Major 7.3 Headscratcher

Well I have a truck here in the shop and I have not experienced this issue before and I can not for the life of me figure it out. I am hoping someone on here can give me some place to turn.

Here is the story:
Truck went to another shop before coming here for the same problem.

ISSUE: Truck rpm oscillates @ idle. Almost sounds like it is going to die at times. Unplug the ICP sensor and it idles perfect, but with 1280 code obviously.

The oscillation started while the customer owned the vehicle. It was not there prior to purchase. It has over 211k miles. Oil level is perfect.

Previous mechanic, replaced trans selector switch, did ICP sensor and pigtail and then also did TPS sensor.

Issue still there, so mechanic unplugged ICP sensor and told the customer his diablo sport programmer wouldn't clear off his PCM. Then he told him his PCM was bad and sent him on his way.

Wellllll... despite the fact I didn't think the PCM was the problem, I swapped it with another PMT1 DPC 422 out of an identical truck. No change.

I checked the resistance at the ICP sensor plug. All were in spec according to fordinstallersupport.com. I also put another ICP sensor in from FORD just to be sure. Swapped in a brand new test IPR. No change, I replaced the pigtail on the IPR because it had **** caked inside of it. No change. I checked continuity from PCM to the 42 pin and the ICP/IPR. They were all <1.

I swapped HPOP's, still does it. I pressurized each head individually. Both held pressure.

Oil looks fine running.

Fuel pressure idle 60.

Fuel quality good.

F250 so no deliminated tanks like in 450/550 trucks.

Icp v is going from 0.66-1.30 @ idle. Psi readings are 453-637.

Ipr says 12-13%

Rpm 630-713

Very noticeable at idle.

Accelerator pedal pos voltage idle .61v wot 3.93

It does it in ANY gear selection. Park, Reverse, Drive, 2nd, 1st as long as it's idling.

The customer has also stated that the truck has been shutting off recently now. Never did before.


To recap, ICP sensor Replaced, IPR replaced, HPOP swapped, Heads Pressure Checked, truck idles perfect without the ICP plugged in. All wiring checked for continuity that concerns the ICP, IPR from the sensor to the PCM. Resistance check also in spec.

What weight oil is in the truck? I had a truck not run for crap because he was running straight 0 weight oil in it.

If that was the case it would still do it with the icp sensor unplugged. Sounds like the sensor, or wiring of that particular sensor, since it acts right when the sensor is unplugged.

Have you tried a whole new/different harness?

Check your 5 volt reference at the icp with truck running.

We had a truck do something similar and it was a bad connection on the valve cover harness. We washed the motor and it just was enough to tweek it so it would idle a bit goofy and all of a sudden studder a bit and sometimes die. Probably not what you have and most likely something you have already looked at...

ford_trck said:

What weight oil is in the truck? I had a truck not run for crap because he was running straight 0 weight oil in it.

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I was going to mention the same thing. Might also try a CPS.

Will it run a contribution test with the idle fluctuating?

Also lean towards CPS.

I don't see how a cam sensor could do this.

I checked the resistance at the icp sensor. Its within spec. I have ohmed the entire harness that has to do with icp.... like the ipr, icp, 42 pin and pcm.

What if you had an injector that was not working right and lowing icp and the system is trying to compensate for the loss in high-pressure oil. Have you tried a kill box and kill injector individually.

silverslug said:

What if you had an injector that was not working right and lowing icp and the system is trying to compensate for the loss in high-pressure oil. Have you tried a kill box and kill injector individually.

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This... could be an Injector issue and unplugging icp sets it to a default reading and it bumps up ipr since it doesn't know an actual icp value and raising your icp is enough for it to idle smooth.

Marty said:

I don't see how a cam sensor could do this.

I checked the resistance at the icp sensor. Its within spec. I have ohmed the entire harness that has to do with icp.... like the ipr, icp, 42 pin and pcm.

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it's possible to have another sensor in the system causing the problem because it's shorting out the 5 Volt reference.

ford_trck said:

What weight oil is in the truck? I had a truck not run for crap because he was running straight 0 weight oil in it.

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Customer changes own oil, rotella jugs in bed. Regardless it has 15w40 in it.

HVFC6040 said:

If that was the case it would still do it with the icp sensor unplugged. Sounds like the sensor, or wiring of that particular sensor, since it acts right when the sensor is unplugged.

Have you tried a whole new/different harness?

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I have not tried a whole new harness but I have tested continuity from one end to the other, there is NO interruption.

Luke@MillerPerformance said:

Check your 5 volt reference at the icp with truck running.

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As stated above, the ICP 5 volt reference is within spec, per fordinstallersupport.com.

Layson said:

We had a truck do something similar and it was a bad connection on the valve cover harness. We washed the motor and it just was enough to tweek it so it would idle a bit goofy and all of a sudden studder a bit and sometimes die. Probably not what you have and most likely something you have already looked at...

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Wiring looks fine, and unplugging injectors individually make the truck run with a miss, but the oscillation is still there regardless.

Tim @ P.I.S. said:

Will it run a contribution test with the idle fluctuating?

Also lean towards CPS.

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It gives me cyl #8 as failing the contribution. However, I understand how a contribution test works and I do not think that has anything to do with my problem.

silverslug said:

What if you had an injector that was not working right and lowing icp and the system is trying to compensate for the loss in high-pressure oil. Have you tried a kill box and kill injector individually.

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Yes the injectors have been removed from the circuit individually. There is no change in oscillation, only a miss until that injector is plugged back in.

mint-6oh said:

This... could be an Injector issue and unplugging icp sets it to a default reading and it bumps up ipr since it doesn't know an actual icp value and raising your icp is enough for it to idle smooth.

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I don't disagree with the above statement that I have put in bold.

Luke@MillerPerformance said:

it's possible to have another sensor in the system causing the problem because it's shorting out the 5 Volt reference.

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Well it doesn't look like I have anything to really go on. I mean the only thing here in the HPOIL system I haven't swapped out is the injectors... but individually disconnecting them from the system hasn't helped the case.

Could be injector o ring bleeding pressure. Idle osscilates with ICP sensor plugged in because pcm tries to compensate and over compensates back and forth....would be my next place to chck

Come on Marty, I know it makes no sense whatsoever. I know you know what you are doing, way more than I do. Spend five minutes and swap out the CPS.

You say you unplug the icp sensor and the idle is perfect, which makes it sound like it is something in the icp sensor circuit.

To prove it. Could you adapt a icp sensor to a porta-power and pump it up to idle psi with a mechanical gauge T'ed in and see if the pressure reading on a scan tool is steady or not. Maybe with the engine running as well to see if that makes any difference.

He isolated/pressurized the oil rails and both held..

It runs smooth when off idle? No other codes? The IVS was changed with the TP assy , but how is the wiring going to the IVS? Can you tap into that signal?

I had a truck doing this same thing but it had 450,000 miles on it. Figured it was injectors because it got better but didn't completely stop after running Hot Shots through the oil and fuel.

does the oil look aerated on the dipstick after running?

is the truck tuned with the Diablo thingy? if it idles fine with the icp sensor unplugged then that tells me it is electical or tuning. if it was mechanical or oil related i would assume it would still idle the same when the icp sensor was unpluged but would still change a lil. if it didnt change at all then that means the icp sensor isn't working and it was already in a default program. if every perameter is erratic I would swap out the cps!!

live life full throttle

Crank sensor . Thats what my diesel buddy says.