The story so far:

Got this truck a few weeks ago - '96 ext cab, 4.0 M50D, about 160k on the clock.

Had a rough idle, hard to start when hot. Poor gas mileage, even for a 4 liter... Seems to have a pretty regular miss or stumble when idling.

Had two codes, P0401, P0402.

Here's what I've done so far:

Sprayed out TB with TB cleaner
Replaced air filter
Replaced IAC
Replaced DPFE sensor
Tested egr valve with vacuum pump - seems OK
Replaced spark plugs - autolite
Replaced wires - autolite
Replaced PCV - purolator
Changed oil
Changed trans fluid
Have run two tankfuls of gas, each with a bottle of chevron Techron cleaner in it.

I sprayed the TB cleaner around the edges of the intake and on vacuum connections looking for leaks - none found

The truck is definately idling better, and the idle speed comes down to 800 - 900 rpm fairly quickly after starting. Prior to IAC replacement, it would hang out around 1500 rpm for a while, then start "hunting" for a bit, eventually winding up at a normal idle. I tried cleaning the old IAC, didn't work much better, and it started making a horrible noise when air was sucked thru it...

Starting seems a bit easier now, and possibly this is normal for these engines? My 2.3L just fires right up all the time. I occasionally drive an aerostar with the 3.0, and it needs a lot more cranking when hot too...

Anyway, as I get more familiar with this truck, I think that miss I hear at idle is also occurring while driving. Engine makes good power, but I think it should be a little smoother. I have not seen any other codes show up though.

I hate to go out and buy an ign coil pack just to test. I was thinking about hooking up my timing light to each wire in an attempt to discern a steady pattern of pulses to each cylinder. Is this worthwhile?

Sure would appreciate any guidance on this!

 

Forgot to mention that I also cleaned the MAF and the air temp sensor

 

Have you reset the computer since you did all this work? If not you definetly out to disconnect the battery for 20 mins or so and let it relearn everything now that you've tuned it up.

Afterwords see if the codes are still present.

That being said, hook a vacuum gauge up to it and see what it's doing, could be a small vacuum leak. How's it go at WOT?

What exactly do you feel/hear/see when it "misses"? Do you have a tach? If so, does it move during the miss? And unusual sounds or vibrations, etc?

As for needing more cranking when hot, I haven't noticed it needing much cranking during any condition. Assuming you mean when the motor is hot, not when the temperature outside is hot, mine starts right up, I can just blip the key to start and it's running, so extended cranking could be something up. I always let the fuel system pressurize before turning the key to start though, so that could have something to do with it.

Hope it helps.

 

may not fix it but its good insurance..fuel filter especially runnig the cleaner through there lossening all that stuff up and getting stuck in there.. and since it has alot of miles on it i would try a decarbon flush/kit whatever you wanna call it but if you just changed your plugs and your oil you may want to wait because that stuff makes a mess of bolth at least till it needs an oil change again....

josh

 

When you changed the plugs did you notice of all the plugs looked about the same? or was one real dark or real light in comparison to the rest? (could indicate a injector with issues) Like Zach said hook up a vacumn gauge and see that it is steady and high enough. Next start it up in the dark and look for fire around the coilpack.

 

Those are good suggestions. All six plugs looked the same. Pretty much like every other used plug I have ever seen... They all had a slightly whitish - grayish appearance on the electrode which I am guessing is due to the techron stuff I dumped in the gas tank?

The computer has been reset - battery was disconnected for the hour I was swapping plugs & wires.

I'll put a vacuum gauge on, and check for discharge corona around the coil pack. How much vacuum is "high enough"?

The troubleshooting charts on the CD are a dead-end for me - They instruct to hook up the engine analyser... which I don't have access to...

My Chilton book outlines a procedure to check DC resistance for the primary and secondaries on the coil pack. Might run through that. Maybe that would show a gross fault in the coil, but if it's breaking down under high voltage conditions, an ohmmeter won't show that...

 

As I had time today, I hooked up my timing light and tried each ignition wire. They all flashed very erratically. I shut off the lights in the garage and looked for discharge corona - nothing.

I bit the bullet and bought a new Borg Warner coil pack. Immediately the hard starting problem seems solved. Idle is a LOT better, and the engine sounds smoother and feels better when pulling.

Still seems to have a slight 'hunting' when idling, though I wonder if thats due to the ongoing EGR problem I'm having.

I'm going to reset the computer as it occurs to me I haven't done that since replacing the coil pack.

 

never mind.
Dave

 

Follow up - as noted in another thread, I found a 1/4" hole in the intake side of my EGR valve, so basically a huge vacuum leak...