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NTC 335 low power

Posted By Tattoo Sunday, November 10, 2019 2:23 PM
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Tattoo
 Posted Sunday, November 10, 2019 2:23 PM
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I have the privilege of pulling the equipment trailer to football games and band competitions for our local high school marching band. My truck is a 1968 K123 with Cummins ntc 335. The entire combination when loaded weighs 45300 lbs. I kind of expected to be able to pull that pretty easily, but when we're on the interstate I have to keep the fuel pedal on the floor pretty much the entire time. On a flat stretch of highway we can reach 75 mph but any kind of an uphill will slow the truck down. If it's a moderate hill, I have to downshift and the truck will slow to 50-55 mph.

The engine coolant temp never goes above 175 degrees. The pyrometer typically reads about 900 under max load and the turbo can max out at 17 psi of boost.

For the Cummins guys out there, what do you think of my numbers? Any thoughts about where I should start to try to wake this thing up a bit?

Thanks.

Bryan
 IMG_20191004_170858.jpg (30 views, 3.71 MB)
Newto
 Posted Sunday, November 10, 2019 4:35 PM
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Hi Tattoo,

your numbers do suggest that its a bit of a slug!!!!!!!

A standard 335 puts out around 28-30 psi of turbo boost......& 17 psi my immediate thought is that you are down on fuel pressure.

Things to check are...

1. Are you getting full throttle at the fuel pump lever?
Get someone to push the throttle pedal to the floor while you watch and see if the throttle lever on the fuel pump breaks at the spring.

If not then the fix is easy......adjust linkages to get full throttle.

2 if you are getting full throttle then you should check the fuel pressure (under load) by fitting a fuel pressure gauge with a flexible line into the 1/8 gas plug on the side of the shutdown valve on top of the pump. Fuel pressure should be around 185 psi....if not you better find a Cummins mechanic to adjust it for you.

3.Does it blow any smoke??...Grey/White on startup indicates a loose injector set.....black under load (without good fuel pressure) may indicate a worn turbo.

4.Also check the usual maintenance items....is the spin on fuel filter only about 3/4 full when you take it off????....if its full its blocked!!!!..get a new one!
and air cleaner.....is it clean and free of dirt and crud???....blocked air cleaner will also contribute to black smoke under load.

There's some homework for you Tattoo......good luck and don't forget to report back....

Cheers,

Newto
Tattoo
 Posted Tuesday, November 12, 2019 2:39 PM
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Thanks for that Newto. You've given me some direction. I like your theory about low fuel pressure under load. I don't have a fuel pressure gauge with that kind of range, but I'll work on getting my hands on one.

I had a few interesting thoughts after reading your post. I'll start by saying that I've owned the truck for probably 4 years now and have been driving it bobtail most of that time. The boost had never gone above 17 psi but I never knew what the correct range should be so I just chalked that up as normal. The lack of power really only became obvious with the trailer on it and trying to maintain highway speed.

Also, I had recently replaced the head gaskets and installed jake brakes. It was the first time I had worked on one of these engines and I struggled with the injector settings. My first attempt resulted in very loose injector adjustment. The truck ran when I was finished but smoked like a locomotive. The interesting part is that it made at least 30 psi of boost that way (gauge was pegged) and more power than I had ever remembered. I've since managed to get the injectors properly adjusted which eliminated the smoke and brought the boost and the power back to what has been normal for this truck.

Anyway, thanks again and I'll post again when I know some more.
Newto
 Posted Wednesday, November 13, 2019 9:59 AM
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Well Tattoo.......you've just about solved the problem!

The loose injector set would allow the injectors to "dribble" extra fuel into the combustion chamber between firing strokes thus giving a bigger bang & more turbo boost.

I suggest you find a Cummins mechanic that can adjust your fuel pressure for you....(you need a different size fuel button) because its obvious now that your fuel pressure is low.

