Sorry, I know it’s a thousand posts on this and I’ve read a thousand posts on it and still can’t figure out why my 84 F150 inline 6 is dieseling, it only does it when the engine gets up to temp. I just recently bought it and have been going through replacing parts that have been neglected. It over heats at idle and crawling speeds, so I put a 180 thermostat on hoping to bring the temp down some (and possibly reducing chance of run on) but that didn’t work so I’m installing a new fan clutch today. I installed new plugs and wires. I used seafoam in the cylinders and gas tank, twice. I rebuilt the carb (thought the float might be off). I’ve also checked timing, it’s sitting dead on 10 degrees btdc.

It has a slight tick that I believe is a lifter. Could a bad lifter cause dieseling? Ive considered replacing the lifters and rods but I read some posts that said it wasn’t a good idea to replace lifters on these engines without doing the cam too. Is there anything else I’m missing or need to recheck?

Also, it’s the 4spd manual so I just put it in fourth and let of the clutch when it diesels. That usually cuts it off and isn’t too aggressive. I bought it as a hunting/farm truck so I’m not that concerned but it would be for it turn off properly.

 

In general dieseling is caused by hot spots in the combustion chamber. Excessive carbon buildup and deposits retain a lot of heat and can ignite any fuel that is still being sucked into the cylinder. Aggressive timing can make it worse but 10 btdc is nothing so that's fine where it is. If your idle speed is too high it give the engine more of a chance to ignite the fuel after the ignition is off. High coolant temps can also make the problem worse so even with the low temp t stat I would still verify with a temp gun. Check your temps at the thermostat housing and also see what the manifold temps are. If you have access to a boroscope I would take a peak inside the cylinders and see how much carbon is on the piston tops. Run a ton of cleaner through it and giving it the business can help clear up some of that buildup.

-Dan F

 

I have to do this same thing on my 83.
I have read that it could be from curb idle speed, engine being too hot, and timing.
This video might help....even if it is brand x.

 

I’ve got aftermarket gauges on it. The idle speed when it’s warm is 800-1000. The temp now sits at 180-190 when driving but creeps up to 210-220 when idling or creeping around. I’ll check the temps with a gun like you said to verify they my gauges are accurate. I have noticed when the temp gauge is reading 180 or so it doesn’t diesel. Higher than that, and it does usually. It may be a ton of buildup and need more treatment, I’m estimating the truck has 277k on it. The guy before didn’t know since the odometer stops at 99k.

thanks,
Eli

 

That is the problem right there!
Lower the idle RPM to 500 to 700 max, better yet check what the factory should be, mine I set to 650 RPPM and my timing is 14* BTDC and I have not run on.

Someone said "hot spots" that can light the fuel without spark but the wrong heat range spark plug can stay hotter than it should.
You should just run the factory heat range and type of plug, COPPER, not them fancy plugs as they can also cause problems down the road.
Yes I run copper in factory heat range, no run on.

Your over heating or running hot could be a few things.
You said you changed the clutch for the fan so you must have factory AC and that should have a factory shroud on the radiator.
No shroud you will run hotter at idle as the fan cant pull air through the radiator.
As a test at idle take a sheet of paper and see if it gets pulled to the grill if not you have a air flow problem and being you did not say over heating when moving / driving when you have air being pushed through radiator.
I do have factory AC with shroud but I never had the clutch fan, it is a 4 blade mounted to the water pump, and the only time I have a "heat problem" is at idle, it is 95*+ and I sit with the AC on full and sitting for 20+ min waiting for people.
Once I start to move the temp comes right back down.

I dont think it would be a bad / plugged radiator because you said it only happens at idle not driving down the road.

You said you checked the timing and it is set to 10* BTDC is that with the vacuum hose pulled and plugged or the SPOT jumper pulled?
Because the idle RPM is so high the mechanical advance could be kicking in so once you lower the idle re-check the timing.

After you get the idle set and the timing checked adjust the idle mixture using a vacuum gauge for the highest reading.
Dave ----

 

my RPMs are 500-700 when the engine isn’t up to temp yet but at at 800-1000 rpm the engine is usually around 190-205 degrees. Should I adjust the idle/air mixture or just the idle screw? I’m new to carbs, I know there’s a fast idle screw and another one that sits right beside it.
It does also cool down when I drive at highway speeds. The new fan clutch didn’t help anything either. I also put a shroud on it, that slows the temperature climb but doesn’t stop it.
The timing I checked with the vacuum hose pulled and plugged.

 

It sound like you have2 things going on, low RPM when cold and too high when up to temp.
I would say let start with the up to temp RPM as that is where the motor runs most of the time.

Motor up to temp set the RPM to the 600-700 max.
Then check the timing to make sure it is still at the 10* BTDC
Then adjust the idle MIX screw not the speed screw for RPM. Now when adjusting the MIXTURE the RPM may change so you may need to go back and adjust the idle SPEED again.

Once you have that set and no more "run on" we can look at the cold RPM being too low.
I hate to start messing with the choke side till the "hot side" is working as it should.
Dave ----

 

Low RPM when cold is probably because the guy before has the choke completely disconnected. I haven’t had a chance to run a new wire from the choke to the alternator. I have to pump the gas to get it to crank if it sits for a while.
I plan on fixing the choke and flushing the coolant system tomorrow if I don’t get rained out.

 

When it's warmed up, lower the idle stop screw. If the choke is wide open straight up, the fast idle adjustment is no longer in play. If the choke is partially shut or shut, then the fast idle speed adjustment works.

 

I was worried as some carbs only have 1 speed adjustment screw and I wanted to take care of the problem that he first posted about.
Then we can look into the cold idle speed and choke settings.

Now if we had a picture of said carb to see if it has 2 speed screw settings we could do the choke now when cold and as it comes up to temp for that setting we can see how the choke works.
Dave ----

 

I followed y’all’s instructions. When the engine was warm I turned the top screw counterclockwise until the engine slowed down to 500 rpm. From there I unplugged the vacuum hose that runs from the distributor to the carb (the engine idle didn’t change any) and checked the timing. It was 6 degrees btdc so I pulled it back to 10 degrees. I was using the small inscribed line on the cam and using the metal tab that’s on the passenger side of the engine. The RPMs were around 700 now. I then adjusted the mixture screw (clockwise) that’s on the other side of the carb in the picture, to bring the rpm’s back down to 600. So far, the dieseling is GONE.

The overheating though, I’m at my wits end. The fan was on backwards pushing lots of air through the radiator instead of pulling. I flipped it thinking that would pull air into the radiator instead. Didn’t work even with a shroud added. Although, it has slowed the temperature climb down at idle. At highway speeds it rides dead on 180 like it’s supposed to.

The fan on backwards

The fan on correctly

 

You mentioned your float adjustment. Did you resolve that? If fuel can leak past the float valve that could contribute to running rich and dieseling. Choke out of adjustment same issue running rich, causing carbon deposits which adds to the dieseling issue. Fan on backwards definitely an overheating issue. What do your plugs look like? Seems you are on your way to solving the dieseling problem.

 

I don’t actually think the float was off but I set to the measurements that came on the rebuild kit. The choke is either broke or the other guy disconnected the spring on the inside. The butterfly stays wide open at all times. The plugs before I did all these things were dry and clean. I replaced them and the wires anyway and they are still dry and clean. After I turned the idle down some like the other guy said the dieseling has stopped. Timing was off some after that so I adjusted it back to 10 degrees. I disconnected the vacuum hose that runs to the distributor and plugged it when I adjusted the timing. I noticed the timing marks or rpms didn’t move any with or without it plugged. The diagram under the hood says to unplug a single wire near the distributor, I can’t find what’s it’s talking about so I’m not really even sure I did that right. It feel like I can hear it ping now but it’s hard to tell with the windows rattling/air blowing.

The fan is on correctly now and it still pushes air through the radiator instead of the pulling it through. It’s almost like the fins are set for the fan to spin counterclockwise instead of clockwise.

 

sounds to me like you are running hot, because you are running a little retarded. 10’ if accurate should be ok, (not necessarily optimal) but these balancers are known to slip, and if the ecm is still in service you have other complications at play. It may help to richen it up a bit and advance your timing a few degrees.

 

If the fan is backwards pushing air through the front, it's the wrong fan, it must be one for a serpentine setup. I see you have the thread on type of clutch/fan combo. You have to be careful about this. If this setup turns backwards, the fan can come unscrewed. That is why on a conventional v-belt setup like you have, they will have embossed on the fan shroud that the threads are left hand. Hopefully you have the correct waterpump and clutch, and just have the wrong blade assembly.

On the choke, your best bet is to go on Amazon and get a manual choke kit. It will come with a cable like on a lawnmower and instructions on now to hook it up. If you put the rebuild kit in the carb, I think you can handle this too. Automatic chokes can be very difficult to setup and get working correctly, even for the experienced carb people. The manual choke works well and is one less thing to break. It may need a little wd40 on the cable every couple of years but that is it.