SOHC V6 cylinder 1 misfire

OBDII shows a misfire on cylinder 1. When I put a timeing light on any cylinder's spark plug wire the timeing light flashes intermittently at idle and goes out at higher revolutions. Are timeing lights supposed to not be able to light at higher revolutions? And what does intermittent ignition on all cylinders have to do with only a cylinder 1 misfire?

Waste spark ignition system

Your Explorer has a waste spark ignition system. The spark plugs are paired and each pair fire at the same time - one on the compression stroke and the other on the exhaust stroke. Current flow on one plug in the pair is in the opposite direction as current flow on the other plug.

Because the pressure is low during the exhaust stroke compared to the pressure during the compression stroke the voltage drop across the exhaust stroke spark plug is only about 1/5 the drop across the compression stroke spark plug. That may affect the reliable operation of your timing light. Also, if your timing light is polarity dependent it may only work on the plugs in one bank but not the other.

Have you checked the spark plug in cylinder 1 for signs of misfire? If you remove the plug for inspection or replacement I suggest that you measure the compression while the plug is out.

Thank you very much, 2000StreetRod. I would never have guessed such an arrangement.
I replaced the #1 plug, but it was hard to get a pressure reading. That made no difference,
It's hard to hold a rubber gauge there and even to screw a threaded gauge in. The best it could get was 110lbs. pressure. This was my first engine check light episode. The car runs normally, but at stop lights it is starting to get rougher as time goes on. The mpg hasn't changed either.

My OBDII also shows:
fuel sys 1 CL
fuel sys 2 0
It has always done this.

STFT B1 occasionally goes out of rangs, (above 5)
STFT B 2 does the same
That was new since this check light episode.
The freeze frame of the initial trouble was taken at about 64 miles/hour.
Does this give any clue about how to eliminate the check light or fix the problem?

low compression

110 psi is very low but it may be because you did not have a good seal. Did you have the throttle plate full open? I usually crank the starter until 5 compressions. There usually is no increase between compression 4 and 5. 180 psi is typical for a warm engine in good condition.

Apparently you have some scan capability instead of just a code reader. STFTs normally have a range of +/- 30%. What do you mean by "out of range (above 5)"? If STFT for one bank is habitually above or below a certain % (maybe 10) the PCM will adjust the associated LTFT to get the STFT near 0. When the LTFT gets above a certain % and the STFT is above a certain % the PCM sets a DTC (lean or rich) depending on the LTFT. I suggest that you read the LTFT for both banks.

This thread may be useful: Engine misfire procedure

2000StreetRod, you are correct. The STFT should deviate no more than 10% for this SOHC engine. Both Bank 1 and 2 do that intermittently at about the same frequency as the timeing light's occasional irregularities to a constant blinking rate. LTFT is pretty constant. I'm now guessing that it's the fuel injector if the compression turns out to be close to 180lbs. First I'll try fuel injector cleaner. Then compression if I can get the threaded connection tight in the cylinder. I think the intake manifold has to be removed to change a fuel injector.
Thank you,
field

Oh, and I changed the fuel filter and fuel pump about 3000 miles back. What a job! Had to get rid of 20 gallons that were in the tank. A friend took them.

Hi 2000Street Rod,

The fuel injection cleaner had no effect on the misfire reading.

1. With the pressure gauge in tight on the OBDII designated cylinder #1 misfire, the needle fluxuated around 65 psi while the engine was running. I've worked only on carburetor engines before and don't know how to crank an injector engine without starting it. Where do you short to the starter?

2. Then the same result surpriseingly happened on randomly chosen cylinder #5 .

The Explorer still drives like it always has.

Dose these readings have a reasonable interpretation?

MY OBDII is an Autor brane D900.
The pressure gauge has no brand on it.

up to 200 miles

Fuel injector cleaner can take up to 200 miles for full effect. I only use Techron.

You can just disconnect the main connector to the coilpack to eliminate spark and pull the fuel pump relay to eliminate injection. I installed a remote crank switch under the hood.

I've never performed a dynamic compression test but one of my books indicates that the readings often are 50 to 75% less than those for a cranking test. If your compression is approximately 65 psi that could be reasonable. The reading should be no less than 85% of the other cylinders.

You are really very helpful 2000StreetRod.

I used techron too and still need more milage to be sure then.

Correction: my OBDII is an Autop model D900 that shows about 20 live data points and the freeze frame of those points when a DTC occurs.

System bank #1 always shows CL (closed loop)
System bank #2 always shows nothing (presumably meaning no closed loop)

This seems counter-intuitive since the cylinder #1 trouble causing the ECL is on bank #1

Does that provide any clues?

I got my 1999 SOHC XLT in 2010 at an auction with 160,000 miles for $1500 and found that it had been owned by a Sports medicine doctor who took good care of it except he must have parked it faceing downhill. Starting in that position causes the fuel to flow to the front of the tank and the pump to suck only air. Then the pump over heats without fuel cooling and burns out sooner.

So it's only two problems in the last four years were replaceing the pump and trying to get rid of the misfire DTC.
Plus the CD player will not accept another CD until about 12 hours elapse since the last one has been removed. Guessing the cause of that over taxes the imagination.

both banks the same

field said:

. . . System bank 1 always shows CL (closed loop)
System bank 2 always shows nothing (presumably meaning no closed loop). . .

Click to expand...

