98 Explorer 4.0 OHV Can't find the misfire! PLEASE HELP!

[K.Elliott]

1998 Explorer Sport 4x4 104,000 miles
4.0 OHV 5 Speed Automatic
I've had a problem with a serious misfire that randomly started a few weeks ago.There are NO codes or CEL
Truck starts fine and idles good, I can accelerate at grandpa speed and all is good.
As soon as I open the throttle 50% or more I get a misfire. Also when the engine is under load it will miss
It wont really get over 55 mph or pull a hill well. It shakes the entire truck and I can hear it popping in the exhaust.
This truck has been seriously taken care of its whole life.
It has all new sensors and parts including:
MAF
IAC
EGR
Air Temp Sensor
O2 sensors,both upstream and the one downstream
Camshaft Position Sensor
Water Temp Sensor
Fuel pressure regulator
Fuel Filter
All fuel injectors have been checked and serviced
Coil Pack
Spark Plugs
Plug Wires
Air Filter

Fuel Rail pressure is 30 psi at idle, 35 at 2,500 rpm
Cylinder Compression is 150-160 psi all cylinders
I pulled all my spark plugs and they all are a nice light tan color indicating a perfect burn. No cracks in the porcelain.
However I pulled a plug on #4 right after I drove it and I had wet gas on the plug. I replaced the plug (which was only 5,000 miles old). No change. Not sure what is causing this or how to fix it. Obviously its dumping to much fuel.
I've work on street cars and race cars for 18 years and I've never had one stump me like this. Hopefully there is a simple fix I've overlooked. This is the second explorer I own. The other is a 94 4.0 ohv in which I totally rebuilt. The 98 only has a few differences in the electronics and obviously this is what is stumping me.

 

[koda2000]

It must be fuel or spark related. Have you tried misting water around the spark plug wires at night to see if you see any electrical arcing? If there is no problem with the plug wires or spark plugs, I'd be suspecting that perhaps the #4 fuel injector is leaking. Have you checked the hole that the #4 on the coil isn't corroded? I once had an engine miss under a load, even after replacing the spark plugs and wires and it turned out to be rust in the coil terminals. I've also found arcing plug wires and cracked spark plug insulators by misting water on the engine.

 

[K.Elliott]

It must be fuel or spark related. Have you tried misting water around the spark plug wires at night to see if you see any electrical arcing? If there is no problem with the plug wires or spark plugs, I'd be suspecting that perhaps the #4 fuel injector is leaking. Have you checked the hole that the #4 on the coil isn't corroded? I once had an engine miss under a load, even after replacing the spark plugs and wires and it turned out to be rust in the coil terminals. I've also found arcing plug wires and cracked spark plug insulators by misting water on the engine.

I have swapped out the coil. All the coil terminals are like new. I could try the water trick, the passenger side will be most difficult to see around all the a/c components. I had the injectors professionally tested and all were good.

 

[Runnin'OnEmpty]

1998 Explorer Sport 4x4 104,000 miles
4.0 OHV 5 Speed Automatic
.....Fuel Rail pressure is 30 psi at idle, 35 at 2,500 rpm
........

That fuel pressure is very low. I suspect you have a faulty fuel pump or a split in the soft hose connected to the pump output.

 

[1998Exp]

That fuel pressure is very low. I suspect you have a faulty fuel pump or a split in the soft hose connected to the pump output.

Methinks you are judging by specs for a returnless system. The '98 has a return system, so 30 at idle and 35 at 2500 rpm (when vacuum drops) seems normal.

 

[koda2000]

Fuel pressure sounds right for a return-type system, which I believe the '98 still was.

 

[boominXplorer]

Spec is actually 36-40 (should sit right at 38). Under load you might be dropping lower than what you see revving it sitting still. Does it fire right up on first crank?

 

[koda2000]

Spec is actually 36-40 (should sit right at 38). Under load you might be dropping lower than what you see revving it sitting still. Does it fire right up on first crank?

