I think my cruise control is crapping out on me

[Crunch]

I usually set my cruise control at about 69 MPH or so, and lately I've noticed that my truck is surging when it is engaged. If I cancel it and hold steady on the accelerator there is no surging; it's a perfectly smooth ride. Is the culprit more likely to be the vacuum pump going out, a leak in the line or something electrical? Other than the surging it seems to be working fine.

Thanks.

 

[cerberusiam]

What year of truck and do you have manual or auto trans? Do you actually have a CC unit? Could just be a leak if you actually have a CC unit and not speed control.

 

[Crunch]

What year of truck and do you have manual or auto trans? Do you actually have a CC unit? Could just be a leak if you actually have a CC unit and not speed control.

It's an '03 with an automatic. I've always called it cruise control, but I'm not sure I know the difference between cruise control and speed control. There is a vacuum pump unit between the firewall and the passenger side battery, and a plastic vacuum hose coming off that. It T's off into the area above the firewall, with the other end running over to, and under the drivers side battery. I don't know what the hose is connected to under the battery.

 

[cerberusiam]

It is connected to the CC unit under the battery that actually pulls the bell crank open for the cruise control. You should see a cable coming from under the battery to the APPS setup. It could be a leaking line somewhere, they get brittle with age and sometimes rub on edges and wear thru or crack. It could be a leak in the reservoir under the cowling, the CC unit diaphragm cracked, CC cable sticky, or the APPS misbehaving, or something in the ECM, or even injectors. Surging on that year with that setup can be a bugger to find. Oh, and bad batteries or an electrical system can cause it also.

Speed control is done in the ECM so no CC unit to worry about, it is all electronic. Manual trans trucks normally would have that but some still had CC, mish mash of configurations at times on the 03-04 trucks. 05 and newer was all speed control as the APPS moved to the firewall and no more cables.

Does the tach move at all or is steady and you are feeling the surge thru the SOP meter?

 

[Crunch]

It is connected to the CC unit under the battery that actually pulls the bell crank open for the cruise control. You should see a cable coming from under the battery to the APPS setup. It could be a leaking line somewhere, they get brittle with age and sometimes rub on edges and wear thru or crack. It could be a leak in the reservoir under the cowling, the CC unit diaphragm cracked, CC cable sticky, or the APPS misbehaving, or something in the ECM, or even injectors. Surging on that year with that setup can be a bugger to find. Oh, and bad batteries or an electrical system can cause it also.

I originally discounted the leaky line factor because the surge does not always occur. I figured that if the line was leaking it would happen all the time. It's been extremely hot here so I haven't been motivated to get outside and listen to the vacuum pump and find out if it is running constantly. We've got cooler weather coming up, so I'll do it then. What I find interesting is that you mentioned the APPS. I threw an APPS code and had the dead pedal a few months ago and was going to replace it, but it hasn't happened since so I've procrastinated. I also considered injectors, but the surge doesn't seem to happen if the cruise control isn't engaged.

A friend of mine had a cat get up under the hood of his '03, and when he opened the hood the cat freaked and broke the vacuum line. Unfortunately for him, it broke somewhere under the cowling and he hasn't been motivated to fix it--for several years. I'm spoiled on things like cruise control, power windows and door locks so I couldn't live like that.

Batteries are only a year or so old, so I doubt they are the problem and the tach seems to be steady as a rock. Based on the information you have provided I would say that the problem is going to be the APPS, vacuum line or injectors. 336K on the injectors, but they still run smooth and other than getting the connectors reseated I haven't had any trouble with them. That said, I still know that I am living on borrowed time in that regard.

 

[cerberusiam]

I woulf look at changing the APPS and FCA if you have to do something after checking for vacuum leaks.

 

[Ozymandias]

question; if you drive with the cruise on - do you see your accelerator pedal going slightly up and down?
is it worse at low load? more downhill and flat then uphill?

had something similar and cured it.

 

[Crunch]

question; if you drive with the cruise on - do you see your accelerator pedal going slightly up and down?
is it worse at low load? more downhill and flat then uphill?

had something similar and cured it.

Haven't noticed the accelerator pedal moving, but I don't remember ever looking down to check it, either. I notice it more when using cruise between 68-70 MPH and running on flat road. Seems to do better going uphill. What did you do to cure it for your truck?

 

[Ozymandias]

please check first if it is moving up and down in a nice frequency when cruising.
otherwise my solution is not your solution.

 

[Ozymandias]

No time to check this?

 

[Crunch]

No time to check this?

Sorry I haven't got back on this. The city has been replacing water mains on my street and it's screwed up my schedule. I haven't even been able to wash my truck because of the daily dust storms. When on level highway and driving while in cruise control I would put my foot gently on the accelerator and try to hold it in place to see how it would react. It didn't seem to move much at all. Since I had an isolated APPS issue earlier this summer I think I am going to start with that. If that fails, I will probably move on to injectors. I've started to notice that when accelerating when driving the truck for the first time in a day it's almost as if the APPS drops back to its idle position, then returns to where it should be. Since my foot is pressing on the accelerator at the time I would think an injector problem would send some kind of feedback in the form of vibration back to my foot. The engine doesn't miss at all, and continues to run very smoothly. It's feels exactly like if you take your foot off the accelerator while driving up a hill. 338K on the clock, so I can't discount injectors. But I'll try the <$200 solution before I try the $3K solution.