Caterpillar GC20K:
High Idle / Vacuum Hose Layout

Hi Everyone,

I recently purchased a used 2003 Caterpillar GC20K (Serial Number: AT82D01065) and I'm am having a problem with a high idle. When the machine starts it takes a few seconds and the idle raises up about 1500 rpm (I'm guessing as it does not have a tach). Not sure if its related but if the machine sits (ex: overnight) it is difficult to start. As you will need to turn the motor over 4-5 times and it will eventually start. There are several other forums related to this issue and most seam to point to a vacuum leak. Reading those other posts I have already checked the two vacuum hoses that go from the throttle body to the distributer and the computer and they are correct, connected, and not leaking. Next to the regulator/converter there is a vacuum solenoid that has three hoses; one hose off one side, and a elbow with two hoses off the other which I'm assuming bypasses or redirects the vacuum when the solenoid is charged. When I got this machine that solenoid was hanging there and those three vacuum hoses were disconnected. Does anyone have a vacuum diagram for this machine or can confirm where those three hoses go? Additionally, I can tell that the converter/regulator was recently replaced as it looks considerable newer that the other components. The regulator is a Impco J (P/N JB) can anyone confirm that is the correct regulator? Does anyone have or know where I can get a service manual? Also I have removed the throttle body to make sure everything is moving freely, and everything seemed to be fine.

Not sure what to check or where to go from here. Any advice would be appreciated.
  • Posted 28 Sep 2021 21:04
  • Discussion started by Bryce_Rico
  • Texas, United States
Bryce Rico
AccuSpec Fabrication
www.accuspecfab.com
Showing items 1 - 4 of 4 results.
Hi swoop223

I have looking into everything that has been discussed and the documents that you have sent me. After all that I'm still dealing with a high idle.

I have possibly located the issue, but I wanted to see what you though as you seemed very knowledgably. I know that the FCVS is supposed to click as it cycles on and off. I have noticed that I also have a clicking noise coming from the throttle body. I had removed the side cover and when the key is on the motor/coil on the side of the throttle body keeps opening the butterfly in the throttle body. With the key 'On' it will keep opening full throttle then it goes down to about 70% throttle, goes back to 100% back to 70%. It will just keep cycling as long as the key is on. Then once I start it it goes full throttle and then will just fluxuate slightly at a high idle.

I figure I may have a bad throttle body or maybe the computer is bad and sending a signal to the throttle body to open when it not supposed to?

What do you think?
  • Posted 27 Oct 2021 02:31
  • Reply by Bryce_Rico
  • Texas, United States
Bryce Rico
AccuSpec Fabrication
www.accuspecfab.com
hello bryce,
generally you can hear it, only when the power/key is on and the engine running.
if you can't hear it then put your fingers on it and you can feel it vibrating.
  • Posted 30 Sep 2021 08:55
  • Reply by swoop223
  • North Carolina, United States
You've been swooped!
swoop223@gmail.com
swoop223,

Thank's for the reply and the info. Regarding the fuel control solenoid valve (FCSV) rattling; I do not recall hearing this so I will defiantly check it out. To clarify, should it rattle any time the key is in the on/run position weather the engine is running or not? I figure if the engine is running it may be hard to hear so I'm just tying to figure out if I would need to put my ear next to it or something.

Thanks,

Bryce
  • Posted 30 Sep 2021 00:01
  • Modified 30 Sep 2021 00:12 by poster
  • Reply by Bryce_Rico
  • Texas, United States
Bryce Rico
AccuSpec Fabrication
www.accuspecfab.com
hello Bryce,
first off these trucks after then get some hours on them do not start very quickly anyway, they always turn over a few rounds before the fire off, has to get the lp gas up to the mixer and that has to pump enough vapor into the engine before it starts up.
The pressure regulator (vaporizer) is a model J in the older trucks but they moved on to another Cobra version of that regulator later on. The biggest difference is the primer button is missing on the new ones and the relief spring inside for the inlet port button assy is a blue spring i think, different pressure regulation. I have interchanged them and have had no issues. Your 2 biggest problems with these regulators are either contamination from oil buildup or antifreeze leaking into them causing the diaphragms to go bad. Generally they will start freezing up after a bit or they just will not put out any vapor or it will be restricted.
Another issue is these trucks have this fuel control solenoid valve (FCSV) mounted in the same area as the regulator and filter assy, when it is operating correctly you will hear it rattling, if you do not hear that it is probably stopped up or bad, it does have vac hoses that hook from it to the intake/mixer vac hoses. This will be shown in the diagrams i have.

send me an email at my address in my signature and i'll send them to you.
  • Posted 29 Sep 2021 06:17
  • Reply by swoop223
  • North Carolina, United States
You've been swooped!
swoop223@gmail.com

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