A lot depends on the accuracy of your pressure testing gauge. In any event 30-35 isn't low enough to cause a misfire.

 

[fast_dave]

Hey K.Elliott,

KODA2000 and 1998 EXP are spot on regarding that a 1998 EXO with a 4.0 OHV has a return fuel system (aka 2 Line Fuel System) and 30lbs at idle is A-OK. I can verify their info because this past weekend I installed a new Fuel Pump (TYC Brand from Rock Auto) in my 4.0 OHV and that's the pressure my new pump was putting out.

You can test your pump further for a Wide Open Throttle condition by having the engine running, the fuel pressure gauge connected, and disconnecting the Vacuum Line from the Fuel Pressure Regulator, located next to the Schrader Valve - your fuel pressure should spike up to right around 40lbs.

If you get those fuel pressures - your fuel system is A-OK for a 4.0 OHV with a return fuel system...

For Reference, my original original fuel pump was getting weak = 28lbs pressure output after 20 years/201,000 miles. But what got me to change it was the following condition: the "check valve" within the pump that holds line pressure for the next start was showing signs that it wasn't sealing after turning the ignition off. The recent cold weather in CA (30's and 40's) made it so that I had to turn the ignition on to the first position, wait a few seconds for the pump to prime the fuel rail, and then turn the ignition key to get a reliable cold engine start the first turn of the key. If I didn't perform that "workaround" operation, on a cold start, I'd have to have the engine spin about 7-8 revolutions before she fired over.



Fuel Pressure Regulator Pic

BUT at the stage you're at, based on the list of parts that you've thrown at your rig ;-)
I'm going to suggest trying to keep this simple -

• Looking at your signature line I see you have 104,000 miles on your Ex.
• You are NOT throwing a CEL - but your rig is bucking like it has a fuel related problem or a ignition mis-fire condition.
• Ignition related Mis-Fires on Ex's Usually/Eventually throw a CEL - and you aren't getting one - so, with that in mind, I've heard, you're getting pretty cold temps in FL lately.
• 100,000 miles is about the life for Upper Intake Manifold Gaskets.
• Air slipping past these upper intake gaskets typically does NOT throw a CEL - hence the reason why guys scratch their heads wondering what is going on.
  
• Any air slipping past these gaskets will cause the computer to go into a RICH condition.
• COLD TEMPERATURES are the condition that makes this happen/rear it's head.
  
• If you haven't installed new Upper Intake Manifold Gaskets, OR, if you have, you need to go back and re-torque them (when the engine is cold).
• The font and rear intake nuts (on the studs) are usually the ones that get loose, since there is only one (1) nut holding down the intake manifold. The "middle section" of the intake is held down with two (2) nuts.

This pic will show/clarify the stud/nut arrangement holding down the intake manifold

And here's a pic of the offending gaskets that have a 100,000 service life - you need (3) total

 

[96eb96]

It wont really get over 55 mph or pull a hill well. It shakes the entire truck and I can hear it popping in the exhaust.
This truck has been seriously taken care of its whole life.


However I pulled a plug on #4 right after I drove it and I had wet gas on the plug. I replaced the plug (which was only 5,000 miles old). No change. Not sure what is causing this or how to fix it. Obviously its dumping to much fuel.
I've work on street cars and race cars for 18 years and I've never had one stump me like this. Hopefully there is a simple fix I've overlooked. This is the second explorer I own. The other is a 94 4.0 ohv in which I totally rebuilt. The 98 only has a few differences in the electronics and obviously this is what is stumping me.

I am going to wager you have a slight crack in the head. It may open under load and a compression test wont show it. Download forscan and monitor misfire counts on Cyl 4. I have the same engine,similar problem, and K-seal fixed it 100%. It can go that way for a very long time. I'm surprised you don't have a flashing CEL, there may be different thresholds per year.

