Engine surging at low throttle


Burkster
10-23-2010, 10:59 PM
98 Jimmy 4.3 106,000 miles, replaced plugs and wires
Engine surges slightly at low throttle, kind like it stumbles slightly. Runs fine other than that. No CEL , runs fine at mid or high throttle under load. Problem seems to show up at 45-50mph in OD when throttle is slightly pressed.It just seems to surge a bit no bucking or such, when throttle is pressed more it seems to go away. Any ideas?

Thanks

old_master
10-24-2010, 06:56 PM
Could be a problem with the EGR valve. Carbon deposits can and will get stuck in the valve and passages in the intake manifold. The passages can all be cleaned out using carb spray. When you reinstall the valve, use a gasket with a screen in it.

http://i561.photobucket.com/albums/ss58/cwhook/2_1357.jpg

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_EGR-Valve-Gasket-Tomco_15470097-P_283_R%7CGRPGASKAMS_271725256___ Usually if the valve doesn't close completely, the SES light will come on and set a DTC in memory, but if the passages are partially restricted, it could cause a surge without turning the light on.

Have the oxygen sensors been replaced yet? Oxygen sensor reaction time slows with age. At 100,000 miles they have slowed significantly. Possibly enough to cause a lean surge. A scan tool can spot this before the SES light even thinks about coming on ;) Might want to at least have them checked, if not replaced because they're due. There are 3 of them on your 98 and it's not a difficult job to replace them.

93gmcjimmy
10-29-2010, 02:47 AM
Old Master does his vehicle have a fast idle valve. I had a very similar problem on a Honda Accord, turns out the fast idle valve was clogged. I took it apart and cleaned it out and problem went away.

old_master
10-29-2010, 03:46 PM
It has an Idle Air Controller, (IAC) on the driver side of the throttle body. The passages in the throttle body and the IAC can be cleaned, but be very careful not to push or pull on the valve, it can/will strip the gears that operate it, very easily. The IAC can be reset but requires a capable scan tool. The cheapie $400 or $500 jobbies that the auto parts stores use are not capable of resetting the IAC.

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