98 Explorer 4.0LOHV Severe stumbling/bucking under load

[hawc]

Hi,

I've got a 98 Explorer Sport 4.0I OHV 5 speed with about 151K on the ticker. In the past few weeks it has started stumbling/bucking during acceleration at all throttle positions. It is worse as the engine is under more load such as going up hills. It's not throwing and codes and the idle is actually very good. I don't see any bad vacuum lines.

It has done this in the past but it was mild and infrequent and usually would go away on its own on the same trip. Now it's persistent and is at the point of being too annoying to drive.

I replaced the plugs, wires, and coil back at 141,000 miles hoping to get rid of the occasional stumbling. It still did the intermittent stumbling as before but it wasn't bad enough to be a concern. So I'm hesitant to suspect an issue there. The wires are NGK and the plugs are NGK fine wire platinums. The coil is an oem motorcraft.

I replaced the fuel pump and filter at 129,000 miles. Cold starts and hot starts are excellent. I don't have a fuel pressure gauge to check the fuel pressure.

In the past few days, I pulled and carefully cleaned the MAF sensor. No change. I also took a quick test drive with the MAF disconnected just see if the hesitation was slightly better. No change.

Also, I don't think this is related to the current issue but in colder weather (50's or lower) over the past few years it does throw a p0171 and p0174. It only does it when the engine is cold. If I clear the codes once the engine is warm the codes never come back on the same drive. I always suspected the upper intake gasket but never messed with it. Again, I'm not getting any codes right now.

Anyone got any ideas as to where to start? Anyone with a similar issue that was resolved?

Thanks

 

[rasouth]

I had a similar issue a year or so ago but mine was the spark plug wires. I did not have the wires routed properly and was getting a short where it touched metal. It did not throw a code, either. At the time I looked into intake gasket, head gasket (because I was blowing water out the tailpipe), vacuum lines, etc. A quick test you can try is to spray starting fluid around intake while engine is running. If engine revs up you have a leak. Some of the intake is hard to get at though.
Check wire connections and proper routing of wires, pull plugs and check for oily residue ( misfire of plug), do a compression check ( should be around 135-145 psi), check the fuel injectors and connections, check oil for a "milky" residue (possible blown head gasket), check EGR for cracked or disconnected hose.
Quite a few people believe that the Autolite spark plug is better for the Explorers.
Good luck finding the problem and keep us updated.

 

[J_C]

You can get a fuel pressure gauge loaner tool at Advance Auto, Autozone, etc. IIRC on a '98 it should be about mid-30's PSI fuel pressure.

If fuel pressure is good I would hook up a scan tool capable of live data and look at fuel trim during the issue, among other live data. If you don't have one it is worth getting, even some inexpensive dongle that links to your cellphone/tablet/etc to use as the display like the BAFX OBD2 on Amazon.

Once it gets colder you can also use the scan tool to check the coolant temp sensor in case it's off, but otherwise yes you may need a new upper intake manifold gasket. If it's only doing it during idle then you might try cleaning your IAC valve and putting a drop of light oil on its shaft at the bushing end, moving the shaft back and forth to work that into the bushing (along with gravity, shaft pointed downward with it sitting to soak in)... at least that's easy and free to do.

 

[hawc]

I just realized that I do have a cheap little ELM327 OBD2 bluetooth tool that I bought a couple of years ago and forgot about. I'll mess around with that later and see what I can find out. Not sure what live data it will give me.

I'll probably order a fuel pressure gauge kit off Amazon. Any recommendations there?

 

[EB4X]

Possibly fuel filter.....cheap replacement to check.....

 

[J_C]

^ hawc mentioned replacing the filter 22K mi. ago. If it's already clogged again, there may be a fuel line or tank rust problem that needs fixed before the new filter is put on.

 

[96eb96]

Losing coolant?

 

[hawc]

I literally like 10 minutes ago just discovered I do have a tiny crack in the radiator just above the upper hose. I had noticed a little weeping at the upper neck last weekend. Thought it was just the hose but the radiator was in the back of my mind. Replaced the hose. Seemed to have stopped the weeping after a test drive. A few days later and I notice it's still weeping a bit. Double clamped it by adding a worm clamp in front of the spring clamp. I noticed today it was still weeping. No way was it leaking past two clamps. With the engine hot I cleaned around the neck and within a minute I saw a drop of coolant.

So this very well might be the root problem. So off I go to do some online radiator shopping. And I guess I'll also take this time to replace the fan which has the infamous cracking.

Stay tuned...

 

[96eb96]

I literally like 10 minutes ago just discovered I do have a tiny crack in the radiator just above the upper hose. I had noticed a little weeping at the upper neck last weekend. Thought it was just the hose but the radiator was in the back of my mind. Replaced the hose. Seemed to have stopped the weeping after a test drive. A few days later and I notice it's still weeping a bit. Double clamped it by adding a worm clamp in front of the spring clamp. I noticed today it was still weeping. No way was it leaking past two clamps. With the engine hot I cleaned around the neck and within a minute I saw a drop of coolant.

So this very well might be the root problem. So off I go to do some online radiator shopping. And I guess I'll also take this time to replace the fan which has the infamous cracking.

Stay tuned...

My concern was more about the cracked head problem OHV engines have.

