Ok, I know theres a few things that play a role in why an engine may not shut off when ignition is turned off, but I'm not quite sure of my next step here.

I have a 350 with Vortec heads, and whatever is in my sig. I have tried base timing anywhere from 0* to about 10*, and I still have this problem. Right now I have it at about 4*. Engine temps are anywhere from 165 to 195, it does it even at lower temps. I'm running 87 octane, which should be fine for my 9.6:1 compression ratio, and I'd rather not use higher octane gas, especially at the current prices. Now, I'm using a set of Bosch Platinums (I forget which part number, cant remember off the top of my head) only because they were the cheapest that would fit with my headers (I need shorter plugs). My question is, can this be from too hot of a spark plug? Does anyone think that switching down one or two heat ranges may fix this problem, or would it cause drivability problems under normal driving conditions? Any help appreciated. I dont like having to put my car in park AFTER I shut it off everywhere I go, seems kinda hillbillyish to me.

Thanks,
Ben

On second thought, heres a pic, so if anyone can identify these plugs for me, thanks. It may help out?

 

I too am interested to know what people might reccomend to cure this. I do know that if you get colder plugs you are in effect retarding the timing alittle so it will make it slightly slower.
BTW, have you dynoed your motor?

 

Spark plug type and heat range has no effect whatsoever on timing. It doesn't slow anything down either.
Your 9.6 c/r is in the 90's octane range.
What is your idle set to?
The plug heat range will only have an effect on turning the engine off if it's the tip of the plug glowing and causing the ignition of the fuel.

 

Guess I forgot to mention, idle is around 1100 in park, around 800-900 in gear. Any lower than that and I cant get it to idle in gear (I think I need a heated O2 sensor). I know I can use higher octane fuel and solve it (have before with previous motor) but I didnt think 9.6:1 warranted relatively high-octane gas.

FirstGen, no dyno pulls yet, but I'm really curious as to what it's putting out. I'm satisfied (for the time being anyhow) with it's power, it gets where you wanna go.

 

Also forgot to mention, it's not running lean either, I recently richened up the idle mixture (maybe a little too rich even), and it persists.

 

Running rich with carbs can cause excessive carbon deposits in the cylinder head's combustion chambers. Is what happens is the carbon heats up enough so that it can ignite the fuel and turn the engine over a few times before quitting.

Try shutting the car off while it is in drive. And what is your timing set at?

 

Shuts off just fine in drive, thats what I have to do now. I havent had these heads on long enough for carbon to be built up, I dont think. I had originally thought that may be the case and I ran some H2O through the engine to try and clear it out, but like I said I dont think that is the case.

 

Oh yes, long shot but worth a try. Whens the last time you went over your engines vacuum connections? PCV and what not?

Do you have AIM? If so, my screen name is in my profile.

 

One way to get around the idle issue is to install an idle stop solenoid. You can use the solenoid to maintain the 1100 rpm idle. When the key is turned off, the solenoid will drop out letting the throttle lever move to the carb idle screw which you can back way out.
I use this on my 10.7 motor.
But back to fuel, the c/r is calling for at least 92 octane if I recall.
Pull the plugs and check with a magnifying glass.
What's the color?
Also, look for small specs ( *****) around the tip of the porcelain. Running the lower octane under load may be mildly detonating and eating away at the pistons without you realizing it.

 

so why do people reccomend that you go to a colder plug when running nitrous on newer computer controlled vehicles? I always thought this had a similar effect as to retarding the timing. I do realize that it cannot actually physically retard the timing...
thanks

 

The heat range is just an index as to how fast or slow the porcelain tip can transfer the heat from the combustion chamber to the shall of the plug. If it's too cold, it will not burn off deposits accumulating on the firing tip. If the heat doesn't transfer fast enough, the tip will glow and set off premature ignition.

 

In very simple terms: a stable combustion process desires a certain level of heat in the chamber as the piston moves up the bore. Anything above the desired temps causing the combustion process to run outside control of the ignition system. Many things can contribute to those temps: piston speed, compression, effect of burnig N2O, blower, tight cam, timing, spark plug, etc... It just so happens that twsiting out a spark plug and running a cooler heat range is the easiest way to tune heat out of the combustion chamber.

 

87Formula4bbl

First of all the 2 plugs in the pic dont have the same reach, which could be part of the problem. Which plug were you having the problems with?

The platinum appears to be the proper reach for a Vortec head while the other plug is for a iron L98 head.

For your application I would recomend that you dump that Bosch crap and run a NGK TR55. The TR55 is the proper reach and proper heat range for Vortec heads and a mild motor.

 

I run 93 octane with my 9.6:1 compression just to be safe.

I had a bad dieseling problem with my motor after I got it together. No matter what I did it would diesel. Finally I went ahead and did a cam swap. I figured my intake gasket was leaking. Well got the cam in and intake back on and it hasn't dieseled since. So I'm assuming I had a bad vacuum leak.

 

Usually going to a larger cam will decrease the chances of dieseling at idle since the increased overlap lowers the cylinder pressure at idle speed.

