Ok my '93 351 (204k miles) is sputtering intermittently under load at mid throttle range. Works fine at idle and at high-full throttle- but sometimes it chokes in the mid range under load- if I back off on the throttle it runs fine.

I took it to the dealer since I don't have a lot of time. They said the plug wires are bad. I just replaced the cap and rotor, so instead of paying them the $180 to change the plug wires I did them myself for $25 and 26 minutes of time. Same problem. I told the service department I thought it might be a fuel injector problem or boost pump problem- they obviously didn't listen.

Since it doesn't backfire and I don't see unburned fuel coming out of the exhaust I think it is a fuel delivery problem and not a spark problem. If it were intermittent spark I would think when it did light all the fuel it would backfire and/or fuel would be leaking out of the exhaust. I changed the O2 sensor, fuel filter, plugs, and the above mentioned cap/rotor/wires. No Engine codes either.

One more thing- I have an exhaust leak at the manifold- could this cause the O2 sensor to detect less exhaust and signal the PCM to defuel? If so why would it be intermittent as the exhaust leak is pretty constant. There is no noticeable drop in intake manifold vacuum before the problem occurs.


Thoughts on this? It is obvious, despite my discussion with the service manager, the dealership just follows a troubleshooting book mindlessly. I keep coming back to injectors- clogged or something.

 

Doubt it is an injector, but it wouldn't hurt to clean them with that mileage on them.

Have seen MANY mass air flow sensors act up like your description.

The exhaust leak will not effect what the 02's read, unless pipe is disconected.

I also agree with what the dealership told you about the plug wires. Happens all the time. Are you sure you have them on correctly?? Greased and tight on the plugs??

 

I did grease the boots and put them tightly on the plugs- the problem was the same prior to changing the wires.

I wasn't sure about the Manifold Air pressure sensor (same as a MAF?- Do I have a MAF?). Since the MAP sensor was connected directly to the PCM I figured it would give a trouble code, although I guess if it thinks it is working correctly even though it is failing it wouldn't signal a problem. I will check it out- thanks.

 

Ok I am thoroughly disgusted with the dealer service dept. I took it to them again after they said they would figure it out and they came back with the EGR ports needed to be cleaned. They assured me it ran good so I picked it up and it has the same problem as before.

I decided after $130 and nothing I would shotgun some things I thought it might be. I checked the fuel pressure on the rail and it read good at idle and at RPM. I have now replaced the wires, cap, rotor, O2 sensor, EGR valve, EGR valve position sensor (codes popped up for this), and spark plugs. I changed the IAC and that seemed to help, although it now idles too high. I cleaned out the fuel injectors and that did it!!!! So now the truck runs really good (pretty good tune up), but idles high. The manifold pressure seems good and I don't think I have a vacuum leak so I was wondering why it is now idling high? Is there something on the IAC to adjust the idle? I don't think so?

Also now that the dealer has messed with it-it is leaking oil. Looks like I will have to take off the intake manifold and investigate. I paid $130 for them to break my truck and waste my time- thanks for nothing.

Thanks for the help gearhead.

 

After the trip to the dealer my truck went from leaving no oil on the ground to leaving an outline of the engine in oil on my driveway like the Exxon Valdez.

Retorqued all the valve cover bolts (valve cover gasket is about 2 1/2 yrs old and has only about 30k miles on it) and changed the PCV valve. Really strange that most of the bolts on the valve cover were really loose. I should have listened to a friend about this service dept.

In the end- the truck runs great now and the oil leak is gone. Old Blue rides again. Now I have to scrub my driveway .