Increased fuel pressure will bring the turbo boost up higher & quicker which will also ( if you have an AFC style fuel pump) open the air/fuel control bellows quicker which should help in acceleration.

cheers,

Newto
Tattoo
 Posted Sunday, November 24, 2019 3:19 PM
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So I spent several hours working on the truck today. I replaced all 6 fuel injectors with reman units because I suspected that I had cracked 1 or 2 injector cups when I was having all that trouble with the injector adjustment. I did discover 1 cracked injector cup that was causing the truck to smoke lightly all the time. The other 5 injectors appeared ok. After the injector replacement, the truck runs noticeably better and does not make so much as a wisp of smoke.

The next thing I did was install a fuel pressure gauge and take the truck for a test drive. I'm attaching a pic of the fuel pump. You'll notice the braided line I installed which runs inside the cab and has the pressure gauge attached to the other end. I was able to hold the gauge in my hand and monitor fuel pressure while driving.

http://forums.aths.org/Uploads/Images/411b3902-9cdd-4a1c-ac94-8ea7.jpg

Here are my fuel pressure numbers...
at idle 0 psi fuel pressure. I was surprised by that. Is that normal?
at 1500 rpm in 8th and 9th gear, pedal on the floor 110-125 psi
at 2000 rpm in 8th and 9th gear, pedal on the floor 140-155 psi
At low rpm there is almost no boost. At max I never saw over 16 psi.
The needle on the gauge flutters a lot at higher rpms, but I think my numbers are reasonably accurate.
1500-2000 rpm is the range where the truck would fall on its face while pulling the trailer.
What do you think of my fuel pressure numbers?
I wanted to pull the button out of the fuel pump to see what it is but I ran out of time.

Thanks.

Bryan
Sunday, November 24, 2019 3:20 PM by Tattoo
Newto
 Posted Sunday, November 24, 2019 10:31 PM
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at idle the fuel pressure is usually pretty low....like 10 to 17 psi so if your gauge is calibrated towards higher pressure readings then idle may not show.

as for your max pressures......you are still down about 30 psi as a rough guide.

Good news is ..........you have an early fuel pump (pre AFC) so you can shim up your fuel pressure by removing the large circlip behind the throttle lever and pull the throttle shaft out.

In the shaft there is a hole which is partly blocked by an internal screw in the throttle shaft.

Usually that hole is somewhere between 1/2 & 3/4 blocked by that screw.Removing the screw and adding shims so the hole becomes larger will increase your fuel pressure.

Once the hole is more than 1/2 exposed the fuel pressure gain drops off and you may need to do a button change for big fuel pressure gains but up to 1/2 way open shims should fix your pressure problems.

The only issue you may have is if Mr Cummins put a ball bearing into the throttle shaft end to stop you tampering with the fuel pressure.
Some balls were soft and could be drilled....some were hard and needed an oxy torch to spit them out.......WARNING!!!.....point the throttle shaft away from humans and delicate receptacles
if you need the oxy torch option!!

Newto




terry sillik
 Posted Monday, November 25, 2019 1:34 PM
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Are you checking boost under a load? terry
Tattoo
 Posted Wednesday, November 27, 2019 1:49 PM
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Yes. All of my gauge readings were taken while driving. Thanks.
Tattoo
 Posted Thursday, November 28, 2019 12:04 PM
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Very helpful Newto! Thank you.

I had the opportunity to work on the fuel pump today and have a Thanksgiving feast with my family. It's been a pretty good day!

I discovered that my fuel pump button is a #27. On your advice, I pulled the throttle shaft and found that the fuel opening is just over 1/2 way open. By eye, I'd estimate that it's 55% open.

There's a seller on ebay offering #10 and #5 buttons with a housing gasket and a Cummins sticker! for $25. I'm inclined to order a 10 button and see what it does for me. Not sure what I'll do with the sticker though.

Maybe open up the throttle shaft a bit too?


What do you think?

Also, I found some great videos that explain the pt fuel system on youtube. Here's a good one.

Tattoo
 Posted Thursday, November 28, 2019 12:57 PM
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After looking around a bit, I'm finding other buttons available online... #15 #20 #22

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