 

[EB4X]

Autozone let's you borrow for free the tester to see if there's exhaust gases in coolant....have to pay for the dye though.....not pricey....not to jump to that extreme conclusion but now that you mentioned radiator leak / possible over pressure. I don't think these heads crack nearly as much as some give impression. I slightly overheated my abused 4.0 OHV in my high mileage Ranger and it was fine afterwards.....

 

[410Fortune]

Sounds like it is getting lean under load = starving for fuel
Check your fuel rail pressure, ESPECIALLY WHEN THE BUCKING/HESITATION OCCURS

It sounds like your fuel pump is getting weak when its hot and the issue is getting worse.
What you describe is a lean condition, bucking and hesitating under load = CHECK FUEL PRESSURE first
You can rent a rail pressure tester for free (deposit), heck you can even use a $3 dial type air pressure gauge made for checking tire PSI, it will work once or twice before the fuel kills it.
I don't care if the fuel pump and filter were replaced yesterday, the first thing to check when a ohv 4.0 does this, is the fuel rail pressure

 

[hawc]

I'll test the fuel pressure shortly. So I should be getting mid 30's psi reading for the OHV 4.0L?

 

[EB4X]

My Ranger manual for exact same engine states:
Key on/engine off: 35 to 45 psi
At idle: w/vacuum hose connected: 25 to 35 psi
At idle: w/vacuum hose disconnected: 35 to 45 psi

 

[410Fortune]

I like to see 36-42, if the rail pressure is down around 30-32 when its cold it can drop below that when warm (failing pump may not be able to make over 35, but it will still run under most conditions)

 

[lobo411]

The lower intake manifold gaskets seem to die around 100-150k on the 4.0 OHV. This may be the cause of your p0171 and p0174 codes. When it's hot the engine swells and a bad gasket can keep the intake sealed. When it's cold, the engine contracts and a bad gasket can't seal the intake. Usually, the most noticeable symptom of a bad lower gasket is a very slight coolant leak from the top front and rear of the manifold.

Also, I know the 1996 and possibly your 1998 4.0 OHV had defective DPFE sensors. If your DPFE sensor (a box mounted on the driver's side of the intake plenum, with two 1/4" hoses going towards the EGR pipe) is an aluminum box, it's the defective kind that needs to be replaced. The redesigned one is plastic. IIRC it causes bucking at low idle or low RPM (mine was that way), but it's a $15 part that takes 5 mins to replace. If you have the aluminum box type, I'd replace it and hope that fixes things.

Also, I'd replace the lower intake manifold gasket if you think it's the original (with 150k mi on it) just on general principles. If it's not leaking yet it probably will soon. Replacing the gasket will give you a chance to inspect things too...you might find a dirty injector, or you might find that one of the small 1/8" hard plastic vacuum lines has broken, or you might find wire damage. But I wouldn't replace the lower gasket until I tried the DFPE sensor.

 

[hawc]

Sorry for the delay. Anyways, stumbling became severe. Finally threw a code. Misfire on cylinder 3. Fuel pressure is 40psi with key on and engine off. 32psi at idle. This seems to be okay for a 98 ohv 4.0 with a return system. Ignition wire visually looks fine. Pulled it out and reinstalled. I haven't pulled the plug yet. That's my next step. I'll probably swap the wire and the plug for new ones and drive it just to see. I really hope it's not the injector. It's only throwing the single p0303 code.

 

[J_C]

I'd much rather replace an injector than a head gasket, though then I'd be thinking.... while I have the rail off to replace one... should I replace them all... at about $48 a pop.

Then I'd be thinking, I can get two sets of injector o-rings for under $10, take the rail off and swap the injector to a different cylinder to see if it's moved the misfire to the new cylinder, or take a multimeter to see if it's getting the power pulse on the connector. Second set of o-rings so when you pull the fuel rail off again, you again have a fresh set to put on... IF you need to pull it off again, the problem could remain on cylinder 3.

I'd also back up a bit and consider there was a lean code for both banks, which a faulty injector would not cause for the bank it's not on. You mentioned upper intake manifold gasket previously, did you ever get around to replacing that?

Certainly you could pull the plug and wire but I don't think that's it because due to the waste spark setup, a misfire on one cylinder from the coil, wire, or plug, should cause a misfire on the opposing cylinder that uses the same coil.

I'd be wondering if the prior problem just got worse and is now causing a misfire often enough to register a code, or if you instead now have a new problem too.

However one last thing about the fuel pressure to rule it out more decisively, is you only mentioned engine off and at idle. I would also want the fuel pressure with the engine at higher RPM.

 

[hawc]

Here's the update: Good News!
Pulled the # 3 plug. Looked scorched. Even smelled noticeably scorched. So out went the Japanese NGK G-power and in went a Mexican made Motocraft SP-500. Problem completely solved. The NGK had just under 10K miles on it. Perhaps it was slightly defective when new and just got worse. Or the cooling system not holding full pressure was heating up the heads a little more than normal. Who knows. Not too worried about it. I'm not going to mess with the other 5 plugs. Hopefully, it's just a one off fluke on that number 3 cylinder. Either way, I'm still considering letting Godzilla loose on Tokyo. Just kidding, I still like NGK's.
So the stumbling's gone and the radiator and fan are done. So I'm .