2.5 surges

i have a 1987 s-10 2wd drive with a 2.5l 4 speed. it has a rough idle and surges when your going down the road. this is what i haven done so far. new plugs, wires, distrubtor cap, rotor button, changed oil and filter, egr gasket,air filter, fuel filter, pcv valve. what else could it be, i have look at all the vacmun hoses and couldn't see any leaks. any help would be grateful.


rodney

Rodney,
welcome to AF!
all this maint. you just did AFTER it started surging?
if this problem started before some of this maintenance,
check the distributor cap. many aftermarket ones are made less than desirable, with a possible crack in a cap, this will definitely affect idle/power surges.

even though you looked at the vacuum hoses, spray some carb cleaner in different areas around your engine bay while running at idle. if the engine idle jumps, when you spray a certain area, you probably have a leak in that vicinity, as the carb spray entered into the system.

if the problem was before all of the items you listed;
on the side of the GMs rounded air cleaner is the MAP (manifold absolute pressure) sensor; the pre-cursor to the MAF, which has electrical and/or vac running to it.
this would be the first item I'd bench test, the second id the throttle position sensor (TPS) which will be located on or near the TBI unit.
AutoZone's have sensor testers, that can TEST both these items, before purchasing, if they are not bad.

it could also be a VSS (vehicle speed sensor), but I think these may have only been on S10s with auto trans. (unsure)

hope this helps,
chris
nrt racing

my 2.5 surges because I disconnected the vacuum from my egr valve. Sometimes when it goes into closed loop the computer doesn't set the code and then it will surge or have an extremely high idle for maybe a minute. Usually my check engine will come on but when it doesn't is will surge. May be totally unrelated to your problem but figured I'd throw it in.

i have changed out the trottle position sensor, and also checked out the map sensor, everything seems to be alright. i will check the cap and rotor button. here is something else i found out with a timing light, the back 3 wires are firing fine, but the number one cylinder has a miss fire every now and then, what causes this..


Rodney

clean and reinstall your IAC valve.

This is a vacuum problem for sure.

Check ALL vacuum hoses in the engine bay. Better yet, replace them all.