High engine speed

[koldcustoms]

1987 Ranger, 2.9L, 5 speed.

The truck starts fine when cold (-10 celsius). It warms up and idles normally. It accelerates through the gears normally. Coasting or slowing down from a cruise, I'll release the clutch and the engine will rev to a steady 2000 rpm with no input to the gas peddle. It'll stay at this rpm for 5 to 10 seconds and then quickly drop to 850.

Full tune up and all fluids changed less then 3500 km ago.

Ignition timing checked and adjusted to 10 degrees TDC.

EEC diagnostics checked and no codes found.

I have found that the cold start air valve is open when the throttle suddenly closes- causing the high idle speed. The motor runs excellent, otherwise.

Anybody encounter these symptoms before? What did you do to fix it?

 

[87Ranger4me]

My 87 used to do the same thing. After cleaning the IAC valve and replacing it along with the TPS I found that it was the engine coolant sensor up by the top radiator hose that was causing mine to do that. I guess it thought the engine was cold and would put the computer into the first start up mode, anyway fixed my problem.

 

[koldcustoms]

Thanks 87Ranger4me, it makes sense. Did you check for codes when you were troubleshooting? Did any come up?

I replaced the throttle bypass valve (free spare), and it cleared up the problem. I learned that it's a regulating type valve. It's control logic isn't simply open/closed. It regulates engine rate of deceleration when the throttle closes. Otherwise it would stall. There is no feedback from the bypass valve to the EEC. Hence, no code.

Problem solved.

 

[87Ranger4me]

Yes, I checked codes and found none, that is was so frustrating. I actually found that solution on this board somewhere.