Looking for guidance on inconsistent / rough idle

[drew1987gn]

Hi. Looking for some guidance. I have a 1983 Hurst Olds. All stock. She'll start right up when cold. Idle is high so the choke is working. After about 5 minutes the engine idle starts to drop then stall. If I try to re start immediately, she will not start. I'll try pumping the pedal, open throttle, no throttle, nothing. I have to wait about 5 minutes before she'll start back up by pumping the pedal a few times while she initially starts back up. At this point I have to keep hitting the throttle because she wants to stall. After a few more minutes I can stop hitting the throttle and she'll idle. Idle will be pretty smooth. I can let it idle for 5-10 minutes and no problem, smooth idle, good throttle response. If I shut the car off, wait a few minutes then start again, she'll start but will want to stall so I have to repeat and keep hitting the gas until the idle smooths out again. This is what I have done:

Replaced the carb gasket (base)
Replaced all vacuum lines
New plugs, wires, cap, rotor, coil
Checked timing (20 degrees)
Replaced fuel filter
Replaced o2 sensor (was original sensor)
Vacuum holds steady just under 17hg when idling fine.
I believe I don't have a vacuum leak between the intake and the heads. The idle doesn't shoot up when I spray carb cleaner.

I did get code 44 a few times, on and off. I did not replace the EGR valve. I did not check the TPS. I cannot adjust the idle mixture because the plugs are still in.

I'm thinking it's something internal with the carb.

Any feedback would be greatly appreciated. Thanks! Andy

 

[pontiac guy]

I think you choke thermostat is bad. It's likely opening too quickly and you are off the mid step of the cam. Run it 5 mins shut it off. Pop the lid. Open the throttle slightly and close the choke. See if it restarts

 

[MontesbyE]

Hi. Looking for some guidance. I have a 1983 Hurst Olds. All stock. She'll start right up when cold. Idle is high so the choke is working. After about 5 minutes the engine idle starts to drop then stall. If I try to re start immediately, she will not start. I'll try pumping the pedal, open throttle, no throttle, nothing. I have to wait about 5 minutes before she'll start back up by pumping the pedal a few times while she initially starts back up. At this point I have to keep hitting the throttle because she wants to stall. After a few more minutes I can stop hitting the throttle and she'll idle. Idle will be pretty smooth. I can let it idle for 5-10 minutes and no problem, smooth idle, good throttle response. If I shut the car off, wait a few minutes then start again, she'll start but will want to stall so I have to repeat and keep hitting the gas until the idle smooths out again. This is what I have done:

Replaced the carb gasket (base)
Replaced all vacuum lines
New plugs, wires, cap, rotor, coil
Checked timing (20 degrees)
Replaced fuel filter
Replaced o2 sensor (was original sensor)
Vacuum holds steady just under 17hg when idling fine.
I believe I don't have a vacuum leak between the intake and the heads. The idle doesn't shoot up when I spray carb cleaner.

I did get code 44 a few times, on and off. I did not replace the EGR valve. I did not check the TPS. I cannot adjust the idle mixture because the plugs are still in.

I'm thinking it's something internal with the carb.

Any feedback would be greatly appreciated. Thanks! Andy

The quadrajet has a high and low idle. After you start it and it warms for a min and the choke opens up it will let you kick down the idle by tapping the gas. Might see if you can kick down to low idle manually that way and see if you get the same results. Dont know if it helps you or not.

 

[79 USA 1]

Could be a slow responding EGR valve. Sticks open and causes a lean condition. I would try disconnecting the vacuum line and plugging it, see if the problem goes away.
I had one on my 86 305 in my El Camino that would act this way. Ended up replacing the valve.

 

[CopperNick]

You did mention the EGR valve throwing a code so with what 79USA1 just offered, might be time to go after that EGR. Might also be useful to take a run at the Throttle Position Sensor, TPS. If it is still wired into the system and sending signals back to the ECU then it could be throwing a bad signal and generating a code from it. Oh, yeah, if you have an external fuel filter, get rid of the one inside the fuel filter housing that the fuel line attaches to at the carb. Two filters are too much of a good thing and can restrict fuel flow, causing the carb to starve.

