Vehicle misfires sometimes,Occasionally dies Tach and speedo go dead will not restart until the key is cycled fully off then back on Odometer reads only lines until restart then quit working all together,Intermittent Heavy Stumble on acceleration not a misfire more of complete loss of power momentarily then comes back on. Set various codes for All the above sensors one at a time without check engine light. Has freeze frame data sometimes. Code and data clear on first key cycle. Crank,TPS,Knock,cam,and MAF sensors all test good. Battery 12.6 volts key on,14.2 volts running,Replaced coils and secondary O2 sensors, EVAP monitor will not set. Never the same code twice. All symptoms clear up for a day or two then come back one at a time never at the same time. Checked grounds and commons for voltage and resistance. I suspect PCM ground or PCM power feed. Possibly on the VSS circuit. Any help would be appreciated. Thank you.
Intermittent misfire.Stumble on acceleration,Dies,Various codes,No service light on 1999 Ford Expedition
by AKnight1 in Fremont, CA on August 03, 2013
3 answers 4 comments
ANSWER by Hal... on August 03, 2013
Check power connections. With vehicle running start moving the harness until you find the gremlin.
COMMENT by AKnight1 on August 03, 2013
Thank you very much for your response. I have done the wiggle test several times. I did find a change in idle RPM once but nothing that caused a miss fire or stumble. When it happened last this afternoon. I noticed the speedometer and the Tachometer went dead even the I was doing over 60 mph. Tach makes sense but not the speedo. I then noticed the odometer was reading only horizontal lines. After trying to restart half a dozen times,I turned the key all the way off then back on. When I cycled the key the odometer was reading fine and the engine started. However the odometer no longer works at all. But the truck is running although slightly rough. There are no codes and this has gone on for more then a month. I also noticed that the fuse for the overdrive was blown twice but is fine now. I am suspecting an intermittent short caused by a wire rubbing against metal from time to time. Or something of that nature. I have more then 30 years experience with diagnostics. But this one has me perplexed. Again I do thank you for the suggestion
ANSWER by goodguy on August 04, 2013
you could have a bad pcm
ANSWER by tramies on August 04, 2013
Have you checked fuel pressure? What is the fuel trim?
Thank you gentlemen for your responses. I have considered the PCM except I have only replaced two of those ever. They just don't seem to fail all to often. That is what Ford has been doing. And they have had the majority come back with the problem reoccurring. I haven't ruled that out yet though. Fuel trim is acting kind of strange. All four O2 sensors are switching properly although the secondary's are a bit slower. They are both brand new. Short term trim is switching negative then positive. But long term seems to stay negative for most of the time. Which could be a PCM problem. I just don't want to throw parts at the vehicle. This morning I found that they intake manifold insulator was dried up and in a hundred pieces. This sit between the upper and lower plenums next to the Knock sensor. And coincidentally as I accelerate the timing is retarding. So is there a problem with fuel trim or a component of determining fuel trim causing a lean misfire. Telling the knock sensor to retard timing. But that doesn't account for the loss of electrical power and the erroneous codes. I think I may have two problems occurring simultaneously. Or one causing the other. Likely electrical. Or am I going down the wrong path? I think you can see my frustration at this point. Again I really do thank you all for your input. The three of you seem to know what you are talking about. And I am far from turning down experienced opinion. And I will go over each of these systems again. If you have any other suggestion's please send them my way. Thank you very much.
COMMENT by tramies on August 04, 2013
After thinking a bit. We had a 2001 Ford van with a 5.4p here. It would Run like crap intermittently. Checked everything. Finally we back probed the TPS And the 5 volt reference was all over the place when it started running bad! 3 volts 5 7 Etc! It was the PCM! Check your 5 volt reference you sound like a smart guy. Found that Napa had the best price. Get the calibration number and go on their site Tom www.pvgarage.com
I will certainly give that a shot. I'm surprised I hadn't thought of that. Thanks Tom for your help Andrew