I have a 96 ranger 2wd, extracab, 3.0L, 5speed. Several months ago I Had a check engine light for egr low flow. Replaced the little box next to the valve(cant remember what its called) solved the problem. Then soon after my engine developed a pulsing miss at low rpm's...(not sure what rpm exactly, no tach, but guess would be between idle rpm, and 1500.) The miss is soo bad that the truck will jerk back and forth as if the engine is completely shutting down, then back on, then off again, and so on untill accelerated above that rpm range. The truck runs great below (lugging), and above that rpm range, and throttle position and gear selection have no effect on this problem. The engine code I had was for a cam position sensor, so I replaced the sensor and the tower it sits on because the tab was broken. This took care of the code, but the problem remains. The engine does not idle properly either... It acts like it wants to die, and then it returns to normal idle... this happens several times a minute.

Now the computer has no stored codes, and no check engine light, but this problem continues. I have replaced plugs, wires, coilpack, Mass airflow, crank sensor, cam sensor, egr selenoid. I have cleaned the egr valve, and idle air controller on top of the intake. I have also looked in the computer at several parameters of the engine operation while its running. Everything seems to be working fine except the downstream 3rd O2 sensor is not registering any voltage and is stuck in the lean condition. I replaced this sensor and that did not fix that problem, it still does the same thing... also checked the wires and all circuits are good up to the computer. That seems to be the only thing that doesn't look right in the computer, but I cant imagine that one O2 sensor would cause this much of a engine miss. I'm stumped, and so are several mechanics in my local area. If anyone has had similar problems and any ideas on solutions please feel free... I'm open to any seggestions at this point.

 

In some ways, I have to suspect there is a problem with the IAC solenoid and/or wiring. Remember, just because a part is new in the box doesn't mean that you didn't get a bad one. Pretty rare, but it really does happen.

On another note, it would sure be nice if you could get a "flight recorder" and hook it up, then activate it when the problem is occuring. The flight recorder records instantanious data (codes) when you push a little button; it's useful for tracking down the cause of tough problems like this.

 

I'll take a look... Thanks

 

It does sound like the IAC valve. Did you clean it or replace it? Cleaning rarely works.

 

I did clean it, but did not replace it. maybe I'll get one.

 

I'm sure the IAC is not the problem... I unplugged it and ran the truck, and the problem continues...

 

Did the rpm drop or did the engine try to stall when you unplugged the IAC? Have you check for vacuum leaks? Hows the PCV system?

 

Yes the rpm did drop, and the engine would almost stall... No vacuum leaks, and PCV system is working properly.

 

Ignition system sounds OK. You say the IAC is OK. So, time to consider some other things. Fuel delivery is always high on the list of possible suspects in a situation like this. So, when is the last time you replaced the fuel filter?

Also, a downstream 02 sensor registering nothing could either mean the circuit is bad, the sensor is bad, or you have a dead CAT. But usually a dead cat will throw a P0420/P0430 code. In any event, you ought to at least get that issue resolved so that you can get back some data from the 02 sensor, seeing as you have the capability of monitoring it. Just as important, you will be able to rule it out as a contributing cause of the misfiring.

 

Thats what it should do when you unplug the IAC and would help rule out vacuum leaks.

 

I would have to rule out the fuel system because, the fuel filter was just replaced, fuel pressure is good at the bleeder valve, and this problem only exists between a narrow RPM range that just happens to be the range where you would normaly cruise at. So lately I have had to gear down and run the engine faster to keep it from bucking me thru the windshield.

I know the wiring is good for the O2 sensor, I checked each wire for continuity between the plug and the computer. And I put a brand new sensor in and got the same readings. I'm starting to wonder if my ECM (comp.) is just messed up and is not throwing any codes. I just hate to buy a new one to find out that it did not fix the problem.

I very much appreciate your idea's, I'm open to any suggestions at this point.

 

That could be the cause but I agree about buying one, maybe you could find one at a junkyard or find someone that will let you borrow theirs, make sure it's the same calibration.

 

I've got nearly the same symptoms on my '99 B2500. The problem RPM range is up to about 2400, particularly if under load. I'm getting no codes. Changed air and fuel filters, plugs, wires; cleaned the IAC, MAF, injectors, throttle body, etc. I'm down to throwing money at IAC, coils, fuel pump, O2 sensors--blindfolded with no codes.

 

Check again for vacuum leaks!!.up and down rpms, it's a common problem related to
vacuum leaks, a code reader would not find a vacuum problem, it only reads electrical
signals from every sensor, a good way to find a vacuum problem, spray carb cleaner
at the gasket area of egr valve intake etc, and see if the engine drops while you
spray it, sounds to me like your engine computer its trying to adjust an air+gas mixture

 

I've got it figured out... finally took it to the ford garage, and it turns out that the cam position sensor was installed with the wrong tool, which put it 180 degrees out. The comp. didn't throw a code, but they could tell it was out of sync when they hooked up the laptop diagnostic equipment. Thanks for all the replies.