3000 Rpm Limit

[Eye Relief]

I am hoping someone here can help me out. I have a 2001 GMC HD 8.1 Allison crewcab. Bought new in Jan 01. I have replaced cam and crank sensor, wheel hub assembly, tailgate straps and transmission NSBU switch. That is all that I can recall at the moment. 165K miles, 3 sets of tires, and new shocks all around and a K&N air filter.

 

Intermittently, I find that I can not generate more than 3000 RPM. Truck always starts fine, shifts smooth, accelerates fine as long as RPM's stay below 3000. Idles and accelerates as smooth as glass. Also, in park, if you depress the accelerator, RPM's rise smoothly and quietly to 3000 RPM's, but no more. At that point the engine sounds great, but RPM's won't rise any higher regardless of pedal location. If you floor the pedal rapidly, the engine cuts at 3000 RPM and holds there.

 

 

Any ideas?

Hooked up my code scanner, but no codes.

 

Thanks in advance for the help

[ryank1]

In park it is not supposed to go any higher than that, it is designed to keep from blowing the engine. As far any other time it could be a problem with the fly by wire system.

[dalejrfan88]

Catalytic converter.

[Eye Relief]

ryank1 and dalejrfan88 thanks for the responses. I have since learned that in neutral there is a rev limiter @3000 rpm, but didn't know it applied to park as well. I questioned the catalytic converter, (boy do I wish both of them were bad) but I continued to observe how the truck was acting over the weekend and have determined that the problem is intermittent, not related to temperature, and also when its acting up, the cruise control will not engage. I am considering the fly by wire, because I have also noticed that at cruising speeds, say 60 or 65, cruise turned off, if I take my foot off the gas, the vehicle is very slow to decelerate, taking over a mile to go from 65 to 55 mph. If I come to a stop however, the idle RMP's are appropriate, around 550, and the vehicle does not want creep more than usual at idle.

 

I am also wondering if the NSBU switch has gone bad yet again. I dug up the receipt today, I replaced it over a year ago. It seems that this problem started after a snow we had last week that left about a foot in the yard. A lot of slush on the road therefor a lot of water spraying up under the truck. Does this make sense to you guy's?

[Eye Relief]

Well,

 

Last night I changed out the NSBU again. It was bad. All the above mentioned problems cleared up as soon as I changed out the NSBU. As stated before, these problems started after about a foot of snow fell last week. The dealer advised me that as soon as the snow's start coming, these parts start going out the door.