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1979 Chevy Silverado stalls warm
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car1104
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Mar 29, 2014, 1:13 PM
Post #1 of 12
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1979 Chevy Silverado stalls warm
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We have a 1979 Chevy Silverado 305-8 cyl. w/auto transmission and air, 140,000 miles. It stalls after warm-up when put in gear and has low power. The first problem was a stall while driving warm. We had no spark so we replaced all the electrics-- plugs distributor cap, rotor, wires, command module, and pickup coil. After this it started but rough idled and stalled. We found a vacuum leak and cleaned the EGR valve, and vacuum gauge said vacuum was good. Automatic choke was closing and it was replaced. The engine would now start and warm up but the low power continued and the fuel pump was replaced. We also disconnected the catalytic converter, not the problem.
Still stalled after warm up and some small back fire. We tried advancing the idle, and the automatic choke from three notches lean to two notches lean. We did not find a fast idle to move up. The fast idle cam has a can connected to it with a vacuum line attached. We did not find and advance the fuel mixture. Results here are poor. The Rochester et120 carburetor (Holly 4160 crossover) did not seem to respond dramatically to adjustments. So should we go ahead with a compression test and look for a sticky valve? This truck has a coil mounted on the distributor, what is the best way to disable the ignition system. Or is the carburetor the problem and needs to be replaced? Or are we missing it?
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Discretesignals
Ultimate Carjunky
/ Moderator
Mar 29, 2014, 1:37 PM
Post #2 of 12
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Re: 1979 Chevy Silverado stalls warm
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Best way to disable spark on those is to unplug the power connector on the side of the distributor.
Definitely go ahead with a static compression test. Make sure the battery is fully charged and the throttle is wide open when you do your test. Write down the numbers and let us know.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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kev2
Veteran
Mar 29, 2014, 1:52 PM
Post #3 of 12
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Re: 1979 Chevy Silverado stalls warm
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sounds like a trip down memory lane- I will try to add ideas-
I am confused which carb is mounted on this?
compression test will be interesting- do you still have Vac gauge on it - what was readngs?
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Mar 30, 2014, 7:10 AM
Post #4 of 12
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Re: 1979 Chevy Silverado stalls warm
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Off the wall sometimes no surprise: Quick read - sorry. This shows a vacuum advance, tell us if not so. Does it look like this.......
If so, lower right in that pic those wires to the module can and will rub to the wall of distributor and give intermittent stalls. Reason is the vacuum advance is moving those and like any wire only take so much and could look good or rub lightly thru insulation and heat would be on the list of them touching for a moment or more and hence head scratching as to why is this doing this. Duh - you go to test and it works and the hunt is fruitless.
If rubbing carefully move them such that they can't. If wear thru insulation perhaps try liquid electrical tape. Not sure if real troubles if you can get those wires new - ask if there's an issue there.
Carb - plain less fussing with mixture from any setting where it was that worked the better. If not happy it probably caught some dirt in a tiny port up to bowl being full of rust mud. Magnet will take that out. Again - if a carb was once right know where you started if you change any adjustment so you can go back.
Vacuum gauge used properly is highly telling of compression or if this picks on a single cylinder. Possible of course for a few things are not all in order confusing which one to attack first.
Suggestion is to check ONE of the possible reasons and if something found stop at that for a bit and test. If you do two things at once you may never know which was the primary reason,
T
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car1104
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Apr 1, 2014, 12:24 PM
Post #5 of 12
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Re: 1979 Chevy Silverado stalls warm
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1979 V8 Chevy Silverado
Compression test March 31, 2014
Psi cyl
175---8 7---170
160---6 5---180
170---4 3---170
180---2 1---165
The specifications for tune up of this truck call
for a 150 psi . and I see some carbon on the plugs.
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car1104
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Apr 1, 2014, 12:35 PM
Post #6 of 12
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Re: 1979 Chevy Silverado stalls warm
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The vacuum gauge is a dashboard type. It has a 18--22 inches range on the vacuum dial which indicates a good vacuum at idle.
My readings were 20 inches at idle.
The Carburetor is a Rochester et120, made by Chevy. It is identical to the Holly 4160 and was installed on mostly four cylinder engines, some six cylinder, and some eight cylinders. This makes me think the carburetor might be not supporting the engine and losing power after warm up and stalling when being put in gear.
I posted the compression result on another reply. Problem there though, readings were higher then my specification sheet.
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car1104
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Apr 1, 2014, 12:41 PM
Post #7 of 12
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Re: 1979 Chevy Silverado stalls warm
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Yes this photo posted is the distributor model.
We checked the wires and do not have a short.
The stall occurred after power goes down after warm.
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kev2
Veteran
Apr 1, 2014, 1:17 PM
Post #8 of 12
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Re: 1979 Chevy Silverado stalls warm
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hearing a issue when hot I would,
>check choke is full open,
> disable vacuum to EGR for testing, easy and fast test
> confirm that plugs spark, distribuator rotor is rotating, again easy simple elemination
> also these ICM's were very prone to failiing when hot,
> observe engine vac did it drop slowly before stall, exhaust issue and possiable gaskets
> check PCV system excess blow by OR major vacuum leak,
> CAREFULLY open gas cap as engine start running rough,
> issue is when engine warms up? - what is ECT at warm up- I suspect it is running time NOT an ect issue, very little is changed (choke*) on this early engine design
* choke and idel step up....
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Discretesignals
Ultimate Carjunky
/ Moderator
Apr 1, 2014, 5:54 PM
Post #9 of 12
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Re: 1979 Chevy Silverado stalls warm
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Another trick you could try is when it is stalling out, put some propane to it and see if it keeps running.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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car1104
User
Apr 3, 2014, 11:48 AM
Post #10 of 12
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Re: 1979 Chevy Silverado stalls warm
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hearing a issue when hot I would,
>check choke is full open, Answer--we moved choke up one notch no improvement
> disable vacuum to EGR for testing, easy and fast test Answer---we checked egr and vacuum it was good
> confirm that plugs spark, distribuator rotor is rotating, again easy simple elemination Answer rebuilt ignition system and set timing
> also these ICM's were very prone to failiing when hot, Answer -- change control module twice
> observe engine vac did it drop slowly before stall, exhaust issue and possiable gaskets Answer -- compression test was good
> check PCV system excess blow by OR major vacuum leak, Answer--- vacuum holds at 20 inches at idle
> CAREFULLY open gas cap as engine start running rough,
> issue is when engine warms up? - what is ECT at warm up- I suspect it is running time NOT an ect issue, very little is changed (choke*) on this early engine design Answer ---ect engine coolant temperature seems normal
We are back to Holly troubleshooting guide, there is a stalls when put in gear caused by a Low fast idle So--- We turned curb idle up 1 turn--- not good enough to solve the problem.
Second Holly suggestion is tight pull-off No clue here---can you give me some details on what a pull-off they are talking about?
Third holly suggestion is clogged jet so is it worth rebuilding and cleaning the carburetor or replacing it?
* choke and idel step up....
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kev2
Veteran
Apr 3, 2014, 1:11 PM
Post #11 of 12
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Re: 1979 Chevy Silverado stalls warm
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I wanted the EGR and the vacuum to it eliminated for testing- no diaphram or hose leaks, no open pintel to cause issue.
The pull off - when engine off and cold, opening and closing the throttle will set choke full closed, starting the engine the dashpot gets engine vacuum and opens the choke plate @1/4" - that measurement is the pull off. As the engine warms the plate will settle in full open. Be sure the dashpot and line is not a vacuum leak.
Did you try venting the tank when issue arises?
what is timing cold and what is it when warm?
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