94 intermittent power loss

[hoosiercheetah]

1994, 2.3L, 138,000 mi. Manual transmission. I just did plugs, wires, air filter, and oil change a few weeks ago. I only drive about 10 miles a day.

This started about a week ago, when we had really cold temperatures.

Engine starts fine. Well, she struggles a little if it's well below freezing. But otherwise starts fine.

While running, the engine sometimes looses power. When it does this, the sound of the engine changes. I think that the rpm does not drop off, although because the power is weak, the engine and truck decelerate. When it happens, the tone of the engine drops low, as if I had shifted up too early and the engine is bogged down. But it's not quite the same as that. It's hard to describe. It will continue to run. It doesn't feel rough or choppy, just lower sound and loss of power.

Sometimes it will do it while the truck is idling. When this happens, it will almost stall out, but so far it hasn't cause the engine to stop.

Based on what little I know, it feels like an ignition problem. Maybe one of the coil packs is failing, so that I'm intermittently losing a row of plugs? Or maybe some kind of ignition module problem? I'm just guessing. I think it's not fuel, because it does continue to run when it's loosing power.

Any help that anyone can offer would be greatly appreciated.

 

[Mark_88]

That was my first thought...coil pack may be failing...

did you try pulling codes? There may be an issue with several things and the codes would be the best place to start.

 

[hoosiercheetah]

That was my first thought...coil pack may be failing...

did you try pulling codes? There may be an issue with several things and the codes would be the best place to start.

I have not pulled codes. The check engine light has never come on. It's the older system, I think it's called eec-IV? I don't have a scanner for it. Would autozone be able to scan it?

 

[Mark_88]

Oh, right...forgot about that...maybe check with them...

The How To Tech section has a document on how to test with the volt meter...

I'm not familiar enough with the older systems to know if they show pending codes or anything...just went through the codes section and didn't see anything about pending...

Here is the test method if Autozone can't check them for you...

http://www.therangerstation.com/how-to/ignition-charging-computers/testing-eec-iv-equipped-engines/

I wouldn't recommend swapping parts unless you are using Junkyard parts...I've swapped in enough "used" parts to know they can work but it can make your life very challenging doing things that way...but it might help to try grabbing a coil pack from a yard if there is one available and see if it fixes the problem.

the other thing I thought about is a damaged wire harness that is shorting out sporadically. Seen that happen and it can take a long time to find the problem...but the best visual inspection is to see if anything looks fried or got too close to a heat source...melted plastic or sheathing is a good indicator...just a pain in this weather though,...

 

[tomw]

Can you swap the coil packs and see if the problem follows? Given an 8-plug system, one side is used 100% of the time and the other is intermittently enabled. If you have misfire problems, it is likely the passenger side coil rather than the other if the coil is the fault source.
I would also check that the coils are grounded properly, which is necessary for them to function properly.
tom

 

[hoosiercheetah]

Can you swap the coil packs and see if the problem follows? Given an 8-plug system, one side is used 100% of the time and the other is intermittently enabled. If you have misfire problems, it is likely the passenger side coil rather than the other if the coil is the fault source.
I would also check that the coils are grounded properly, which is necessary for them to function properly.
tom

So the passenger side (exhaust side) plugs are the ones that actually run the engine?

How would I test the coils for ground?

More information: I was just under the hood while it was idling. Whenever the bog-down happens, the engine does run rougher and shakes around a bit, and I'm getting a strong fuel smell in the exhaust. So I'm definitely thinking ignition. I found some instructions online for troubleshooting the ignition module, but it wants me to disconnect the fuel injectors one by one, in order to determine which cylinders aren't firing. I can't get to the fuel injector connectors to disconnect them. Is there another way to determine if the module is bad?

 

[hoosiercheetah]

Okay, so I probed the signal wires of the coil pack that feeds the exhaust side plugs, with the engine running and the problem happening. At one of the signal wires, I was getting .20 volts steady. At the other wire, it was jumping up and down between .23 and .25 volts. I think this means that one side of the coil is getting signal from the module, but the other side is not. Does that sound right?

 

[tomw]

Quote:At one of the signal wires, I was getting .20 volts steady. At the other wire, it was jumping up and down between .23 and .25 volts.

The signal wires actually provide ground to the coil. There is battery power 'there' all the time with the ignition ON, and the computer provides a ground, completing the coil primary side circuit. When the computer wants to fire the coil, it removes the ground. When the ground goes away, the coil(primary winding) magnetic field collapses. When the field collapses, the secondary(high volt) windings are cut by the field lines of force(theory says), causing electron flow through the wires, and it SPARKS!!!.
So, you should see fluctuating ground(low/zero) voltage on one of the coil leads, BUT you'll see voltage on the 'other' end of the winding from the ignition source. Monitoring the volts on the switching side of the coil is not straight forward, unless the engine is still, but you report no flux on one connector, which indicates it is not going to fire anything.
Ground is provided to the coil by the mounting bolts. Sounds contradictory to what I just typed, but for some reason the coils like to be grounded. If all the bolts are there, and the coil mount is not rusty, don't worry too much about it.
I would try swapping coils before replacing one.
tom

 

[hoosiercheetah]

I put in a new ignition module today. I let it idle up to temp and drove it around the block twice, and the problem seems to be solved. I'll report back after a few days.

Thanks everyone for your help and input!

 

[hoosiercheetah]

Quote:At one of the signal wires, I was getting .20 volts steady. At the other wire, it was jumping up and down between .23 and .25 volts.

The signal wires actually provide ground to the coil. There is battery power 'there' all the time with the ignition ON, and the computer provides a ground, completing the coil primary side circuit. When the computer wants to fire the coil, it removes the ground. When the ground goes away, the coil(primary winding) magnetic field collapses. When the field collapses, the secondary(high volt) windings are cut by the field lines of force(theory says), causing electron flow through the wires, and it SPARKS!!!.
So, you should see fluctuating ground(low/zero) voltage on one of the coil leads, BUT you'll see voltage on the 'other' end of the winding from the ignition source. Monitoring the volts on the switching side of the coil is not straight forward, unless the engine is still, but you report no flux on one connector, which indicates it is not going to fire anything.
Ground is provided to the coil by the mounting bolts. Sounds contradictory to what I just typed, but for some reason the coils like to be grounded. If all the bolts are there, and the coil mount is not rusty, don't worry too much about it.
I would try swapping coils before replacing one.
tom

Yeah, the magneto ignition on an aircraft engine works basically the same way, except that there's a rotating permanent magnet instead of an external trigger circuit. Very reliable tech.