Dieseling & Spark Plugs
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
Dieseling & Spark Plugs
Assuming I've checked ALL other common sources for dieseling (and I have) would it help to try a colder spark plug? Moving from a 5 heat range (NGK) to a 6? Will it really "remove approximately 70°C to 100°C from the combustion chamber" as quoted from NGK's website?
Thanks,
Trace
Thanks,
Trace
Drifting
Joined: Feb 2005
Posts: 1,438
Likes: 2
From: Melrose Florida
I don't doubt that you've tried everything, and the plugs may help the situation.
In my experience, dieseling is always caused by one of two things:
1. Vacuum leak
2. Incorrect timing
Changing the plugs may help with the run-on, but would just mask the problem.
Steve
In my experience, dieseling is always caused by one of two things:
1. Vacuum leak
2. Incorrect timing
Changing the plugs may help with the run-on, but would just mask the problem.
Steve
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
Timing is has been checked.
However, vacuum interests me. I have a new interesting theory. Quick history:
Originally a stock ZZ4 with a Holley 650 Vacuum secondary. No dieseling. Ran for about 4000 miles in this form.
I pulled the heads and replaced them with AFR 195's angle milled from 65 to 58 cc's to match the original ZZ4's 10:1 compression. Also installed a CompCams 280HR Magnum cam. Performer Intake and Holley 650. Car dieseled after a hard run and/or especially on hot days. Ran for less than 200 miles like this.
Recently I replaced the Performer Intake with a Performer RPM AirGap and the 650 Holley with an Barry Grant 750 Double Pumper. Car still diesels. So far I have about 200 miles like this.
Recently I read on AFR's website that if you get their heads angle milled (they did the milling at their factory) that "angle mills require a .120" thick intake gasket." Both times I pulled the intake (the Performer and the Performer RPM) they recommended FelPro's 1205 intake manifold gasket which is only .060" thick.
Could this be my problem? That I'm having a small vacuum leak from somewhere because of incorrect intake manifold gasket thickness?
However, vacuum interests me. I have a new interesting theory. Quick history:
Originally a stock ZZ4 with a Holley 650 Vacuum secondary. No dieseling. Ran for about 4000 miles in this form.
I pulled the heads and replaced them with AFR 195's angle milled from 65 to 58 cc's to match the original ZZ4's 10:1 compression. Also installed a CompCams 280HR Magnum cam. Performer Intake and Holley 650. Car dieseled after a hard run and/or especially on hot days. Ran for less than 200 miles like this.
Recently I replaced the Performer Intake with a Performer RPM AirGap and the 650 Holley with an Barry Grant 750 Double Pumper. Car still diesels. So far I have about 200 miles like this.
Recently I read on AFR's website that if you get their heads angle milled (they did the milling at their factory) that "angle mills require a .120" thick intake gasket." Both times I pulled the intake (the Performer and the Performer RPM) they recommended FelPro's 1205 intake manifold gasket which is only .060" thick.
Could this be my problem? That I'm having a small vacuum leak from somewhere because of incorrect intake manifold gasket thickness?
Drifting
Joined: Feb 2005
Posts: 1,438
Likes: 2
From: Melrose Florida
It sounds like you know a lot more than I do about high-performance parts!
However, I have heard about port mismatch between heads and intake manifolds, especially if machine work has been done on heads or block.
Also, a vacuum leak around the intake manifold would cause your dieseling.
Steve
However, I have heard about port mismatch between heads and intake manifolds, especially if machine work has been done on heads or block.
Also, a vacuum leak around the intake manifold would cause your dieseling.
Steve
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
Not too sure about that.. lol... anyways thanks for the help, I'll eventually change out that gasket when I get the time and motivation, until then I'll live with the problem I suppose.
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,408
From: At my Bar drinking and wrenching in Lafayette Colorado
Timing seldom affects run-on after engine shutdown (a.k.a. "dieseling") - there is no spark after shutdown, so where the timing is set is irrelevant.
The most common cause for run-on after ignition shut down is excessive high idle rpm. GM had this problem and ended up using the idle solenoids to solve the problem: The solenoid would close the throttle after engine shut-down to eliminate run-on. You can do the same by making sure your idle rpm is as low as possible - if you're idling above 750, you're a good candidate for run-on.
If your car originally came with the idle solenoid (most C3 vintage GM cars did), make sure the solenoid is installed and adjusted correctly. A correctly adjusted idle solenoid will eliminate run-on.
