M809 series truck slow to return to idle

[1958 M274]

I have a M816 wrecker that is slow to return to idle when I let off of the accelerator. When you accelerate, then let off the throttle to change gears the rpms "float" a little and it takes 3 or 4 seconds to return to idle. That doesn't sound like a long time, but when you're trying to shift gears going up a hill it is!

I know this is a symptom of air in the fuel system, but I don't think that's it. The truck starts easily, runs well, and there is no lag in throttle response when accelerating. It runs the same way on either fuel tank. It also smokes noticeably more, both idling and driving down the road, than other Cummins powered 5 tons that I've been around. I drove it over 100 miles home and it ran well except for that.

After searching the forums, I ran across someone that had similar problems, and he found that he was getting a constant fuel spray into the manifold from the cold start system. I cracked open the bleeder screw by the nozzle in the intake manifold with the truck running and didn't get any fuel out, so that isn't the issue.

I also checked and lubed all the throttle linkage and everything moves freely.

Any thoughts?

Thanks,
Joe

 

[73m819]

My 819 did the same thing, my hand throttle would work out a bit

 

[Floridianson]

Yea you can check hand throttel.
I will guess and say you have an air leak somewhere. If your lucky it will be in the fuel lines somewhere.
If your not lucky it the PT it's self .Don't know on this model pt but tac seal can leak, throttel shaft I believe can leak. Start with the fuel lines and the O ring for the filter. Replace all the fuel slector rubber lines if you have not done that yet. Brake lose all metal fitting and go for a retight.
You can if you like pressoreize the fuel line system and look for leaks. When you are shure it is not the fuel lines. Take the PT in and while there at it have it pumped up a bit.

If you have a remote clean tank use that while it is sitting on top of the motor/ inerfender. Rules out air leaks in the lines real fast

 

[jcappeljr]

I have had a few 5 tons do that,most likely sucking some air.

 

[Floridianson]

Maybe Will will chime in here as he is pretty hot on the cummins

 

[FormerNewMVGuy]

There was another thread concerning this and the primer pump line was connected to the presurized port on the ip, should be on the inboard port on the ip

 

[jatonka]

I will go with Floridians thoughts, I had air leaks and removed their effects by using a can on top of the cab to test the system. I found the rubber lines to be leaking. JT

 

[Westech]

Mine did that same thing.. started right up.. ran great but long response time for Idle... Replaced all the rubber fuel lines and presto... perfect.

 

[Floridianson]

There was another thread concerning this and the primer pump line was connected to the presurized port on the ip, should be on the inboard port on the ip

If there was a lot of black smoke at idle and slow to return it could be the primer is on the wrong port on the PT. By unlocking the primer the gage should not read anything. Ask me how I know. There was the thread I believe you were talking about and I said that mine showed pressure when running an a open primer. Wrong again I was but someone got me/us straight as to the correct port. Good thought though but I will stick with air leak if his machine is not throwing a lot of black smoke at idle.

 

[MyothersanM1]

Brother Will Wagner probably has the 100% correct answer. However, check your fuel return lines that they are all up to snuff; not leaking or pinched. I have been told in the recent past that this is where this kind of problem rears its head.

 

[Ferroequinologist]

Definitly air in the lines. A wrecker has two tanks, like a m818. There are tons of hoses under the cab for the tank selector. EMD567 was having the same issue- fuel lines looked great- but once he changed them we could bend it and see the line was full of crakcs that went all the way through.

Only other thing, is to make sure that when you let off the throttle, that the spring is returning it to idle at the pump- a weak spring or binding rods from the petal can also delay a return to idle. Have someone hold the petal to the floor, as you watch at the pump, and have them let off it quick- it should snap right back to idle.

but my money is on air in the fuel lines.

 

[EMD567]

I had 3 air leaks on the supply line. THe 814 ran like crap, would not idle worth a **** when hot, and in a hard pull, the power would just go away. Finally replaced the flexible fuel line from the filter to the IP, and a brass fitting near the tank. Now the cummins runs so sweet, and idles so nice, it's hard to believe it's the same truck.

 

[MyothersanM1]

I was amazed when I did all my fuel lines on the 818. The vehicle (I) took on a whole new positive outlook on life.

 

[1958 M274]

My 819 did the same thing, my hand throttle would work out a bit

Only other thing, is to make sure that when you let off the throttle, that the spring is returning it to idle at the pump- a weak spring or binding rods from the petal can also delay a return to idle. Have someone hold the petal to the floor, as you watch at the pump, and have them let off it quick- it should snap right back to idle.

As I stated in my original post, I've checked and lubed all of the throttle linkages. I also checked both the hand throttle from the cab and the one from the controls on the bed and everything works smoothly without binding.

There was another thread concerning this and the primer pump line was connected to the presurized port on the ip, should be on the inboard port on the ip

If there was a lot of black smoke at idle and slow to return it could be the primer is on the wrong port on the PT. By unlocking the primer the gage should not read anything. Ask me how I know. There was the thread I believe you were talking about and I said that mine showed pressure when running an a open primer. Wrong again I was but someone got me/us straight as to the correct port.

That's the thread I was referring to where someone was getting fuel sprayed into the manifold from the cold start system. I've checked that too. The primer pump line is connected to the correct port and I'm not getting pressurized fuel at the manifold with the truck running.

Good thought though but I will stick with air leak if his machine is not throwing a lot of black smoke at idle.

As I stated in my initial post I am getting noticeably more smoke, both at idle and while driving down the road, than other 809 series trucks that I've driven.




To anyone that has experienced air leaks in the fuel system... Did any of you only have the symptom of your truck being slow to return to idle, but otherwise it ran great?

I've experienced air leaks before on a 813 and a different 816. Both of them were hard to start, and throttle response was delayed (you step on the accelerator when starting to drive off and it took a few seconds to have any power), in addition to the "floating" rpms that I'm having with this truck. The truck I'm currently working on runs really well. It starts easily, is not lacking in power at all, and throttle response is quick when accelerating, even after idling for an extended period (that's where I really noticed the air leaks on other trucks).

Due to the lack of other symptoms and the additional smoke, I was leaning towards it getting excessive fuel somehow. Would a restricted fuel return line (say if a rubber hose at the selector valve had swollen up inside) cause my problem?

 

[Floridianson]

That would I believe increase rail pressure. Guess you could check that easey enough just take off the return line at the motor and let it drain into a buckett right.

 

[EMD567]

Just undo the return line where it goes from the flexible hose near the frame rail. Point the hose into a 5 gallon container, crank it up, and see how it runs. I did this to confirm that I didn't have a problem with my IP sucking air back thru the return line.

 

[mslogr]

I agree with the air in lines and clogged return. But if you have clogged filters it will do exactly what your saying.

 

[R Racing]

I had a very slow return to idle. But no power loss and I had alot more smoke , But mine was also a hard starter. Replaced the rubber line and I couldnt believe the difference ! Cured all my trucks ills

 

[Floridianson]

I rember on one of the trucks I had a cracked primer line that gave me problems. I took the line off and pluged the PT port.

Just as a side note the USMC 817's they did not use the PT port but drilled and taped the incomming fuel elbow that screws into the backside of the PT for the primer line. Go figure. They also have a clutch peddel lock out so they wont start inless you push in the clutch.
O well hope you find it.

 

[1958 M274]

Thanks for all of the replies and suggestions! It will probably be a week or two before I'll have some time to work on the truck again, but I'll let y'all know what I find.

Joe