1996chevy astro van idels at high speed intermitenly seeems to be worse when the van has been running for a while and also when the humidity level is up

that could be a bad tire or an out of balance wheel the firs thing i would do is get a new/used tire from the local junk yard and see if that fixes it if not try a wheel from the junk yard or you could just put the/a full size spare on it i believe that should fix it let me know hopfully this helps

You need to get it into a repair shop

You can't ride around for days,weeks or months
with a check light on

You know that tells you there is a system failure

You have a driveability issue as well as fuel consumption

No one can tell you what is wrong ovet the internet,has to
be checked out & testing performed

1999 CHEVROLET TRUCKS ASTRO VAN 4.3L 6-cyl Engine Code [W]
CAPACITIES: Automatic Transmission, 4L60-E Initial Fill..........4.7 liters
Automatic Transmission, Total Fill 4 speed 4L60-E..........10.6 liters
To check fluid level, transmission should be hot and engine running at idle. Pull the transmission dip stick out "red" wipe clean and re-insert. Check level of transmission fluid as indicated on the dip stick using the second reading and make sure the Astro is parked on leveled ground. Add Dexron III or VI as needed and do not over fill.
Thank you for using Fixya and good luck

Feel it throughout van, or just up front? Tire balance has been checked? That is rear wheel drive correct?

check idle air control,

No,the oil ratings 10w30 and so refer to how the oil viscosity reacts to hot and cold.As far as running rough,when was the last time is was tuned up,cap and rotor if applicable,spark plugs and air filter?Is it carburated or EFI?

It is perfectly normal for the idle speed to increase with the AC on. This is done to ensure that adequate coolant flow is available for the increased strain the compressor puts on the engine.

If the idle increase is too high, there is a safety problem. There is an idle adjustment that can be made to reduce RPM to factory specification when the AC is on.

I have a 2000 Astro had the same troubles.I saw that the sides of the coil were burned looking.In between the bar that goes around it. This started around 110K It would run badly when wet or high humidity.Then throw up the check engine lite.Put on a new coil so far the problem has no reocured.

Try cleaning the trotle body at the butterfly,they tend to carbon up and not let proper air mixture at idle,tailtail sign some times is feels like the throtle pedal sticks or pops when depressing it.

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100 % it will be the main blower fan for the heater/aircon has to seperate bits so the heater bit has gone