Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Hi my name is XXXXX XXXXX Your complete satisfaction is my goal. Please provide as much information about your concern as you can
Is this miss able to be duplicated if you put it in drive and hold the brakes, then raise the rpm?
Is the miss a jerk type miss , or does it feel more like a surging?
Can you do anything to get it to duplicate?
Neither...feels like from 8 cylinders to 7...it might run like that for a half an hour or half of the day or half of a week....there is no pattern.
Is this a California emissioned truck? Fed emission trucks did not have a computer controlled carb.
There is a solenoid on the rt top of the carb,but that is for the accelerator pump when cold. temp switch for this is on the thermostat housing.
If this truck has an ECM it will be located behind the glove box above the heater duct.
Can you get me the carb number off the left side of the carb.( example 17084226)
Air pump would not cause the concern, but if this is a computer controlled carb, it has to have the computer operating to deliver fuel correctly. There will be an oxygen sensor also in the exhaust manifold, if this is computer controlled.
This truck does have spark control and a knock sensor. The knock sensor should be located in the rt side block drain in front of the starter. There should also be a four wire connector back by the distributor. Let me know if it is connected or if someone has jumped A to C in the connector.( this bypassed the EST control)
Please give me as much information about how this truck is equiped now. Let me know what is still on the engine, and what has been replaced by after market components. ( ignition parts, carb, egr valve, intake)
What is the timing set to? 4 deg BTDC? 8 deg BTDC? ( all this information should be located on an emission sticker on the radiator shroud. timing spec; fed or calif emission; vacuum diagram)
If you could upload a picture of the engine compartment, that may help me see what you are dealing with. I think you are right , someone has got something messed up by bypassing the components for the emission controls.
The carburater was replaced a month ago but it didn't make the truck stop missing...according to the highway robber it would...the carb number is XXXXX under that number is XXXXX HcF...if the middle letter is a c...it's weird looking. Rochester Quadrajet. Their is a diamond shaped tag on the top of the carb that reads Autoline Products, made in Canada. The number on the diamond tag is C 9637. The emission control label is XDJ, 5.0 Liter, exhaust emission system: E1G5.7T4HHCX, the evaporative emission system is: 4D4D-8...designed to run at low altitude. The timing is (deg BTC @ RPM) 4deg @ 500 (DR). Behind the glove box is a small module with a 7 wire jack...the number on the unit is BLX 889011. I can't find any proof that the emission controls are California but an autozone employee thought so on account of the PCV valve has 2 hose routs...the top hose goes to the charcoal cannister and the side hose goes to the L port on the carb and to the Deceleration valve. The EGR valve seems to work as described in the "Haynes Repair Manual" (the one I've been using). The knock sensor in front of the starter, if that's what it is, has one wire leading up the side of the engine block and leading to the ECM route.
If you turn the key on do you hear a clicking noise from the carb? This would indicate it is a computer controlled carb. But from the info you have given, I believe this is a FED emission truck.
Give me an idea on how smooth the idle is? then rev the engine to 2000 rpm, and describe how smooth the engine runs. ( I am trying to get a good idea of how this engine runs with no load. Remember I am not there, so you need to describe as much as you can.
Indications of poor running, are the engine should idle smoothly Not rough at all, no intemintent missing or shaking. Same would go for the 2000 rpm no load check.
Tell me the best you can how it sounds and runs.
Did you have a chance to look back by the distributor for the four wire connector. Has it been modified in any way. ( this is to the EST in the distributor) or has someone replaced this distributor.
What is the spark plug installed. ( needs to be R43TS set at .035) If it has .045 they are too hot and if the EST is still connected will cause the knock sensor to retard timing.
Also inspect the hose to the distributor vacuum advance. Is there a double delay valve installed. It will be very noticeable. one hose "y" to two valves and then "y" back to one hose just as it goes into the advance.
Is your deceleration valve still hooked up. It will hook near the left front of the engine, one hose will go to the air cleaner and one to the pcv hose, then one to a port on the carb.
The more you tell me the better mental picture I am getting on what is occurring with your truck. You are doing a great job supplying information. We should be able to figure this out. I have worked on many of these trucks, and know them very well.
Yes the knock sensor has only one wire( blue) and goes up to a module .
Good info of description of how it is running.
The first thing to do is to get rid of those champion spark plugs. I have never had champion spark plugs preform well in a chevy engine. Get some good A/C delco spark plugs R43TS. The reason we gap them at .035 instead of .045, is that in 1985 GM had issues with ignition coils failing. They reduced the gap to .035.
Second We need to check the operation of the decell valve. To do this, remove the hose from the air cleaner. and rev the engine up quickly and let go of the throttle. You should hear the decell valve suck air in, and the idle stay up for just a couple of seconds. This will verify it is working. If not , then replace it. It does serve a purpose , and does not effect power or performance other then coming down to an idle, and to reduce HC during decell.
Check for water in fuel. If you have water in the fuel tank, it will work its way into the carb, and cause the idle jets to get too much fuel and not enough air. Rev the engine up and put your hand over the carb opening. This will suck fuel from all ports in the carb, and pull the water through. If this helps , when the condition occurs, then pursue a fuel quality concern, ( stuff still in tank, not all will work its way through)
Look at the egr valve when this condition occurs, Is it pulling open, or sticking open??
Verify timing is at 4 deg BTDC. And make a jumper wire , and connect terminal A to C at the four wire connector at the distributor. ( GM recommended this back in he late 80"s, due the system retarding timing too much, and causing drivability concerns)
This bulletin also recommended the change from R45ts plugs to R43TS plugs.
Make sure the plug wire are routed correctly on the left side of the engine. #7 and #5 cannot be run side by side. This will cause cross fireing. Route #7 at the far back, then #3 and #1 wire, then bring #5 over the top, so it crosses over the #3 and #5 wires.
