Missfire Madness

[Bill-93XJ]

Well, I thought I had it under control.... famous last words.... My '93 4.0L 6cyl w/ AW4 is all bone stock. I had experienced it running rich and sputtering a bit lately. I have 153,000 on the clock and this baby has been taken care of pretty well. Never abused. I have changed fluids religiously and there is normal wear and tear, but mostly suspension/body squeaks and such. I have recently replaced the fuel pump and believe that I have good pressure on the rail. I recently replaced the CPS sensor and it 'should' be good (new part anyway). It was running a lot smoother and not missing at all, but still I had the feeling it was running rich. I last changed out the o2 sensor at 70,000 so I figured it would not hurt. Yesterday, I put a new BOSCH o2 sensor in and it seemed to be SO much better, smooth, idling nice and no miss, no hesitation. I drove it for about a half hour and it was great. This morning's drive to work was great (about 8 miles one way). On the way home tonight, I was one mile from home and it started acting up. Bad miss. Not wanting to run. Under load it would lurch. Put her in Neutral and it would sputter. Bad business. I checked the codes (OBD I) and all I get is 14. i think that is for the o2 sensor?

Anybody got any ideas? Is my original factory ignition finally crapping out?

TIA Bill-93XJ

 

[94XJ2door5speed]

Don't know but if you fix it let me know. I can't fix mine and ive got the same problems.

 

[branch]

code 14 is "MAP (manifold absolute pressure) sensor voltage to low/high. Basicly your map (NOT MAF) sensor is either bad or disconnected. That could easily cause the problems you are talking about.

 

[McQue]

code 14 is a MAP voltage fault

 

[Bill-93XJ]

Well, it looks like the MAP 'may' be the culprit. i think that I remember reading somewhere of someone cleaning on with carburetor cleaner? Does this sound like a good thing to try first? Why would this sensor create such intermitent symptoms....??? Anybody got a handy link to the voltage/resistance readings for this beastie..???

 

[CharlesS]

IF the port where the MAP (Manifold Absolute Pressure) sensor receives manifold pressure on the throttlebody is dirty or partially obstructed; the ECM will have nothing to read from and back off the signal voltage to the fuel injectors....
Check to see if the throttlebody needs a good cleaning....
Could be a cheap fix...

 

[Bill-93XJ]

If a system had been running rich for quite a while, could there be a flake or blob of carbon that could have dislodged? This might explain the intermittent nature of my miss if there is some blockage floating in the vacuum line....???? What ya think about that theory...???

 

[Bill-93XJ]

OK. Sorry, but I might have posted something that was not totally right. The way I was reading the codes, I thought that I saw 1 then 4 flashes... Upon further review, I think it might be 5 and 5, the end of the readout signal from the comp. The spacing is not exactly uniform. I was reading in a page about the codes that they can do that sometimes. After this '5-5' there is nothing more so it appears to be the 'end of message' signal or whatever you call it.

I unplugged and replugged the MAP connector a few times just for grins anyway and it made no difference.

Here is the latest symptoms: Cranks fine. Fires up. Runs for 5-10 seconds. DIES. I turn on the key and can hear the fuel pump running beforehand. I really think that I am getting fuel pressure on the rail. I have a new frikin CPS and O2 sensor. As a matter of fact, this squirrely behavior seemed to happen after the new O2 sensor. It seems I have spark enough to start and run for 5-10 seconds.....

Anybody been through this scenario before....???? TIA Bill-93XJ

 

[McQue]

Never been through this scenario, but it sounds like your new 02 sensor is flaking out as soon it heats ups...which usually only takes 5 to 10 seconds...

 

[Bill-93XJ]

Well, that would fit the timeline of events.... Ever since the O2 sensor replacement it has run OK, until today on the way home from work.... Could the sensor crap out so quickly (catastrophic and sudden failure) and would its failure cause the engine it DIE so abruptly....???? THANKS GUY !!!

 

[birchlakeXJ]

55 is the end signal so it doesn't look like you do have any codes.

You may have an 02 sensor that suffers from "infant mortality". If you can conclusively link the symptom to the sensor replacement, then I would concentrate on that component. You'd be surprised at how much stuff craps out basically right out of the box. Maybe try a different brand of sensor this time although I have had pretty good luck with Bosch in general.

As far as fuel pressure, the only way to verify it is to get a fuel gauge on the schrader valve on the fuel rail and get some numbers. Anything less is guesswork.

Good luck and post back what you find!