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Run-on/dieseling causes

Just lately, and only occasionally, when I shut her down (fully warmed up) she'll run-on for perhaps a scant second...as though one cyliner is still chugging along. No, I do not have the ignition delay relay hooked up, so the pumps and MSD ignition are both shut off at the same time with no timed ignition continuing to burn any remaining fuel.

I'm guessing that I have one injector pintel that's not staying closed when the fuel system pressure drops with the pumps off (i.e., residual pressure bleeding fuel through a faulty injector), and igniting within the cylinder. It would seem that either hot glowing carbon deposits or the tip of the hot spark plug is sufficient to ignite that small amount of fuel seeping past the injector.

Doing some research, I see where someone commented on reducing the plug gap to remedy the run-on. What's the logic with that approach? I run with a .040gap. The only logic I could see would be to run with a step colder plug.

Looks like I may have to pull and check the plugs, check all the injectors.... Fun fun.

Comments anyone?
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Old 09-22-2011, 08:26 AM
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Quote:
Originally Posted by mark houghton View Post
snip...either hot glowing carbon deposits...snip... Looks like I may have to pull and check the plugs, check all the injectors.... Fun fun.

Comments anyone?
I think you should install a water methanol injection system, drive the car hard with lots of boost. This will clean off any carbon deposits, and if the problem goes away you have confirmation of one of your diagnoses.

A lot more fun than checking all the plugs and injectors
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Old 09-22-2011, 09:13 AM
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I have noticed similar behavior in my '88 930 over the past few months. If I give it some gas at shutdown (maybe rev it up to 1500-2000 rpms) and then turn the key off, it shuts down without dieseling. Not sure why, but it works for me. I'm interested to see what you find out...
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Old 09-22-2011, 09:16 AM
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Quote:
Originally Posted by mark houghton View Post
Just lately, and only occasionally, when I shut her down (fully warmed up) she'll run-on for perhaps a scant second...as though one cyliner is still chugging along. No, I do not have the ignition delay relay hooked up, so the pumps and MSD ignition are both shut off at the same time with no timed ignition continuing to burn any remaining fuel.
Just an idea

If some current are "stored" into the high voltage coil . It can create a spark even if the MSD is off .
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Old 09-22-2011, 09:24 AM
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I believe that can be caused by a leaking fuel injector. Mine was doing that before it spilt a couple injector blocks. 3 months in the shop later, I have rebuilt a fuel distributer, a WUR and pretty much gone through the entire injection system. My mechanic called today to let me know that I may also have a bad valve.

It is always something!
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Old 09-22-2011, 12:55 PM
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You can lower your system pressure by a small amount with a thinner shim and it should stop 'dieseling'. You can also jumper bypass the delay-shutdown relay and should solve the problem.

I replaced all my injector blocks and injectors and still had this problem whenever I raised my system pressure. I believe that in my case it is probably my fuel head. Another indicator in my case is going rich under boost even when the signal line to the WUR is disabled.

So check your injectors and blocks but keep in mind it may not be them causing it.
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Last edited by Helmsy; 09-22-2011 at 04:31 PM..
Old 09-22-2011, 04:29 PM
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Mark, before I tore the "Sled" down for painting, When I rolled her out in the spring I would always start out by using a can of BG44k injection system cleaner in the tank. That stuff does wonders and I never had any issues with the fuel system, period. When I start her up for the first time I'll run a can through on the first tank. You can find it on Evil Bay but be sure you follow the instructions on the correct mix, it's very strong stuff.

Cole
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Old 09-22-2011, 04:54 PM
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A little run-on is normal based on threads I've read. DO NOT, and I repeat, DO NOT use BG44k. It ruined my WUR.

BG 44K Fuel System Cleaner

Actually since my daily driver has been in the shop for at least a week and I've been driving the 930 daily the run-on has gone from a short dieseling to nothing. Maybe you just need to drive it more. But if you want to rebuild the WUR then you might as well run that BG44k because it does clean well, causing my WUR rebuild.
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Old 09-22-2011, 06:08 PM
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Hey friends, thanks for the input so far.

You can also jumper bypass the delay-shutdown relay and should solve the problem.
Helmsey, read my original post closely. I'm not using that relay, my issue has nothing to do with the factory ignition cut-off delay.

A little run-on is normal based on threads I've read. DO NOT, and I repeat, DO NOT use BG44k. It ruined my WUR
"Normal" run-on is just normal run-on where the engine continues to run "normally" for a couple of seconds with cars still using the ignition cut-off delay relay. My run-on is not normal...as in, not engineered into the equation. Man, I've driven this car more this summer than I ever have, at least 3 times a week. It's running flawlessly and loving every minute of it (me too).
And, there seems to be a bit of disparity on the use of BG44K. I'll have to research and form my own opinion. Unless it eats rubber things, I can't see how it would harm anything - WUR's included. Interested to hear Bill's experience with it. Oh...and I do make a habit of running a bottle of Chevron's miracle/mystery/overpriced/probably snake oil Techron through at least one tank each season...most recently about a week ago.

Funny, every post I've ever read on dieseling/run on, there always seems to be some confusion on the normal Porsche delayed ignition cutoff vs. dieseling which can be caused by things like crappy fuel, crappy timining, overheating engine (remember, I'm running with a boat-load of advance, so my car runs probably cooler than most), burned valve seats, seriously carboned up combustion chambers, etc. Surely (don't call me Shirley) there are causes that I've missed. Keep the comments coming!