FYI, I had a Holley that was notorious for doing that. Turned out to be a combination of humidity and barometric pressure. Any kind of cold or high humidity or both together and that carb would stumble and stall. Even with the mill at running temperature it would still barf and croak.

Don't think Q-Jets are all that finicky but you might be looking at a blocked idle circuit inside the carb due fuel degradation/fuel sitting and turning into goo. Only cure for that is to pull the carb down, tank it in some carb cleaner, use an air gun on the passages, and rebuild it. Hope your carb has that cute little tag still hanging off one of the body screws as that tag number is about the only way to get the absolute correct kit for that unit. They are about the Model T of carbs and a bazillion of them got assembled and installed by the factory over the decades.



Nick

 

[Clone TIE Pilot]

For Qjets the ID is stamped on thd mainmbody casting, should begin with 170. If yoj have an analog dwell meter you could connect it to the green plug and see what the air fuel mixture dwell is at idle. The computer Qjets run at a fixec valud when cold, ignoring the O2 sensor which is known as open loop. When the engine warms up the computer begins to vary the fuel mixture solenoid dwell according to O2 sensor readings,,this is closed loop. Besides checking the fuel mixture dwell, also check the idle speed in drive and the TPS output which should be about .48 volts at closed throttle. Also check for main nozzle dripping at idle. Furthermore check your base ignition timing as that affects idle quality.

A common enough problem with Qjets are worn throttle shaft bores which cause vacuum leaks. It also causes inconsistent closed throttle position every time you open and close the throttle blades. You can rebush a Qjet with either DIY Teflon bushings or a brass bushing kit depending how worn your baseplate is.

 

[drew1987gn]

Hi! Thanks for all the feedback. I plugged the vacuum line to the egr valve. Same problem. Then I checked the TPS with my meter and it’s reading 1.14 choke open and throttle in idle position. So my plan is to replace the TPS and adjust if needed. I got a rebuild kit also from Quadrajetparts.com. The carb number is stamped on the side so I should have the correct kit. Now the problem I’m running into is I can’t get the horn off! All the bolts are out. I spent an hour carefully trying to pry the horn off the body. Only the horn where the secondaries are is somewhat moving. The primaries and front are not budging at all. It must be that the gasket is so stuck after almost 40 years. I don’t want to force or pry too hard as I’m afraid I’m going to warp the horn or break something. Something that should be so simple is now a headache. I tried to spray carb cleaner where there’s a little separation hoping the gasket gets a little wet. Any other suggestions?

 

[MontesbyE]

Hi! Thanks for all the feedback. I plugged the vacuum line to the egr valve. Same problem. Then I checked the TPS with my meter and it’s reading 1.14 choke open and throttle in idle position. So my plan is to replace the TPS and adjust if needed. I got a rebuild kit also from Quadrajetparts.com. The carb number is stamped on the side so I should have the correct kit. Now the problem I’m running into is I can’t get the horn off! All the bolts are out. I spent an hour carefully trying to pry the horn off the body. Only the horn where the secondaries are is somewhat moving. The primaries and front are not budging at all. It must be that the gasket is so stuck after almost 40 years. I don’t want to force or pry too hard as I’m afraid I’m going to warp the horn or break something. Something that should be so simple is now a headache. I tried to spray carb cleaner where there’s a little separation hoping the gasket gets a little wet. Any other suggestions?

Sometimes you need to tap it with a hammer and the two will split.

 

[CopperNick]

If you carefully work a dull putty knife into the seam that you have managed to separate, and then ever so gently pry on the handle, you might be able to encourage the two halves to part company. Failing a putty knife, a kitchen butter knife will work but only if you can sneak it out the door before your wife gets wise to the loss. BEFORE you go prying on anything, make absolutely sure you have removed all the retaiining screws for the top and take a peak down the throat to make sure there is nothing lurking in any of the shadows. Also, there is a special spray for gaskets termed a "gasket remover". It is supposed to be used to soften or melt gasket material that has refused to come free the easy way. Be careful with it as it is, to some degree, acidic or caustic and the fumes or contact could injure you. (W65.com)



Nick