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Looking to buy a truck - missing sensor? help needed

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MrGoldman

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Hi all,
I'm looking at a GP truck, listed as a non runner but I'm hoping its an easy fix.

It's missing some sort of sensor on the top by the valve cover that spews oil when cranking. Cranks healthy but does not start. GP guys said a few fuel lines were disconnected and some valve cover bolts were loose. Any ideas? Should I run away from this one? I can go look at the truck and mess with it before the auction time to see if I can get it running.

I'm a fairly competent mechanic but not too familiar with these newer engines, I fix up a lot of surplus Drash units and generators and re-sell them currently.

This would be a toy for me.

Thanks!
 

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NDT

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Looks like C7. You need to decide if the disassembly was due to one of two reasons: either uncle needed a part for a different truck OR the mechanics looked inside and said OMG it’s trashed, off to DRMO with you.
 
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GeneralDisorder

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..... based on the injector connector I think it's a 3126.

Fuel system and high pressure oil system pulled apart..... smells like HEUI system problems which would make sense for that engine..... that seems to be a real common theme with them. I bet it wouldn't start so they were chasing fuel delivery and/or injection actuation pressure (IAP). They might have monkeyed with the injectors, etc. The grunts don't have the experience or knowledge to get real deep into HEUI system diagnostics typically.

Reassemble it and check IAP with a scan tool. Needs to hit 870 psi to start and stay running. If you can't get sufficient HEUI IAP then HARD PASS on that truck. Needs $10,000 in parts.
 
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GeneralDisorder

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MrGoldman said:
You are correct. Its a 3126.

which sensor do I need to purchase to plug that hole? Any easy to rent / cheap scan tools ?
Click to expand...
That's the IAP sensor that is measuring the oil pressure produced by the HEUI pump. Between about 500 and 3000 psi.

Not really any good "cheap" options for what you are going to be most interested in since this is J1939 heavy duty world and even in that world the HEUI injection system is an outlier in terms of how it functions and what you need to look at. You really need the special tests that CAT ET can run so you can see IAP, IAP duty cycle, and be able to increase and decrease the IAP in real time to see how the pump behaves - not to mention run cylinder cut-out tests, etc.

Those engines also do suffer from the transducer plague and it could be a situation I have seen before where the boost pressure transducer gets stuck at zero from sitting and that breaks the fueling system feedback loop so the truck won't go more than about 20 mph. Doesn't throw a code or a check engine light either so that would explain why the grunt parade pulled it all to pieces and left it that way. But that's a left field WAG.
 
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Ronmar

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In electronic, C7 in the A1R trucks(04ish and up). The 3126(99-04ish) has had its share of issues, but there are a fair number of them out in the wild operating, so…

The 3116(pre 99) is all mechanical if you are into that sort of thing:) Also a lot of those out and operating…
 
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GeneralDisorder

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MrGoldman said:
Sounds like this truck might not be the one. this batch and the past have been going for $5-6k.

I guess I could buy the sensor, Looks like its $60 or so and if it doesn't run i'm not out much. Would this work? https://www.fridayparts.com/injecti...GpKQ_BKkf-wvLTw1UpamRNHJzrpOM7lYaAivaEALw_wcB

Which engine is regarded as the best in these trucks? 3116 ? seeing a lot of 03-04v trucks on Gov Planet these days.
Click to expand...
That is the IAP Solenoid (regulating valve) that goes into the pump, not the sensor that you need.

C7 is the best choice and the newest - 2004.5 / serial number 100,001 and up. These trucks are known as "A1R" trucks although the data plate still says A1. The C7 was adopted over 20 years ago now, and is still in use in the A1P2 and the new FMTV A2. It has the most support, the most parts availability, and the widest adoption and knowledge base. Parts and surplus for these trucks will be available for another 30-40 years as the Army is still buying trucks that share 90% of the parts and schematics with the A1R. Of these trucks - the best and most desireable are 2006+ trucks with all the MANY upgrades they got like the high pinion rear diff, slide out battery tray, WTEC-IV transmission, and many more too numerous to list.

The 3116 is the second general favorite. Favored by some for it's lack of computer - although the trans has a VERY outdated computer so not "entirely mechanical" as some seem to suggest. You are just as screwed with a dead WTEC-II controller...... They have a couple issues like the oil line of death. They are underwhelming in terms of performance, are now 30 years old, many parts (not just engine) are getting difficult to source and the CAT dealers and heavy equipment shops have little experience working on them due to age and brain drain.

The 3126b is the early first revision of what later became the C7 and all the mistakes were made learning how to build reliable HEUI engines with these engines. They were only made for 5 years and most of the important and expensive parts do not interchange with the C7. Spare ECM's for example tend to be less reliable and twice as expensive. Roughly 50-75% of these trucks seem to suffer from HEUI system meltdown, main oil pump issues, and ECM failures seem also relatively common (I program them for folks in the community)...... definitely the least desirable in the opinion of many.

Personally - the only one's to own are the C7 trucks. But everyone has to choose their comfort (and frustration) level.
 
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