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!tom · Posted: Wed Jun 16, 2010 4:56 pm Post subject: It diesels

I tried searching, but had no luck.

I've got an interesting problem.

Lately, when I try to shut off the car, it diesels. After it eventually stops, it takes way more cranking to get it to start the next time, which is often 9 or so hours later.

So, I now will shut it off then let out the clutch in order to stop the engine.

Imagine my surprise then when, with the car in 1st gear, it shot off backwards when I let out the clutch!

This is not an isolated incident; I'm not keeping statistics, but it seems that much of the time when it diesels, it's running backwards, but not all the time.

So, there's two issues here.

First, it would be nice to get it to stop dieseling. Why does it diesel? My understanding is it's due to either plugs that are too hot a heat range, or carbon buildup, right? I have plugs 1 heat range hotter than stock in the car now to stop them from fouling; she burns a bit of oil (valves I expect), and the hotter plugs seem to help out in this regard. These plugs have been in for almost 4 years, and this dieseling issue has popped up in the last month or two, so I'm guessing the heat range is not the sole cause. Would a SeaFoam bath (or whatever it's called) help?

Secondly, I don't understand how it can diesel backwards. To diesel, it needs fuel, but not ignition (as it self-ignites). If the engine is running backwards, then I would think it would draw air in from the exhaust, and pump that into the intake, which would force the AFM to shut off all fuel flow to the injectors. Am I missing something here? It's not like it's just happening for a second or two either; if I let it run its course sometimes it'll go on for 30 or so seconds; it seems like an eternity.

You may recall that this has only started since I've had my fuel pump relay issue so the fuel pump is running through this entire time, but like I said, no fuel should be getting to the injectors with the AFM shut.

What's going on? What's the best way to stop it?
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78 924 NA
5-lug

Rich H · Posted: Wed Jun 16, 2010 5:08 pm Post subject:

Is it dieseling or is it running on?
Dieseling is pretty extreme and very noisy (And very bad!) it means it's burnign somethign for fuel as as soon as the pump stops the fuel will too (Pretty much) so it's eaither butning fuel or oil but it'll be going KLONK-KLONK-KLONK as it detonates the mix. Very bad for everything.

Running on is ususally an electrical snag, if the engine keeps running normally or perhaps a bit rougher then you need to fit a diode in the wire fromt he alt lamp to the alt. What happens is that there is enough leakage current getting through the alt lamp to keep the 15 line live and the relays all closed so you can't turn the car off. get any old general purpose diode (I think I fitted a x4001 1/2 amp as I had one around) will do. That will stop the leakage current and stop the running on. Assuming it's nto somethign more fundemental! and there are no bodges in the fuel system wiring. There was a 924 tech bullitin IIRC to do this.
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

DOCO · Posted: Wed Jun 16, 2010 11:59 pm Post subject:

desiling can be very hard on an engine.its useually caused by the idle being to high.yes the moter is running backwards.first just try backing off your idle to about 7 to 8 hundred rpm and see if this helps.
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Doco "where am i going and why am i in this handbasket"author unknown
79 924 N/A "Webster"
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"WEBB STR"

!tom · Posted: Thu Jun 17, 2010 1:10 am Post subject:

Yes, it is dieseling. I had thought the running on term referred to the same thing, but it's definately not an electrical snag.

The fuel flow should continue I had thought due to the accumulator. Also, as I've been bypassing the fuel pump relay, the fuel pump is running throughout the dieseling. Fixing this is very high on my priority list, and this may be enough to solve the issue.

However, how can it diesel backwards? By burning oil?

And the idle is a little high. I'll try adjusting it down a bit.
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78 924 NA
5-lug

Rich H · Posted: Thu Jun 17, 2010 1:44 am Post subject:

If it's dieseling it's not reliant on the inlet valves drawing in fresh charge or the spark so it might well run backwards...

Bypassed fuel pump is not good.

Try this: get a normal auto relay on with 4 pins on the base. Get behind your fuseboard and look at the back of the fuel pump relay socket and pull out the topmost center spade connector socket thing (Will need a small screwdriver in the front to bend the tang out the way) now get a 6" piece of wire with a spade sockets on each end and push one into the topmost slot. put your normal relay in the socket now ensuring all the pins are in properly. The relay will now operate the fuel pump when the ignition is on.

I would suggest you have a leaking injector or cold start valve allowing fuel into the inlet manifold when everything else is off. that provides fuel for it to 'diesel' as the pump is always running then it will continue to provide fuel.

Is it klonking while it diesels? It's pretty horrible to listen to - like an old knackered diesel...

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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

!tom · Posted: Thu Jun 17, 2010 2:19 am Post subject:

My fuel pump relay is fine; the issue is there is a wiring fault going to pin 15, which is the top most pin you're referring to.

It seems the most straigthfoward way to stop the dieseling is to stall the engine once I turn it off.

Wouldn't a leaking injector (or CSV, as it's an injector too) cause hot start problems? I have no hot or cold start problems, other than after an extended dieseling session.
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78 924 NA
5-lug

Slam · Posted: Thu Jun 17, 2010 2:41 am Post subject:

http://en.wikipedia.org/wiki/Dieseling

Hot plugs and crankcase vapors would do it.
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'88 944 Auto - may or may not resurrect

!tom · Posted: Wed Jun 30, 2010 11:45 am Post subject:

Problem fixed when I sorted the issue that stopped the fuel pump relay from working. So, a working fuel pump relay cuts the fuel pump, eliminating the fuel that caused the dieseling.

I still don't understand how it can continue to diesel when running backwards, because it seems to me it would pump air from the exhaust into the intake, and there would be no fuel for it to diesel with.
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78 924 NA
5-lug

Rich H · Posted: Wed Jun 30, 2010 4:59 pm Post subject:

I don't understadn how you manage to get it to run backwards...

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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

Mike924 · Joined: 12 Aug 2004 Posts: 2601 Location: IoW UK

The reason it can run backwards is that there is no timing relation with engine position when the 'ignition' occurs.

For example, imagine the engine running in the correct direction, it sucks in a lungfull of mixture, starts to compress it, but before it gets to the top, *pop* it ignites, expanding gas now reverses the piston direction and the engine turns backwards. If there happens to be another cylinder with some unburnt fuel, that can go off as well, but it's difficult to see that anything will happen after that point...
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

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Rich H · Posted: Wed Jun 30, 2010 8:38 pm Post subject:

No what I don't get is that the engine is running normally forwards then somehow, as it's slowing down, it manages to stop the flywheel and then start running backwards.

It is possible to get some big old single pot 2 stroke diesels to bounce off the compression to do this (this is how you reverse them) but I don't see how you are gettign enough compression to then detonate the mix.

Sounds like a rather poorly engine...

Back OT a bit: a leaking injector would cause hot start problems, the pressure in the lines would leak away and you'd get flooding, gas in the oil and hot start problems.
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

I've seen engines with large amounts of carbon buildup cause hot spots and run on. This was more common with carbed engines that still had fuel in their bowls.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black