'94 Ranger, 4.0, 65,000 miles. I'm cruising along at 55 mph and anytime I touch the accelerator, it bucks and misses. Sometimes it will also miss at slower speeds as you lightly accelerate. It also sometimes misses constantly for a few minutes and then clears up!

MAF sensor was replaced a month ago due to a knocking problem, I also tuned it up then, new plugs, wires, air and fuel filters.

I've replaced the coil pack and the crank position sensor, still no good. There is no check engine light and no codes are posted. (The only thing that may be unusual is that the O2 sensors read "rich" at idle but the diagnostic computer says they are functioning fine)

Any suggestions?

 

How do your plugs look?

 

Welcome to FTE!!!

I'd be carefully inspecting for vacuum leaks. This would include the lower intake gasket (retorque the mounting bolts). Also, did you gap the plugs properly and remember to not use Bosch plugs? What brand of plug wires did you use?

 

Maybe its your fuel injectors acting up. Try cleaning them...just an idea. Good luck.

-hooD

 

O2's should read rich at idle until the engine warms up. That's when the heater circuit kicks in at 600 degree exhaust temp.

So, all I can tell you is to check through all the other good tips that are already given.

 

Well from your descriptin of it stumbling when you open the throttle, makes me suspect the air/fuel mixture goes lean, if so, the question is why????

Seeing as how you've just replaced the MAF, it's not likely thats the problem, UNLESS this problem began AFTERWARD.

If so, I might suspect the new MAF or it's electrical connector, wires, pins, or sockets.

Look for connector contact corrosion, pushed back or bent, pins or sockets, cut or loose wires, loose connector not plugged in or fastened properly.

If none of that fits, then I'd begin to suspect the throttle position sensor.

Maybe it's mounting is loose, or it's worn internally, such that it's electrical resistance is not changing smoothly & it's being untruthful to the computer, about true throttle position & movement.

If it's output is erratic the computer gets confused about what the throttle's position was & is doing & then for how long to fire the fuel injectors, to keep the A/F mixture in trim.

So check it's mechanical mounting & make shure it's fasteners are tight, if so, then pull it's electrical connector & check it's electrical resistance over it's full range of travel.
It should change SMOOTHLY, not jump around or have dead spots.

If that all checks out, maybe it's a dirty throttle body, if it's never been cleaned, or maybe the throttle plate area is worn & sticking.

Just some more ideas to ponder.

 

Dont forget, check the injector wiring harnesses. Mine came off on number 2, I spent weeks and hundreds of dollars to chase it down.

 

Thanks for all the input.. Here's what I've done..

I've checked the spark plugs, they are dark, almost black which tells me the engine is running rich. The spark plugs are Autolite platinums and they are gapped properly. I don't recall which brand of wires I used.

I've used fuel injector cleaner in the tank and I just checked the injectors and they are all plugged in.

When I checked the O2 sensors, the engine was hot and it still reported a rich condition.

I replaced the MAF sensor before I had this problem and I just checked the connections, no corrosion. I also checked the Throttle positioning sensor. On a 20K olhm scale it read .56 at rest and gradually rose to 4.00 as you opened the throttle.

I haven't checked the intake manifold, I think I will need an upper intake gasket set before I start.

Any other suggections? Thanks again, Nick

 

Oh, one more thought, if it were the intake gasket, wouldn't that make it run lean?

Thanks,
Nick

 

Yes. Any intake leak will suck in air, causing a lean condition.

Any back firing or poping?

 

I wouldn't call it back firing, popping is probably closer to what it does occasionally.

 

Ok, so the problem began after the MAF & crank sensors were replaced, the plugs indicate it's running rich, the diagnostc scanner says the O2 sensors are funtioning ok, but the reading is on the rich side & there are no trouble codes.

Well then it would seem you have two clues, that seem to confirm it's running rich, sooty plugs & rich O2 readings on the scanner.

Seems to me two things you've changed could have a impact on this.

The MAF sensor & the crank position sensor.

Are you shure you've installed the crank position sensor correctly????

If it's not, the injectors & spark timing are likely off.

Did this spitting problem begin immediately after you replaced this sensor????

Are you shure the MAF is the correct one for your engine???? Did this spitting problem begin immediately after you replaced the MAF????

Have you checked the fuel pressure since changing the fuel filter????

What I'm getting at is, if you can determine exactly when the problem began, then maybe you'll be able to tie it back to something, in the last operation you did, assuming all the things weren't changed at the same time!!!!

 

You may wnt to check the fuel pressure, if it is high it will run rich.. I am not sure what type of fuel regulaor you have but they can sometimes give a problem if they are the vac type. Also try to remember if the problem came with the maf change, you may want to check the part number to make sure someone didn't give you the wrong one.
Dave