So what do the experts think?

[koda2000]

This morning I finished installing the new fuel pump (with new strainer and fuel filter) in my '97 Sport. The one I removed was the original '97 OE fuel pump with 260,000+ miles on it, so it was overdue for changing.

I tested the new pump before completely reattaching the fuel tank and heard it run, so all good there. Finished reinstalling the fuel tank, refilled the tank 1/2 way, primed the pump and still - no start...

I knew this was a possibility, as the old pump worked one day, no start the next 30 degree morning, then after me fiddling with fuses, relays and electrical connections (and the temp reaching the mid 50's) it suddenly decided to start and continued to start normally, several times a day for the next 6 days. It even started right before I dropped the tank to change the fuel pump.

Today, after numerous attempts to start the engine (hearing the f/p run each time) I decided to remove the cam position sensor and test it. I don't know if my test was valid, but I set my VOM on continuity, attached the leads to the 2 terminals and ran a magnet over the bottom and got a continuity chirp, so I'm thinking it's working.

Next I pulled a plug wire, installed an old spark plug in the wire and grounded it to the battery with my jumper cables, turned the ignition key to ON and jumped the fender-mounted starter solenoid to spin the engine. Saw no spark.

So, I apparently have an intermittent electrical problem. I swapped the PCM relay and pulled the PCM plug to see if I saw any corrosion (didn't) wiggled the coil connections and retested. Still no hint of a spark.

Lastly, I decided to check the connection on the crank position sensor. It had started raining and without having the front end up on ramps I could barely see or touch the plug, but I did push on it with a finger. Then I jumped the starter solenoid with the key ON one last time - saw spark and then the engine started running on the 5 connected cylinders. I then reattached the spark plug wire. Started the engine again and drove the truck up on ramps. I plan to do further examination of the crank position sender/plug tomorrow (when the rain is supposed to stop).

BTW: When I got the engine to start last Monday, I never went near the crank position sensor.

So what is your best guess as to what this apparent intermittent spark related electrical problem is?

 

[colintrax]

You reconnected the cam position sensor correct?

Have you checked the coil is receiving a signal? If not I'd start there.

 

[koda2000]

You reconnected the cam position sensor correct?

Have you checked the coil is receiving a signal? If not I'd start there.

I had it running afterwards, so yes I reconnected the cam position sensor. I'm pretty sure the coil was not receiving a signal. The problem with these types of intermittent electrical problems is that once it suddenly fires up and runs perfectly, it ends the trouble shooting session.

Edit: I just noticed that my BATT gauge looks like it's lower than mid point when the engine is running. I think I remember reading that a bad alternator might throw a "dirty" signal and that might screw up the signal from the cam position sensor. I'm going to go check the voltage at the battery while running and see what that says.

Update: Even though the gauge does appear low (I don't normally drive this vehicle) the voltage measured across the battery with the engine running is 14.5-14.6 volts. So I guess that rules the alternator out, or does it?

 

[colintrax]

I understand, intermittent problems are a pain in the butt. Next time it doesn't start I'd back prob the coil just to verify.

I'd say the battery and alternator are fine. Guess it wouldn't hurt to swap them in a different explorer and see if that cures the problem, and causes problems for the other.

 

[drdoom]

Koda I would guess the CKP sensor plug.

 

[koda2000]

Koda I would guess the CKP sensor plug.

That's what I'm thinking/hoping. I'm going to start there tomorrow. Right now it's starting every time, but I can't trust it as it fails intermittently. I'll let you know what I find (if anything).

This is the vehicle my son-in-law drives 45 miles a day for work. So it has to start reliably. I don't want to have to drive all the way down to where he works if it fails on him. If it's not the CKP sensor/plug I don't know what I could do about it anyway. So far it's only failed in the driveway (twice), which is convenient.

I've done some reading, but it's not clear to me which sensor (CKP, CPS) does what (fuel injectors vs coil). It seems like the CKP is responsible for spark, but it's a bit murky.

 

[colintrax]

For what it's worth, when the camshaft sensor went bad in my 4.0 OHV the truck still ran. It was beyond rough and sucked down gas. But it drove for a couple thousand miles before I replaced the sensor (and now destroyed plugs)
Not sure if that gives any insight to how the SOHC controls spark though

 

[1998Exp]

The crankshaft sensor (whatever the proper acronym to describe it...) initiates the spark. Because of the waste-spark system, it does not matter which cylinder is on compression and which on exhaust. However, to avoid spraying fuel into a cylinder on the exhaust cycle, injection is timed by the camshaft (half the speed of the crankshaft).
Correction: it's into manifold next to the cylinder, not the cylinder itself (no direct injection here), but you get the picture.

