Engine dieseling diagnosis…

[dkraven]

I’ve already been reading up on it and have some things to try, but wanted to rope you all in for the benefit of wisdom and experience!

Just got the fuel line fixed so my 77 Sierra Grand fires right up. If I idle it and turn it off this issue doesn’t happen, but if I even drive it around the block the engine runs on for 2 “kathunk kathunk” strokes after I turn it off. Previous owner says the engine is newer and only has 40,000 miles on it.

I’m starting off by getting some Sea Foam to spray in the carb and see if cleaning up the insides is what does the trick. Also read the EGR is a common cause so I might take that off, give it a shake, maybe try to clean it. Hoping it’s not vacuum related issues because I’ve read that’s hard to pin down.

One other piece of info, first trip around the block there was some smoke coming from where the driver-side header attaches to the exhaust downpipe. I had recently changed the oil and spilled some and assumed it was that, but if there’s a leak where the header meets the downpipe is there any way that could be involved in dieseling?

I’m definitely new at this, only know what I’ve gotten from you folks and from messing with the truck the past few months, but I’m encouraged by my progress so far and hope this isn’t too big a deal.

 

[Grit dog]

Dieseling or running on can be caused by several things. Individually or in combination with each other.
Assuming it’s an old low compression engine, it’s not likely it needs higher octane due to running more timing advance than a typical old poochy small block.
But timing could be an issue. Ensure baseline and advance is right and working right.
The other main cause can be carbon buildup in the combustion chamber. The rough edges of the buildup get red hot and act like miniature spark plugs for a brief moment after shutting it off.

I’m sure others will chime in with more in depth knowledge as well.

 

[Bextreme04]

I’ve already been reading up on it and have some things to try, but wanted to rope you all in for the benefit of wisdom and experience!

Just got the fuel line fixed so my 77 Sierra Grand fires right up. If I idle it and turn it off this issue doesn’t happen, but if I even drive it around the block the engine runs on for 2 “kathunk kathunk” strokes after I turn it off. Previous owner says the engine is newer and only has 40,000 miles on it.

I’m starting off by getting some Sea Foam to spray in the carb and see if cleaning up the insides is what does the trick. Also read the EGR is a common cause so I might take that off, give it a shake, maybe try to clean it. Hoping it’s not vacuum related issues because I’ve read that’s hard to pin down.

One other piece of info, first trip around the block there was some smoke coming from where the driver-side header attaches to the exhaust downpipe. I had recently changed the oil and spilled some and assumed it was that, but if there’s a leak where the header meets the downpipe is there any way that could be involved in dieseling?

I’m definitely new at this, only know what I’ve gotten from you folks and from messing with the truck the past few months, but I’m encouraged by my progress so far and hope this isn’t too big a deal.

Start simple. Adjust the timing. Then, adjust the idle mixture screws and hot idle speed. It is best to have a digital timing light with a tach to do this with. If your idle mixture or timing is way off, you could be holding the throttle way open to keep it running at idle and also it could be dumping a bunch of fuel in there, since it would be bypassing the idle circuit.

To adjust the timing get the engine nice and warmed up and then disconnect the vacuum line from the distributor and plug it. Then, with the idle around 650-750rpm, adjust the timing to be somewhere between 4-12 degrees BTDC. Once you have done that, adjust the idle mixture screws. I would personally do this by adjusting one screw in until it lightly bottoms out and then out 1-1/2 turns. Then do the same to the other one. Then you need to screw them each out 1/4 turn at a time until the idle rpm peaks and start going back down again. Adjust the hot idle screw while you are doing this to make sure it stays within 100rpm of the 750rpm you started with. Once the idle rpm has peaked and then started coming down again, tighten them back up to the peak rpm you saw. Once you hit the peak, turn them in until the idle drops by 50rpm from the peak. This will give you a slightly lean-of-peak idle. Now, connect the vacuum line back up to the distributor. Your idle RPM should shoot up since the vacuum advance should be adding 9-12 degrees of timing at idle. Now do your final hot idle adjustment to bring it down to the desired 750rpm or so. This value will drop some when it goes into gear, so you don't want it too low.

 

[SirRobyn0]

Anything that causes to much heat in the combustion chamber will cause this. Sometimes pinging and dieseling are present, it just depends.

Decarb is a great idea, it likely won't fix it, but it's a good idea for any high mileage carbureted motor. A/C Delco X66A if you can get it is I think a little better than seafoam, but seafoam is a good product.

Set the timing.

Most importantly set the idle speed. If it'll do it after you run around the block check that the fast idle is coming off properly.

Check for vacuum leaks, and that the fuel mixture is correct.

EGR valve, while very import on these motors won't cause dieseling typically.

