91 High Idle Problem

Replaced the valve seals 3 days ago. Drove it within an hour after the valve seal change & it ran fine. Washed the engine the next day. I didn’t run the engine until about 3 hours after the wash. The truck now has a high idle. When I did start & drove it, I was getting a flickering “Service Engine Soon” light. I have not had the “Service Engine Soon” light since. It has been driven about 75 miles since the valve seal change. I changed the Idle Air Control today. It did not help. Had the codes read today & there were none. While I have the truck idling, the rpm’s will sometimes drop, then rise & then return to a stable rpm but it’s still high. I did not disconnect the battery before I changed the valve seals. I have pressured washed the engine before with no problems, this wash was just a water hose wash, so I don’t think I have a water issue.

Any Suggestions??

Thanks,
Willie

check vacuum hoses you might have nocked one off

Checked all vacuum hoses. They are all connected & none appear to be cracked. Since my 1st post, I’ve disconnected the battery to hard re-boot everything. Still having the same symptoms. The spark plugs were changed with the valve seals 3 days ago.

Willie

Check the TBI gasket. I searched forever one time to diagnose a high idle only to eventually find a tiny sucked in spot on the gasket.

It's dark now where I'm located. I'll check the gasket in the morning. To get to the gasket, I assume that I unplug everything nessary from the TBI & then un-bolt the TBI from the intake manifold. Did you replace the gasket or remove the sucked in spot?

Thanks,
Willie

It's dark now where I'm located. I'll check the gasket in the morning. To get to the gasket, I assume that I unplug everything nessary from the TBI & then un-bolt the TBI from the intake manifold. Did you replace the gasket or remove the sucked in spot?

Thanks,
Willie

It's a simple gasket (one piece) just like a two barrel carb and takes literally five minutes (remove the linkeage and enough wires and hoses to lift the unit up some so you can clean the old gasket off both surfaces but you can leave the fuel lines attached to the TBI unit, it will move around enough with them still on.) Since it's cheap and easy, it doesn't hurt to try. But before you remove the TBI you could try to retorque the TBI to manifold bolts just for the sake of trying. Just be mindful you're dealing with aluminum.

Recheck the ground connections bolted to the rear of the intake/head on the right side, and the connections to the distributor. There should have been some stored error code if the MIL was on. Since it was flashing, you may have been losing distributor reference pulses intermittently, which would set a "12". Remember that a code 12 is not a "System O.K." code as some believe, but indicates that distributor reference pulses are not occurring, and may be an indication of the problem. If you're not even read a 12 when scanning for error codes, you have an ECM problem.

Tightened the TBI gasket this morning & I didn’t notice any change. Neighbor had an old, removed Throttle Position Sensor that I plugged into my truck. I didn’t install it. There was some improvement. I replaced the Throttle Position Sensor & the TBI Base Gasket. One of the 2 or both was the fix. The truck is now idling properly.

Thanks to everyone that helped out on this.

Willie