I think the PCM is either in open loop or closed loop for both banks. My SCT Power Flash X3 lists only PCM loop status.

Try disconnecting and reconnecting the crankshaft position sensor connector a few times to clean the contacts. Because of its location it is exposed to water and road grime.

After all those working attempts to solve the #1 misfire problem, it's finally fixed. So easy.

I disconnected the battery for several hours. After reconnecting it the check engine light stays off. OBDII shows no DTCs!

Hello 2000StreetRod,

Apparent coolant leak

My 99 SOHC 4.0L engine has been leaking about a quart or two a week. No dripping is visible anywhere. So I borrowed a pressure tester and pumped it up to the 16psi listed on the radiator cap. Water then seemed to pour all over from the front of the engine, but no particular leak site was visible. After that I could not pump more than 5psi no matter how fast and hard I pumped. It looked like the first pumping had unsealed something. But driving the vehicle after that again showed no dripping anywhere. This seems like an inconsistency, how does that copious leak reseal itself? Could the coolant pressure always be way below the 16psi radiator cap limit?

Immediately after normal driving, there has never seemed to be any significant pressure released when the radiator cap is removed. High pressure release is expected for most cars. There are warnings about that everywhere.

There is no white smoke in the exhaust.

The temperature gauge has always and still is centered in the middle of its range while driving.

But there is still and unfound slow leak.

defective radiator cap?

Perhaps your radiator cap is defective preventing pressure from building. Also, for an engine with no leaks the coolant reservoir in conjunction with a functional radiator cap will eventually purge all air from the cooling system making it more effective and eliminating the obvious release of pressure when the cap is removed.

The hand pump pressure tester has proven there is an external leak when the cooling system is pressurized. I don't know why it didn't start to leak until 16 psi but then continued to leak at only 5 psi. Maybe there was a lot of air in the system that had to be purged thru the leak until the coolant reached the leak and became obvious.

It is not advisable to continue to drive the vehicle with a known coolant leak. The thermostat housing location is high relative to the engine block. If the coolant level drops enough that the engine coolant temperature sender is not immersed in coolant it will not provide an accurate temperature reading on the gauge in the instrument cluster. The gauge may indicate a cool engine when it is actually overheating.

It is difficult to purge the air from the SOHC V6 cooling system. Some members have overheated their newly rebuilt engine due to air in the cooling system.

Other than loose or split coolant hoses and a failed radiator the most common coolant leak is from a worn seal in the water pump. Water leaks out the weep hole in the underside of the pump. The next likely leak source is a failed thermostat housing. The plastic housing distorts and splits with age. Also, there can be leaks between the plastic housing and the brass threaded inserts for the temperature sensors.

Thanks again 2000StreetRod for a comprehensive reply.

Your 2 quoted paragraphs below are the most likely to lead to a solution.

"Perhaps your radiator cap is defective preventing pressure from building. Also, for an engine with no leaks the coolant reservoir in conjunction with a functional radiator cap will eventually purge all air from the cooling system making it more effective and eliminating the obvious release of pressure when the cap is removed.

The hand pump pressure tester has proven there is an external leak when the cooling system is pressurized. I don't know why it didn't start to leak until 16 psi but then continued to leak at only 5 psi. Maybe there was a lot of air in the system that had to be purged thru the leak until the coolant reached the leak and became obvious."
***********************************************************************

After a water refill to the reservoir, driving the car soon causes the radiator to fill completely. I don't understand how the reservoir refill system works. The rubber tube from the reservoir enters the radiator above where the radiator cap sealant ring seals the coolant in the engine/radiator system. It would seem that would prevent both air and water from entering or leaving the system, unless the pressure exceeded 16psi which it apparently never has in my system. I understand how a cooling engine could suck something back, but it would have to suck back through a tight radiator cap.

There was a lot of air in the system after the 16psi hand pump blow out when only 4 or 5psi caused more leaking. All that was done with the engine off. But everything seemed to be resealed when the engine was turned on again except for the very slow lead that shows up again after days when the reservoir is lower again.

So, I'll try for a new cap to see what happens.

two valve/spring sets

There are two valve/spring sets in a radiator cap.

One set is for relative pressure of the radiator compared to atmospheric. The other is for relative vacuum of the radiator compared to atmospheric. Even if the radiator doesn't build enough pressure to open the pressure relief valve it will still be greater than atmospheric. When the radiator cools enough vacuum may be created to suck coolant from the reservoir into the radiator. Eventually, all of the air in the radiator is replaced with coolant.

Another feature I would never have guessed. I will have to look more closely at apparently inconsistant objects in the future.
Thank you,
field

Field, just curious if you have recently physically examined the spark plugs? Of course since everything started with #1 that's the one I would check first. When it leaks coolant was it in the area of the front head by cylinder #1 ? I too was under the assumption that if no white smoke came out the tail pipe then it wasn't a bad head gasket. On my 4.0 SOHC I chased a misfire problem and thought it was a bad injector. Upon taking off the intake manifold to change the injector, I was surprised to see a coolant leak in the area of Cylinder 1. It was a bad head gasket that allowed the coolant to slowly leak into the head intake valve but didn't leak into the engine oil at all. The PCM had been compensating for the misfiring of a wet spark plug from the leaking head gasket. Mine's also had a mystery slow unknown coolant leak before that I couldn't find earlier. I may be way off in your case, but I just wanted to let you know what I experienced.