I have a 1998 ford f-150 4.6 and I'm hoping someone can give me some advice. It started missing just here and there but now it's missing all the time. I will give you a list of things I have tried. First off I did talk it and have it hooked on diagnostics and it showed a miss fire on #4 and #7 . this problem has been going on for about 6 months. I have changed spark plugs and wires 4 times and also coil packs and 2 injectors and it's still missing. When I give it gas it seems to shake and miss more and also going up a hill to push the gas. Also I have checked for moisture and it seems to have no moisture. Any advice would be greatly appreciated cause I'm at my last with it. Thanks

 

Have you done a compression and leakdown test? Did you use genuine Motorcraft ignition parts?

 

Yes they did that the day that I had it hooked to diagnostics. They said it was fine and that I should change the spark plugs and that should do it. I bought the spark plugs from them and changed them for the 4th time and still no change they said that the throttle body should be cleaned but it wasnt bad enough to worry about right now and that it wasnt causing my problem.

 

Any information would help. Thanks

 

What brand are your plugs, wires, and coils?

You should clean the throttle body. The 4.6 has a habit of plugging the EGR passages.

 

We changed them all to motorcraft and when we took it and hooked it diagnostics the man came out and said we should change the spark plugs again and it should take care of it and he did say he recommended getting the trottle body cleaned later on but that wasnt causing our problems right now. But I say it probably is the throttle body needing cleaned. I've actually had 2 mechanics tell me that the throttle body wouldnt be causing my problems. It's been in the shop now for over a week and they still haven't found nothing.

 

Okay update on this problem. Just took it back to ford dealership and had the throttle body cleaned and had a vaccum hose replaced that was corroded. Well it didnt change a thing still missing on #4 and #7 very rough when driving. They said they found a little antifreeze on the spark plug, I don't know what that could be because it doesn't smoke at all. Please any help would be appreciated. I'm really tired of putting money in this truck and no-one has a clue to what is wrong with it.

 

Test the radiator fluid for hydrocarbons. Other than that spray a little starting fluid around the intake manifold at the 4/7 cylinder area. See if that picks up the tempo a little.

What do the plugs from those cylinders look like?

 

Well a couple times I pulled them they had black on the ends but other than that they have been clean. The guy at the ford dealership said he found a little antifreeze on the #4 when he pulled it but I have never seen any antifreeze on them when I pulled them and I've pulled them quit a few times.

 

Does it miss when its cold and idle rough? Or does it seem to miss when its up to operating temp?

 

It idles rough when it's cold and at temp.

 

It almost sounds like the intake manifold gasket might have a slight leak at the rear where the coolant exit is located. It might not be pulling air from outside.
Hows the coolant level been holding over this time period?

I would still perform a hydrocarbon test. It's relatively inexpensive and will tell you if your getting combustion gasses in the coolant. Your mechanic telling you he found coolant on the plug isn't great news. However was the coolant sitting in the plug well? Because it could just be one of the heater hoses leaking coolant onto the plugs causing it to misfire.

When you changed out the plugs and wires did it still miss immediately after the fact or did it run good and then pick up the miss again? The connections where the heater hoses connect to the heater aren't the greatest and you might check to see if they look like moisture has been accumulating under them.

 

The coolant level has been good. I'm going to check the heater hoses tomorrow and make sure that's not it. I will let you know tomorrow what I find out. I'm not sure if he said in was in the plug well or not. I don't think it was. I'm going to check that again tomorrow as well. Thanks for all the info. I will definitely check those things and let you know if I find anything. Thanks again.

 

Just another quick thought but I'm not really thinking this would be it, how long ago did you replace the plug wires? and in combination with this are your valve covers leaking any oil. if your plug wires get saturated with oil it can also cause a short and misfire.



Suppose I should justify why I think it might be a bad intake gasket, Being that they performed a leakdown test and it passed would make me think that its not a headgasket or a cracked head but that if coolant is entering its coming from above that. The most likely thing I can think of is the plastic/rubber intake gasket is old and brittle. little coolant making its way into the two closest cylinders.

Hope it all works out for you and it turns out to be something simple.