Ford Ranger / B-Series :: 2000 4.0L - Cylinder 4 Misfire Sometimes

I have a misfire mystery going on in my girlfriends 00 ranger. What is going on, is there is an intermittent misfire on #4. I have an ultra gauge hooked to it, so most of the time when it does it, it will flash a pending code. A lot of the time, the pending code will go away, but sometimes it will set the PO304 code.

First I tried a tune up. New plugs and wires, air cleaner, cleaned out the throttle body. This cured it, for a short while. Then we were back to the pending code / setting code for #4.

When I pull the plug on #4 it is sometimes wet with gas. If you put a new plug in it runs fine for a while. In fact the last time we took a several 100 mile trip with it right after plug change, and it ran great. The pulled plugs re always in good condition with no burn, oil, chips or glazing. I have been using the standard autolites that are in advance auto's computer for that engine. No issues with the other 5 so I figure they re good enough.

The day after we came back I get the call... Honey its doing it again. Often to the point of having no power at all.

But it will clear itself usually, if not a new plug for #4 will cure it immediately.

So I thought something is killing this plug.... Or flooding it. SO I ran 2 cans of sea foam though it, put in a new plug and an new wire. It fixed it for a week then it did it again.

Ok I think... maybe the coil pack is bad either killing the plug or just not firing it at all? So since the pack has never been change in its nearly 300K life (to my knowledge, shes had the truck since about 60K) What the heck.... a new coil pack for the old thing.

I have not had the chance to clean the MAF but we are talking misfire on one cylinder, every now and then with no real conditions that set it off. More often than not its a pending code that you can even hear or feel and other times its a full blown miss that drags the truck down. Well for a short while until it clears up or I replace the plug.

3 cans of seafoam surely would have cleared a sticky injector. I realize it could be a bad wire to that injector but I have not had the time to tear the fiddle bits off to get to it to clean and test it... or just replace it.

Honestly Ive had a lot of cars and trucks, I don't remember ever having to replace an injector. Ive had bad wires to them but they misbehave ALL the time. This problem is so intermittent its freaky.

She says it does it mostly when its cold or not totally warmed up. Almost always, by the time the engine is hot it goes away.

And hot for this truck is over 185F, which I think its too cold to be running and the thermostat needs changed BUT she thinks cool=good. I keep telling her no, cool is often not good, the engine needs to be up to the operating temp.... which to me means 190-200. Her ranger runs between 170-180 all the time. I doubt thats causing the problem but still.... I think new thermo is in order.

My thinking is since it runs so cool all the time, the issue might be related to a spike in the temp over 180 ish, that gods forbid is opening a crack somewhere to suck air or lose compression.

And a compression test is my next step on #4 if/when it does it again. Now I have had cracked valves before, I don't recall them acting this way. After all sometimes the thing will go 100s of miles under different driving conditions and run fine. I don't think a cracked valve would be that nice.

Shes getting @18 MPG mixed driving, when it runs t runs like a bear. It always fires right up. There is no smoke, white or black. There is no water in the oil, no oil in the water. No audible exhaust leaks. No ticks knocks or oddball noises. For its age and miles almost 300K it runs perfectly.

Well until it has its intermittent issue with the misfire, that isnt bad enough to actually set the code.