Cruise control can't keep up - low idle?

GENEG

MEMBER
1990 PONTIAC FIREBIRD
138,000 MILES

When I set my cruise control it tends to have trouble holding the set speed, esspecially when there's even a slight uphill slope. On level ground it's will hold the speed initially but will drop off anyway. One thing I found remedies this is if I tap the gas. This seems to 'reset' things and it I'll hold the speed again for a little while. The fact that tapping the gas helps leads me to believe that if the idle was set higher, there would be no problem. The engine idles at 500-600 rpm. I do not know the correct idle setting for this vehicle. Do you think the idle may be the issue & if so what is the correct idle setting for the 1990 firebird formula; it's a fuel injected 305 chevy small block.

Tuesday, September 20th, 2011 AT 3:05 PM

5 Replies

JACOBANDNICKOLAS

MECHANIC
110,114 POSTS

In park it should be around 900rps's. As far as the problem, check for vacuum leaks. That is my first guess for the low idle as well as the problem with the CC.

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Wednesday, September 21st, 2011 AT 3:39 AM

GENEG

MEMBER
3 POSTS

The car had a bad ECM that caused it to idle high so I messed around with the idle screw alot & definitely left it screwed too far in. The new ECM has many issues fixed but I never re-adjusted the idle screw thinking 500-600 was ok. I will readjust it to idle at 900 in park, think that should solve it. Thank you!

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Wednesday, September 21st, 2011 AT 4:09 AM

JACOBANDNICKOLAS

MECHANIC
110,114 POSTS

I don't think that will eliminate the CC problem. I really believe you have a vacuum leak. That would cause a low, rough idle and cause the CC to lose vacuum pressure and slowly drop speed. That is my guess at this point. As far as the idle, 900 in park or neutral and it should be around 6-700 in gear.

Let me know what you find.

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Wednesday, September 21st, 2011 AT 4:11 AM

GENEG