Dieseling and Slight Backfire - 250I6

[Mike-P]

Hi Everyone,

Ever since I picked up my truck, I've noticed that it has been dieseling slightly (Idling a bit after ignition is turned off, 2-3 seconds). Additionally, when I am gearing down, I thought I was hearing a slight backfire in the engine compartment. So, after running some sea foam through the oil/fuel and changing the oil and filter, the problem still persists.

Today, I thought I would check the vacuum on the carburetor as perhaps the air/fuel mixture was slightly off and perhaps the truck was running lean. After getting under the hood, I noticed some black exhaust on my block. I'll attach a picture, it appears as though my exhaust manifold is either cracked or not sealed properly. Anyhow, going to look at replacing the manifold.

Would it be fare to assume the dieseling/backfire is coming from this?

Thanks for your input,

Mike

[firedemon]

timing could be too far advanced

[geezer#99]

Actually timing could be retarded and the throttle is too far open to maintain an acceptable idle. Open throttle lets fuel vapor to be drawn into the intake where it's ignited by a hot spot.
Turn your idle down and if it doesn't want to run then increase your initial timing.

[RustyBucket]

intake leak... Had a '71 G10 van 250/auto that ran me crazy DIESELING. Cured it by tightening the intake/exhaust manifold bolts. worth checking

[joeminnesota]

Sounds and looks like you have a couple problems. That looks like an exhaust leak to me. I have had someone loosely cap the tailpipe with a thick rag while you listen near the leak at the exhaust manifold. if it is leaking it will be more pronounced while the tailpipe is capped.
I also think the idle is a little high. That alone could cause the dieseling at shutdown.
I dont think the exhaust leak would cause the dieseling.

Good luck and keep us posted!

[Tiger Joe]

I'll agree with whats been said already. exhaust leak most likely isn't causing the dieseling.

what rpm is your idle? my dads 79 350 is having carb issues and will randomly idle higher than normal, and if it does that, it will diesel at shutdown.

[Gmc.guy]

Wiggle your throttle shaft. If there is wear there it will cause a vac leak and cause a lean condition and cause this as well

[RichardJ]

Intake and exhaust is the same 1 piece gasket. If the exhaust is leaking that badly, it is almost a given the intake is leaking as well.

Avoid the Felpro for this. They're crap. Use the cheapest all-fiber gasket you can find and don't over tighten. leave the bolts through the heat stove loose until the manifolds are snugged up.

[CST10]

I had a deiseling issue on my 250 engine. I tired everything, carb adjustments, timing, gaskets......everything.

One day I installed an HEI distributor. Issue went away. Don't know if it was the distributor or the timing. I did set the timing right back to where it was before I took the old one out.

[HO455]

Dieseling is caused by fuel being drawn into the engine and being ignited by glowing carbon deposits in the cylinder. When emissions became an issue the manufacturers started leaning the idle circuits out and revised the timing settings both of which made dieseling a problem due to hotter cylinder temperatures. To fix the problem they installed idle stop solenoids to hold the throttle in the idle position when running and when the vehicle is shut off the throttle plates are allowed to completely close preventing fuel from being pulled into the engine. Many times the idle stop solenoid is missing from our trucks thus the throttle plates will never be fully closed when the engine is shut down. Having an intake vacuum leak makes the engine run lean and hotter cylinder temperatures result with the glowing carbon deposits. Worn throttle plates may not seal properly allowing fuel to be pulled into the intake. Old engines will use oil which build up deposits in the cylinder. Just one vicious cycle. Hope this helps with your trouble shooting efforts.

[Mike-P]

Hey Guys,

Appreciate the opinions and feedback. We've had a break in the weather here in Vancouver, so I spent the afternoon removing the Intake/Exhaust Manifold. As suspected, the exhaust port nearest to the front was burnt right out. The good news, the Exhaust Manifold isn't cracked, so just a clean up and new seal should do the trick.

Question for your experienced guys, any advice on cleaning up the exhaust manifold other than sand blasting. I don't have access to a sand blaster and I was hoping there might be an alternative method of prepping for paint. Also, best way to clean up the ports on the block?

Thanks again and here are some photo updates.

[Mike-P]

Originally Posted by Mike-P

Hey Guys,

Question for your experienced guys, any advice on cleaning up the exhaust manifold other than sand blasting. I don't have access to a sand blaster and I was hoping there might be an alternative method of prepping for paint. Also, best way to clean up the ports on the block?

Thanks again and here are some photo updates.

Scratch the sand blasting note, I found a local do it yourself place I've decided to use. After a little online research, I think a wire brush will do the trick for ports on block, but any other advice would be appreciated.

[RichardJ]

Be sure and replace the missing stud. There should be two. If it was actually missing, most auto parts will have one. 3/8-24 / 3/8-16 Double End Stud about 2" or slightly less.
You will need them to hold the gasket and support the manifolds while you install the bolts and straps.
Don't know about other guys, but I've had bad luck with the gaskets that are metal clad on one side.
With the fiber gaskets, snug the bolts/nuts evenly and then recheck after first heat cycle. After a week or so, check and snug up again.

[1970 charlie]

while the intake is off pull the carb off and check the base to bowl ,mine was lose and a real bad vacuum leak same symptoms as yours, just food for thought.

[Mike-P]

Originally Posted by RichardJ

Be sure and replace the missing stud. There should be two. If it was actually missing, most auto parts will have one. 3/8-24 / 3/8-16 Double End Stud about 2" or slightly less.
You will need them to hold the gasket and support the manifolds while you install the bolts and straps.
Don't know about other guys, but I've had bad luck with the gaskets that are metal clad on one side.
With the fiber gaskets, snug the bolts/nuts evenly and then recheck after first heat cycle. After a week or so, check and snug up again.

Hi Richard,

Nice eye, thanks for catching that. Extremely helpful!

Maybe you could help me with one more thing, while removing the manifold. I've had a few bolts brake... I'm comfortable extracting or drilling/tapping new ones. Do you happen to know the sizes of the two below?

[Mike-P]

Managed to get everything back together. I ended up ordering a new exhaust manifold and sandblasting and painting my intake. I decided to go with black as everyone seems to paint their intake the same as the engine block, I wanted to change it up. Everything seems to be running a lot better and I was able to do a short drive today, didn't notice any backfiring, but definitely need to play with the carb a bit.

Thanks for all the help along the way.

[Wrenchbender Ret]

Nice work.