5.9L V8 Gas - Suddenly no power

svdsinner · Aug 25, 2017

My '98 2500 just surprised me with a wierd issue. I had just finished hauling an 11000# trailer from a family camping trip with the horses. Ran great. About an hour into unloading, I unhitched the trailer and ran into town for a pizza. About a mile from home, the engine abruptly started misfiring on all cylinders. It idled fine, but wouldn't rev beyond about 1500rpm. I pulled over, shut it down and took a look. Seemed like the engine was mildly over heating (210F normally doesn't go above 200F), and a bit of coolant had leaked out the radiator cap. After 10 min, I started it back up and it ran fine like nothing had happened.
I took it for a test drive the next morning, with similar results (ran fine until suddenly unable to rev nuch past idle)
ECU showed nothing but misfire codes, and showed a slightly elevated engine temp (208F)

After that, it starts fine, idles fine, and seems to rev fine in my driveway. However, the symptoms return within a block after leaving the driveway.

My guess is either a weak ignition coil, or some kind of ECU limp mode from the engine running hot. I'm going to check the ignition coil on my multimeter this evening.

Will multimeter testing detect a flaky ignition coil or just a dead coil? Is there a better way to smoke out a flaky coil?

If the ignition coil checks out as good, what other culprits should I check out?

Are the mildly high engine temps likely related? (Could the ECU be detecting it without logging a fault and putting the engine in limp mode? No temp faults are getting logged in the ECU)

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WilliamS · Aug 25, 2017

If its running hot enough for coolant to get past the cap there is an issue somewhere.

It drives fine when its not hot, no lack of power or anything? How long does it take to get hot, 2 mintues, 5 minutes, 10 minutes, is it based on driving? Im basically trying to figure out what in the cooling loop is bad, thermostat, pump, or fan.

Yeret · Aug 25, 2017

Two things come to my mind right away: plugged cat or faulty upstream O2 sensor.

Plugged cat will totally kill your power and since exhaust is basically getting stuck in the manifolds, they get quite hot and that extra heat will get absorbed by the engine. Also, if enough exhaust backs up into the cylinders, the fuel/air mixture becomes diluted enough to where it won't ignite. Easiest way to test the cat is let the engine reach operating temperature, then put the tranny in neutral and try to rev the engine past 3,000 RPM. If it hits a wall there or earlier, plugged cat is almost certainly your problem.

Alternatively, remove the upstream O2 sensor from the cat, tie it away securely and go drive around. If the engine runs better, it also means a plugged cat since removing the sensor provides a (small) exit for the exhaust.

Now, the upstream O2 sensor itself can cause a similar problem as well. Easiest way to tell if this is bad is let the engine completely cool off. When the engine is cold, start it and drive around before it hits operating temperature. If it runs fine cold but then starts running like crap as the engine warms up, a faulty O2 sensor is a good bet.

Do the catalytic test first. If that checks out, do the O2 sensor test. If that checks out...well, back to the drawing board, LOL.

If you have a multimeter, the ignition coil is easy to check. Simply remove it from the engine and set your multimeter to "resistance" testing. Look at the connector end and you'll see two tabs. Place one probe on each of these. You should read 1 ohm, the closer, the better. Then keep, one probe on one of the tabs and place the other on the terminal of the coil. You should read somewhere between 6,000 and 9,000 ohms. Less than 6,000 or more than 9,000, replace the coil. Keep it genuine OEM here as I've read lots of stories of people having issues with aftermarket coils, especially "hot" ones (MSD, Accel).

svdsinner · Aug 25, 2017

Yeret said:
Two things come to my mind right away: plugged cat or faulty upstream O2 sensor.

Plugged cat will totally kill your power and since exhaust is basically getting stuck in the manifolds, they get quite hot and that extra heat will get absorbed by the engine. Also, if enough exhaust backs up into the cylinders, the fuel/air mixture becomes diluted enough to where it won't ignite. Easiest way to test the cat is let the engine reach operating temperature, then put the tranny in neutral and try to rev the engine past 3,000 RPM. If it hits a wall there or earlier, plugged cat is almost certainly your problem.

Alternatively, remove the upstream O2 sensor from the cat, tie it away securely and go drive around. If the engine runs better, it also means a plugged cat since removing the sensor provides a (small) exit for the exhaust.

Now, the upstream O2 sensor itself can cause a similar problem as well. Easiest way to tell if this is bad is let the engine completely cool off. When the engine is cold, start it and drive around before it hits operating temperature. If it runs fine cold but then starts running like crap as the engine warms up, a faulty O2 sensor is a good bet.

Do the catalytic test first. If that checks out, do the O2 sensor test. If that checks out...well, back to the drawing board, LOL.

If you have a multimeter, the ignition coil is easy to check. Simply remove it from the engine and set your multimeter to "resistance" testing. Look at the connector end and you'll see two tabs. Place one probe on each of these. You should read 1 ohm, the closer, the better. Then keep, one probe on one of the tabs and place the other on the terminal of the coil. You should read somewhere between 6,000 and 9,000 ohms. Less than 6,000 or more than 9,000, replace the coil. Keep it genuine OEM here as I've read lots of stories of people having issues with aftermarket coils, especially "hot" ones (MSD, Accel).

I think you nailed it. The engine won't rev past 2500 in neutral and the input to the cat is significantly hotter than the output. The cat is definitely bad. Is 150k a reasonable age for the cat to go bad? IOW, how hard should I hunt for another problem that made the cat go bad? (Not interested in buying another cat a year from now by ignoring an issue causing the issue) Should I do more than check the plugs and compression on the cylinders to feel secure that there are no other issues?

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David H · Aug 26, 2017

Mine went out at 145k. Not uncommon. Age and a bad plenum gasket were contributing factors.
Good time to install a high flow exhaust and read-up on the plenum.

Yeret · Aug 26, 2017

I think there's a sticky in this section referencing the intake plenum. This is a notorious weak point in the OEM intake manifold that will cause a LOT of problems. Read it and absolutely do it if you determine that yours is (likely) bad. If you are leaking oil through the plenum, it will burn in the cylinders and trash the cat in fairly short time.

As far as the cat itself is concerned, OEM units are said to be pretty good, flow wise. Me, I went with a universal Magnaflow which is a one in/one out design as I read a long time ago flows a bit better than a two in/one out design. Any competent exhaust shop will be able to weld it all up.

Whatever you go with, make sure you're not burning oil or you'll just plug it up in no time.

Mm88swrt · Aug 28, 2017

If the cat is the issue, one could always punch it out or delete it with a piece of pipe. If you're outside of emission boundaries that is!

5.9L V8 Gas - Suddenly no power

svdsinner

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WilliamS

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Yeret

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David H

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Mm88swrt

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