big chevy "deiseling" when shut off

[choptop]

Ok, this isnt actually our type of square body truck, but is very close. We have a 90 C60 schoolbus at church in which I get to drive every sunday morning. Recently it has be "deiseling" after shutting off the ignition, and is getting worse. The thing is my 86 is doing the same thing. I have not ever driven one of these trucks with all of the smog pump and related junk on them and will be removing it on my 86 soon. What is causing this? I have never had any of my other trucks do this, but I have seen other peoples truck do it. I was wondering if it is excessive carbon built up on the pistons thst is causing an ignition ignition source, but I am not really sure. Any ideas or suggestions?

[TexasRed]

Well, is it carb'ed or TBI? I know that TBI was usually the norm after 86, but occasionally a carb sneaks outta the factory.

With a carb it's usually the idle speed is too high, the ignition is too advanced, or the choke is not closing. With TBI it may be that the fuel injectors are leaking a little gas into the engine to keep it running.

[Prong]

Could be cheap (low octane) gas. Mine does this on that 85 octane stuff.

[choptop]

The bus is a 2 barrel carb, and my 86 has the 4 barrel q-jet. I'm having issues getting the bus' idle right. If it idles around 900rpm, when its put in gear, it dies. This goofy thing has more junk on it than the truck from Sanford and Sons. We are looking into the option of removing the 2 barrel intake and putting on a 4 barrel and removing the air plumbing and smog pump. I cannot figure out why it dies when put in gear at 900 rpm, its gotta be around 1500rpm to work, then does fine. Its gotta Allison tranny (a 540 i think) that has some shifting problems, but i think if we can get the engine right, that will straighten out.

[Lt.Del]

TexasRed is right... ensure your throttle return spring is closing the throttle plate when shutting off....it seems air /fuel is getting in causing the engine to run on. 
Retard the timing a hair and see what that does.   You can throw a cooler spark plug on there as well. 
Decrease engine rpms to around 600 or so...if it wont idle there, you have vacuum leaks or carb needs rebuild or something similar.

It is possible some carbon or soot is staying in the combustion chamber keeping too much heat in there, therefore, keeping the temp high enough for combustion after the spark actually leaves the chamber.  A quick check of your plugs should display the symptoms---sooty or carboned tips--check when engine is hot. Carb may be too rich if that is the case.

An increase in octane  will work as well since higher octane needs higher temp to combust. all else fails, turn the engine off in Drive--but these ideas just hide the real problem.

[DnStClr]

all else fails, turn the engine off in Drive-

LOL
 Won't the kids on the bus just love that..

[choptop]

Ok, I got removed both air pumps and plugged the exhaust holes. I reset the timing to 8 degress btdc, and it is running ok, but still have an issue getting the engine to idle in gear. The rpms drop 300-400 rpms when it is put in gear. So I have the idle set at 1200 to drop to 750 rpm in gear.I have pluged the vacuum lines, and cant seem to find anymore. I think I need to replace the modulator on the tranny because it has tranny fluid in it and the hose is swollen, not lekaing though. I think the carb may be messed up, because if I tighten up the aircleaner too much, it changes the way it runs. May be time for a 4 barrel upgrade. But, I have a new issue. It runs better, and drives better, but when I got back from a test drive, shut it off to see if its still deiseling, and it did, but not as bad. The problem is when I tried to restart it, it didnt wanna turn over. Let it cool, and it fired right up. Shut it down, and would not restart. I believe I have a starter issue. My question is, what year model chevys used the started that bolts to the face of the tranny, isntead of the bottom of the block. It is a 90 C60 bus, autozone probabaly wont have a clue on which one I need. Any idea??

[frogman68]

Ok, I got removed both air pumps and plugged the exhaust holes. I reset the timing to 8 degress btdc, and it is running ok, but still have an issue getting the engine to idle in gear. The rpms drop 300-400 rpms when it is put in gear. So I have the idle set at 1200 to drop to 750 rpm in gear.I have pluged the vacuum lines, and cant seem to find anymore. I think I need to replace the modulator on the tranny because it has tranny fluid in it and the hose is swollen, not lekaing though. I think the carb may be messed up, because if I tighten up the aircleaner too much, it changes the way it runs. May be time for a 4 barrel upgrade. But, I have a new issue. It runs better, and drives better, but when I got back from a test drive, shut it off to see if its still deiseling, and it did, but not as bad. The problem is when I tried to restart it, it didnt wanna turn over. Let it cool, and it fired right up. Shut it down, and would not restart. I believe I have a starter issue. My question is, what year model chevys used the started that bolts to the face of the tranny, isntead of the bottom of the block. It is a 90 C60 bus, autozone probabaly wont have a clue on which one I need. Any idea??

My 70 GMC C25 350 bolted like that starter . Starting issue when warm could be the timing

[TexasRed]

Sounds like a vacuum leak. Try replacing the modulator. The base gasket on the carb may be leaking and causing it to leak too much vacuum. That's why it drops the idle so much when you place her into gear. Try spraying a little carb cleaner near the base. Check your battery cables to see if it's them causing a slow start. Or maybe your battery is heatsoaked? Also, check your oil level.

[choptop]

Thanks frogman68, I'll go there if they cant find it any other way. I did some research and found that there were several other modles that used it up into the late 80s, so we'll see . The timing was set a 8 btdc. When it does this, I can move the distributor any where, and it doesnt help until it cools off.

   TexasRed, I do believe the modulator is bad, but I plugged the line, and it didnt change anything. I did check the carb base gasket, but couldnt find anything. The odd thing about this, is that when I tighten or loosen the air cleaner, it affects the way it idles. I'm gonna try to get a better look at it later, and see if maybe someone over tightened the wingnut, and may have cracked the housing or something. I know the battery cables are good, but the battery is questionable. I didnt think about testing it yesterday because it had started many times now prob. The reason I was thinking starter instead of timing, is I started and stopped it several times, no problem, then, drove it around the block, and it wouldnt restart.  I really wanna change out this 2 barrel for a 4 barrel anyway.We'll see how today goes.

[Lt.Del]

I started and stopped it several times, no problem, then, drove it around the block, and it wouldnt restart

Your starter solenoid is heating up, right up under the exhaust manifold...my sub did that, even with the heat shield in place.

I replaced the starter and solenoid and no problemo any more.