Cruise Control / Throttle Malfunction 1986 PT 38'
Frank Steltz VB#375

Last August while traveling home from the Summer FMCA Rally I had occasion to apply the brakes and shortly thereafter press the "Resume" cruise control and the thing I knew I was doing 75 MPH and climbing, WOW WHAT POWER!!! I applied brakes and the cruise dropped out and then acted normal. Occasionally thereafter it would be slow responding and sometimes overrun slightly (not serious). The throttle response grew increasingly lazier especially on cold start up.

On the way down to "winter wanderings" this January the throttle started not returning to closed throttle after either "cruise" shutdown or release of throttle while manually throttling. This condition could be hazardous to your health (read life threatening). It was hanging at approximately 1500 RPM which relates to about 50 MPH. A tremendous amount of load is added to the braking system under these conditions and could easily result in a late and/or long stop (read crash and serious if not fatal injury on somebodyís part).

After much studying of the cruise control (Booklet form # SD-10-1) and throttle systems I identified the root cause and specific problem.

The root cause is the presence of a solid rubber mud flap on the rear bumper.

The specific problem is dirt in the single action, spring return air throttle linear actuator (air cylinder).

The conditions creating this problem are the following: (1) Engine radiator fan blowing out (by design) (2) Rubber rear bumper flap creating a high pressure area under the engine compartment and with the help of the fan drives all the dirt and debris up into the engine compartment.

The use of single action (spring return) air device, requiring the presence of vent holes in the spring side of the cylinder to allow that side of the cylinder to aspirate in response to normal cylinder / throttle variations in this HOSTILE environment. This resulted in the "inhalation" of significant amounts of fine dirt (three vent holes (3) are about 0.020 dia.) into the cylinder causing binding of the piston assembly thereby not allowing the usually adequate springs to close the throttle.

I remedied the dirty cylinder condition by removing the cruise control/throttle actuating cylinder and dismantling it by removing the transducer (remove snap ring) and flushing the cylinder with WD 40, squirting some silicone oil spray in the air side of the cylinder and reassembling it (approximately 1/2 hour). I tested it by building air (electric compressor) and having my partner (read wife Jean) operate the throttle (engine off) and observing the throttle action. It operated flawlessly. I then did a live test (engine running) and it worked fine. At the same time I removed the slack from the throttle cable. I now have instant response of throttle at the engine.

Previously I had to move the pedal about half down to get engine response.

My recommendations for all PTís that have a radiator fan that blows out, an air throttle as described in the referenced booklet and a rear full length rubber flap is to remove the rubber flap and remove and dismantle the throttle cylinder using the Cruise Control System booklet (form #SD-10-1) for reference, and clean t as I described. I personally am aware of two other owners who have experienced this condition and are unaware of the "root" cause and remedy. The rear rubber flap MUST GO. Even then the condition continues to exist, due to the fan blowing out, albeit reduced.

Respectfully submitted, Frank Steltz VB#375