Delayed power?

[NH8970]

Just bought a 98 Newmar and tested it out last week. I was in the mountains and the power was very slow to react when I put the pedal down. I could count to 10 before the engine would start pulling and then it would accelerate up the hill. Before I felt the power kick in there was black smoke just falling from the tailpipe. Then when the power came the smoke changed to a normal grayish color and was forcefully coming out of the tailpipe. What could cause this?

[CharlesinGA]

What engine? how many miles? has it been sitting a very long time?

Charles

[Skip426]

1998 was a change over year for Cummins , the 8.3L engine changed from a mechanical injection pump ( Using a P-7100 pump ) to an electronic controlled fuel injection system CAPS ( Cummins Accumulator Pump System ) .
Troubleshooting your issue would be easier if we know exactly what system you have and if your coach is built on a Spartan chassis.

Looking at the engine from the rear access , do you see the two solenoids ( white arrows ) in this picture.

[NH8970]

Spartan
Cummins (C8.3)
Allison automatic World Transmission

Those solenoids aren’t there.

The title is a 1998 but the owners manual copyright is 1997. The black smoke while floored with low power leads me to think it’s a boost issue. The waste gate looks to be controlled by connection to the fuel pump but I don’t know enough to tell if the fuel pump/waste gate interface is mechanical or electrical. I’ve got an OBD scanner but haven’t found a port to plug into. Do engines like these have OBD ports? I took pictures but unfortunately can’t upload them directly from my phone. I’ll add them after I get them uploaded to a URL to link to.

[Skip426]

The C 8.3 Cummins is strictly mechanical , no diagnostic connection .

Even if there was , it's a different connection and software than OBD II .

Solenoids are on the CAPS pump.

With black smoke then acceleration ; the injection system is suppling fuel but there isn't enough ( turbo ) air supplied to burn it properly .

Be sure to reduce the size of any pictures you try to upload to iRV2 ; 1000 Pixels works best for me .

EDIT: There maybe a diagnostic connection for the Allison transmission ; but only for the tranny.

[NH8970]

Originally Posted by Skip426

The C 8.3 Cummins is strictly mechanical , no diagnostic connection .

Even if there was , it's a different connection and software than OBD II .

Solenoids are on the CAPS pump.

With black smoke then acceleration ; the injection system is suppling fuel but there isn't enough ( turbo ) air supplied to burn it properly .

Be sure to reduce the size of any pictures you try to upload to iRV2 ; 1000 Pixels works best for me .

Here’s some pictures

[NH8970]

I forgot to add the mileage, it has 180k on it.

Another thing that crossed my mind is the slope of the road. The fuel tank is up front and I’m wondering if it’s having issues drawing enough fuel from the tank until it gets help from gravity on the upslope. The soot-black smoke when I first floor still makes me think it’s a boost issue though.

[bubby124]

Do you have an exhaust brake it could be sticking closed

[NH8970]

Originally Posted by bubby124

Do you have an exhaust brake it could be sticking closed

PAC brake on the intake side of the turbo. I’ve never had one before. I guess restricted air could be the problem. And I did use it on the downhills out of habit even though it probably wasn’t needed.

[bubby124]

Originally Posted by NH8970

PAC brake on the intake side of the turbo. I’ve never had one before. I guess restricted air could be the problem. And I did use it on the downhills out of habit even though it probably wasn’t needed.

Pac brake should be on the exhaust side but if it sticks it will affect power

[NH8970]

After a 1000 mile trip I’ve determined that the PAC brake is the issue. I never used it and never had a problem.

[CharlesinGA]

I've posted this before.

If the PacBrake is sticking, you might just consider replacing it with the newer PRXB unit which is more effective at braking than the original one.

Here is the chart of RV PacBrake Upgrade part numbers by engine and connection type.

Here is a short PB video about it. https://youtu.be/dUxtftraI2o?si=UkLB2NVYzirl0A5X

One minute short video https://youtube.com/shorts/F1-q-rh-N...iFD14kSihgpe1-

Charles

[Ray,IN]

Buy a small container of high-temperature silicone oil and lubricate the exhaust brake moving joints, including the butterfly valve bushing.
It can't be moved by hand unless you're stong enough to overcome the spring return, or the linkage is disconnected from the air cylinder. If it can't be moved by hand then, it must be removed from the exhaust pipe, cleaned and restored to working order.

[SafariBen]

It sounds like your pacbrake is moving, just slow to move, or possibly the electrical is slow to disengage? I'd use a lube, any lube, even soapy water, and a hot wire or momentary switch like another member suggested, and turn on the solenoid while watching the pacbrake, with the engine off. It should almost slam open and closed. Work it several times while lubing the shaft and linkage, then start the engine to dry if you used water. If the issue still persists when you use the pacbrake, then I'd investigate the electrical side, but couldn't tell you where to look exactly, other than I would try to follow the wire from the pacbrake air solenoid to the front. It shouldn't be as hard as it sounds as it should be identified and carry that same ID through the harness to a connector at the front, then add a switch there to turn off the pacbrake manually to be sure it's off in those cases.