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91.5 12 Valve running hot in 2000 F350 (Fresh rebuild)

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Hello,

Back in September 2021 I started rebuilding a 91.5 12 valve out of a D350 to put into a 2000 crew cab F350. I did a full rebuild on the engine and replaced just about everything. I will try to make a full list below. Just started driving it around earlier this month and its around 100 degrees down here in Texas. Whole ac system is new as well, running down the road with ac on seems to run around 180. This is using an autometer gauge with probe at rear of head. Once traffic slows down (normal rush hour traffic coming home from work) the gauge slowly starts rising getting up to 210 or more before speed picks up and slowly the temp starts to fall. I have swapped from a napa t stat to cummins 180 with no change at all- neither have temp swings like so many seem to have. Cummins t stat is installed with jiggle pins at 12 and 6 o clock. It gets up to 180 and stays there until it can't cool anymore and starts climbing. When it starts to get hot the upper and lower radiator hoses will be only have a delta T of 10-15 degrees (upper hose around 205 with lower around 189). I have tried 2 different fan clutches and am getting lockup now- verified by sound and feel of airflow being pulled through intercooler/rad. 7.3 fan shroud is installed trimmed down. I also have 2-12" pusher fans in the front bumper aimed at lower half of condenser (mounted before finding out first fan clutch wasn't locking up at idle). I installed another autometer temp sender at the front of head and hooked it to gauge to see if maybe the sender was off and it seems to read a little bit hotter as its right by the thermostat so I think the gauge is ok. Engine runs great otherwise but this has me stumped. The cooling stack is identical to the one in my 7.3 which will idle in 100+ weather with the A/C on indefinitely and never move the needle past 180 and that is with a 345K mile ac condenser that looks like hell vs all the new parts in this truck. The 7.3 is a stick so hard to compare idle egt vs the auto behind the cummins but the 7.3 will be around 300 egt at idle with the ac on where as the cummins will run 350 or so with ac off. Then once you put it in drive it will jump to 550 then add ac on and its over 600 and in drive at idle there is some visible black smoke out of the exhaust. Even in park it started creeping up to 210 the other day as I was hooking up my trailer though. I am running out of ideas, cooling systems are pretty simple lol and many people I have read about with the same or similar setups as well as the cummins in the dodges claim that they never budge over 180 either unless getting worked hard. The only thing left I can see is maybe the egts are too high? and the black smoke at idle in drive? overloading the cooling system at slow speeds? If anyone has any ideas or sees anything I might be missing I would really appreciate it. I am ready for a vacation from working on this thing for the last 9 months and would like to drive it to the beach as soon as I get this cooling problem fixed!



Things of note-
EGT at idle seem a lot higher than my 7.3.
In drive with ac on egts on the cummins are between 550-660
in drive at idle I have a bit of black smoke out the exhaust
fan seems to be pushing air OUT the sides of the shroud - I have already trimmed more shroud to let more air out the back (towards engine) but it still throws air out the small gap between shroud and radiator


Engine---91.5 12 Valve out of D350
VE pump rebuilt Farrell Injection
Duckys 5x12 injectors new bosch bodies
Hamilton 178/208 cam
Rebuilt HE351 with 12 volt on electronic wastegate making 30+ psi
cummins high volume low pressure lift pump
bored .020, New pistons, bearings, oil pump, valve job, 60lb springs
New cummins head bolts with slightly thicker Mahle head gasket accounting for piston protrusion
Many other things I am surely forgetting...

Cooling system-
New 7.3 radiator
(New A/C condenser and trans cooler in front and cleaned intercooler while it was out - no airflow blockage to radiator)
New 12 valve fan clutch, fan, and hub assembly
New gates water pump
New napa thermostat changed out to cummins 180 thermostat (currently running cummins)

Trans-
Fresh rebuild 4r100 from Dieselsite
billet triple disc converter, billet input, etc
trans cooler lines run to radiator first and then into huge 6.0 trans cooler in between condenser and intercooler
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Discussion Starter · #41 ·
I originally had a Napa stat in, have it sitting here it just says motorad 180F. I read about a lot of people having issues with aftermarket t stats so I called cummins, gave them the engine serial and they gave me the part number below which I ordered from Geno’s. The cummins t stat has a piece on the back that fits into the block (bypass?), the Napa did not have that though there was zero change in cooling. Also the cummins one has 2 jiggle pins that I placed at 12 & 6.

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The jiggle pins is to help with the temp swings because of the big Dodges radiators.
 

