Let’s talk P400

I have a coworker that is selling his 1998 Cheyenne extended cab truck with a P400, nv4500, long box with only 175k. $8,000

It is very clean with a nice matching topper. Here’s what he did to it...

1998 Cheyenne 175k nv4500.
3rd owner
Stage 1 towing tune from quadstar
Heath HD lift pump
Diamond eye 4" exhaust
Wicked wheel 2 compressor wheel.
Hand crank windows
Long box
Ex cab
Cloth seats
Rubber floor mat


My questions...

Buy it?
Good truck?
P400 any good?
Average lifespan of a P400?
What would be must have upgrades?
Upgrades to avoid?
Safe egt and boost levels?
What would be healthy p400 compression ratios?

The P400 is/was AM General Navistar premium HD edition of the 6.5, but does require much accommodation to fit into a pickup (it was never designed or intended for passenger-type vehicles). First, verify that it is a P400. If so, it will have a very modified sump (or the truck must have a significant lift), and an obvious deep skirt, cross-bolt main block. They aren't common, and were/are very costly to get hands on. Otherwise, it's a common GM 6.5, Navistar 6.5, or an Optimizer. The entry price for a new P400 alone is north of the vehicle asking price. Likely the best 6.5 ever designed. The were only ever sold as complete long blocks. There are some extensive articles in the Member's Area of TDP, if they are still posted (I didn't look, but might be revisited upon request).

Stage 1 towing Quadstar towing tune? No personal experience, but no positive rumors, either. Coin flip.
Heath HD lift pump? No better than OEM. The lift pump isn't/wasn't an issue.
Diamond Eye exhaust? Good.
Wicked Wheel 2 compressor wheel? The GMx compressors were never a problem. The restrictive turbine housing, on the other hand, is. Improving the compressor wheel is polishing a turd.

Hand crank windows? Deal breaker.

Ok! Thanks for that advice! I will check and verify it is indeed a p400.

Personally, I like hand crank windows! They work better in the winter.

Plus one on hand crank windows...
I have never had a hand crank window give me any issues since i have begun laying my hands on vehicles
But i have wasted many hours of my life fixing powered windows.
Sounds like a great motor to have in the stable

I had a p-400 in a 95 burb. It is a solid motor. They modified the pan to fit into the truck. If I recall correctly you had to modify the pan for 4x4 models only. In all other regards the engine used the same 6.5 accessories.

The P-400 was designed for a different fuel system capable of increased horsepower. If the rumors were correct it was designed for the up armored Humvee. When they added additional armor to the vehicle they added weight and needed more power to keep it moving. I never upped the horsepower and stayed with a factory fuel system. I did have a chip from Heath Diesel but it was mild and designed for the ATT turbo. I do not think he sold many of those chips. The engine was bullet proof. Now with that being said, anyone can kill any engine at anytime just by being stupid.

Again, if my memory was correct the factory P-400 was 21.5 to 1 compression (maybe 20.5 to 1). When I ordered mine I was able to get 18 to 1 if I wanted. I chose the stick with the 21.5 to 1 due to I was not looking for more power, just reliability and better winter starts.

Mine had the timing gear set, ATT turbo, big pre-fuel filters with pre-fuel pump and a Diamond Eye exhaust. I put just under 500,000 miles on that engine with 2 transmissions. All my miles were highway and very little towing. I cannot remember the EGT but they were never an issue and my boost pressure with the ATT turbo would rarely go over 15 psi. I was after economy and long life. On the highway I was always driving posted speed limit plus 4 mph. The truck ran great and returned a solid 18 mpg.

Keep the oil changed, use good oil, make sure the cooling system is up to the task, keep the glow plug system in good shape, monitor pre-injector pump fuel pressure, and keep the PMD out of the engine bay. In the diesel world the 6.5 got a bit of a bad rap. It was never a powerhouse. It's purpose was to have respectable towing capability while returning decent mileage. If you could accept that then the 6.5 worked well.

If the P-400 was not abused you should get a lot of miles out of it.

Good Luck

Brian

I'd be curious to hear of the mileage on the P400. That engine was always rare, but it's now uniquely rare... AMG only made a few of them and they never actually went into full production. I heard a couple of years ago that the last 200 or so P400s would be assembled from parts on hand... when they're gone, that'll be it for new P400 production... So... If the engine is nice, not abused and low mileage, it alone would be nearly worth the asking price of the truck. Jim