New bus with no power.

[mikerob643]

Ok so I have been looking to buy a bus for a while. Yesterday the right one came up for sale in eastern LA so I left work in flagstaff az and drove the 6 or 7 hour drive to check it our
1996 thomas vista 3800 with 80xxx miles on it . not a bit of rust , all rubber looking decent and overall really clean, all fluids looking good and it drove well.
I bought it .
It drove great for the first few hundred miles . holding 60 or 65 if wanted all the way till the needles area were it started loosing its get up and go. I can't pinpoint an exact time or incident that caused it, I just started noticing it wouldn't go over 45 ish . engine was still going and temp was look in fine at 170 . I just couldn't get her to get moving at all.

Has this happened to anyone ? I'm in Kingman arizona now. Going to try and start wrenching and diagnosing the issue

[rensch26]

What engine? What transmission? Sounds like a transmission issue to me. Maybe struggling to transfer power to the rear wheels. Check your fluid levels.

[mikerob643]

7.3 l diesel , I believe dt408 and 4 speed automatic allison transmission

[mikerob643]

It pushes through the lower three gears alright but then when it get to 4 it just sit at 1900 or 1800 rpms and won't go over it.

[PNW_Steve]

Hmm.

If I understand correctly, it pulls like it should and shifts properly until you hit around 1800 rpm in 4th gear.

I had a similar situation that turned out to be a partially plugged fuel filter.

Mine would run up to governed rpm going downhill but not on level ground.

[PNW_Steve]

Note: the 7.3 is a V8 engine and the DT408 is an Inline 6.

[mikerob643]

Yup you got it exactly it pulls to 4 gear and 1900 rpms pretty fine but then won't go past. Checking the filter and fuel pressure now. And it is the 7.3 l diesel . not the inline dt408. That was from some quick Google searching trying to give everyone more info .

[brokedown]

Is it making extra noise? My T444E had a exhaust back pressure valve failure, which for practical purposes disables the turbo and keeps you from making any power or revving higher than around 1900rpm. It would also spew black smoke when you pshed the pedal down, and was a lot louder.

[PNW_Steve]

Edit: I misread your reply. Disregard my comment.

Thankfully someone smarter than I am showed up

[cadillackid]

+1 on checking the exhaust backpressure valve.. and also the fuel filter.. since you once were going 55-60 it doesnt sound like a computer speed limiter

the fiuel filter on these is a pretty easy change.. the engine is a T-444E by navistar standards.. the fuel filter should be in stock at any navistar dealer..

also make sure you didnt suck a vacuum on the fuel tank.. loosen the cap and make sure you dont get a "whoosh" of air in.

fuel filter in a brown circular canister up on top of the engine. the cap unscrews and the filter element and cap come off the top.

also make sure your engine oil is full and that you arent dropping oil pressure.. Low oil pressure can cause runability issues with that engine.

-Christopher

[mikerob643]

Fuel filter is clean but I replaced anyways.and it's full on oil and holding pressure .ill look into the pressure valve failure though and see. Also I'm thinking it might be this california DPF exhaust filter thing

[mikerob643]

The oil level is fine and so is the oil pressure. Do you know how to check the back pressure valve? Doing a little Googling I found a video on how to clean it but not how to diagnose if it's bad or not. Also I am starting to think it may have something to do with this California DPF filter that goes on the exhaust of diesels in California.

[mikerob643]

Sorry about the double reply my internet is pretty spotty here on kingman

[cadillackid]

The EBPV is supposed to be on cold weather busses as a way to help them warm up. From what I understand it’s possibke to disconnect the linkage and manually activate the lever and tie it off. My bus didn’t come with one since it came from Texas .. add on DPF filters were notorious for causing issues.. on A 444e there shouldn’t be any connection to the add on DPF and the computer so if you replaced it with a pipe it might help things. I don’t know how those were cleaned. It had to be a manual process of some sort?

[brokedown]

Here's a photo of mine. The valve is on the pipe as it leaves the turbo. There is an actuator that is supposed to automatically open and close it and when it stops doing the right thing it really runs bad!

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[Johnny Mullet]

Does it accelerate fine and then seems like something stops you from going any faster like a governor? Seems like a low speed but the speed limiter might be set for that speed and can be turned off or changed with the proper scanner.

[cadillackid]

I would’ve thought that except he said it was running 55-60 for awhile before it got slower otherwise id be thinking computer limited

[mikerob643]

Hi guys wanted to thank everyone who commented with suggestions they were all great starts and good things to look for .
It ended up being the California dpf . it was a aftermarket filter that needed manual cleaning every xxx number of miles and I was over that number . ended up taking the filter out of the system to get it home , going to try and figure out what the long term solution for it will be tomorrow.

Thanks again for all the help , so happy the bus is home !!!

[mikerob643]

Broke down thanks a ton for the photos too they really helped tract down were that valve was so I could check it off the list

[brokedown]

Glad you found it!

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