If they are all bunched together, cross firing will occur. ( engine will have an intermittent miss)
After all this, Correct plugs, wires routed correctly and boots not torn or no holes. Timing at 4 deg btdc, and four wire connector bypassed A to C. Make sure the mechanical advance weights are not stuck, the rotor should snap back when turned. If you have an adjustable timing light, make sure both the mechanical and vacuum advance are working.and vacuum hoses are connected like the diagram on the fan shroud indicate.
You can now adjust the carb.
To start, turn the idle mixture screws in till they lightly seat, and back out 2 turns( starting point). Engine should be idling around 700 rpm in park (XXXXX
With your RPM meter connected, use a propane torch( do not light, just use the gas) to en richen the fuel, till you get the highest idle and smoothest idles speed. With it running at this point turn the idle screw down to keep the idle around 700 rpm. Then remove the propane enrichment ( This is not s final idle, we just use this as a base for adjustment of mixture)
The idle will drop, or it should drop. Start turning out the idle mixture screws equally 1/4 turn at a time, until the engine is back to idling at 700 rpm. ( Note if you en richen the idle again, it should go down/ not up . This is an ideal idle mixture. then back the screws in till the idle is smooth but about 25 to 50 rpm slower. En richen the fuel again with the propane. It should go back up to 700 rpm.
This is a basic way to adjust a carb using a rpm drop method utilizing propane. as long as the mixture needles are turn the same, you should be getting around a 25 -50 rpm drop after the propane is removed. Then adjust the idle speed screw to 550-600 rpm in drive.
Some parts stores may still carry a kit for adjusting the old computer controlled carbs. In this kit is an float level gauge, that can be used to check the float level with out removing the top of the carb, and while it is running. Ideal float level is around 13/32 to 14/32 . It is a good idea just to check this if you can find the gauge to do so,. ( it is put down the slot behind the primary choke plate)
If this engine is still missing, after this. Make sure the basic compression is good. Take all the readings, and the lowest should not be lower than 70% of the highest.
Then do a running test. ( remove one plug at a time, and connect the compression tester, and start the engine. ) readings will be about 1/3 of a normal compression test.
But this will give you an idea of how well this engine is operating while running. These readings should be equal with in 10 psi. If not , you could have weak valve springs, or a worn camshaft. Then it is removing the valve covers and checking the camshaft lobes for lift, or just looking at the rockers and see if you can notice any one that is not moving as much as the others.( hint cylinder #7 and #5 , and #6 and #8 were the worst for camshaft lobes rounding)
I know this is allot of info, But if I had this in my work stall, this is the process I would follow based on the info you gave me.
Take it one step at a time, and it should lead you to your root cause of your concern.
Feel free to ask any follow up questions until you isolate the concern and are satisfied.
I am here to help you, so please just ask if you need anything clarified.
It may be the type of pump they are installing. I do know that if the lever bushing wears, it will cause a slight tick noise that can be picked up by the knock sensor. This is another reason we disconnect the ESC system by jumping A to C at the four wire connector.
But if this concern has been occurring for the last 8-9 yrs, I doubt this is the case. May just be the quality of the pump.
Let me know if you need further assistance. Happy to help.
The knock sound was it like Spark knock/ rattle type noise?? If it is this is not caused by the knock sensor,, but would be picked up by the knock sensor and timing would be retarded as far as it could.
Spark knock or detonation, Is caused by too high a combustion temp, or from timing too far advanced( stuck distributor weights) The EGR valve helps with this by lowering the cylinder temps by 500 deg F( this also reduces Nox emmissions) So the EGR valve has to open and have flow to work.
Other things that can raise cylinder temps, are the air cleaner door not opening when engine temps reach operating temp under the hood. This draws in hot air right of the exhaust manifold. Another thing is the EFE valve in the exhaust manifold stuck closed,. this will direct exhaust gas up and under the intake when it is stuck closed. So make sure all of these components are operating.
If is not a rattle type knock that goes away when you take pressure off the accell pedal.
Then it could be a rocker arm or lifter making the noise. Any noise the engine makes will affect the ESC. The ESC is only there to retard timing in the case of spark knock/detonation. for every knock it hears the timing will retard 2 deg.
You can actually see this work with a timing lamp. Look at the timing marks and use a small hammer to tap on the exhaust manifold. If the ESC is working the timing will retard until you stop tapping.
Older engines make lots of noise, so this is why GM decided to bypass this system, and uses a lower heat range plug.
I am sorry i did not make it clear which side of the plug to jump. Yes it is a shunt you are putting between terminal A and C of the distributor side. Since this is the control circuit in the Ignition module. All were doing is making it connect so the engine will start.
That is the coolant temp sensor for the gauge or temp light. Should have a green wire attached.
Make sure you follow the diagnostic i gave you earlier, and set the timing and carb as I described. All these little things work together.
Good question. The timing is very easy to set. First connect your timing light to #1 cylinder ( front left side) and then loosen the 9/16 bolt on the clamp at the bottom of the distributor ( there is an inexpensive, off set wrench to do this) Then remove the vacuum hose from the vacuum advance.
The balancer has a line on it and you may want to find it first and clean it off, use some paint to highlight it. Then look behind the water pump and the timing tab should be visible looking straight down. The big "V" is TDC the points are 4 deg each The crank turns clock wise, so advance would be before the TDC mark.
Once you have everything ready , then start the engine and adjust the timing to 4 deg BTDC. moving the distributor counter clockwise advances timing and clockwise retards timing.
When done adjusting tighten distributor bolt and recheck to make sure it hasn't moved.
Reconnect vacuum hose and remove the timing light.
If you need more info , Just let me know.
Thanks for asking