'00 F150 stalling, missing under load...

Customer brought in '00 F150XL 5.4L Auto. He said the truck occasionally stalls (intermittently), and also has a "sputter" that gets very pronounced under load, like going up hill, or climbing the local bridge. No check engine light, when I hook up the scanner, no codes present, except for the "trip monitor" or whatever not complete. While the scanner was connected, I had the truck in drive with parking brake engaged and foot on the brake pedal. I gave it some gas and it started missing slightly. The more throttle I gave it the more harsh the miss got, but still no codes. The engine seems very lacking in power. After this little "loading test", I shut it off and heard the catalytic converter "sizzling". Had the exhaust backpressure checked for possible clogged converters, no dice, converters were good. I did notice that both "#2" (bank1 sen2 & bank2 sen2) O2 sensors were slow. Truck has less the 55k miles, most of which was spent at speeds lower than 30 mph. I have thought about checking the sp plugs, but I dont think that would cause intermittent stalling. When it does stall, it starts right back up like nothing. Maybe the fuel pump is getting weak?? and "internittently" cutting out?? Any ideas? Anybody?

That one sounds like a challenge for sure. Normally, worn spark plugs misfire under a load, but the mileage seems to rule that out, except that I've heard that some plugs don't make it to the manufacturer's 100,000 mile expectancy. I'd pull a couple of plugs to check the gap and condition. Always use OEM specified plugs on replacements as some others cause problems - unless the engine is burning oil and fouling plugs, then a hotter heat range plug is needed.

Next comes the fuel system. Check the injector fuel rail pressure. I'm not sure what this truck is supposed to have, but near 40psi is common. Check it under load too. When was the fuel filter last replaced? Maybe it's plugged and the engine is starving for fuel under high load? Besides a clogged filter reducing fuel flow, the fuel pump could be weak, but not usually at 55k miles. That leaves possible partially plugged injectors - try an injector cleaning or known-good gas additive like Techron for immediate results.

Lastly, on my list, would be a cleaning of the MAF sensor. Dirty MAF's make the engine run lean, maybe too lean under a load in this case. Lean run conditions are not reported as a malfunction on some computers. A real clean tailpipe (no black soot, just metal showing inside the pipe) is a tell-tale sign of excessive lean burn. Nice for emissions, but bad due to power loss, engine ping under loads and going up steep inclines.

Slow O2 sensors don't usually cause a misfire like this to my knowledge.

Be sure and let us all know when you find the problem - thanks!

Mike

Did you check the fuel filter? Or maybe you have carbon buildup?

Missing under a load is almost always ignition wires. Start it up after dark, pop the hood and look for sparks grounding out on the engine.

You said you used a scanner on the vehicle. Did you try the Global OBD II mode ? If you use the OBD II mode, go into mode 6. This gives you the pass/fails of monitor components. The 50's tests will isolate a misfire. Mode 6 works extrmely well on diagnosing Fords. Go to www.motorcraft.com (http://www.motorcraft.com) website.
You have to do some looking, but there is a section in there that explains the TID's, Test Identification, & CID's, Component Id., along with the criteria for each monitor system.

My '01 F150 has a coil-pack(?) on each plug. At 135k it began to have a misfire like that. I had one replaced and that cured it. This kind of a failure does not show an error code & I was lucky - I found (word-of-mouth) a mechanic that knew just what it was and just which cylindar it was.

You should not need that at 55k, but what do I know.

Now at 148k I can feel it happening again - so it's probably time to get the other 7 done.

cvt

My 02 4.6L had this or a very similar problem like that, if not the same at only 47K. It happened to be a fouled #3 spark plug. and I did not get a ses light either. After new plugs it ran fine.

I have a 88 f-150 with 302 it misses as soon as throttle is applied but it idles perfect and if you put her to the floor it does not seem to miss either?? I have put pump units in both tanks, changed filter on rail, it has new distributer, wires , plugs, tps sensor, egr valve,new coil pack but it still has a miss and it seems to be consistant when you hold trottle and watch RPM gauge its like every 3 seconds, my next step was to remove clean/rebuild fuel injectors and do pressure check any suggestions???????????? david