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vwsquare2
Samba Member
Joined: February 16, 2016
Posts: 104
Location: Pennsylvania
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 Posted: Sun Jun 20, 2021 10:41 pm Post subject: Dieseling/Run On |
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Over the past few weeks, I've experienced very brief (< 2 seconds) of dieseling/run on after I switch off the ignition key of my 1973 Squareback with the original D-Jet fuel injection. I can only think of three causes:
1. Carbon Build Up on the spark plugs. I haven't driven the car much of the past year due to the pandemic and the trips I do take are only about 10-20 miles.
2. Too much advance.
3. Idle too high
Is there anything else?
I pulled the spark plugs today to check their condition. The pictures below are of the plug pulled from cylinder 3. All the plugs looked like this. I think this looks pretty good after 4 years and about 1500 miles.
The plug I pulled from cylinder #4 had a gap of 0.028". This matched what the owner's manual says the gap should be (0.028"). Below is a photo of that page.
Plugs in cylinder's 1 and 2 had gaps of about 0.024" while #3 (pictured above) had a gap of about 0.028". That got me wondering why. I checked my Bentley which says the gap should be 0.024". This is different from the Owner's manual. I also found a post on Samba stating a gap of 0.028".
Before I reinstall the cleaned up plugs and move on the checking timing and idle, what should the plug gap be? Or is anywhere between 24 and 28 O.K.?
Thanks. |
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sjbartnik
Samba Member
Joined: September 01, 2011
Posts: 5994
Location: Brooklyn
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| Posted: Mon Jun 21, 2021 3:44 pm Post subject: Re: Dieseling/Run On |
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Dieseling should not happen at all on an FI car UNLESS you have a leaky injector (or more than one).
Dieseling needs fuel to happen, it's when fuel/air mix is ignited by a hot spot in a cylinder rather than the spark plug and so on a carb car that continued rotation of the engine can suck in a bit more fuel/air from the idle circuit and keep running on for a bit.
VW started putting solenoid-activated plungers in the idle circuits of their carbs to avoid this phenomenon. When they key turned off, plunger would pop in and seal off the idle circuit to prevent fuel flow.
With FI, the fuel flow should stop as soon as you turn the ignition off since that cuts power to the injectors.
However if you have leaky injectors they can still dribble fuel and that could allow for run-on.
So I think your problem is not likely to be in the spark plugs but rather you're going to have to find out how/why fuel is still getting into the cylinders to allow for run-on.
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vwsquare2
Samba Member
Joined: February 16, 2016
Posts: 104
Location: Pennsylvania
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| Posted: Tue Jun 22, 2021 10:29 am Post subject: Re: Dieseling/Run On |
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O.K. so checking for too much advance would only be useful if I had carbs that didn't have the solenoid activated plungers. My fuel injectors should do the same thing as the carb plungers unless they are leaking so I'll have them checked.
Any opinion on what the spark plug gap should be, 0.024 or 0.028"? Not my main issue but as I have the plugs out, I'd like to gap them correctly.
Thanks |
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21512
Location: Oklahoma City
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| Posted: Tue Jun 22, 2021 11:05 am Post subject: Re: Dieseling/Run On |
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| sjbartnik wrote: |
Dieseling should not happen at all on an FI car UNLESS you have a leaky injector (or more than one).
Dieseling needs fuel to happen, it's when fuel/air mix is ignited by a hot spot in a cylinder rather than the spark plug and so on a carb car that continued rotation of the engine can suck in a bit more fuel/air from the idle circuit and keep running on for a bit.
VW started putting solenoid-activated plungers in the idle circuits of their carbs to avoid this phenomenon. When they key turned off, plunger would pop in and seal off the idle circuit to prevent fuel flow.
With FI, the fuel flow should stop as soon as you turn the ignition off since that cuts power to the injectors.
However if you have leaky injectors they can still dribble fuel and that could allow for run-on.
So I think your problem is not likely to be in the spark plugs but rather you're going to have to find out how/why fuel is still getting into the cylinders to allow for run-on. |
^^^^^^^THIS!
I have NEVER seen ANY injected vehicle "run on"....except.....always an exception.....when I had an engine whose cold start injector was connected improperly and spraying when it should not have been.....and once on a CIS injected engine with an injector with a cracked pintle.
In both cases....as sjbartnik noted...there was fuel entering the combustion chamber when there should not have been.
This is also important. When its a single injector....like with the CIS system....the "run on"...was more of a violent lurch. Maybe 2-3 complete revs. This is because only one cylinder had fuel it should not have had.
On the type 4 with D-jet with a bad cold start valve .....it ran on for like 5-8 seconds...because all four cylinders were being fed from the leaking cold start valve.
In both cases...since the residual line fuel pressure was now gone...they were both hard to start. Ray |
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vwsquare2
Samba Member
Joined: February 16, 2016
Posts: 104
Location: Pennsylvania
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| Posted: Tue Jun 22, 2021 5:45 pm Post subject: Re: Dieseling/Run On |
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Ray, a lurch is a good way to describe it. It doesn't last long, just a second or two at the most so I'll check for a leaking injector.