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I originally had a Napa stat in, have it sitting here it just says motorad 180F. I read about a lot of people having issues with aftermarket t stats so I called cummins, gave them the engine serial and they gave me the part number below which I ordered from Geno’s. The cummins t stat has a piece on the back that fits into the block (bypass?), the Napa did not have that though there was zero change in cooling. Also the cummins one has 2 jiggle pins that I placed at 12 & 6.

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That should be the correct thermostat, the disc on the back closes off the coolant bypass once the thermostat has opened up if installed correctly, the 2nd gen stat is shorter and won't fully close off the bypass allowing hot coolant to recirculate back through the block, at idle this would cause a loss of some coolant flow to the radiator
A 2nd gen stat in a 1st gen the disc would hit the block before the stat could fully open, not good.
 

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Discussion Starter · #44 ·
So just to update everyone, my plan for now is to pull the injectors and have them tested (hopefully sometime this week or next) and then if those check out, buy a pump timing adapter to verify timing. I also have the old injectors that I pulled off the engine before rebuilding, I saw the engine run with those injectors but they are pretty damn dirty looking. I was contemplating cleaning them up and throwing them in just to see if there was any change.

Destroked 450- Is there anyway you could confirm whether you have any air being blown back forward through the radiator towards the bottom corners? I know it sounds weird, but feeling my 7.3 uniformly sucking air through the rad with a constant 30 degree delta t and the cummins truck sucking and blowing air all around with a constant 10 degree delta I feel that is part of my problem along with the high EGT’s being the major contributor.
 

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So just to update everyone, my plan for now is to pull the injectors and have them tested (hopefully sometime this week or next) and then if those check out, buy a pump timing adapter to verify timing. I also have the old injectors that I pulled off the engine before rebuilding, I saw the engine run with those injectors but they are pretty damn dirty looking. I was contemplating cleaning them up and throwing them in just to see if there was any change.

Destroked 450- Is there anyway you could confirm whether you have any air being blown back forward through the radiator towards the bottom corners? I know it sounds weird, but feeling my 7.3 uniformly sucking air through the rad with a constant 30 degree delta t and the cummins truck sucking and blowing air all around with a constant 10 degree delta I feel that is part of my problem along with the high EGT’s being the major contributor.
I find a fan flows best if the fan blades are placed half in the shroud. Viewing from the side you will see rear half of fan. Front will be in shroud. Too far in shroud the air exiting fan to rear will strike shroud creating turnbulence.I have made a couple shrouds out of fiberglass for swaps. Have 93 using 93 radiator with all front accessories from 94(including fan, fan clutch and mounting. Puts fan off center to one side so had to fabricate a shroud to fit it. Only gets to 180 on hot day with a load.
 

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Antonm, It looks like you are still in the process of your swap and your other trucks are sticks? I was curious if you had any input on EGTs at idle in drive as they seem really high to me 550-660? from what I can find on the forums it seems like 450 or so should be normal.
Looks like 450 is about right for EGT at idle while in drive (at least on a P-pump engine, I assume the VE would be similar). The below pic was taken after about a 45 minute drive with the ambient temp in the low 80's (early morning temps here in TX). Truck was idling in drive with the AC on.

Font Gauge Auto part Technology Display device
 

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Discussion Starter · #47 ·
Awesome thanks! Sounds like I am running about 100-150 degrees hotter EGTs at idle in neutral and drive compared to most 12 valves.
 

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Discussion Starter · #48 ·
So after breaking my stock trimmed 7.3 fan shroud while removing it I ended up driving the truck for a couple weeks with no shroud. There was no difference in cooling compared to the 7.3 shroud. I took some measurements of where my 7.3 f250 fan sits (1/2 in/out) of the shroud and decided to make a fiberglass shroud for the cummins. It needed to be about 2.5 inches deep vs the 5 inch deep 7.3 shroud. I could stick a piece of paper on the front of the radiator with the 7.3 shroud on or even without the shroud and I would get air being sucked through in the middle of the radiator, but blown back toward the front of the truck at the bottom and sides of the radiator. With the fiberglass fan shroud allowing the Cummins fan to sit about 1/3-1/2 way out of the shroud I am getting suction (paper stuck to rad) through the radiator everywhere. In traffic/normal driving conditions it stays right at 180 now. I have seen it get up to 190 after purposely leaving it in drive for 3-5 min parked where before it would have been up to 200+ in that amount of time. It definitely runs cool enough to where I could let my girlfriend drive it to work now and not worry about it running hot. We are going to take it on its first trip at the end of the month, a 500 mile round trip to padre island national seashore so I feel much better now that I don’t have to worry about the temp creeping. I still think the EGT’s are about 100 degrees too high at idle I neutral and drive but that is something I think I will have to try to track down after we get back. Truck runs great otherwise. Couple pics of the truck and shroud mock up. Thanks for everyone’s help!


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