1970 Kenworth K100

[rlp in Co.]

I just got this truck running and have taken it for a couple of test drives. The problem is, it is very hard to shift because the engine is very slow to idle down. Sometimes the truck slows down faster then the engine and you have to go back in the lower gear. At first I put an extra spring on the fuel pump but that was not the problem. It must be something wrong inside the fuel pump. Tomorrow I'm going to put a fuel pressure gauge on it and see if the fuel pressure is slow to come down. Has anyone heard of this problem before and knows what is wrong?
It is a cummins 335 HP, small cam engine. It has high flow injectors that takes it to 575 HP.

 

[willie in mn]

Couple things come to mind.
Are you using clutch or "floating" it? If doing it one way, try the other.
How high are you running in each gear? Yours is a Cummins, each has its "sweet spot". Cummins, Mack, Cat like it better if shift at around 1550-1600.
Had an awful time when first switched from Detroit 8V71 & 8V92 to Cummins 14 liter(855 cu in), took a good 1/4 to 1/2 mile to get the feel of it. Never rattled another tooth until I retired 9 years later.
Others may have different ideas. HTH
WJ

 

[rlp in Co.]

It doesn't make any difference if I float the gears or use the clutch. The RPMs just won't come down fast enough. It is impossible to shift at 1600 RPMs. If I take it up to 2100 then sometimes it will shift. I counted around seven seconds for the engine to go from 2100 to 500.
I drive a 96 Volvo with the super 10 transmission. It is very easy to shift. I shift it at 1200 and 1550. This old Cummins seems to like higher RPMS. I feels like full power comes in around 1500 to 1800 RPMs, but I could be wrong. Maybe somebody knows where the peak HP is on this engine.
Going out now to put a fuel pressure guage on it.

 

[terry wh]

if the truck sat for awile i would say your pump is sticking on it in side the pump there is a piston that moves you might be bringing the pedel back but the piston in the pump is not comeing back it might come out of it you might try some type of cleaner its been maney years since i drove a 335 the one i drove was geard for 62 mph at 2100 but you could shift it around 1800 thelast cummins i drove we ran it around 1800-1900 rpm that was it power band

 

[Old Sarge]

If it has sat for a long time the comment on the fuel pump is on target. It could well be the piston in the fuel pump is sticking. Possible rebuild is in order.

Course it has been 30+ years since I drove one equipped with a 335.

 

[rlp in Co.]

Thanks to all for the ideas. I got the jake brake working today and it shifts good with the jake on. I will try the cleaner idea too. I'm going to get some new fuel lines and fuel return line next time I go to town. I thought maybe the fuel return line might be partially pluged up. They both are frayed and need changed anyway. The truck has been setting for a long time.
Now I have another problem. When I shift from 9th to 10th, the splitter won't shift down and it goes into 11th. Then when I give it some gas, it shifts down. I tried preselecting the splitter and also shifting the splitter in neutral, but either way it won't shift down till I get it in 11th and then step on the throttle. Looks like I'm going to be learning a lot about rebuilding transmissions. I have twisted every bolt on the truck except for the engine and transmission. The engine was rebuilt and has about 80,000 miles on it. It really has the power. When I pour the coal to it, I can see by the shadow that it is pouring out some black smoke till the turbo gets spooled up. I would like to find an anaroid valve to put on it. From what I understand, it holds the fuel pressure down till the turbo pressure builds and you still have almost as much power but no smoke.

 

[rlp in Co.]

Something must be wrong, like you say, a piston sticking or something. I will try some cleaner.
I had an idea that maybe the power band was higher on this engine than the Detroit that I drive. At 65, the RPMs were around 21 or 22 hundred. I haven't had the courage yet to see where it is governed out at. What is it supposed to be governed at, and how high can you safely go?

 

[sixtyninegmc]

In no particular order, I would not run it over 2100 RPM, most of the 855`s are governed there. 855 cummins drop RPM`s notoriesly slowly, many people shift them on the jakes for that reason, I would guess turning it up as far as you did compounded the problem. On that note, most I know that have ran hopped-up cummins have found that reliability and true pulling power on the 855s drop off after about 430-440 horse. Also, in my experience I got the most out of these engines when you run them to the governor (2100) when you really need power, or else you drop to much speed in the shift and it falls out of the powerband in the next gear.

