90 5.7 TBI idle surging

[Bright Idea]

1990 Suburban manual transmission 5.7 TBI. Runs good but after warmup at idle it surges probably 300-500 rpm. only annoying at this point but am concerned as to where this is going. It is not consistent in that sometimes it is worse than at others. No Codes. I am thinking possibly the Idle Air Control Valve ?
Any other possibilities that I should look at ?

 

[gmbellew]

check for vacuum leaks.

also, hook up a scan tool if you can and see what the sensors are doing. I use aldldroid and a Bluetooth aldl reader with my android phone to see the sensor parameters

 

[Matt69olds]

I agree. The idle air control solenoid only does what it’s commanded to do by the PCM. If the computer is commanding the IAC to hunt around at idle, you need to find out why.

As mentioned, inspect all the vacuum lines/fittings. Is the engine getting up to the correct operating temp? EGR valve hanging up partially open?

 

[45acp]

The 5.7k has a problem with the gasket between the tbi and intake manifold leaking( I sold a lot of them in the 90's). I would spray some carb. Cleaner at the base of the tbi and see if it surges. Good luck!

 

[Bright Idea]

No apparent leaks at the TBI base or in the vacuum lines. Did the relearn procedure and things are much better. It also seems it could be coolant temp related. After a longer consistent drive it seems to idle down and remain steady and with short trips it has more of a tendency to hunt around. Coolant temp sensor was replaced within last year. Temp guage ( for what it is worth) seems to be on the cool side. Possibly thermostat not closing or too low of opening temp ? Maybe just not getting warm enough on the short trips ? Possibly the IAC valve is sticky ? It appears to be the original.

 

[jrlichina]

On my 1990 V3500 454 TBI I had a similar issue which turned out to be the magnets in my pickup coil were broken up and causing random firing issues which manifested in surging idle. So take you cap off and see if you have any pickup coil failure/degradation.