| Thottle lever actuator [message #67237] |
Fri, 11 December 2009 23:47  |
vernonite
Messages: 5
Registered: June 2009
Location: United States
Karma: 0
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Junior Member |
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I had a problem that my 1978 GMC MH engine (403) would die when I exited the interstate and arrived at a stop light. It would start up ok and run fine until the next stop sign and die.
While searching for possible idle related causes I found several problems: 1) The PCV valve hose was not inserted in engine valve cover. 2) Cracked and brittle vacuum hoses. 3) Two of the thermal vacuum switch hoses reversed. 4) Vacuum operated Throttle Lever Actuator was leaking air. Initial thinking was that the TLA was activated when the A/C is turned on to compansate for compressor drag on engine. A simple bypass was to plug the vacuum line and increase the idle speed about 200 RPM's, but the increased idle sometimes leads to dieseling when ignition is turned off.
After weeks of searching for a replacement TLA I was eventually refered to a place in Texas that rebuilds them.
REBUILDERS SPECIALTY INC. 15049 Valleyview Rd. Forney, TX 75126 972-564-4141
They were very helpful and rebuilt my TLA for $15 (just like new). Much better deal than finding another old unit with aged diaphragm.
I installed the rebuilt TLA and followed the adjustment procedure in the GMC Maintenance Manual page 6T-9. Basically it says "adjust stem of actuator to provide 0.020 inch clearance between actuator stem and throttle lever". I started the engine to verify its operation. To my surprise and disappointment turning on the A/C did nothing!
After more research, I discovered that the signal to increase vacuum to the TLA occures when the transmission is moved to drive position (not when turning on A/C). Part of the emission control system, its purpose is to control the rate of throttle closing during decelleration.
Its been an interesting experience, however I now believe the probable cause of the engine stall was the PVC hose.
John Wolever
Camp Verde, AZ
928-567-3805
[Updated on: Fri, 18 December 2009 23:33] Report message to a moderator |
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| Re: Thottle lever actuator [message #67238 is a reply to message #67237] |
Sat, 12 December 2009 01:21  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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That is a good find. I do not have one on my 455 carb but a lot of GMCers do have them.
The information you posted would be a good addition to the GMC Motorhome International parts list.
You might want to send the information to Billy Massey. His email is: budb at Budb dot net
Thanks for the information.
Ken B.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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