What causes Dieseling? Need Tech Help

mustvid · Dec 16, 2004

Can someone explain the reason dieseling happens? I have a 92 5.0 with box stock AFR 165's heads & Ed Curtis cam plus all the bolt on's you can do. it's got an AOD tranny with 3:73 gears. Timing is set at 16 degrees. I'm running the AFR recommended Autolite 3924 plugs. The heads and cam were installed in the last 45 days and I have been trying to get the tune right. It does not deisel when the engine is cold. Once it's at 190 is when it will deisel. I usually have the tranny in drive and it happens when it shifts into third around 1500 RPM 25-30 mph. It only last for a few seconds. I have taken it to the track and had the timing at 18 degrees and it was real bad at high RPM and WOT. Once I backed the time down to 16 it ran fine at WOT and high RPM. It only does it now at part throttle and low RPM. I have searched many threads on different web sites and I can't find an explanation for while dieseling happens. Someone told me that advancing the timing only effects the spark in relation to top dead center. Is this happening because of to much timing? Is it because it is to hot a spark or to cold? Should I try a colder plug? I did try running mid grade gas but it made it worse. Once I put premium in it has become fainter but is still there. It is driving me nuts because I don't understand why it happen. Once I can understand why then I will know what I can do to fix it.

tmoss · Dec 16, 2004

What was your piston deck height for all cylinders? Clean the piston tops real good when adding the heads?

Your more than likely getting pre-igntion because the gas mixture is igniting before the spark. Or it could be two flame fronts colliding.

mustvid · Dec 16, 2004

tmoss: What was your piston deck height for all cylinders? Clean the piston tops real good when adding the heads?

Your more than likely getting pre-igntion because the gas mixture is igniting before the spark. Or it could be two flame fronts colliding.

You are the guy I was hoping to hear from. The pistons were fairly clean when I removed the stock heads. I cleaned them real good before installing the new heads. I did have to have the heads milled .004. However, I used ROH gaskets which are real thick. Ed Curtis sold me the cam and he didn't think this would make any difference. We did clay the pistons to check clearance. We had about a 1/4 inch of clearance. The funny thing is that it didn't do this right after the install. Do I need a colder plug? Does fuel pressure setting have anything to do with this? I plan on taking to the Dyno soon to better tune.

tmoss · Dec 16, 2004

mustvid said:
tmoss: What was your piston deck height for all cylinders? Clean the piston tops real good when adding the heads?

Your more than likely getting pre-igntion because the gas mixture is igniting before the spark. Or it could be two flame fronts colliding.

You are the guy I was hoping to hear from. The pistons were fairly clean when I removed the stock heads. I cleaned them real good before installing the new heads. I did have to have the heads milled .004. However, I used ROH gaskets which are real thick. Ed Curtis sold me the cam and he didn't think this would make any difference. We did clay the pistons to check clearance. We had about a 1/4 inch of clearance. The funny thing is that it didn't do this right after the install. Do I need a colder plug? Does fuel pressure setting have anything to do with this? I plan on taking to the Dyno soon to better tune.

If the chamber and pistons were clean, then carbon buildup might not be a big factor. .250" is a TON of clearance and seems to indicate that your deck height (position of the top edge of the piston relative to the top of the block) was positive which means the top piston shoulder was down below the top deck (top deck being above or "positive" to the piston. This is not a good quench situation and lends itself to preignition. Most builders want zero deck (piston even with top of block) or negative deck (piston above the deck) which promotes better flame fronts and flame quench. Did you check deck height when you did the install? Does backing off timing help the situation, and if so how far did you have to go?

mustvid · Dec 16, 2004

Tmoss:

Since I perceived this to be a bolt on project and I'm not really familiar with deck height I guess the answer is no. As to the change timing, I just lowered it to ten degrees and will take a test ride and see. Why would it do it only when the engine is at operating temp (190) rather than when it is cold. What would I need to do correct a deck height problem?

88-GT · Dec 16, 2004

I always associated dieseling with exactly what a diesel engine runs on, thats why it only does it when its hot, I know a few of my friends with HP BBC's, like around 500 HP diesel out a lot, and from what I hear is normal, dunno if thats true or not but one way to minimize it is to shift the car into reverse and then shut the engine off, just a little trick. Good Luck.

04sleeper · Dec 16, 2004

Did you get the spring upgrade kit? Weak springs can cause this problem as well. Try to run all the low octane fuel out and put it higher test.

jocko · Dec 16, 2004

i think you mean pinging cause i thought dieseling was when you turn off engine and it keeps running,i had the same problem on a stock motor and reducing the timing fixed it,not all engines can run the same amount and it does it more when is hot cause it is easier to ignite the mix wich in your case is preigniting.

jocko · Dec 16, 2004

i think you mean pinging cause i thought dieseling was when you turn off engine and it keeps running,i had your same problem on a stock motor and reducing the timing fixed it,not all engines can run the same amount and it does it more when is hot cause it is easier to ignite the mix wich in your case is preigniting.

dastang2 · Dec 16, 2004

you are correct, dieseling is when your timing it to out of adjustment or you have a crapload of carbon buildup. after you turn the key off it will try to keep running

tmoss · Dec 16, 2004

mustvid said:
Tmoss:

Since I perceived this to be a bolt on project and I'm not really familiar with deck height I guess the answer is no. As to the change timing, I just lowered it to ten degrees and will take a test ride and see. Why would it do it only when the engine is at operating temp (190) rather than when it is cold. What would I need to do correct a deck height problem?

Deck height can only be changed by changing pistons or milling the block deck. Safe to say you don't know what your deck height is, so no sense guessing.

Your car has preignition when it's hot because that's when chamber temperatures get high enough that any carbon or sharp edges will ignite a second flame front. Does the car diesel when you shut it off? If so, then that pretty much tells you that you have carbon or a sharp edge that is a problem.

mustvid · Dec 16, 2004

What I mean by dieseling is also referred to pinging. However, it kind of sounds like a diesel.

Tmoss: Car does not diesel when I turn it off. Lowering the timing to 10 degrees did eliminate the pinging. I guess I can add a little more timing until it does it again then back it off two degrees. Will running a colder plug allow more timing? I guess the deck height will be the stock AFR heads less .004 right? Should I have measured the deck height with calipers before installing? What would I do I found out the deck height changed from stock? I guess that means I have increased the compression correct?

HISSIN50 · Dec 16, 2004

dont forget that leaky injectors can cause dieseling. Tom is spot on about the preignition (carbon, etc acting as a glow plug).

good luck.

tmoss · Dec 16, 2004

mustvid said:
Tmoss: Car does not diesel when I turn it off. Lowering the timing to 10 degrees did eliminate the pinging. I guess I can add a little more timing until it does it again then back it off two degrees. Will running a colder plug allow more timing? I guess the deck height will be the stock AFR heads less .004 right? Should I have measured the deck height with calipers before installing? What would I do I found out the deck height changed from stock? I guess that means I have increased the compression correct?

You can try running a colder plug if you want to run more timing. Deck height varies from engine to engine and piston to piston on any engine. My stock readings were zero to .009" negative deck (piston top out of the bore). The only way to know what it is is to measure it with the heads off and each piston at ACTUAL TDC as measured. Methods of finding actual TDC are on some of the cam manufacturers web sites. When you have the piston at TDC you measure with a good dial micrometer how far the piston is below or above the top of the block (deck). It can also be exactly even with the block, called zero deck. Remember, you use the top edge of the piston as the place to measure the piston relative to the top of the block.

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