What's the best way to fix an engine run-on problem? I have an MSD 6AL box and an MSD Mechanical Advance distributer on a newly built 406. I am running less that 12 degrees initial timing, but when the engine warms up it wants to keep going after I cut the ignition. I do have my Spal electric fan controller tied to the ignition wiring, but they tell me the fans will not provide feedback after cutting the engine.

Is there something I put inline on the MSD box or my stock ignition wire.

Thanks!

 

How fast is your idle? If it's running too fast when you shut it off, it could run-on.

 

Idle is 1040. That was set on the dyno - didn't experience any problems on there, so I'm assuming it must be electrical in my car... some sort of feedback through the ignition wire. I was reading on MSD web site that many problems are with older cars and external regulators. They do say if you have an internal regulator the problem might be solved by inserting a resistor in the small red MSD wire running to the ignition. Was wondering if anyone has used this approach??

 

I had the same problem after installing dual spals & my MSD box.

Here are some ideas...

I moved the trigger wires for the fan relays from the ignition circuit to the accessory circuit and tied it into the fuse panel.

Installed diodes in the alternator wire (as suggested by MSD) and also in the trigger wires between the fan and ACC circuit. I was definitely getting feedback into the MSD box from the fans.

That solved my run-on problem. Hope this helps you a bit...

--Jack

 

Jack - thanks for the feedback! I'm going to take a stab at it this weekend... first I'll try the diodes or a resistor in the red ignition wire to the MSD box, if that doesn't do it, I will relocate the "ignition" wire for the spals.

 

This might sound stupid (I'm not a mechanic by any means), but isn't it possible that you're simply using spark plugs with the wrong heat rating? I used to have an '84 Firebird that did that; it kept running (REALLY rough, mind you) after I killed the ignition. The plugs got so hot, they could still ignite the fuel in the cylinders without spark being provided by the distributor - I've heard it referred to as "dieseling."
Just a thought.

 

Interesting. I never heard of that before.

Most likely the MSD Boxin this case though. See, the MSD boxes need very little power to keep it going. With the older cars the volt regs either inline or interal to the alt provide a slight feed back allowing the MSD Box to stay powered with the key in the off position. MSD boxes for these case require a diode be installed. If the regulator is internal then the diode needs to be place inline on the smaller, usually brown, wire going back the the charge light from the alt. This diode will only allow the power to flow one way cutting the power to the MSD Box completely when the ignition is turn off at the key.

I like to install the diode with quick disconects. this way if the diode, god forbid, burns out, I can scrap it quickly and run the wires direct to get where ever I have to go. I will just need to open the hood and disconnect the wires once I get there. You just need to make sure you crimps are really good on the quick disconects of the diode.

 

Idling at 1040 sounds high to me and I'm thinking somewhere between 650-850 RPM but what do I know. Anyways, when I changed my distributor, "while I was at it" I decided to recurve my new distributor and ended up with way too much advance due to too light of springs. This caused my engine to run-on after it got really warm.

 

if engine run on is rough when the car is shut off then it's dieseling. that idle speed does seem high which will cause dieseling. also what octane fuel are you using and what is your compression ratio?

 

You can install an anti-dieseling solenoid. You just set it up to COMPLETELY close the throttle when the car is off, and set to give you your 1040 (or whatever) idle setting when energized (whenever the ignition is hot). An engine can't diesel without air....

 

If it keeps running after you shut the ignition off then you need to install a diode in the alternator circuit to fix the problem with the MSD box.

If it diesels after shutting it off then lower your idle RPM to 800

 

Mine has trouble running at 900 rpm. 950 to 1000 is about as low as I can go. I just let the clutch out when I shut it down.

 

I think this crappyass ethanol mixed fuel is contributing to some people having this problem. I have an 82 Toyota truck (DD and parts runner...hehe)with 260K on it (I have had it since it had SEVEN miles on it) and it has NEVER "run-on" or "dieseled" or whatever you wanna call it until just recently. The timing is perfect, the heat rating on the plugs has been the same since October of '82, my idle is 700rpm, etc. Nuthin has changed except for this "earth friendly summer blend ethanol mix GASOLINE". I'm from the government, and I'm here to help...

 

Dieseling is only a function of octane rating. Ethanol blended gas ususally has higher octane and is less likely to either knock or diesel. It does tend to percolate worse than pure gasoline. It is noticable with a Holley carb, although I use high test ethanol exclusively and put up with with the occational pecolation problems.

 

[QUOTE=Chilli]This might sound stupid (I'm not a mechanic by any means), but isn't it possible that you're simply using spark plugs with the wrong heat rating? I used to have an '84 Firebird that did that; it kept running (REALLY rough, mind you) after I killed the ignition. The plugs got so hot, they could still ignite the fuel in the cylinders without spark being provided by the distributor - I've heard it referred to as "dieseling."
Just a thought.[/QUOTE


really?

 

Actually, this was quite a common occurrence back in the days of carbureted engines. A contributing factor was fuel soaked carbon buildup on the piston tops. The fact that a carb WILL feed fuel (and air) as long as the engine turns also was a contributing factor.
It wasn't uncommon to find engines that would 'run on' for a minute or so after being turned off. Some would even run BACKWARDS.

With fuel injected engines the situation is a little different.

But even fuel injection requires the basic 'Three' to make the engine run. And one of those is ALWAYS available.
Even if you have two there's still the third one (Fuel) that should be missing in this particular case.

When the ignition is turned to the OFF position there should be no +12 volts to the injectors. The negative side of the injectors is 'grounded' thru the injector drivers in the ECM in syncronization with the ignition pulses from the distributor pickup coil.
The ignition switched +12 volts is also routed to the ECM and is switched off with the key.
The ECM also has a continous +12 volt source that is NOT switched off by the key.
The ECM will not fire or drive the injectors without pulse feedback from the distributor pickup coil.
I suppose the MSD box is supplying this signal to make the ECM work properly.

Even if the ignition stays active and supplies HV for the plugs, where does the fuel to make the engine continue running come from?

Sounds like some electrical 'issues' need to be worked out including the switched +12 volts to the ECM.

If the injectors cannot be pulsed with the key off (as is the normal situation) where is the fuel coming from?

The 'blocking' diode in series with the alternator field wiring sounds like a patch more than a good design even if it does work.

Making sure the switched and unswitched 12 volt sources are routed correctly after the MSD box installation would be #1 on my 'check list'.

Lowering the idle is not an option on a lot of these engines with more duration than stock. They just will not idle down into the 3 digit realm.
But they should stop when the fuel supply is turned off.
Even if the ignition is still hotter than the 4th of July.

 

Uh Oh.... Sorry, I believe by your avatar picture you may have a carb. engine.
Please disregard the previous post.:o :o

 

I sometimes have dieseling due to low octane gas so I just leave the AC on and that stops it right now

 

I have the idle set at 1040 because that is where it did best on the dyno. Idles like a top at 1040 - my cam is a 242/248 at .05. Compression is 10.4:1. This is a 406 that dynoed at 528hp and 551 ft/lbs torque, so the engine builder said to set the idle whereever it wants to be - on the dyno that was 1040.

There were no problems with run on, on the dyno, so I think it is an electrical problem in my car. I might be able to set it a bit lower (maybe 950 -1000). The plugs I have in there might be contributing, they are the race plugs that were used on the dyno, so I will keep that in mind, too.

So, where do I get one of these diodes? Straight from MSD?

Thanks!

 

A diode usually comes with a MSD box. Here is a pict from their website...



You can get that diode (1a-100v) at any Radio Shack. Just make sure that little line on the diode is oriented towards the Alternator.