engine at wide open throttle! (Page 1/1)
john carter MAR 06, 12:32 PM
I just installed a new vacuum canister (87 GT auto) for the cruise control function. It is now working, but the engine keeps accelerating beyond the speed I set and keeps climbing! It's like I stabbed the gas pedal to the floor! I climbed to 85 MPH real quick. Scary. When I touch the brake pedal, the system dis-engages. Any help or advice is greatly appreciated. Thanks
theogre MAR 06, 01:40 PM
The servo &/or vac lines still has problems. Do Not use cruise until fix.

Sevro has valves like to get crap in them or go bad for other reasons.
Plus Fiero has extra "dump valve" run by brake switch that work by itself but likely not your problem.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Notorio MAR 08, 03:11 PM
Just to be clear, the engine still runs normally AFTER your repair, if you run in a non-cruise control mode. But if you then engage the cruise control it wants to keep accelerating, yes? If that's true then you might try isolating the various vacuum lines associated with the system. You have a vacuum leak and want to see where it is.
Notorio APR 09, 11:04 PM

quote
Originally posted by Notorio:

Just to be clear, the engine still runs normally AFTER your repair, if you run in a non-cruise control mode. But if you then engage the cruise control it wants to keep accelerating, yes? If that's true then you might try isolating the various vacuum lines associated with the system. You have a vacuum leak and want to see where it is.



Hi John. I got your PM and took another look at your issue. Please confirm my understanding (or lack thereof) in the ABOVE question, and then also update us all on what else you have tried since your original posting. Thank you!
john carter APR 13, 11:36 AM
The car runs and drives normally' BUT, if I press the "set" button, the acceleration is wide open, and uses the kickdown function to speed up. If it tap the brake pedal, or use the slide switch on the stalk, it dis-engages. No SES signal, or any other problems. I really havn'tt done anything else....I'm at a loss. Thanks
john carter APR 13, 11:41 AM
I also replaced the Servo. The dump solenoid clicks when I test it.
theogre APR 13, 07:21 PM
Replace w/ another Used Servo? Often are "Bad" too for same problem.

VERY Carefully take off cover & valve out to Check both control valves & clean them.

Solenoid cores get crap/rust & won't move right.
seals get crap won't seal or Fail in other ways.

Servo Bleed/Vent valve stays open until the control box is active.
Then Iffy Vac valve keeps sucking in the bellows.

You Might be able to test in the car following charts in factory service manuals etc but can't find link where to get right now.
But those charts are mainly look for electrical problems.

If you want to make a "test" light for the control box under the D-side carpet... (or use any other "12v" test light)
Back Probe the plug for Lt Grn wire w/ Sewing Pin etc then connect small bulb like #94 or "12v" LED (LED + correct resistor) & Ground other side.
If can do same for Dark Blue vent wire. Make sure 2 new wires don't short each other or to the car or can "fry" the box.
Making the w/ long wires so can put bulb easy to see will driving.

If the box is working right, the Lt Grn light turn On for a bit when push Set button then Turn Off then On/Off as needed. If turns On then Off then likely Servo problems. More if Vent wire stays On. The box "sees" the servo pulled too far tries to vent but can't when vac side don't seal right.

If the Lt Grn wire Light stays On... then the box has wiring, control "knob," or servo problem(s) or is you problem.
Examples:
● Box Doesn't see the bellows move because box, servo or wires have problems. Servo has "feed back coil" to tell the box the bellows moved.
● see https://web.archive.org/web...fierocave/cruise.htm covering the control "knob" on the column. Maybe shorted or other problems there keeping resume/accel wire "On." (Gry/Blk wire on the box near the others.)
can check that w/o driving or I-key On w/ Ωmeter on that plug.
john carter APR 13, 07:51 PM
The servo and cannister are new. Obviosely, I don't know why, but the cruise works the same as with the old parts. Any help or advice is greatly appreciated. Thanks
Vintage-Nut APR 14, 10:26 AM
John,

My first question: Do you have a printed copy of the factory service manual?
If 'No' - this link will help but to me, buy one for troubleshooting reasons- https://fieroinfo.com/manua...M_Service_Manual.pdf

Section 9B / CRUISE CONTROL / Starts on Page 1149

DIAGNOSIS / Page 1152
"Improper operation can be caused by one or a combination of mechanical, electrical, and vacuum problems."

Read this page and follow the manual........Good Luck!

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Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles