what would cause a motor to diesel, but only after its warm?

I can make short trips all day and the temp never gets to the "N" in normal range...but if i go for a run down the highway and get it up to "N" or "O" (NEVER GETS ABOVE THAT), it will diesel when i shut it off.

If i drop a bit of timing out it runs like crap.
Idle is set to 750-800 in "P", and 500-550 in "D"

After a highway run, if i let the motor run for a few secs, then shut it off it wont diesel as bad but you can still feel it trying to.

plugs, wires, cap, rotor, Ig module, carb rebuilt, and stator all have <2k miles on them.

Motorcraft stock replacement plugs, all other parts are stock replacement.

THanks, Rob

 

if you doo a lolt of short trips and very few hiwy trips there could be a nice big pice of carbin in one of the cylinders that needs to get burned out, but the only way i know of how to do that is to run her realy hard for a day, you know pull a heavy traler or rev up to 3,000-4,000 rpms in sperts to realy rase the cylinder temp.

 

by the way waht year is the pickup, if it is fule injected and not caburated this shouln't hapen.

 

'81 300-C6, 2.75 rear

i do get on it a bit more then i prolly should, rev to 3500+ before it shifts

it is carb motor, i dont have any carbon build up on the tail pipe (that i have noticed)

 

I don't know about the 81 but my 83 302 had a anti dieseling solenoid that would close the throttle blades when the ignition was shut off.
regards
rikard

 

An engine will almost never diesel when cold. An '81 should probably have an anti-dieseling solenoid, as rikard mentioned. On carburetors equipped with this solenoid, the solenoid is what actually holds the idle while the motor is running. As soon as you shut the truck off, the solenoid retracts, and allows the throttle plates to shut completely. This closes off any path for air to enter the motor, and the motor can't diesel. Hence the name.

Does your motor ping at all? An '81 should have an EGR valve. Engines equipped with EGR are set up from the factory with lean jets and an over-advanced timing curve. If the EGR valve goes out under these conditions, the combustion chamber temperatures will increase (has nothing to do with the temperature gauge on the dash). If this happens, the motor is much more likely to diesel. I ask if it pings, because pinging and dieseling often go hand-in-hand. However, even if it's not pinging, it can still diesel as I've described.

Other common causes of dieseling include high curb idle and a vacuum leak.

 

it HAD an egr, smog pump, bunch of vacuum lines, connectors, etc...i took all those off, put a block off plate where the EGR was, and retarded the timing until i no longer had any pinging on the highway. I had a problem with pinging on the highway under 1/2-2/3 throttle but got that adjusted out with the timing.

I will drop the curb idle back a bit and see it that helps...

what does the anti diesel solenoid look like, and where is it located?

 

On my 83 302 it was located near the throttle linkage on the FoMoCo 2150 carb. It was silver cylinder with a black cap and a wire. The idle speed was set by a screw that was part of the plunger coming out of the cylinder. About 5" long by 2" round.
HTH
rikard

 

nope, dont have anything like that...it s 300 - six though

 

I would check idle speed and make sure its not set too high, and also check that its not running too hot , sometimes if your timing is too far advanced it will run hot. Does the engine ping when accelerating or under load?

 

nope, no pinging....i need to put a REAL temp gauge, along with a trans temp gauge on

 

There is your problem. Engines with EGR are tuned completely different than their pre-smog predecessors. Changing the ignition timing was a band-aid fix that's only robbing you of power and mileage. You need to change the timing curve, not the base timing. Furthermore, timing is only half of the deal. Engines with EGR come with leaner jets to compensate for the modified combustion chemistry. When the EGR is removed, the engine runs lean. Running lean increases the combustion temperature which leads to dieseling.

You're going to fight the dieseling problem for as long as you're running without EGR, unless you retune the motor with an adjustable vacuum advance, different mechanical advance weights and springs in the distributor, and different size jets in the carburetor. However, this is going to be a painful trial-and-error process that will, at best, bring your performance back to where the engine was before you started ripping stuff out. The anti-dieseling solenoid can help, but the fact is, if you're running an EGR engine without EGR, it's going to diesel and ping every chance it gets.

FYI, removing EGR is a waste of your time and a step backwards. It's a common misconception that EGR is "smog junk." It cools the combustion chamber by displacing air\fuel mixture with inert exhaust gas. This does not hurt performance at all and in some cases can actually increase gas mileage. I have no idea how this myth got started about EGR robbing your motor of power, but who ever started the rumor years ago was very misinformed.

 

the only reason i removed all of it, is because the EGR was non functioning, and had a mess of vacuum leaks...

so i need to find a pre smog dizzy?

 

You can if you want.
I think a better alternative would be to have your distributor re-curved.
Remove your distributor and take it to a speed shop.
I found it kind of funny when I took mine in to have it tested and re-curved.
When I told him it was off of a 300 I6, he gave me the reaction of-
"Ahh.... well.... ok..... ahh.... whaaa....?" It was a speed shop, after all.

Anyway, this is the best way to know what your putting back in your truck.
If you find a pre-smog distributor,... then you have a pre-smog distributor.
What does that mean? I dunno.
If you have your distributor spun and checked,
you'll have numbers to use for further diagnosis.

If you go this route, leave your vacuum advance on your distributor.
They will check that for you also, telling you how much
your system is retarding your ignition at any given vacuum pressure.

I don't know what a "new" pre-smog distributor might cost you.
The checks and adjustments I mentioned above will probably cost you less than $50.

Let us know what happens,
Murph.

P.S. Then you can look into your carb jets.
P.P.S. Do you still have those emissions components to reinstall?
P.P.P.S. Yikes, good luck!

 

I wouldn't buy a new distributor, because you don't know what kind of calibration you're going to get. Because the same distributor covers a broad range of years, I'm not sure you can specify what kind of calibration you'd be getting. You might get one from a newer model or an older model, or if there's a "one size fits all." I'd hope you'd get a distributor from the same year as the vehicle you state, but it's hard to know for sure. I'm not sure if anyone would really be able to answer that for you. Remanufactured components are a mess. In that case, you'd probably have to have it checked like Murph is saying anyway. So the new distributor is a bad idea. Even with a new distributor, you'd still have the problem because you'd need new jets, and that's a battle I wouldn't want to fight. Good luck finding those.

It's much easier to go back to an EGR setup. A new valve and ported vacuum switch will cost less than anything you'd have to do involving the timing curve. I imagine you still have the spacer plate and manifold pipe installed. Cleaning out the spacer plate wouldn't be a bad job. Personally I think this is the most straightforward, direct solution to the problem.

Don't forget to check for vacuum leaks as well. I'd do that first just to eliminate anything obviouis. See if you can lower your idle speed some as well. But at the end of the day, an EGR motor missing the EGR valve is just asking for pinging and dieseling problems. If you can get the motor to stop dieseling without fixing the EGR system as I've described, then you should probably buy a lottery ticket while you're on the lucky streak.