the plug wires wasn't snapped quite all the way on.

Just sayin'. If you did the plugs, I assume all the wires are on properly.

It is possible that you have a valve not seating well causing the lower compression reading. 100psi should still fire okay even though its lower than ideal. Can you check all 4 and compare, also do wet test. I would guess you have a dirty spray pattern on your injector or a vacuum leak causing the miss, but verify compression before going deeper is a good idea to me.

Should still fire although its low, unplug your EGR valve see what that does .

Not sure if those are adjustable valves on that or not? Peak at under hood decal should give specs if they are. Good place to start.

Unplug MAF sensor , see if it changes. Sometimes they can be cleaned by spraying carb spray on wire element , other times just replace

I would do a running compression test. That will tell if you more about how the cylinder is breathing. 100psi is too low and is probably your problem. I would guess that engine should be in the 150 to 180 range.

The last time I ran into misfire and funny running it was the coil(s).

RT (my two cents)

I like to use dielectric grease on the spark plugs tip and then install the wires. Keeps the spark from jumping down the plug.

The problem is, on that 3.0, several things could set the misfire code. Id tend to think 100psi is enough to fire good, but I don't know. What I do know, the CPS can cause a misfire code, bad headgasket can cause a misfire and the low reading on that 1 cylinder, bad plugs or wires can cause it, a couple of sensors can cause, Bad head or valves, and of corse the coil pack. First thing that comes to my mind, is did you use plain ordinary type plugs, or some kind of platinum plug or E3s or something like that? Those engines hate those and will burn a valve. (Seen it more than once on the 3.0)

Bob, you never did say what engine you have in it. IIRC, it could have the 2.3 4-cylinder, the 3.0 V-6, or the 4.0 V-6.
So, I will start with the 4 cylinder. This engine used 2 spark plugs in each cylinder. So, the likelihood of a problem with the coil packs and spark plugs is somewhat reduced but still worthy of checking out. With 100 psi of compression, a head gasket is not likely. Possibly an intake manifold leak, dirty or weak injector, or a corroded connector on the injector.
For the 3.0 V-6 - These engines do have a tendency to flatten out the valve seats with high mileage. 100 psi is too low, but should not be causing a misfire code. Again, check over the basics. Look closely at the coil pack for carbon tracks or any sign of arcing. Also check the wires for any spots that may be rubbed through. Tired spark plugs can cause misfire. On rare occasions, I have seen intake manifold leaks, and in 20 years I saw exactly ONE cracked head.
Now, for the 4.0. Overall, this has been an excellent engine that is nearly impossible to wear out. It did have a few bad habits in the earlier days. There was a tendency to crack heads. I replaced quite a few on the earlier ones. 1991 to 1993 were the most affected years for that. Also, there was an issue with intake manifold gaskets getting out of place and drawing down into the valley under the manifold. Again, the obvious things like the plug wires, coil pack, and plugs are the most likely cause. I also have found corroded connectors at the fuel injectors.

Now, for the general items.....
EGR will generally cause rough running in general when it malfunctions - not a miss on a single cylinder. So, I call baloney on that one. Not likely your issue.
MAF will also affect ALL cylinders. So, I call baloney on that one too. MAF sensor is the main mixture control on the engine. When it fails, it generally does NOT set a trouble code except for a major electronic failure. More often it goes out of calibration and causes a rich or lean condition for THE WHOLE ENGINE. Not just one cylinder.
Spark plugs. The engine was designed to use platinum plugs. Hundreds of thousands of those engines have run billions of miles on platinum plugs. Now somebody wants to tell you that the engine does not like them and wants a copper core plug???? AHEM. I don't think so.
You need to concentrate on the things that are affecting #3. Injector, plug, wire, coil pack.

(quoted from post at 04:58:39 04/15/15) Bob, you never did say what engine you have in it. IIRC, it could have the 2.3 4-cylinder, the 3.0 V-6, or the 4.0 Now, for the general items.....
EGR will generally cause rough running in general when it malfunctions - not a miss on a single cylinder. So, I call baloney on that one. Not likely your issue.
MAF will also affect ALL cylinders. So, I call baloney on that one too. MAF sensor is the main mixture control on the engine. When it fails, it generally does NOT set a trouble code except for a major electronic failure. More often it goes out of calibration and causes a rich or lean condition for THE WHOLE ENGINE. Not just one cylinder.
Spark plugs. The engine was designed to use platinum plugs. Hundreds of thousands of those engines have run billions of miles on platinum plugs. Now somebody wants to tell you that the engine does not like them and wants a copper core plug???? AHEM. I don't think so.
You need to concentrate on the things that are affecting #3. Injector, plug, wire, coil pack.

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Exactly! What he said. I have 178K on 4.0 ohv and going good.

Well, they may have been designed for platinum's, but the ones Ive pulled out of 3.0 from Ford all have the same part number - Motorcraft AGSF52C. Thats a non platinum plug. Ive had 3.0s from 89 up to 2003, and they all take the same plug. Not disagreeing with you, Just know the last set of platinum plug we put in my sons 03 ranger didnt last 9000 miles before they started fouling out, and the plugs in it now have gone over 30000 and hasnt missed a beat yet.

Thank you to everyone for your suggestions. I didn't have time to mess with it today. My next move will be swapping the coil with another truck that doesn't skip. If that doesn't work I will go for the injector.