4.0 SOHC misfire/ Shudder from May to July

[JustAnotherMember7]

I am having problems with what seems to be an engine misfire on my 4.0 SOHC from late may to the end of july the last few years I've had the car.

I can run air conditioning just fine but the engine will randomly start shuddering at idle (dips down to 400RPM), moderately reduced acceleration, may run rough at higher RPM, and the problem will randomly start and stop on a cold and warm engine. It starts one day, and can "magically go away" a day later, or a month later. (These symptoms usually start towards the end of may and can not be recreated consistently, only about 50% of the time, and problems usually persist until the end of july).

Turning the air conditioning system off *sometimes* will help reduce shuddering, but the above-mentioned problems have a 90% chance of continuing even after A/C is turned off (completely off, not just to vent and floor which seems to be a A/C "hybrid mode").

Although I can run the A/C all I want in April, most of May, August-October and not have ANY of these problems. This has been a reoccuring problem the last 3 years, during the same months, same symptoms, goes away as quickly as the problem shows up.



Here is what has been done the last few years I have owned the vehicle in terms of repair

Jan-May 2019

• 1st Engine replaced with a 2nd engine pulled from a different explorer (timing chain componenets looked good, no other wear problems noticed).

• New spark plugs installed
• Wiring Harness COMPLETELY rebuilt and covered
• All fluids changed
• All filters changed
• Conventional oil used, later changed out with full synthetic (based on E.F. posts)
• Spark plug wires tested
• New battery
• New fuel pump
• New fuel filter
• New tires (pulled from boneyard cars)
• tires mounted and balanced
• Sway bar bushings and end links replaced
• R 134-a added to A/C system

October 2019

• Front and rear shocks changed

2020 and 2021

• Oil changed 2020
• Fluids topped off 2020
• Safety inspection performed 2020
• Oil changed 2021
• Battery corrosion cleaned from terminals2021
• Positive battery post loose wire fixed 2021
• Upper control arms changed 2021
• Lower ball joints changed 2021
• Front wheel bearings replaced 2021
• Rear Wheel bearings replaced 2021
• Rear wheel seal replaced 2021
• Rear diff. oil changed 2021
• New brake pads (front and rear) 2021
• New brake rotors (front and rear) 2021
• 2 wheel Thrust alignment performed 2021

I have put about 5000 miles on the car, run 87 octane fuel (from same gas station) and there is NO engine light on.

My current plan is to remove spark plug wires, coat them with some dielectric grease, inspect spark plug porcelain, inspect spark plugs, measure resistance of spark plug wires, remove and reconnect the computer wire harness, and remove and reconnect the main engine wire harness connector (large square-shaped, on left side of engine).

What in the ever living world could be going on? I plan on ordering the OBD-2 adapter so I can use forscan and check EVERYTHING but am at a loss as to what is going on.

 

[EB4X]

You're stating issue "seems" to be a misfire but also have this "shuddering at idle (dips down to 400RPM"
Going with shuddering / idle dropping to barely staying running at idle I would check / clean IAC and replace gasket(s) --(my ohv has TWO IAC gaskets...nor sure about your SOHC)
I would inconsistently have stalling / dropping idle at stops until I replaced the "other" -- rubber o-ring style gasket that's embedded into the plastic intake manifold.....had new IAC and new cardboard gasket already on.

 

[donalds]

^^^ to add
Same symptoms
Turns out it was a coolant temp sensor

 

[rasouth]

I would start with the easiest fix and work my way to the most difficult. With that said, like EB4X said check IAC and gaskets, but also spray some starting fluid around intake plenum to see if engine revs up any. If it does you have an intake leak. Check all vacuum hoses, clean air flow sensor, check EGR.

 

[J_C]

What in the ever living world could be going on? I plan on ordering the OBD-2 adapter so I can use forscan and check EVERYTHING but am at a loss as to what is going on.

Yes, hooking up a scan tool is the next thing I'd try, although I too, have had dropping idle, especially when the A/C compressor kicked in, due to the IAC valve needing cleaned out. If it hasn't been cleaned recently, it's going to need done eventually so might as well do it now, being a really easy part to pull off to do that. I also like to prop it up on end and put a drop of light oil on the shaft where it enters the bushing to relube it, leave it sitting like that for an hour so more oil creeps into the bushing.

 

[JustAnotherMember7]

I would start with the easiest fix and work my way to the most difficult. With that said, like EB4X said check IAC and gaskets, but also spray some starting fluid around intake plenum to see if engine revs up any. If it does you have an intake leak. Check all vacuum hoses, clean air flow sensor, check EGR.

I'll go ahead and recheck everything and try the starting fluid and clean the IAC valve (with throttle body cleaner?)

