new to the site. i've been reading these threads fro a few hours now and you guys seem to be the people to talk to about my issue. i have a 98' xlt 4x4 4.0L, and my problem seems to follow alot of guidlines as everyone elses. Rough cold start, but goes away after a minute, the only difference in mine is that my misfire also happens under load and open loop conditons ie.. 3,000-3,500 Rpms, and usually under wide open throttle. now the problem can become intermittent, and in fact dissapear after running it for a bit. The issue im getting which i wouldn't be surprised if they were un related is a lean condition not enough to set a DTC but still sitting anywhere from 12 to 17%+ on long term fuel trims. I've tried almost everything. Ignition wise everything is new ( plugs,wires,coil ) i even threw a new MAF at it for **** and giggles but no change in the numbers. I've smoked the motor thinking the intake manifold gaskets as they are ntorious on these things and nothing showed. I also gone as far as to disconnect the Cam Postion sensor (knowing that both the sensor and the shaft are notorious on 3.0L, but nothing) and then run the vehicle hoping that reading the Crank sensor alone would remove the misfire. Nothing. I have rulled out EGR, Crank PS, Cam PS, compression, and all ignition components. I have also ran mode 6 in my scanner which tells me that nothing but Cyl 4 is missing although not enough to set the DTC it definetely is bucking and missing under load. I did have to add a little coolant this summer but nothing radical, I have also not noticed any in the oil or in the cylinder during diagnosis. I have not ran cylinder leak down yet but in my experience I just dont see the emminet signs of a bad headgasket. So know that i have bored you with all that i'll get to my question. I noticed in one of your threads you said that the lower intake manifold mounting bolt might work their way loose. Did you have this misfire under load issue with you vehicle as well? Also you mentioned something about sending out used oil for a lab to check for coolant, what lab, how much did it cost, and what was the turn around time?

 

Welcome again to FTE.
Good idea to begin your own thread.

Lets recap your problems & see if we can list things & begin to see a pattern.

You have a 98 4.0L 4X4 that has a rough running cold start, that goes away in about a minute.

You also have an intermittent misfire, in open loop operation, during wot (3- 3.5K RPM), that can dissapear after running it a bit.
The engne dfinately misses & bucks under load.

No CEL lit, but your scantool shows #4 cyl is misfiring, or is at least suspect, even though the CEL isn't yet lit, or flashing to indicate a misfre.

The scantool says LTFT is +12-17%

You've checked compression & the EGR system & smoke tested the engine for intake manafold leaks & all are ok,

You've disconnected the CMP & CKP sensors with no effect on the miss, or rough cold start.
You've thought about, but not yet done a cyl blow down test.

You've replaced the plugs, wires, coilpack & MAF sensor, all with no affect on the miss or rough start.

So, back to basics, a misfire can be caused by a number of things, so you May have more than one problem.

So lets begin with what you know for sure & that is that the scantool is telling us the Long Term Fuel Trim is definately out of whack & #4 cyl is at least suspect, or misfiring.

SO, a bunch of questions, is LTFT bad for Both cyl banks????

What do the upstream O2 sensors PID look like for both banks????

Have you checked fuel pressure & pressure leakdown at KOEO & KOER????

Have you had your scantool do a "Cyl Balance Test"????

Did you use OEM plugs, wires & coilpack & were they all tested before installation????

Did the rough cold start & misfire under load come about together, or suddenly after some event, or slowly over time.

How many miles on this ride????

 

Alright another peice of info this misfire only started happening 5-6 months ago. No major work or changes at that time. The only thing I noticed before this misfire issue was that i was getting high idle on cold start/ cold mornings.

LTFT- reading lean on both cyl banks

STFT or upstream O2 sensors- are both switching fine and within range (centering on 0) didn't watch voltage but will check'em again tomorrow and get more definetive numbers posted.

Fuel Pressure is within in range at idle but havent checked at WOT. will probably check it this weekend

Mileage: 130k

Wires- were motorcraft till I changed them two weeks ago. Had the problem before and after. New set of wires are NAPA brand. Resistance is less than 6 ohms and i have done wire swap on cylinders and misfire is still isolated to cyl #4

Plugs- were autolite platinums. when I bought the truck 10 months ago I switched them to Autolite Double platinums. I have done a plug swap between cylinders #4 & 6. Misfire stays isolated to cylinder 4. Today i went and bought a Bosch fusion iridium plug to try and the cylinder still misses.

Coil Pack- was factory, i bought a new one from kragen last week to try. No change in the missfire.