If all else fails, you can change the ignition timing to run on manifold vacuum instead of ported. This will cause the idle speed to increase, you then reduce the speed using the screw to the desired idle speed. Because the throttle plates are closed a bit more it'll be much harder for the engine to diesel. This isn't really a fix, it's more of a patch job. Dieseling isn't good for a engine, and sometimes high mileage engines just want to do it, because things are no longer what they once were, but please be sure to check all the other boxes before patching it up by changing the vacuum advance. But it's old trick we sometimes have to use.

 

[1STLS1]

The things I would be looking at first would be to see if the throttle blades are shut at idle and the air is being drawn from the idle circuit of the carb. If the throttle blades are open just a little, it will continue to draw fuel through the venturies and make hard getting rid of the dieseling. Another point is old quadrajets tend to like to leak internally and if the throttle shafts are worn out will also keep feeding it until it stalls.

I have heard great things about Seafoam but have never used it. I normally when I would have an issue that I suspect dirty combustion chambers, I would get a bottle of water, run the engine to 3000 rpm and trickle a steam into the carb. You will get this funky brown exhaust for a while but it clears quickly.

If none of these are the issue, then get into timing and such. I said this before on the board, I am a fork lift mechanic and have to deal with super high hour, worn out motors on a daily basis that run on is a common issue.

 

[dkraven]

Sea foam in the fuel tank and I sprayed carb cleaner into the venturi while running. Got a timing light and going to have a go at checking the timing. Harmonic balancer is rusted all to hell but it has a groove in it which I'm assuming is the mark for timing.

 

[dkraven]

Got to work on the timing today and though I had success in accelerating the timing from somewhere below 0 (my gauge doesn't go down) to 12, I fowled up my choke messing around with the carb and it's stuck open. It seems that might mess with the timing since it's full open. I'll try to get video of the choke problem, it seems the rod going into the spring is stuck so maybe the spring is broken?
Stuck choke

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[Bextreme04]

Got to work on the timing today and though I had success in accelerating the timing from somewhere below 0 (my gauge doesn't go down) to 12, I fowled up my choke messing around with the carb and it's stuck open. It seems that might mess with the timing since it's full open. I'll try to get video of the choke problem, it seems the rod going into the spring is stuck so maybe the spring is broken?
Stuck choke

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That is a divorced choke thermostat. There is a spring inside the housing bolted to the intake manifold. Once the engine gets a bit warm, that spring gets warm and holds the blade open like that. It will have no effect on the timing though. You should have been setting timing and idle mix/idle rpm with the engine warm... which will be with choke fully open like that. Once you have set all of that, then you can wait for it to fully cool, at which point that spring should have relaxed and allow you to set the choke by pressing the pedal all the way to the floor once and then slowly releasing it. That will allow the choke flap to snap shut and put the throttle shaft onto the high idle cam. Start the engine at that point and then adjust the high idle cam to have the idle at 900-1200rpm. I usually set it to go to about 900 right on startup and then it will climb on its own to 1200-1300 while it warms up. Then you can tap the throttle and it will drop off the high idle cam and snap the choke flap open to settle down to the hot idle.

 

[dkraven]

That is a divorced choke thermostat. There is a spring inside the housing bolted to the intake manifold. Once the engine gets a bit warm, that spring gets warm and holds the blade open like that. It will have no effect on the timing though. You should have been setting timing and idle mix/idle rpm with the engine warm... which will be with choke fully open like that. Once you have set all of that, then you can wait for it to fully cool, at which point that spring should have relaxed and allow you to set the choke by pressing the pedal all the way to the floor once and then slowly releasing it. That will allow the choke flap to snap shut and put the throttle shaft onto the high idle cam. Start the engine at that point and then adjust the high idle cam to have the idle at 900-1200rpm. I usually set it to go to about 900 right on startup and then it will climb on its own to 1200-1300 while it warms up. Then you can tap the throttle and it will drop off the high idle cam and snap the choke flap open to settle down to the hot idle.

This is helpful, and I've figured out some of this from watching videos. I'm realizing my fast idle cam might have a problem as when I pull on the throttle linkage or press the pedal the choke doesn't snap shut. It'll shut if I take hold of the fast idle cam and move it back into place. That fast idle cam locks into place in two places as I move it down and the only way to release it is to pull on the throttle. Before I get to tuning the fast idle, I need to figure out if that whole assembly is working properly and it seems like it isn't.

 

[Bextreme04]

This is helpful, and I've figured out some of this from watching videos. I'm realizing my fast idle cam might have a problem as when I pull on the throttle linkage or press the pedal the choke doesn't snap shut. It'll shut if I take hold of the fast idle cam and move it back into place. That fast idle cam locks into place in two places as I move it down and the only way to release it is to pull on the throttle. Before I get to tuning the fast idle, I need to figure out if that whole assembly is working properly and it seems like it isn't.