What really makes me scratch my head is that the car has great idle besides shuddering issues that occur may-july.

I don't think I had this on the list but the throttle body and MAF sensor were cleaned in 2019 as well.

 

[JustAnotherMember7]

I would start with the easiest fix and work my way to the most difficult. With that said, like EB4X said check IAC and gaskets, but also spray some starting fluid around intake plenum to see if engine revs up any. If it does you have an intake leak. Check all vacuum hoses, clean air flow sensor, check EGR.

Possibly. I'll check and clean everything tomorrow.

My main concern is that when this shuddering occurs and I revv up the engine, there are 2 seperate things that can happen (reguardless of whether A/C is on or off) number 1 is that the engine will revv up and the misfire will be noticeable OR the engine will not revv up properly (the limit is usually about 1900 RPM) and will act sluggish (random length of time) and eventually, the RPM will "pop" up above it's 1900 RPM limit and run at higher speeds. The second variable occurs regardless of throttle body position (partially open, halfway open, or full throttle).

***This only occurs after the rough idle issue starts in may-july, the rest of the time there is not shuddering, misfire feeling, or poor idle***

 

[BenDT]

Definitely spray for vacuum leaks. I suspect something IAC, temp sensor, TPS related. make sure nothing is leaking, can’t say that enough especially warmer weather where hoses are soft. A dirty iac with a failing tps could definitely give some funky readings. IAC May be able to function just barely in spec but the tps May not differentiate the excess air from throttle input. Can also say that a new coil does wonders for a sohc engine. Even when they run “fine” if they are due for one go ahead and pony up. The mpg improvements midrange will pay for the maintenance itself. Failures are rare from what I understand but just a thought if nothing seems to be jumping out, they too act up when warm more so if they do fail.

 

[JustAnotherMember7]

Definitely spray for vacuum leaks. I suspect something IAC, temp sensor, TPS related. make sure nothing is leaking, can’t say that enough especially warmer weather where hoses are soft. A dirty iac with a failing tps could definitely give some funky readings. IAC May be able to function just barely in spec but the tps May not differentiate the excess air from throttle input. Can also say that a new coil does wonders for a sohc engine. Even when they run “fine” if they are due for one go ahead and pony up. The mpg improvements midrange will pay for the maintenance itself. Failures are rare from what I understand but just a thought if nothing seems to be jumping out, they too act up when warm more so if they do fail.

I do have an extra coil pack from the engine swap so If have verified everything else is good, I can just swap out the coil pack and see what happens. Both are OEM though. I highly doubt it is the coil pack, but it could be something worth trying AFTER the IAC is cleaned and I check for vaccuum leaks, check spark plug wires, check spark plugs, e.t.c.

I'm off to the store in just a minute and will update y'all on my progress. Thanks.

 

[J_C]

You're stating issue "seems" to be a misfire but also have this "shuddering at idle (dips down to 400RPM"
Going with shuddering / idle dropping to barely staying running at idle I would check / clean IAC and replace gasket(s) --(my ohv has TWO IAC gaskets...nor sure about your SOHC)
I would inconsistently have stalling / dropping idle at stops until I replaced the "other" -- rubber o-ring style gasket that's embedded into the plastic intake manifold.....had new IAC and new cardboard gasket already on.

Oh my SOHC, I noticed the way it's made, looks like the silicone o-ring style gasket will seal fine by itself, as long as it is in good condition still, but if fatigued or ripped, it can be removed and a traditional fiber gasket put on in its place, or if not fatigued or ripped, you could even put a fiber gasket on in addition to leaving it on, but if it is fatigued or ripped, don't use both the old o-ring gasket and a fiber gasket as the old o-ring gasket may then make it leak.

Referring to other random posts...

Spare parts to swap on like a coil pack, I'd wait until you hook up a scan tool to see if any readings look out of bounds. What does "spark plug wires tested" mean? If they are the original wires, now ~20 years old, I'd have replaced them with the plugs, and would not manipulate them at all to test as they can get brittle and be fine until disturbed, and likewise I wouldn't disturb them to put a different coil pack on. In other words I would replace them the next time I touched them, again if they are the originals, or over a decade old (depending on mileage and environment).

 

[JustAnotherMember7]

7/29/2021 UPDATE:

I have been driving the vehicle for the last month after cleaning the IAC valve. I waited to post any results so that I could test the vehicle.

The poor idle happened again yesterday and the engine actually threw a code and the check engine light came on.

Here is the code: P0172

I have been getting 87 Octane fuel from the same station (Maverick) for the last 3 years.

About 3 weeks ago I put in fuel injector cleaner (into 17 gallons) and have been running the same tank of gas since then.