I've been having a problem with my '99 B4000 4.0L with auto trans and 107k missing and hesitating terribly on acceleration. Not getting any engine codes except an occassional #6 cylinder misfire. I've done all the usual with plugs, wires, filters, etc. as detailed in other threads and this week replaced my #6 injector. Unfortunately, that did not fix the problem. About the only two remaining suggestions to check are the crank position sensor and the PCM itself.

Question, any way to check the PCM to see if it is faulty before replacing this $700 item? I sort of suspect it may be the culprit now as when I unplug the negative battery cable while working on the vehicle or after checking and clearing a code from the PCM it seems to run better for a short time. It also appears to run better and sometimes with no problems at all until it warms up and comes out of the closed loop mode.

 

What does the #6 plug look like compared to the others?

 

Has the oxygen sensor ever been replaced?? Is your truck equipped with EGR?? If yes, try disconnecting the vacuum line that actuates the valve, drive it a few days, and observe if anything's different. [the codes should identify these things, but who knows??]

 

The #6 plug looks no different than the others. When the problem first started, I replaced plugs, wires, filters, etc. and did a general tune-up. The plugs had about 60k on them. The problem persisted and getting the #6 misfire code only, I removed the #6 plug thinking I may have cracked the insulator or the gap was off. I checked it and actually switched it out with another plug from another cylinder and still only got the #6 misfire code.

In response to the other note about the EGR valve. I have checked the EGR valve and it appears to be operating. The valve is sealed so I can't actually feel for it opening and closing like you can with some valves but I have removed the vacuum line on it and gotten a fault code with the line off, reconnected it and cleared the code with no other faults so I felt pretty sure my EGR was working okay.

 

Any idea on an approximate cost for a new PCM (ECU or whatever the proper term). The Mazda dealer quoted a figure around $700 for the part only and I've seen a couple on-line dealers quoting around $650 but in checking the forum I've seen comments from folks about buying them for under $300 and as cheap as $175 for 4.0L Rangers which is the same engine.

One thing I have learned is that there can be a big difference either way for Mazda or Ford parts. Generally, the Mazda dealer is pretty close to the same and at times even cheaper on the same part as my local Ford dealer. Virtually all engine and drive train parts are packaged as Ford parts and are identical.

 

tlk401, it sounds like you have done all the right things to try and resolve this problem... I wish I had more insight to offer...

As for the PCM, why not try a junkyard? Also, I think you can find PCM "rebuilds" at RockAuto for a little over 100 bucks w/core.