TH400 Uphill Problems, Severe Bogging/Lugging

Good afternoon Steel Soldiers,

I’ve scoured the cucv tranny threads and not come across anything similar to this yet. My M1009 sat for about a year before I got her started this April. Only new parts from that repair are the starter, flexplate, and some bolts for the U-joints on the rear shaft. The T converter to block bolts are torqued to spec, and the driveline is as it should be.

Since getting her going she ran/drove excellent periodically for about 300miles. a few weeks ago while driving in 1[SUP]st[/SUP]-2[SUP]nd[/SUP] I started having power issues and bogging down. Initially it was only uphills and mostly while in first gear. Slowly but surely the problem has intensified.

Basically, the engine runs like a top and there is nothing noteworthy about idling, shifting into gear, or being in neutral/reverse. But I have a hard time getting going over 25mph (and really have to feather the pedal to do so) and I certainly can’t go more than 10-15mph up any grade at all. The last time I did that I limped up the hill with my hazards on, and then once topside the problem went away but then the truck died. After a systems check, she started right back up and drove home fine other than the bogging issue. It bogs down, accelerator doesn’t acquire normal response, and the engine sometimes seems (by sound/feel) to run slower than if it’s out of gear and simply idling.

AFT’s good, there were literally no shavings in the fluid when I changed the fluid/filter 750miles ago, and I have installed a new modulator as the old one looked crappy and I was having this issue. I can tell it needs the vacuum lines being replaced because for rubber they feel an awful lot like pex… and I will do so Saturday, however I am not too confident that those are my entire issue.

I don’t see anything obviously awry, but I did find a clear 4inch tube (roughly 1/8”OD) coming from the front end of the case near torque converter which I assume is a vent line. It is not clogged but I will look into replacing it as it does have some scale/whatever buildup inside which constricts the ID to even less space than normal.


Anyone with more experience than myself have some TH400 troubleshooting advice or had any similar issues? Thanks in Advance.

Change your fuel filter.... mine was doing the same after sitting for almost 3yrs. Night and day difference after changing mine.

As said by LT67 (It has nothing to do with the gearbox.) Service the engine.

Check your fuel (lift)pump pressure before changing the filter, if pressure low then decide if it's the pump or possibly a clogged filter sock in the tank. Personally my money is on the lift pump 1st, and the fuel filter2nd, and the sock last.

These trucks do not even need a lift pump to run and run well, joy of a mechanical diesel. It's very likely a fuel filter issue and really no reason not to replace a fuel filter. This is not a tranny issue at all. Before replacing the filter I would sample some of the fuel and see how it looks, if the fuel is bad, drain tank then replace. If the lift pump is bad, the truck will not restart after the stock filter change (can't prime the system).

These trucks do not even need a lift pump to run and run well, joy of a mechanical diesel.
Then explain this.
I was once out strolling riding about one very hot summer's day in my M1009. When it decided to let me sit is a big field of tall Mexican grass. I sat there in the sun and felt it caressing my face. As I feel asleep and dreamed.
My fuel pump had gone out as the fuel was leaking out of the weep hole. The truck would not start. As I removed the pump and waited for the roll back. The arm / lever had broken off the diaphragm. It was a genuine GM pump. Nothing no make or brand is infallible to wear and attrition. That nearly blew my mind. The fact that me an over fed long bearded leaping gnome could have a fuel pump fail. But there I was dead along the road. Naked to the world. I was taken to a place. The local GM dealer. It was placed high upon a lift. its underside naked to the world. In front of every technician. There were long ones, tall ones, short ones, brown ones, black ones, round ones, big ones and crazy ones. I said drop the lift change that pump. And from that point all these young guys were lost. they never seen such a crazy contraption. I actually paid for the part and had it replaced with my help and instruction. I was 2 states away and it cost me $175. total with everything. True story. The names have changed to protect the innocent. And all I remember was Momma told me not to come. That ain't no way to have fun noooo. Have a Great Day.

cucvrus said:

These trucks do not even need a lift pump to run and run well, joy of a mechanical diesel.
Then explain this.
I was once out strolling riding about one very hot summer's day in my M1009. When it decided to let me sit is a big field of tall Mexican grass. I sat there in the sun and felt it caressing my face. As I feel asleep and dreamed.
My fuel pump had gone out as the fuel was leaking out of the weep hole. The truck would not start. As I removed the pump and waited for the roll back. The arm / lever had broken off the diaphragm. It was a genuine GM pump. Nothing no make or brand is infallible to wear and attrition. That nearly blew my mind. The fact that me an over fed long bearded leaping gnome could have a fuel pump fail. But there I was dead along the road. Naked to the world. I was taken to a place. The local GM dealer. It was placed high upon a lift. its underside naked to the world. In front of every technician. There were long ones, tall ones, short ones, brown ones, black ones, round ones, big ones and crazy ones. I said drop the lift change that pump. And from that point all these young guys were lost. they never seen such a crazy contraption. I actually paid for the part and had it replaced with my help and instruction. I was 2 states away and it cost me $175. total with everything. True story. The names have changed to protect the innocent. And all I remember was Momma told me not to come. That ain't no way to have fun noooo. Have a Great Day.

Click to expand...

Rick, you missed your calling, you should have been a "Bed Time Story Teller". Or are you practicing for your new gig, "Grand Kid Sitting"?

Will you have whiskey with your water
Or sugar with your tea
What are these crazy questions
That they're asking of me
This is the craziest party
That there ever could be
Oh, don't turn on the light
'Cause I don't want to see.


-Three Dog Night

cucvrus said:

These trucks do not even need a lift pump to run and run well, joy of a mechanical diesel.
Then explain this.
I was once out strolling riding about one very hot summer's day in my M1009. When it decided to let me sit is a big field of tall Mexican grass. I sat there in the sun and felt it caressing my face. As I feel asleep and dreamed.
My fuel pump had gone out as the fuel was leaking out of the weep hole. The truck would not start. As I removed the pump and waited for the roll back. The arm / lever had broken off the diaphragm. It was a genuine GM pump. Nothing no make or brand is infallible to wear and attrition. That nearly blew my mind. The fact that me an over fed long bearded leaping gnome could have a fuel pump fail. But there I was dead along the road. Naked to the world. I was taken to a place. The local GM dealer. It was placed high upon a lift. its underside naked to the world. In front of every technician. There were long ones, tall ones, short ones, brown ones, black ones, round ones, big ones and crazy ones. I said drop the lift change that pump. And from that point all these young guys were lost. they never seen such a crazy contraption. I actually paid for the part and had it replaced with my help and instruction. I was 2 states away and it cost me $175. total with everything. True story. The names have changed to protect the innocent. And all I remember was Momma told me not to come. That ain't no way to have fun noooo. Have a Great Day.

Click to expand...

Your showing your age! And I'm showing mine cause I recognize it. Good job

No problem send the truck and failed pump here and I will explain it. Those ips can pull full simple fact... will you prime a system off that NOPE, if you have any sort of air leaks will it still run NOPE... if the system is sealed and the pump fails will the truck still run, YEP... and the OPs truck is *running* and it happened *slowly*... this all points to a filter, I had a filter plug and I quickly "slowly" lost power, I thought I blew a tranny or something as I could not climb a hill with my plow UP on driveways I would plow DOWN... finally I started to notice alot of smoke behind me...

Changed filter... shes running great ever since...

Could the OP have another issue SURE... should the OP check fuel flow after the filter... YEP... its free a filter is cheap ISH if stock CHEAP is aftermarket like a fm100 housing...

A lot of blue smoke, loss off power, going up hills, junk in the tank. Sometimes after being stuck and knowing that the fuel tank had umm fuel station gunk in it I would pull the primary filter, K30 Civi primary and secondary, and dump fuel into jar to check and to see how bad. I carried a 1 gallon fuel can in an old dairy crate and refill.

Finally got tired of that drill and pulled the tank and tossed it into the scrap after the ugly sight of RUST. I feel if it is just gunk it can be cleaned and reused. I was going to open a new in box tank but opted to drop other tank to look at what was in it after not using it for 15+ years. Dropped it half full, opened it up and seen nothing but real sulfur diesel. Not a rust spot or crap in the tank. Put back together and cured the problem.

New filters, Wix brand and spot check every year, dump into glass jar, look at, refill and put back on, start truck. I like to do when truck has been warmed up as it is less strain on the starter. When I know I will be fording water, not floating and sinking, I carry extra filters in plastic ziplocks as water is very hard to remove from filters, although the water petcock at the bottom of the primary filter does help

Remove filter, primary filter and secondary if equipped dump into glass jar and observe the contaminates. Rubber, dirt, water, rust all have a tale to tell.

Good Luck!