Finally, a spirited 20 mile blast home from work this pm, several times opening the WG on my favorite twisty back country road yielded immediate shutdown when I got home. Seems to happen most often when shutting down after a docile morning commute. Probably need to "blow the carbon out" more often.
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Last edited by mark houghton; 09-22-2011 at 06:37 PM..
Old 09-22-2011, 06:30 PM
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You can't have run-on (combustion) without fuel. You're probably right that an injector is leaking.
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Old 09-22-2011, 06:48 PM
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You can't have run-on (combustion) without fuel. .
And you can't have run on (combustion) without a spark
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Old 09-23-2011, 01:44 AM
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Actually you don't need a spark if the conditions are right, which is why it is sometimes called dieseling.
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Old 09-23-2011, 03:17 AM
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i didnt read this whole thread but saw the title and wanted to respond.

i would shut my car down and i swear it would run for like 5 seconds.

replaced the fuel accumulator and it stopped right away, i shut car off, and car shuts off, i start car and car starts right away now without a hiccup.
Old 09-23-2011, 03:34 AM
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Actually you don't need a spark if the conditions are right, which is why it is sometimes called dieseling.
Which conditions ???


Of course Diesel engine doesn't need any sparkling !

I do not believe a such low compression engine can make dieseling while we try to stop it. But I do believe because it internal switching power supply & voltage regulation, the MSD can keep working during very short period even if you turn if off.

Even with new and perfect injectors, the CIS always leave some unburnt fuel in the intake path. Porsche fitted the car with a temporised relay on purpose !
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Old 09-23-2011, 06:21 AM
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Which conditions ???


Of course Diesel engine doesn't need any sparkling !

I do not believe a such low compression engine can make dieseling while we try to stop it. But I do believe because it internal switching power supply & voltage regulation, the MSD can keep working during very short period even if you turn if off.

Even with new and perfect injectors, the CIS always leave some unburnt fuel in the intake path. Porsche fitted the car with a temporised relay on purpose !
So....it could be a combination of a weeping injector (akin to an aging prostate) as well as a symptom of MSD electronics design is what you're saying Thierry. If that is true, then presumeably it won't make any difference if I were to hook back up the ignition cutoff delay relay to delay killing power to the MSD (I may just do that anyway for kicks and giggles). I wonder if dieseling is more prevalent for MSD users vs. stock CDI users?
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Old 09-23-2011, 09:34 AM
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Quote:
Originally Posted by mark houghton View Post
So....it could be a combination of a weeping injector (akin to an aging prostate) as well as a symptom of MSD electronics design is what you're saying Thierry. If that is true, then presumeably it won't make any difference if I were to hook back up the ignition cutoff delay relay to delay killing power to the MSD (I may just do that anyway for kicks and giggles). I wonder if dieseling is more prevalent for MSD users vs. stock CDI users?
I would think that the relay would be better left on. Dieseling is destructive combustion to the engine where the normal run on is spark ignited and will stop when the remaining fuel in the intake tract is burned off. Porsche added this for a reason so why not utilize it. My car runs on slightly for about 1 second as well after I raised the system pressure slightly. before this with below spec system pressure it shut off immeditely. It does not worry me but a dieseling engine run on is very rough and destructive.
Frankly I dont know how gasoline could be made to diesel without a spark, looking for someone to offer an explanation on how this could happen.
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Old 09-23-2011, 10:12 AM
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Fred, I concur with using the delay ignition relay. IF there's gonna be fuel in there, might as well ignite it properly. Mine has never been hooked up when the PO installed MSD, but I can fix that easily.

Here's Wikipedia's list of dieseling reasons:

This condition can occur for a multitude of reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
A carburetor that does not close entirely (or a leaking fuel injector in our case) can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
Incorrect timing.
An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.
An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turnover and combust more fuel and lock itself into a cycle of continuous running.
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Old 09-23-2011, 10:51 AM
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Quote:
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So....it could be a combination of a weeping injector (akin to an aging prostate) as well as a symptom of MSD electronics design is what you're saying Thierry. If that is true, then presumeably it won't make any difference if I were to hook back up the ignition cutoff delay relay to delay killing power to the MSD (I may just do that anyway for kicks and giggles). I wonder if dieseling is more prevalent for MSD users vs. stock CDI users?
With a 2 channels scope ( with internal memory / recording ) it can be investigated .

1st channel connected to the non permanent 12V, second channel connected to the high voltage coil output wire ( through an induction probe ).

With this , it is possible to see if some sparkling event happen or not after you turn off the key.

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Old 09-23-2011, 12:05 PM
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My experience with BG44K was that I ran a can of this through with a full tank. The car felt like it had a pep in it's step. Pretty nice. Then I filled it back up ~16.5 gallons of Shell 93octane and got about a half tank out of that. Car started blowing out clouds of black smoke then quit along the road. Towed it home and it started but barely and idled extremely rough and blew black smoke and smelled like a lot of unburnt fuel coming out. Was it due and gonna happen anyways and this was just a freak coincidence or is this the direct result of the BG44K? I suppose we'll never know but it's very strange. You need to decide for yourself. However, I personally will steer clear of this product from now on.
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Old 09-23-2011, 02:47 PM
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