 

Thanks for all the posts guys. About the spark plug picture, thats from when I found out that I actually needed a longer-reach plug for the Vortecs, and when I bought a new set to fit I took a quick picture to show the difference in length between my old ones and new ones. I'm running it now with the Bosch's (left side). Looks like I'll just have to step up the octane level, which I guess woudlnt be a bad thing because anything more than about 6* base, it will ping under heavy load sometimes. Maybe I'm missing out on a bit of power here and can run more base with better gas (I still have the LG4 computer-based equipment too). I guess I'll try it out and see how she runs (I'm almost sure this will fix my problem, I just wanted it to be my last resort with gas prices as they currently are). Once again, thanks for the replies, and I'll let ya know if this helped cause I'll be filling up today, I'm empty.

 

Is everyone blind?!

First thing to check for a run-on condition is for a high idle.

He stated that the idle in park was at 1100rpm. Thats WAY too high for QJet car. I can't believe you missed the obvious: It shuts off fine when in drive. The only difference between park and drive is about 250-300rpm. You're dropping the engine speed down to shut it off by leaving it in gear. Still, 850-900 is still a bit high for idle, much less in gear. I have a much more radical cam than you, and my car idles fine in park at 800, and in gear at 700.

Get that idle down, and you'll fix the run-on problem as well. The high idle is not the 02 sensors fault. After you make sure the choke is operating properly, you need to adjust that carb...

I still can't believe everyone overlooked this. Its like asking why you have a flat tire. Then you post you have a huge hole in the sidewall. Then some genius suggests checking your valvestem, or maybe the bead isn't seating

 

I see the idle's high but as said it can't be lowered with the non-stock cam. This is why I in particular posted the idle stop solenoid.
I realize non stock is non stock and there's only so much the factory setup can tolerate.

 

Overlooked that for some reason, that is way out of line for that little cam he got.

Sometihng out of tune for sure.

 

My winter car had a similiar problem. High idle and it would diesel when you would turn it off.


Turned out I had 3 vacum leaks. Fixed them and the idle went down and no more dieseling.

 

I agree. Looking back at that duration, it should easily idle well below that number.
With my 238 degree I idle lower than that.

 

Shuts right off with premium, as tested today. I'm going to find an empty lot and see if I cant get this carb running a little bit better. I think my O2 sensor is not giving a signal at lower rpms. What will happen is after cruising around, if I come to a light or any stop, it will idle fine for a few seconds (sometimes its fine even for a while) but more often than not, it will start running rough and the car will want to buck slightly (when holding the brakes on), feels like the torque convertor is catching then uncatching from the rough rpm's (not staying steady), but other than that, it cruises perfectly, and even at around 1100 rpms in park, it will idle smooth as all get out.

 

In response to the vacuum leak question, I'm 100% sure there are no vacuum leaks. I've checked time and again, everything is sealed up (Have most all new rubber hose-to-whatever boots too and new vacuum line). I even just recently relocated my MAP sensor to near the carb, with about 8" of new correct size vacuum hose (I thought the old one may be a bit plugged, could hardly blow through it), so I just went ahead and moved it while I was at it. I thought it may not be getting a good signal and therefore may have had my timing off a bit for whatever reason.

 

Maybe the idle circuit can't supply enough fuel.
Can you adjust the fuel enough for it to start running rich at idle or are the screws maxed?
Also, I'm not that familiar with the GM feedback system, but does it do anything at all on idle?

 

Right now the idle mixture screws are about 3 turns out (maybe 2.5, cant remember). Either way, I'm not sure about this. I dont want to turn this into a carb-tuning thread, but I have the screws about 3 out, and i have to run the IAB out a bit or else I have a big flat spot if you give it any gas off idle. Seemingly it idles at about 50% duty cycle, but does fluctuate some. Majority of the time it stays around about 52% IIRC.

edit: I've read of people running 6-7 turns out on the Mixture Screws, but I dont see how. Seems almost to the point of falling out to me.

 

Well, I went out and did a little tuning. I pulled the Mixture screws out to 4 turns out and reset the IAB. 50% duty cycled obtained. I reset the base timing to 8* advanced and idle speed to about 900rpms in park. Idles decent in drive at around 600 or so rpms, but gets a little choppy sometimes (gauge fluctuates by about + or - 50rpms). I think I may need to step up to a heated O2 sensor, but I'm going to wait until I can find a way to see if the O2 sensor is sending a signal at those low rpms first (WinALDL?). Anyhow, I now have good throttle response again, and its running good. Thanks for the help guys! Next tank I'll probly fill her up with midgrade and see how it does, and if it runs fine still I may step back down to 87 again, which would be nice! Thanks again!

-Ben

 

With your compression I think 87 octane is alittle low.

As for the idle...mine idles at 1000rpm out of gear and 750 in gear....

Never diesels.

 

More often than not a car diesels because too much raw fuel is being dumped into the intake when you shut the key off. In laymens terms, something on the carb is out of adjustement.

 

The cam timing you have and the vortec heads generate a lot of low rpm compression. You probably are on the verge of detonation if it not for the computer contolled spark. 87 octane is probably borderline insufficent. Tuning is going to be your only hope

Sorry for being late to the party. You probably now already now know what I told you. But this is not all that unexpected.

 

Ya, I resolved the problem a long time ago through tuning the carb. I've since ditched the carb and am now in the process of wiring up my RamJet setup. I like being able to see exactly how the car is running by looking at numbers (on a computer screen)!