It looks to be mostly free in your video. I would disconnect the jesus clip from the thermostat rod and see if the whole assembly moves freely when disconnected from the choke thermostat.

I'm thinking either someone didn't know what they were doing and overtightened the high idle screw, or you have a bad choke thermostat. Its also possible that the rod came out of the thermostat coil and is bound up inside the housing. You can take the thermostat housing off the intake manifold by removing a single screw/bolt and inspect the whole thing. Just remove the screw holding the housing to the intake manifold and remove the jesus clip that holds the rod to the carb linkages and it will come right off.

 

[dkraven]

Alright, def have the timing set at 12, checked it today. And I took a video of the dieseling complete with vapor? shooting out the venturi. It didn't thunk twice in this video but it did the previous time. I also had screwed in both idle screws then backed them out a total of 3 turns to get the rpms down. The video starts at the bottom of the exhaust manifold because I had exhaust or smoke coming from that joint as well, didn't show up in the video but I'm still curious if a leak at that joint could be involved. Also the dieseling only happens when the engine has been running and is pretty hot.

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[Ricko1966]

The things I would be looking at first would be to see if the throttle blades are shut at idle and the air is being drawn from the idle circuit of the carb. If the throttle blades are open just a little, it will continue to draw fuel through the venturies and make hard getting rid of the dieseling. Another point is old quadrajets tend to like to leak internally and if the throttle shafts are worn out will also keep feeding it until it stalls.

I have heard great things about Seafoam but have never used it. I normally when I would have an issue that I suspect dirty combustion chambers, I would get a bottle of water, run the engine to 3000 rpm and trickle a steam into the carb. You will get this funky brown exhaust for a while but it clears quickly.

If none of these are the issue, then get into timing and such. I said this before on the board, I am a fork lift mechanic and have to deal with super high hour, worn out motors on a daily basis that run on is a common issue.

Exactly what I was going to say it has to get fuel and air to diesel anyway you covered it. I've always thought sea foam was like snake oil. I use water myself ,have for over 40 years. I suggest it once on here and everybody acted like I told him to douse it with gasoline and throw a match in. Water works great. O.P. is this truck stone stock all the way down to the fuel pump. If your fuel pressure us too high it can diesel.

 

[dkraven]

Exactly what I was going to say it has to get fuel and air to diesel anyway you covered it. I've always thought sea foam was like snake oil. I use water myself ,have for over 40 years. I suggest it once on here and everybody acted like I told him to douse it with gasoline and throw a match in. Water works great. O.P. is this truck stone stock all the way down to the fuel pump. If your fuel pressure us too high it can diesel.

New mechanical fuel pump that I replaced when dealing with fuel line issues. I think I made it worse by raising the idle some so that's good info, but before the curb idle was all the way out, which is odd.

How do I check the throttle blades with the carb attached?

 

[Ricko1966]

Have you checked for vacuum leaks yet? Do your mixture screws have any affect on Idle? If the EGR is stuck open it can create a problem,don't just take it off without testing it because you add a new variable to the problem. If you have a vacuum pump great,if not get a meat tenderizer syringe apply vacuum to the EGR valve with a piece of vacuum hose and your vacuum pump/syringe feel for the valve to open as you apply vacuum and close as you release vacuum you can feel the valve move through the cutouts in the bottom. If it's working your idle should change big when it opens.your base idle stop adjustment back the screw all the way until the throttle plate is completely closed then turn the screw in 2. Turns. That's a preliminary you will have to play with it to keep your base idle down as you make other adjustments. I think @bextreme already provided that info to you.

 

[dkraven]

UPDATE:

Dieseling issue resolved! Of course have a new problem. The engine seems to hesitate when I depress the throttle lightly. If I give it a good punch it goes fine, but it feels like it’s going to stall if I’m too gentle with the throttle. I’m hoping this is something obvious to someone with more experience.

Here’s what happened with the dieseling:

Decided to rebuild the carburetor so I could understand it better, get it clean with new gaskets, and also put some more epoxy on the bottom welds. Got the carb back together and on the truck, dealt with some leaking fuel because the fuel inlet wasn’t tight enough, and then it started up! And dieseled when I shut it off. Realized I had the curb idle screw in wayyy too far, backed it out as far as I could where the engine wouldn’t stall out, and lo and behold no more dieseling.

After that I used a vacuum gauge to dial in the timing and fuel mixture screws, keeping the rpms around 750, and it’s running good. Idle is fine. Now I just have to figure out why it feels like it’s going to stall without a little punch to the throttle when driving. Is this something to adjust in the throttle linkage somehow, or something on the carb?