1996 Explorer 4.0 OHV Rough Running and Rough Idle

[Desertoasis]

I have searched this forum dozens of times in search of answers on the cause of my 1996 4.0 OHV running rough. Actually this is my brother-in-laws vehicle which I take care of. 278k miles and still running. So, I encountered a rough running condition recently which occurred rather abruptly but had a history of P0304 codes that turned into P0174 P0153 P0300. It was seriously missing at idle and at speed but would occasionally come out of it and run smooth. Did the gamut of tests. Plug and wire was first. Checked wire for ohms and again with spark plug simulator tool. Ok as wires were recent. Checked plug in wire while cranking and fine spark. Recent Autolite AP605 so figured ok. Figured injectors so dove in. Bought rebuilds off EBay for $82 delivered! What a deal and 19 lb/hr too. They needed it as originals hadn't been touched in 278k miles. Checked injector harness with home made noid light (light bulb) and ok. Back together with new gaskets and really clean fuel rail and plenum. No vacuum leaks for sure. Wish people would quit saying it's a vacuum leak if running rough. Maybe minor stumble or miss but no way vacuum leak has one dead cylinder of rough running. Fire it up and still runs rough! OMG WTF! I can't believe this. Check each cylinder while running by pulling plug wires off coil. Oh BTW recent new Motorcraft coil. Kind of strange bur #4 had less effect on running than other cylinders. Hmmmm. But I already checked that. Hmmm. Well decided to go for new MAF and O2 sensor left bank. MAF was original so why not. Had put new O2 sensors two years ago but hey, one could go bad right. NOT! I was so frustrated this morning I wanted to torch this POS (er, fine classic). I really thought I had an internal problem. Could not face having to do motor internals so started over with approach I wish I had done originally. Pulled #4 plug and swapped with #6. Oh forgot, did compression check along the way and 120 in 4, 5, 6. Running rough still but pulled #6 wire and still rough. #4 wire pulled and definitely RPM loss. Finally, that plug originally in #4 was bad. OMG I have never ever I mean in 40 years never ever had a plug fail like that. I checked it as good but driving, heat, whatever had intermittent failure. Mind you this thing has throw P0304 for two years and resetting it made it ok for months at a time. Then this abrupt symptom of rough running which sometimes would run smooth. Bottom line is I spent many hours over a week and spent over $200 to find out it was a $3 plug. I feel like a dumbass because I know I'm better than this. So, test and retest a second method if possible. At least it runs again and looking to cross 300k soon!

 

[OjJR]

Been there, done that. Own two explorers of that generation. Both 4.0 OHV engines, 96 and 99. One thing I have learned these Fords are finicky when it comes to plugs. I only use Motorcraft now. Another thing. The coils are finicke also. Check with multimeter (see net) and if they are just a little out of factory spec. replace. It won't run right.

 

[96eb96]

Another thing to check on these engines are leaks thru the EGR valve. They wear out and create a bit of a lean condition. Run propane or brake cleaner under the valve and sense the idle or monitor trims..

The guy who sold me my truck was an ex ford tech turned salesman and WARNED me to buy motorcraft as much as you can. He said if he had a dollar for every aftermarket set of coils, plugs, sensors he replaced after a customer or a shop brought the problem in he would be retired. Maybe things improved since then but it is still important. Remember that their parts are meant for dealers and if it fails Ford must pay for the repair again.

 

[Desertoasis]

I couldn't agree more with you on the use of Motorcraft parts. I try but when they are sometimes 3X or 4X the cost of decent aftermarket it's hard. And since this is a charity case for me it's even harder! Over the last three years I've put $2500 into a vehicle that's worth less than $1k. Oh well, it's my hobby and therapy and it keeps peace in the family!

 

[Desertoasis]

Another thing to check on these engines are leaks thru the EGR valve. They wear out and create a bit of a lean condition. Run propane or brake cleaner under the valve and sense the idle or monitor trims..

The guy who sold me my truck was an ex ford tech turned salesman and WARNED me to buy motorcraft as much as you can. He said if he had a dollar for every aftermarket set of coils, plugs, sensors he replaced after a customer or a shop brought the problem in he would be retired. Maybe things improved since then but it is still important. Remember that their parts are meant for dealers and if it fails Ford must pay for the repair again.

Interesting on the EGR. I still have a slight stumble off idle but clears up almost immediately. Drives fine and idles ok with maybe a slight burble. EGR? I'll have to try your test and check fuel trims. But EGR is original it appears. I've had it off and cleaned and checked function when vacuum applied but I'm sure it's tired out.

 

[96eb96]

Interesting on the EGR. I still have a slight stumble off idle but clears up almost immediately. Drives fine and idles ok with maybe a slight burble. EGR? I'll have to try your test and check fuel trims. But EGR is original it appears. I've had it off and cleaned and checked function when vacuum applied but I'm sure it's tired out.

It will pass all vac and obd tests but the pintle shaft becomes worn (like a valve guide) and lets in air. When I got the new egr valve and blew into it no air went thru. The old one felt like it was open. Your total trims at idle should be at least 10 positive if that is an issue. This is a case of OBD accounting for aging components, but not setting any codes. The egr valve should be changed, because you are not running optimally.

Slight stumble off idle could be your MAF. Even a reman one can be bad, so check the BARO parameter and it should correspond to your altitude (159Hz +- 4 at sea level, lower as you are higher).