Runs great when cold, power loss in closed loop, neg trims

[kolb]

2002 Explorer EB 4.0 AWD. 230K miles. In the mornings, when the truck is cold, it runs really strong. Pulls through the RPMs strong and has good throttle response with nice shifting. Once warmed up there is a marked drop in power and the shift quality isn't as good. Slight inclines require a downshift for enough power where cold it would not. Higher RPMs the truck just kinda wheezes. No CEL but have negative fuel trims with the left bank higher at a -7. If I do a KAM reset, the truck runs and shifts great both warm and cold for about 3 days then slowly goes back to its old ways. Also seems worse on hot days which makes me think it's a fuel pressure problem like partial vapor lock. Have been thinking about pulling the injectors and puting them in my sonic cleaner as the negative trims indicate it is getting fuel from somewhere (stuck injector?). Any help or ideas would be greatly welcomed. I tested the fuel pressure at idle and everything appeared fine but I'm not sure if the pumps in these can get weak when they warm up or not. It's the original fuel pump. Also, when it's acting up (weak power) it gets worse fuel mileage. After a reset the mileage is better for a few days then back to lower mileage.

 

[colintrax]

My guess is bad O2 sensor(s)
In open loop the truck runs of preset parameters. When it goes into closed loop, it adjusts the AF ratio based off the O2 sensors. A bad sensor could throw off the AF ratio once warm.

 

[kolb]

I had considered bad O2 sensors but they oscillate when it's running. Perhaps they can oscillate but give bad readings? It HAS been a long time since they were replace (100k +/- miles) so it sure wouldnt hurt. Is there any brand other than Motorcraft that's any good or should I just stick with Motorcraft?

Thanks for the help.

 

[Mustang Guy]

I had considered bad O2 sensors but they oscillate when it's running. Perhaps they can oscillate but give bad readings? It HAS been a long time since they were replace (100k +/- miles) so it sure wouldnt hurt. Is there any brand other than Motorcraft that's any good or should I just stick with Motorcraft?

Thanks for the help.

I was having a similar issue so I tried Bosch O2's on my 2002 Explorer 4.0 and I had a CEL in a couple days and it ran like crap. I ordered Motorcraft O2's from Amazon and it ran perfectly. BTW, I've found Amazon to be the best place to purchase auto parts from, especially Motorcraft. Prices are usually less than the Chineses knockoffs from AAP or AZ.

 

[kolb]

Ok, I'm sold. I'll order some Motorcraft up today and post back with results after they come in. Thanks for the help!! BTW - it has Bosch O2's in it right now so I find your coment about them very interesting.

 

[colintrax]

Ok, I'm sold. I'll order some Motorcraft up today and post back with results after they come in. Thanks for the help!! BTW - it has Bosch O2's in it right now so I find your coment about them very interesting.

Bosch is a good brand, but for some reason Fords do not like Bosch parts.
Anyways good luck, I hope this fixes your problem

 

[swshawaii]

Aren't Bosch injectors OE? Many Ford forums say they are.

Bosch, an OE manufacturer, is the worldwide leader in innovative fuel injection technology and design.

As an OE Supplier Bosch provides the exact flow and original spray pattern for optimal performance

http://www.rockauto.com/en/moreinfo.php?pk=976392&cc=1377660&jsn=431&jsn=431&jsn=431

 

[colintrax]

Aren't Bosch injectors OE? Many Ford forums say they are.

http://www.rockauto.com/en/moreinfo.php?pk=976392&cc=1377660&jsn=431&jsn=431&jsn=431

I believe so, I think they make the fuel pump as well? I'm just saying as a general rule, Bosch parts don't mix well with ford's. Bosch is a quality brand none the less and I wouldn't hesitate to throw Bosch parts at a Volvo

 

[swshawaii]

Especially their +4 spark plugs. This one's an EF classic:

http://www.explorerforum.com/forums/showpost.php?p=2654702&postcount=165

 

[kolb]

swshawaii - That's funny. I've never understood those multi-gap-split-tailed electrode plugs anyway. Law is, there's only ONE path of least resistance, yet somehow, these plugs split the arc into 2, 4 or more arcs??? Whataload.

 

[swshawaii]

BUT WAIT... THERE'S MORE!!!

http://kalecoauto.com/index.php?mai...9&products_id=30&zenid=EIlwsn9nmUAQwXbApvp8f0

 

[colintrax]

swshawaii - That's funny. I've never understood those multi-gap-split-tailed electrode plugs anyway. Law is, there's only ONE path of least resistance, yet somehow, these plugs split the arc into 2, 4 or more arcs??? Whataload.

The theory is it'll use whichever has the least resistance. Kinda like, "well it ain't perfect but 1 outta 4 should be good enough."

 

[apenland01]

BUT WAIT... THERE'S MORE!!!

http://kalecoauto.com/index.php?mai...9&products_id=30&zenid=EIlwsn9nmUAQwXbApvp8f0

OMG that's hilarious!! This is total thread hijacking, but I kinda like it......

 

[shucker1]

We have been using 4 prong "Aviation Style" plugs on our "Big Bore" natural gas fired slow speeds for years without any issues.

Just saying.

 

[kolb]

shucker1 - You Louisiana guys will use anything to plug a "big bore" hole....just say'n. Lol (Just kidding)

Ordered the Motorcraft O2s. We'll see what they do.

 

[kolb]

Well, I'm back. I replaced both upstream O2's with Motorcraft units and while it seams to run a little better, its still a LOT more peppy when cold and in open loop. I haven't checked my fuel trims yet but I will. It's almost like it's trying to vapor lock when it gets warm. Throttle response is lazy and it seems to have a slight lean out of wheezing at anything above 5K rpm. Makes me wonder if a cat is plugged but I checked for back pressure at the EGR sensor and didn't get any pressure build up. Maybe it's a weak fuel pump? I'm kinda at a loss.

 

[kolb]

After thinking about this for a bit something has occurred to me. This truck has a 4.0 SOHC with an intermittent primary timing chain rattle. Could the rattle be setting off the knock sensor which is pulling timing and causing it to be lazy?