Running rough at higher rpms when hot - Ford Forums

markedman450 · 07-30-2007, 08:58

Car runs good when cold of course. But after 30 minutes of driving if i even try to mash the gas or accelerate hard to pass someone, once the car goes above 3500 RPMS, it starts to really miss or buck. Ive had this car for over 30k miles and it never did this. Had to take it to a Ford Dealer because not alot of garages have the tools to work on the 4.0 SOHC. They say it was a blocked pcv valve system and replaced all of it and put new intake manifold gaskets on. Problem is still there. Put new plugs and put a new coil pack and problem is still there. Wires although a cheap pep boys brand look ok, all have same ohms resistance and are only 25k miles old. ALready had a new fuel pump and filter put on 7 months ago. Im gonna check the MAF sensor but if anyone has any other ideas please let me know.

backlash1 · 07-31-2007, 04:33

Hi
I'd start by looking at the spark plug wires. You probably measured their resistance when they were cold and not when the car was actually misfiring. Resistance in most materials increases with temperature, so you could have a lead (or even a coil) that checks out ok when cold but it will fail the test when hot. I know it's a pain in the @$$ to check parts when they're hot so here's a simple test. Get a timing light. Any will do. Drive or warm the car up until it starts to miss. Hook up the timing light as you normally would to check timing but, instead of leaving the pick up on lead 1, try it on every single lead while you press the trigger and watch the light while it flashes. On every good lead you will see the light flash at the same rate. On the shitty ones the light will be erratic and/or missing sometimes. You may need someone to load the engine up while you check, so don't stand in front of the car, do each bank from the sides. Let us know how you go. cheers
Marc

markedman450 · 07-31-2007, 09:06

Def. gonna get new wires. Just not sure which ones. Id like to get Motorcraft racing wires but they dont have any for my explorer. I sprayed the MAF sensor and it seemed to help a little but its still having a miss. Ford dealer since they didnt fix it the first time are gonna look at it and all labor is free. They think its another air sensor controlling fuel to the car. Haha i dont care i just want my explorer back at full power. I plan on getting an Accel coil pack, a better set of wires, and good NGK plugs.

markedman450 · 08-01-2007, 13:05

So from what the dealer is saying its def. the catalytic converters...problem is they think its only one of the two. What are some good ways if any to determine which one it is. I figured drive around for an hour, get under it and see if ones changing colors from being clogged or which one is hotter. I do know that cylinders 3, 5, 6 seem to be blowing through plugs especially 5 if that helps determine anything. I figured if one is clogged, it could have the o2 sensor messing up the fuel/o2 mixture and causing the plugs to burn out.

Any other ideas?

backlash1 · 08-02-2007, 03:58

Well
I could be wrong, but in my experience when the cats are blocked the car feels like there's no power when accelerating, regardless of engine temperature. And you don't necessarily get a misfire, it just feel like the engine doesn't want to rev. If you don't trust what they're saying, you can remove the cats (when they're cold!) and look at the inside, you'll see if the ceramic honeycomb is broken or not, and if pieces of it are coming out when you shake it. Shake it in your hands, don't drop it, hit it or bang it against a hard surface because you WILL damage it. You can also drive your car without the cats and see if it feels the way it should, but it will be extremely loud and you could make your neighbors angry :) cheers
marc
ps: If your plugs are burning out too quickly this is usually due to the engine running too lean (ie: blocked injectors, manifold leaks, etc) and causing the temp inside the combustion chamber to be too high. Investigate this issue, as it could also lead to cat failure.

ShadySlim · 03-08-2008, 20:49

Hey, My '06 Explorer is doing the same thing... If the temp is over 60 degrees it has no power (no get up and go) I have had it at the Ford dealership many many times. Still no fix.. my check engine light never comes on so they say nothing is wrong....