Major 7.3 Headscratcher

Bustedknuckles · Sep 26, 2014

Marty said:
Ebps reading 17 @ idle. Absolute reading. Does this with tune and without tune.

Lpo is fine, so is icp off idle or with sensor unplugged.

I inspected icp sensor face and checked resistance, no marking and was the same from one 7.3 to another. This is a stock 7.3 and the cam gear hasn't been off. I doubt it has cam gear walk for that reason too.

Weird. Guess ill keep watching this then cause I'm out of guesses... I'm a tech and find threads like this fascinating!

boggerr · Sep 27, 2014

Marty, there is no logged codes?
When it died on the guy does it shut right off or does it slowly die out and does it start right back up?
Any difference hot or cold?
Does the owner remember exactly when this started, after oil change, after work done to the truck ?
I can't remember, when you disconnect the ICP sensor on a good running truck does it kick the idle up a bit?

Tom S · Sep 27, 2014

Only odd ball thing that keeps popping back in my mind was Charles with the cracked pickup tube for the oil pump.

ghohouston · Sep 27, 2014

Tom S said:
Only odd ball thing that keeps popping back in my mind was Charles with the cracked pickup tube for the oil pump.

exactly why i asked if the oil looked aerated

mandkole · Sep 27, 2014

what is the oil pressure at idle/running? don't see where that was noted. If the truck runs fine at rpm and when oil demand is higher, oil supply quality seems ok.

boggerr · Sep 27, 2014

boggerr said:
Easy fix, change cap and rotor.

Check oil pressure at the hpop resevoir.

Marty said:
Ebps reading 17 @ idle. Absolute reading. Does this with tune and without tune.

Lpo is fine, so is icp off idle or with sensor unplugged.

I inspected icp sensor face and checked resistance, no marking and was the same from one 7.3 to another. This is a stock 7.3 and the cam gear hasn't been off. I doubt it has cam gear walk for that reason too.

Tom S said:
Only odd ball thing that keeps popping back in my mind was Charles with the cracked pickup tube for the oil pump.

Tom, I was thinking pretty much the same thing. Almost sounds like a oil problem on the low pressure side. Either something is clogged or cracked.

How about adding extra oil or jacking the rear of the truck up to rule out pickup tube

TyCorr · Sep 27, 2014

boggerr said:
Tom, I was thinking pretty much the same thing. Almost sounds like a oil problem on the low pressure side. Either something is clogged or cracked.

How about adding extra oil or jacking the rear of the truck up to rule out pickup tube

This^ jack er up

Obsledpuller · Sep 30, 2014

Disregard

golfer · Sep 30, 2014

an effing VIDEO would be nice...

Marty · Oct 1, 2014

RPM oscillation @ idle LOL!

After doing some more searching, I have some more possibly "key" information.

Took the belt off, problem went away!
Checked all pulleys, all were great, reinstalled belt.
Restarted, problem there.
Removed fan clutch, problem still there.
Removed the alternator wires while running. BAM, oscillation went away.
Alternator producing 14.1v @ low end of the oscillation, 14.4v @ high end of the oscillation.
Swapped alternators same problem.

I will try and look @ it more tomorrow. But basically as long as the alternator power wire is unplugged the truck idles without issue. I am going to figure out what the readings should be @ the plug... I'm figuring that whatever controls the alternator plug is bad, causing it to load/unload the alternator, giving us the oscillation. I will hopefully know more tomorrow.

bluedge8 · Oct 1, 2014

Good work!

Tom S · Oct 1, 2014

Great diagnosis.

Marty · Oct 2, 2014

rpm-inc · Oct 2, 2014

Just throwing this out there. I had a truck where the volt gauge went bad and the alternator wouldn't charge. Different cluster and it started charging again. Maybe something is wacky with the gauge?

Layson · Oct 8, 2014

Get it all figured out!

MOE · Oct 10, 2014

Any end result?

Marty · Oct 12, 2014

No, the truck is still at the shop.

The alternator is charging just fine. Everything checks out electronically.

Unplugging the ICP bumps the idle/icp/ipr up which masked the problem. This isn't an ICP related problem.

If I unplug the main power wire to the alternator then the truck doesn't have an issue. If I unplug the orange wire on the 2 pin plug then it doesn't have an issue. In both cases, the truck isn't charging, obviously. If I give 12v power to both alternator plug pins, it still has the problem. This isolates the under dash fuse panel or instrument cluster as being a potential problem.

The truck has been backed outside. I guess I will try and get back to it asap, but for now I don't really have any direction to head.

Bustedknuckles · Oct 12, 2014

Marty said:
No, the truck is still at the shop.

The alternator is charging just fine. Everything checks out electronically.

Unplugging the ICP bumps the idle/icp/ipr up which masked the problem. This isn't an ICP related problem.

If I unplug the main power wire to the alternator then the truck doesn't have an issue. If I unplug the orange wire on the 2 pin plug then it doesn't have an issue. In both cases, the truck isn't charging, obviously. If I give 12v power to both alternator plug pins, it still has the problem. This isolates the under dash fuse panel or instrument cluster as being a potential problem.

The truck has been backed outside. I guess I will try and get back to it asap, but for now I don't really have any direction to head.

Did you check alternator AC ripple? I see it all the time on older GM cars. The diodes start to go bad and cause rpm surge for some reason.