As for your splitter sticking, before you rebuild the trans I would take a look at the air-shift actuator on the tranny, and all the lines to it, and some of them have a inline filter in the airline to the splitter that will plug up. If the splitter sticks only going into 10th (or 7th, as opposed to 7th-over, what you would call 11th) it is only because of the different load on the powertrain making it harder to shift at that point, the splitter is the same gearset that makes every other split.

That said, much of this is my opinion, hope some of it helps, don`t take any of it as gospel.

Josh

 

[terry wh]

i think it set at 2350 but not shure but if you run it at lower rpm the engine will last lomger i would try to run it at 2000 or less to keep the engine better try to run some air tool oil in the shifter

 

[terry wh]

just remember the older engine are set up for higher rpm for pulling not like the newer one you can run around 1200 to 1500 the tork range are higher on the older ones you will have to drive it to see were it will pull the best and not luging the last truck i drove was a 1981 pete with a 400a cat you had to run that one between 1900 to 2100 other wise it would smoke so bad that was the power band you will run it you will find the best were it will run and mialage out of it

right now im putting a 353t detroit diesel in a 1959 f100 4x4 short box they want me to run it at 1900 to keep it from melting down if i ran it lees and pulling it would heart it but the rpm is set at 2695 on the tag

 

[rlp in Co.]

I would like to have a truck like you are building. I always liked that body style. I always like to hear those Detroits run too. What kind of transmission are you going to use and what gear ratio rear end will you run. Do you have any pictures? Put some before and after pictures up if you can. I want to take a picture of the KW one of these days. Keep us posted on the progress that you are making. I and others, would be very interested. I love trucks! ha
That was a good sugestion that you made about putting oil in the air lines. I should have thought of that.

 

[rlp in Co.]

I was going to look for a filter on the air line today but I forgot. I got distracted by putting some heated mirrors on today. I got one on; took longer than I thought because I had to modify the brackets quite a bit because it is larger than the old one. I still have to finish the other one. It had a broken wire inside the mirror and I now have the mirror all apart. They were off of a freight shaker cabover that a guy GAVE ME! I took the front axle off of it and put it under the KW because mine didn't have front brakes or power steering. It is the old air cylinder type which I still need to find a cylinder for.

 

[sixtyninegmc]

usually if they have a filter it is in one of the fittings, pull the fitting and there it is. You can pull some of the actuators off the tranny with no other work, I`m not sure on the older 13 speeds. Sometimes you can clean up the piston and put new o-rings for cheap.

 

[terry wh]

i have a chevy nv 4500 5spd and a 208 tc and 411 gears if you go to the web site i told you my handel is 1964reo

 

[Rick L.]

check the return fuel line if it is plugged or crimped the engine won't idle back down properly. i worked for a freight co. and we used to put visegrips on the return line to get some extra speed out of the truck, but this made it really hard to shift because they wouldn't idle back down.

 

[rlp in Co.]

I hope that you are right about the fuel return line. I'm going to check that out as soon as I get a chance to work on the truck again. I had never heard about the vice grip trick.

 

[jtannsteven9]

(quoted from post at 19:06:30 10/03/11) I just got this truck running and have taken it for a couple of test drives. The problem is, it is very hard to shift because the engine is very slow to idle down. Sometimes the truck slows down faster then the engine and you have to go back in the lower gear. At first I put an extra spring on the fuel pump but that was not the problem. It must be something wrong inside the fuel pump. Tomorrow I'm going to put a fuel pressure gauge on it and see if the fuel pressure is slow to come down. Has anyone heard of this problem before and knows what is wrong?
It is a cummins 335 HP, small cam engine. It has high flow injectors that takes it to 575 HP.

Hey all,

I've been following this discussion on the 1970 Kenworth K100 with interest. I had a bit of a similar situation with my rig. It's a tricky beast when it comes to shifting, especially with that slow idle down. Reminds me of when I was dealing with my own truck's quirks.

About your K100, it sounds like you're on the right track suspecting the fuel pump. I've seen a few old Cummins behave that way. It's like trying to dance with two left feet, isn't it? When I had a similar issue, the guys at WWTC were pretty sharp at diagnosing it. They know their way around these older engines.

If you're ever near WWTC or looking for some advice, might be worth vist them, you can see where they are here: https://wallworktrucks.com/truck-repair . They've got a good handle on these classic workhorses. Just a thought from one truck enthusiast to another.

Keep us posted on how it goes with the K100. Those old Kenworths are something else, aren't they?

Safe travels!