As far as fuel pressure under load, upstream O2 PID, and cyl. blalnce test I'll have to run them tomorrow and get back to you.

Thanks for the interest in this issue I gotta figure someone else has had this issue.

 

With the high idle problem just prior to this miss I would still suspect a vacuum leak in the vicinity of number 4 cyl. The only other thing I can think of would be to swap that fuel injector with another one if you can do that on that motor with out tearing it completely apart.

 

OK more good feedback.

130K miles on this ride, LTFT is way + for both cyl banks, so the computer is trimming in more fuel over the long term, says it's trying to compensate for a lean condition, thats affecting both cyl banks.

Smoking the engine didn't show an intake leak, so with 130K miles, maybe that leaves us looking for a simple vacuum leak source, say something like a sticking, or stuck open PCV valve, or it's connecting hose, or a loose fitting vacuum line or fitting to the vapor recovery, EGR, or brake systems, or maybe an acting out, sticking, or partally clogged fuel injector & seeing as how you also have indications that #4 cyl may be acting out & you have what feels like a misfire under load, maybe #4 fuel injector belongs on your suspect list & swapping it into another cyl to see if the misfire follows, as wtroger has suggested is a good trouble shooting option.

All the above assumptions said & in case you have more than one problem & so we don't overlook something & all are singing from the same hymnal, I meant to ask, what the before cat O2 sensors PID numbers & switching speed look like & it would be helpful to know what the new MAF sensor & STFT numbers are too.

More thoughts to ponder.

 

I think that #4 injector should be very high on the suspect list. I agree, switch it to another cylinder and see if the problem follows. Otherwise you may have a sticking valve.

For the record, used the Napa Belden wires, JUNK. They use cheap manufacturing methods and they break down from standard engine heat. They break down faster on waste spark systems. I used them before, got about 3000 miles out of them. Likewise, the Bosch plugs might have the right gap, but they have the wrong heat range and displacement. They can cause or exaggerate problems even when they are new. Put the Autolite Double Platinums back in.

 

Get a used oil analysis (UOA) done. It will tell you if coolant is mixing in with the oil, which could be a sign of bad gaskets (head/lower intake) or maybe even a slightly cracked head. Cheap and easy to do: Blackstone Labs

 

sorry i have not been able to finish running anymore test or post results. picked up a bad case of poison oak and havent been able to get around to it. i'm looking at doing an injector swap as soon as i can but it is a bit of a pain in the *** on this motor. i will post new info asap

 

Well scanner was identifying wrong cylinder. Finally set DTC for cyl 6 miss. But has anyone experienced a miss/bucking at open loop (wide open throttle) only at highway speeds. Only changes since last post are new intake plenum and fuel rail gaskets. Also new MAF. Fuel pressure checks out at 68 psi. Everything else is checking out.

 

Is it now #6 cylinder that the scantool is flagging with a miss????

Well a miss under load at WOT might suggest a spark plug or ignition wire breakdown problem, or maybe a misgaped plug.

You could raise the hood at night & have a look see for arcs & sparks, or maybe do a wet down test of the suspect wire & plug, with a spray bottle of water & see if you can induce some fireworks, or a rough idle.

Did you pull the battery B- cable, to wipe the comuters memory after the repairs, so it could begin to rebuild new fuel trim tables with the new parts in place????

Is the LTFT still way plus????

 

scan tool is not showing any miss now. before the scanner pid was identifying the wrong cylinder. as far as the miss. plugs are a year ol ( autolite double plats ), the wires are 3 months old but had the miss with the old set of OEM wires. also just replaced intake gasket and fuel rail manifold gasket yesterday. so i wiped the KAM to clear lean codes and relearn paramiters. The random part is sometimes I can run it down the highway and not get the miss/bucking. after first drive of the day. sometimes it there sometimes not.

 

I had a cracked #6 spark plug insulator that would cause a really bad miss under load, especially with damp conditions present, so with that info the Dealer found the problem with the wet down test.

 

Thats funny you say that cause two weeks ago when we had heavy storms was the fist time she's run perfect in awhile. just touchy blue oval bitches. thinking about upgrading to E3's. heard anything on them.

 

I just replaced the OEM plugs with the specified Motorcraft finewire AGSF-22FM in my 99 4.0L & noticed improved/quicker starting.
At 100K miles it'll get a new set of Motorcraft wires too!!!!

 

might try new plugs again. the wires bug me cause the resistance is fine. oh well. i'll just keep digging. What did those plugs cost you?