I need to put the spark plugs back in. Do you (or anyone else) want to weigh in on my question about what the plug gap should be? The Bentley calls for 0.024", the owner's manual calls for 0.028" and I find 0.028" in a few posts here so I'm leaning towards 0.028".
From what I've read, the smaller gap is better for starting and idle while a wider gap is better at higher RPM. As I have FI and Pertronix electronic ignition, I think my idle will be fine with the wider gap.
Thanks. |
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21512
Location: Oklahoma City
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| Posted: Tue Jun 22, 2021 8:36 pm Post subject: Re: Dieseling/Run On |
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| vwsquare2 wrote: |
Ray, a lurch is a good way to describe it. It doesn't last long, just a second or two at the most so I'll check for a leaking injector.
I need to put the spark plugs back in. Do you (or anyone else) want to weigh in on my question about what the plug gap should be? The Bentley calls for 0.024", the owner's manual calls for 0.028" and I find 0.028" in a few posts here so I'm leaning towards 0.028".
From what I've read, the smaller gap is better for starting and idle while a wider gap is better at higher RPM. As I have FI and Pertronix electronic ignition, I think my idle will be fine with the wider gap.
Thanks. |
Your plugs look damn good actually. Very uniform, tan electrodes...black ring common with unleaded fuel.
The stock gap on that plug is 0.024". Yes...you could get away with 0.028". Its not a hige gap change....but if all is stock including coil and points....stay with 0.024". It will be more reliable at high rpm as points get a little carbon or coil starts to age.
If you went to a pertronix and a pertronix 42kv coil...easy 0.028" or 0.032". Ray |
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vwsquare2
Samba Member
Joined: February 16, 2016
Posts: 104
Location: Pennsylvania
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| Posted: Wed Jun 23, 2021 12:07 am Post subject: Re: Dieseling/Run On |
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| I already have the Pertronix ignition but with an original style coil, not the 42KV Pertronix coil. Two of the plugs were 0.028" but the other two were 0.024" when I pulled them. They all looked like the plug in the picture so I'm guessing the gap didn't matter much. I didn't install the plugs new; the only reason I think they are different is that someone assumed they came pre gapped. I haven't driven far in the last 4+ years, ~1500 miles. |
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21512
Location: Oklahoma City
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| Posted: Wed Jun 23, 2021 6:53 am Post subject: Re: Dieseling/Run On |
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| vwsquare2 wrote: |
| I already have the Pertronix ignition but with an original style coil, not the 42KV Pertronix coil. Two of the plugs were 0.028" but the other two were 0.024" when I pulled them. They all looked like the plug in the picture so I'm guessing the gap didn't matter much. I didn't install the plugs new; the only reason I think they are different is that someone assumed they came pre gapped. I haven't driven far in the last 4+ years, ~1500 miles. |
Actually.....Bosch plugs DO come pre-gapped.....or are supposed to. The part number you sre using comes pre-gapped to 0.024.
Make note.....this does not mean I trust anyone! I check the gaps out of the box for every plug I install. 
But I do that for a reason. When I first started checking new plug gaps.....it was just out of curisosity.
A long time ago.....I started noticing that an occasional plug was way off...usually too small of a gap. Luckily ai noticed that the box was kind of wrinkled on one end....and the lottle cardboard cone on the inside was flattened.
What happens is that sloppy parts stocking people had DROPPED the box. Likely the whole bulk box. You will notice that spark plugs come with a little cardboard cone around the electrode end. This is designed to prevent a minor drop of a sparkplug box from dinging the electrode. But if you have a dozen in a bulk sales box and you drop that box.....all bets are off with that much weight.
This is purely speculation....but I am betting that whoever installed your plugs found two of them tight....and regapped them ti either the wrong gap or got sloppy.....or.....and I find this common.....used one of those damn crappy ramp style ring gauges.
Like this one
https://www.amazon.com/OEMTOOLS-25352-Style-Spark-Gauge/dp/B000BYM508
The rather wide electrodes on the Bosch Super plugs like yours covers roughly 2 to 2.5 gauge numbers. So.....it always gaps electrodes with one side wider than the other....which means the smaller edge of the electrode will ALWAYS be the only area to arc.
So if they were also a little sloppy and centered the electrode on 0.025" instead of 0.024"....They could have a real world gap on one edge of 0.027". And.....arcing on one narrow side will rapidly wear the gap on that edge.
Check them carefully to see if any of them are slanted. Ray |
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Bobnotch
Samba Member
Joined: July 06, 2003
Posts: 22410
Location: Kimball, Mi
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| Posted: Wed Jun 23, 2021 12:38 pm Post subject: Re: Dieseling/Run On |
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I check them all, irregardless of make. Doesn't matter if it's NGK, Bosch, or AC Delco, I check them all.
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