...
I've done some reading, but it's not clear to me which sensor (CKP, CPS) does what (fuel injectors vs coil). It seems like the CKP is responsible for spark, but it's a bit murky.

 

[Explodr94]

If this doesn't narrow down to a sensor, I have had a few repairs boil down to faulty replacement part. Rare, but it happens.

 

[koda2000]

If this doesn't narrow down to a sensor, I have had a few repairs boil down to faulty replacement part. Rare, but it happens.

I haven't replaced anything related to spark. I had a '71 Road Runner that had a first gen of Chrysler's transistorized voltage regulator. The regulator went bad and I replaced it 3 times. All went bad w/in a month. 4th replacement finally worked. So, yes it does happen.

 

[koda2000]

The crankshaft sensor (whatever the proper acronym to describe it...) initiates the spark. Because of the waste-spark system, it does not matter which cylinder is on compression and which on exhaust. However, to avoid spraying fuel into a cylinder on the exhaust cycle, injection is timed by the camshaft (half the speed of the crankshaft).
Correction: it's into manifold next to the cylinder, not the cylinder itself (no direct injection here), but you get the picture.

That's what I thought. Thanks for clearing that up for me.

 

[koda2000]

Update:
I removed the crank position sensor this morning (a 3 band aid job) cleaned 19 years of dirt and grease off it, tested it with my VOM and a magnet and it seemed to be okay. There was no corrosion on the plug and the wires looked okay and undisturbed. I thought about going to AutoZone and getting a new one for $15.99, but other than the grease and dirt build up on the old one, I don't see how it could be bad. I cleaned the wheel on the crank damper, put some dielectric grease in the plug and put everything back together using a business card as a spacer between the sensor and the reluctor wheel. It started right up, but that doesn't prove anything.

I'm going to let my son-in-law start driving it again tomorrow and we'll see what happens. Time will tell (fingers crossed...). At least I can tell him what to wiggle if it acts up on him.

Thanks for the help & suggestions.

 

[koda2000]

Well, (not to jinx it, but...) it's been about a week and a half and since removing, testing and cleaning the crank position sensor and re-seating the electrical connector it's been starting every time. So I claim a cautious victory.

 

[drdoom]

Good job Koda.

 

[rb142]

The crankshaft sensor (whatever the proper acronym to describe it...) initiates the spark. Because of the waste-spark system, it does not matter which cylinder is on compression and which on exhaust. However, to avoid spraying fuel into a cylinder on the exhaust cycle, injection is timed by the camshaft (half the speed of the crankshaft).
Correction: it's into manifold next to the cylinder, not the cylinder itself (no direct injection here), but you get the picture.

Right idea but not quite correct. Everything (spark and fuel) is timed from the crankshaft sensor. That is the source of engine position. The only thing the cam sensor does is to distinguish whether you are on the first or second rev of the cycle. Without the cam sensor, it will guess which rev it is on and keep running. The cam signal (on these systems) is not accurate enough in position to time anything. So the cam synchronizes to the revolution, but the rotational position within the rev comes from the crank, and that position controls spark and fuel.


Koda -- check the chassis ground connections too.

 

[1998Exp]

Thanks for the correction. Live and learn!

Right idea but not quite correct. Everything (spark and fuel) is timed from the crankshaft sensor. That is the source of engine position. The only thing the cam sensor does is to distinguish whether you are on the first or second rev of the cycle. Without the cam sensor, it will guess which rev it is on and keep running. The cam signal (on these systems) is not accurate enough in position to time anything. So the cam synchronizes to the revolution, but the rotational position within the rev comes from the crank, and that position controls spark and fuel.

 

[decipha]

my sister daily drives a 96 explorer, a few years back she had an intermittent no start issue, it would appear at random times, she's been stuck at a gas station before due to a no start as well as in front friends and familes houses, etc.. its even killed on her a few times when driving, until eventually it wouldn't start at all, her boyfriend diagnosed the fuel pump was faulty so i went over there to give him a hand replacing it and when i turned the key on to put it in neutral the pump primed ???

upon further investigation the fuel pump relay was on its way out, replaced it and hasn't had a problem with it since about 2013-ish