Cruise control performance question

[Sld_Gld]

My C4 is a 1985 with automatic transmission and has 115,000 miles on it. With only a few minor exceptions, the vehicle is completely stock. When driving on a level surface with the cruise control engaged, the engine pulsates. While not enough to observe a variation in rpm on the tachometer, the engine pulsates as if I've pressed very slightly on the accelerator. Might there be any action I can take to eliminate the pulsating?

 

[Hib Halverson]

Not quite understanding the use of the term "pulsate".

Do you mean the engine is misfiring at light throttle tip-in situations?

Or, do you mean the engine RPM rapidly fluctuates.

 

[DonB]

Make sure you have good vacuum connections at the manifold vacuum source and at the cruise diaphragm (pancake) including any other hose connections. Check the vacuum hose(s) for kinks and cracks too. Any loss of vacuum will cause it to "hunt" of fluctuate. A good way to find a vacuum leak is to spray a little bit of carb cleaner(Gummout), using the straw, on each junction. Just a quick shot. When the engine RPM increases, that''s the spot that's leaking(or sucking air). The stuff IS FLAMMABLE, so be careful.
I'd also check the engine manifold vacuum at idle to be sure there isn't a more serious cause of the fluctuation. On a vacuum gauge, it should be around 18" and somewhat steady. The needle CAN wander a bit but sharp pulsations are an indicator of a potential problem. Let us know what you find. Good Luck.

 

[Sld_Gld]

Not quite understanding the use of the term "pulsate".

Do you mean the engine is misfiring at light throttle tip-in situations?

Or, do you mean the engine RPM rapidly fluctuates.

Surge as DonB has referred to in a subsequent reply to my question is a better description of the problem. I'll be checking for the potential fixes as he has suggested. Thanks for your help!

Dennis

 

[Sld_Gld]

Make sure you have good vacuum connections at the manifold vacuum source and at the cruise diaphragm (pancake) including any other hose connections. Check the vacuum hose(s) for kinks and cracks too. Any loss of vacuum will cause it to "hunt" of fluctuate. A good way to find a vacuum leak is to spray a little bit of carb cleaner(Gummout), using the straw, on each junction. Just a quick shot. When the engine RPM increases, that''s the spot that's leaking(or sucking air). The stuff IS FLAMMABLE, so be careful.
I'd also check the engine manifold vacuum at idle to be sure there isn't a more serious cause of the fluctuation. On a vacuum gauge, it should be around 18" and somewhat steady. The needle CAN wander a bit but sharp pulsations are an indicator of a potential problem. Let us know what you find. Good Luck.

Thanks for your help, Don. I'll let you know what I find based on the information you have provided.

Dennis

 

[Sld_Gld]

Thanks for your help, Don. I'll let you know what I find based on the information you have provided.

Dennis

While checking the vacuum hoses for a potential leak, I discovered the electrical connector was not making proper contact with the pins on the cruise diaphragm (pancake). Just took a test ride...hasn't been performing this smooth while in cruise control for quite some time. Thanks again for your help!

Dennis

 

[DonB]

You gotta love an easy and free fix...Great job. Congrats.