Hello all. Having strange problem with my '94 MAzda B4000's engine (4.0 litre). It runs fine when cold, but when it warms up, while running on road under part throttle, it loses power and runs rough. When this happens the engine light pops in and out sporadically. Things clear up somewhat when I floor the accelerator, but once I ease back it's back to the same routine. Idle is interestingly always fine.

I tried to pull some codes from the via the engine light, but didnt come up with anything coherent. I'll throw out the numbers here in case anybody can figure it out. I couldnt really figure out where tyhe pauses here, so this is just gonna be a raw readout.

5 2 2 5 2 2 1 1 7 6 1 7 7 5 4 3 1 7 6 1 7 7 5 4 3

I just did regular tune up (plugs, wires, filters) in a (futile) attempt to fix the problem. I figure the issue must be sensor-related but am not sure which one might be suspect in this situation.

Anybody ever have a problem simlar to this one, or can decipher the jumbled numbers into some kinda code? Thanks for any help.

 

I have the same problem with my 95 4.0 ranger. Runs rough on a warm restart, but "clears up" after I mover fast than 30mph.

 

You may have a vacuum leak at the lower intake manifold. The
lower intake manifold bolts were known to loosen up. To verify
this, use carb cleaner to spray where the lower intake, and heads
come together. Spray with the engine hot, and at idle, if there is a leak, you will see a change in RPM. Other symptoms of this problem will be unexplained coolant and/or oil loss.

 

Thanks BOb. I'll try that tomorrow morn.

 

Sorry for taking so long to get back here on this.

Bob, I wasnt really convinced I had a vacuum leak since the rough running only occurred after warmup, so I didnt really check the lower intake .

Thinking more that my problem could be sensor related, 2 of them came to mind that in function were affected by cold and hot. These were the Engine COOLANT TEMPERATURE (ECT) sensor and the OXYGEN sensor. After ohm and voltage readings on the ECT checked out I moved onto the oxy sensor.

Since Haynes was vague about checking voltages that emanate from this sensor when operating temp is reached (which I imagine was intentional since amplitudes vary depending on many running conditions), I simply pulled the plug on it to see what happened.

Well, surprise surprise,,,,, 95% of the sputter problem went away. After reading Haynes it started to become clear to me why this may have happened. Simplified for brevity,,, when cold, the engine runs in closed loop mode which virtually ignores any output from the oxygen sensor. At this time the engine runs based on programmed variables to set fuel/ air mixture. Once things heat up, open loop kicks in and real world variables via the oxy sensor are used and mixture is set by this.

Potentially in my case, a defective sensor may have been giving corrupted info which in turn upset the mixture. By unplugging the sensor, I estimate, the system was locked in a closed loop setting. No voltage was received from the sensor so things were still running on the closed setting's.

Since things arent exactly 100% solved and I am concerned about loss of fuel economy/ pollution issues, I'm going to remove the oxy sensor and take a look at it. Does anyone know if
these things can be cleaned at all? Haynes says not to use solevants to clean them and I imagine a harsh wire brush might remove crictical platnium/ ceramic coatings. I would think gasoline itself could be used to clean it since the sensor invariably comes in contact with it in the exhaust. Maybe this coupled with a nylon brush might bring some life back into it. Good idea??

I know replacements ones are cheap enough, but I guess I'm still not totally sure this is my problem, and I've already spent some $$ on parts already which I thought might correct things.

 

Well, I found out there were actually 2 oxy sensors in my truck. One in the left header and one in the right. I replaced both of them and I no longer have the problem. For some reason though, the engine light still comes on after everything warms up.

It seems that these sensors need to be replaced all at once. When I 1st only replaced the right side, the truck actually ran worse previously.

Thanks for all the posts.

 

Try resetting your PCM by disconnecting your battery for 10-15 minutes.....

 

TUrns out the second sensor wasnt plugged in all the way when I checked it. While I repluged it, took your suggestion Bob and disconnected the battery for about 1/2 hour.

Runs well now and the engine light doesnt come on any more while running. Can't tell you what a good feeling that is!!

Thanks