matrophy Posted October 6, 2019 Report Share Posted October 6, 2019 I am using JustAnswer to get some troubleshooting help for my 86 720 with a carbureted Z24. I had major surging at idle and found a couple of vacuum hoses that were cracked. I replaced all vacuum hoses under the hood and now I have a high idle of around 1800 rpm at operating temp. Idle starts out at around 900 and them gradually goes up. Adjusting idle screw has no effect. The online mechanic said I likely have another vacuum leak, butterfly stuck open or my IAC valve is no good. I can’t find any reference that shows that I have an IAC valve. Do these trucks have them? Quote Link to comment
datzenmike Posted October 6, 2019 Report Share Posted October 6, 2019 (edited) If carburetor there is no IAC, that's for fuel injection engines. When a carburetor engine is started cold the choke will engage. With the choke on, a linkage pulls the fast idle cam into position so the throttle can't close all the way and the engine revs to warm up faster. So either the choke is remaining on for some reason or the fast idle cam is sticking. Choke. With the engine fully warmed up take just the top off the air filter and look at the choke flap... open... or closed... Fast Idle Cam. Find the plastic cam and see if the throttle is closed on one of the steps... Spraying with WD-40 and moving all the parts while holding the throttle 1/2 open should loosen it up. With throttle open open and close the choke flap and the cam should move freely. Edited October 6, 2019 by datzenmike 1 Quote Link to comment
matrophy Posted October 8, 2019 Author Report Share Posted October 8, 2019 Mike, the high idle seems to be working OK per your description. I did spray around the carb with carb cleaner and the idle increased when I sprayed the rear of the carb base. I removed the carb to replace the gaskets and it looks like there are 3 gaskets - 1 between the manifold and a metal plate the has a recess that fits into the manifold (photo attached), 1 between this plate and the heater and 1 between the heater and the carb. The gasket kit only comes with 2 gaskets. Should there be 3? I replaced every vacuum hose under the hood and I see that there is a plunger of some sort on the back of the carb that looks like it is supposed to have a vacuum line attached but putting my thumb over what looks like a place for a hose while it was running had no effect on the idle. Am I missing a hose for that part? 1 Quote Link to comment
datzenmike Posted October 8, 2019 Report Share Posted October 8, 2019 The plunger is the FICD (fast idle control device) actuator, to speed the idle when the engine is loaded down with the air con compressor. The vacuum line goes to a solenoid that applies engine vacuum to it to make it work. The vacuum line should come from near the one way valve on the brake booster hose. If the actuator is not holding the throttle open then this is not the cause of the fast idle. preventing 1 Quote Link to comment
matrophy Posted October 8, 2019 Author Report Share Posted October 8, 2019 If I don't have air conditioning, is it possible that it has been disabled? I am reasonable sure that there is no hose fitting on the one-way valve. Any comment on the number of gaskets between carb and manifold? 1 Quote Link to comment
datzenmike Posted October 8, 2019 Report Share Posted October 8, 2019 The carburetor or entire engine may have been swapped from a truck that had air conditioning. If the idle went up when spraying carb cleaner probably you have an air leak through a loose carb or bad gasket. If there were 3 gaskets replace with three to be sure the seal is tight. 1 Quote Link to comment
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