I have some rust on mine, so I decided to take that sealer route, I have no issues. New heads are inexpensive and an easy repair.

Also, hold down the gas pedal all the way when its off, crank the engine, and see if it is smooth. This is the test mode (sense vibrations) and the truck will not start. Do it when it is hot.

 

[koda2000]

I am going to wager you have a slight crack in the head. It may open under load and a compression test wont show it. Download forscan and monitor misfire counts on Cyl 4. I have the same engine,similar problem, and K-seal fixed it 100%. It can go that way for a very long time. I'm surprised you don't have a flashing CEL, there may be different thresholds per year.

I have some rust on mine, so I decided to take that sealer route, I have no issues. New heads are inexpensive and an easy repair.

Also, hold down the gas pedal all the way when its off, crank the engine, and see if it is smooth. This is the test mode (sense vibrations) and the truck will not start. Do it when it is hot.

Interesting hypothesis. Could be, knowing the propensity for the 4.0L heads to develop cracks. .

 

[Runnin'OnEmpty]

Methinks you are judging by specs for a returnless system. The '98 has a return system, so 30 at idle and 35 at 2500 rpm (when vacuum drops) seems normal.

My apologies. I was indeed thinking of the returnless system.

 

[1998Exp]

A lot depends on the accuracy of your pressure testing gauge. In any event 30-35 isn't low enough to cause a misfire.

To clarify, fuel pressure is regulated against manifold pressure, not atmospheric air. The regulator has a vacuum connection for that purpose. At idle the manifold is at high vacuum (about negative 10PSI), so you should see about 10 PSI less than at full throttle, when manifold vacuum is very low. If you want to test more accurately, disconnect the vacuum tube from the regulator. With no vacuum connection there shouldn't be much change - if any - between idle and full throttle. If it drops significantly at full throttle, the pump is weak or there is an obstruction somewhere.

 

[Terkins]

I can accelerate at grandpa speed...
As soon as I open the throttle 50% or more I get a misfire. Also when the engine is under load it will miss
It wont really get over 55 mph or pull a hill well. It shakes the entire truck and I can hear it popping in the exhaust.

Clogged catalytic converter? Backpressure test?

 

[Dono]

My 4.0 OHV had exactly the same symptoms.
What I learned was that I could replace the spark plug (yup, just the one), and be fine until the first time I was hard on the gas. Then I'd be be back to the misfire until I changed the spark plug again
I can't remember examining the porcelain on the spark plug, but I think when I'd hit the gas, coolant would spit on the spark plug and crack the porcelain. Once that happened, the plug would not fire properly and I'd be back to a misfire.

I'm going with cracked head. Mine was. Right between the valves. Very small crack. I hope I'm wrong, but it sure sounds like the same thing.

 

[96eb96]

My 4.0 OHV had exactly the same symptoms.
What I learned was that I could replace the spark plug (yup, just the one), and be fine until the first time I was hard on the gas. Then I'd be be back to the misfire until I changed the spark plug again
I can't remember examining the porcelain on the spark plug, but I think when I'd hit the gas, coolant would spit on the spark plug and crack the porcelain. Once that happened, the plug would not fire properly and I'd be back to a misfire.

I'm going with cracked head. Mine was. Right between the valves. Very small crack. I hope I'm wrong, but it sure sounds like the same thing.

Yup after the K-seal business it would still misfire at certain times. I pulled the plug and it had a nice crack. Switched plugs and the problem moved to the next cyl. I had a coupon, got the new Denso TTs in my sig, runs better than new. I was playing in 4 low the last few days offroad in the snow, lots of high revs getting stuck, no problems at all.

 

[K.Elliott]

Update! I pulled the heads off and both had cracks. They cracked between the valves. Why? Who knows. However i bought two replacement heads and just finished assembling the motor this evening. She runs like a champ. I want to thank everyone for their input and for the folks who said cracked head you were right.