The most common cause for run-on after ignition shut down is excessive high idle rpm. GM had this problem and ended up using the idle solenoids to solve the problem: The solenoid would close the throttle after engine shut-down to eliminate run-on. You can do the same by making sure your idle rpm is as low as possible - if you're idling above 750, you're a good candidate for run-on.
If your car originally came with the idle solenoid (most C3 vintage GM cars did), make sure the solenoid is installed and adjusted correctly. A correctly adjusted idle solenoid will eliminate run-on.
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
She does idle around 750 or 800 RPM... I've been avoiding the idle stop solenoid solution because I was afraid of clearance issues with my drop down air cleaner base and the difficulty in finding correct brackets... I guess I will finally nip this issue in the butt and just get one and make it work somehow. I'll keep you guys posted. Thanks as always.
Trace
Trace
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Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
9 times in 10 the primary throttle blades are too far open at idle. Your cam (280H) has a low vacuum signal at idle in a 350 (probably about 10 hg) and you've adjusted the idle speed to compensate. But now the TB's are too far open and pulling fuel from the transition slots and heat in the chamber is lighting it off even after shut down (dieseling).
First make sure you're running manifold vacuum to the vacuum advance on your distributor. If not, do so, which will raise your idle speed then you can reduce the idle speed at the carb which will close off the primary TB's.
If you are running MV and she diesels, you can crack the secondary TB's at idle on your Holley which will raise the idle speed but then you can reduce the idle speed at the carb which will close the primary TB's and prevent the dieseling. There's a secondary TB adjusment on your Holley on the passenger's side near the TB shaft.
Some like to drill holes in the primary TB's which allows them to be more closed at idle and to cover the transition slots but this is a last resort.
Good luck.
First make sure you're running manifold vacuum to the vacuum advance on your distributor. If not, do so, which will raise your idle speed then you can reduce the idle speed at the carb which will close off the primary TB's.
If you are running MV and she diesels, you can crack the secondary TB's at idle on your Holley which will raise the idle speed but then you can reduce the idle speed at the carb which will close the primary TB's and prevent the dieseling. There's a secondary TB adjusment on your Holley on the passenger's side near the TB shaft.
Some like to drill holes in the primary TB's which allows them to be more closed at idle and to cover the transition slots but this is a last resort.
Good luck.
Last edited by 73, Dark Blue 454; Apr 1, 2008 at 12:34 AM.
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
I see what you are saying, I will give that a shot as well before I go the idle-stop solenoid route.
I have gotten the idle down around 700 too... but adding to my troubles I have a non factory A/C compressor that doesn't like to idle that low when engaged... pointing again to some sort of solenoid...
I have gotten the idle down around 700 too... but adding to my troubles I have a non factory A/C compressor that doesn't like to idle that low when engaged... pointing again to some sort of solenoid...
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,408
From: At my Bar drinking and wrenching in Lafayette Colorado
9 times in 10 the primary throttle blades are too far open at idle. Your cam (280H) has a low vacuum signal at idle in a 350 (probably about 10 hg) and you've adjusted the idle speed to compensate. But now the TB's are too far open and pulling fuel from the transition slots and heat in the chamber is lighting it off even after shut down (dieseling).
If you are running MV and she diesels, you can crack the secondary TB's at idle on your Holley which will raise the idle speed but then you can reduce the idle speed at the carb which will close the primary TB's and prevent the dieseling. There's a secondary TB adjusment on your Holley on the passenger's side near the TB shaft.
If you are running MV and she diesels, you can crack the secondary TB's at idle on your Holley which will raise the idle speed but then you can reduce the idle speed at the carb which will close the primary TB's and prevent the dieseling. There's a secondary TB adjusment on your Holley on the passenger's side near the TB shaft.
This is absolutely correct - make sure you do not have more than .020" of the primary side transition slots exposed below the throttle blades - crack the secondary idle up to reduce the primary side idle speed to limit transition slot exposure. If you need specific instructions on this setup, drop me a note for my Holley and BG setup papers - they show how to do this.V8FastCars@msn.com
Lars
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
One of the GM "tricks" was to put a 'false' load on the drivetrain when the ignition was shut down. Since you have A/C, you can do the same thing. Just rig up a timed relay to turn ON the A/C compressor clutch for about 3 seconds when you shut the engine down. The compressor load will 'kill' the engine and the timed relay will shut off after the 3 seconds. That will stop the dieseling. I'd also recommend that you follow-up with the 'corrections' to your idle condition as suggested above.
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
I've actually been doing the "false load" trick.. by turning on the a/c before shut down... I've noticed it helps and makes it less common (definitely shorter dieseling).. but it hasn't stopped completely.
