Dieseling problem i think??

[HotRod929]

Hi,
Every once in a while when I turn off my truck it will try to keep going for a second. It sounds like its dieseling from what my friends have told me. I wanted to make sure with you guys and see what I can do to fix it. It started to happen more often when I decided to try a lower octain gas (89) versis the usual 93 that I use. Thats what the previous owner told me to put in it but it was getting kind of expensive. Let me know if I can fix this. Thanks a lot.

HotRod

[jmj4]

had the same thing happen to me. started dieseling after dropping octane to 87 from 93. after going through endless trials and errors (cleaning carbon w/ water down throat of carb to adjusting timing) i finally figured out what was causing it. guess if i'd set down and thought of what i'd done different since the truck started doing it, i could've figured it out without the headache. anyway, i too was trying to save some pennys. just started paying more for the gas and driving it a little less.

[HotRod929]

So basically I have to stick with 93 octain gas. THeres nothing I can really do about it?

HotRod

[jmj4]

people told me its the combustion of carbon built up on the side walls of the cylinders. this carbon gets so hot that it will actually cause the cylinder to fire with no spark. thats why it continues to run with no fire from the plug. this can damage the engine if it gets to bad because it can cause the engine to basically run backwards and not in a synchronized manner. like i mentioned, you can adjust the idle screw up to get the engine idling at about 1700 rpms and slowly pour water or tranny fluid down the thoat. this cause the carbon to break up and shatter off the walls of the cylinder and is blown out the tail pipe. however, it didn't solve the problem for me. i then proceeded to by some engine cleaner (i think it was called "greased lighting) which did break up alot of carbon. so much that i had to take all of the plugs out to clean them because the darn things were covered in the stuff and was miss firing terribly. i also tried adjusting the timing. from what i understand, retarding the timing will make the cylinder not as hot as say, advancing it. also, running the carb too lean will cause the cylinder to be hotter than if it was more rich (which these engines were developed to run rich... decreasing further in your mpg). anyway, i'm learning and not an expert by any means, but this has been my experience. anyone else is welcome to jump in with advise. like i said, i just went back to running 93 octane and it solved the problem

[HotRod929]

Is it safe for the engine to pour water or especially tranny fluid down into the carb? This just seems like not a good idea to me but Im new at this stuff. I really dont want to break anything on my truck. Can you give me some more info on doing this? Sorry for doubting you...Im just nervious about something like this. Thanks again

HotRod

[bigjimzlll]

depending on your compression ratio and cam, you should be able to retard the timing a couple of degrees and fix your problem..you will lose a little performance..but thats what you get for being cheap J/K

[HotRod929]

I dont like the idea of looseing performance...sounds bad to me. I might try the water/tranny fluid thing if i find out more about it and am not such a wus about it. Ill probably go back to 93 gas anyway. THanks

HotRod

[Zkast]

Check your basic tune. Hows the idle? Ignition timing? Coolant tempurature? High idle, advanced ignition timing and overheating/running hot will cause this problem. Any one or combination of those things listed. Also, improper heat range on the spark plug will do this too.

If the engine has many miles on it and maybe using a bit of oil also, it could be carbon deposits causing hot spots in the combustion chamber which act as an igntion source for the incoming fuel charge. Basicly it ignites the fuel mixture before the spark plug has a chance to.

Water AND Trans fluid will do basicly the same thing. There is also a product called SeaFoam that does this too. What your doing is SLOWLY pouring a SMALL... SMALL amount of water/trans fluid down the intake at 2000 rpm. This fluid vaporizes inside the cylinder and will often break up any carbon deposits on the head/piston.

There have been MANY imports (mainly toyota) that come in knocking like a rod is coming out at any moment. First thing to do is try the "mix" to see if its carbon deposits. 75% of the time its carbon.

Here's how I do this procedure:
Pull a small 5/16's vacuum hose off any constant vacuum source (usually the vacuum control for the HVAC) and place it into a quart of 1/2 transmission fluid and 1/2 water (Shaken very well) and allow the engine to pull the fliud in at its own pace.
If you use any larger vacuum source, your in danger of hydrolocking the engine!!!! With a small 5/16's hose this is the same thing as a bad modulator valve on a automatic transmission thats gone bad. Run the engine at 2000rpm until the bottle is empty then shut if off!!! Let the engine sit for 5 minutes or so to let any remaining oil soak into the carbon. Start the engine and go on a highway drive to clear things out.

[HotRod929]

Thanks Zkast. What is the HVAC and where is it? Sorry im so dumb. THanks again.

HotRod

[Zkast]

oops my bad (heating/ventilation/airconditioning system)
Usually there is some type of vacuum hose coming from the rear port runner on the intake manifold that has a hose. You can grab the hvac hose OR unhook the vacuum for the modulator valve (transmission). basicly any smaller direct vacuum port will be fine. Closer to the carb the better.

Now that I think of it... try to get one off the carb. If you pick the rear ports off the intake it will only saturate 4 cylinders at best.

[1-PU70]

Thats why they put an idel selonoid on the sid of the carb for. When you shut off the truck the idel would drop so there wouldnt be any run on.

[Zkast]

I can't speak for our year of truck but on the 73 - 87 trucks and cars GM used a Idle soleniod to raise the idle when the A/C is turned on. I havent looked at the schematic for the 67-72 yet. Anyhow, A properly tuned gm stock V8 should never run on.