engine and drivetrain How to stop tii run-on?

[Guest Anonymous]

If I set my 72 tii idle speed too low, the engine dies when dropping down to idle. If I set it at spec (900 RPM or so), it occasionally dies coming down to idle, and it occasionally runs on when I switch off the ignition. If I set it high enough that it never dies, it usually runs on at shutoff.

What causes run-on? Since the ignition is switched off, there is no spark, eh? So it must continue to get fuel? Can I add something that will definitively cut off the fuel?

Disclosures: Very strong engine rebuilt by Ben Flores in 2002. E30 M3 rods, JE 10:1 pistons, Schrick 284 cam, Pertronix, red coil, otherwise stock. This problem has existed since the rebuild.

[Guest Anonymous]

your timing being off can cause excessive heat at the motor (vs exhaust) that will cause a motor to diesel, or run after turning off the ignition.

I just experienced this after timing my non-rebuilt tii to specs. Car was very hard to start and low on power

[Guest Anonymous]

URL: http://www.geocities.com/karlkarlb/BMW.html

high compression pistons sometimes make run-on difficult to eliminate. I would check timing too.

[Guest Anonymous]

I can't help too much with fixing the run-on (dieseling) that you report, but here's something important to do in the interim:

You need to stall the car in gear (letting the clutch out) as you key off the ignition. Time this action so there is no oppurtunity to "diesel."

It can be very bad for the engine when this dieseling occurs, as during the sporadic firings of the dying engine, there is no control of ignition timing.

A single cylinder may very well fire on its own way BTDC and force the crank to turn BACKWARDS.

This can be dangerous to the valvetrain, as the chain drive is made to have the tensioner work only in the normal rotation. Herky-jerky, maybe backwards spinning diesel action is better avoided, so use the clutch to bring it to a stop as you key off.

[Guest Anonymous]

every 5th time, totally random. I figure it needs a good tune up. To be safe as John suggested when I turn the key off I have it in 4th or 5ht gear and just let the clutch out at the same time--I really don't like having to do that though.

[Guest Anonymous]

I am careful not to let the run-on occur. I get a really clean, quick shutdown if I load the clutch slightly, say 100-200 RPM drop in idle speed, in gear just before turning the key off.

The car has been tuned numerous times since the rebuild with various fuel mixture and timing settings, so I don't attribute the run-on to state-of-tune. Operating temperature, as best I can measure it, is normal. There is no hint of overheating.

Is run-on the same thing as dieseling?

(A minor irritation is trying to set the idle speed for use with the A/C. At the best-compromise idle speed without A/C, it dies every time with the A/C on.)

[Guest Anonymous]

Curt,

The key indicators in your message are the 284 cam and JE HC pistons. It is consistent that this condition has occurred since the rebuild because the Tii does not react well with those particular changes. One or the other might not be so bad, yet with both the combination persists.

I have seen this over and over for years. The most recent example was with an "isky" camshaft. They just do not like anything other than a stock bumpstick.

Retarding the timing will get rid of the run on condition some of the time, yet it will still occur and there will be a significant power loss. No good.

The only full proof way to get rid of it permanently is to remove that cam. There is an alternative though.

If octane booster is added to the fuel mix, it will for the most part alleviate the problem. It is a pain to carry around and add when filling up although better than the alternative at this point.

Try it and report back.

You could also run some kind of race gas 8^)

[Guest Anonymous]

I'll get a tank of race gas on my next trip past Sears Point and see how that works. Meanwhile, I'll try octane booster.

Is this happening because the mixture is being ignited by something other than spark? How does the cam contribute?

[Guest Anonymous]

How could retarding the timing affect run-on? The ignition is off at that point, so spark timing is irrelevent. Or something.

[Guest Anonymous]

With high compression engines with modern (stock) fuel the plugs maybe glowing almost red hot (causing the diesling). Run one or two settings cooler plugs should help... if not retard the ignition timing as well.

WH

[Guest Anonymous]

What advance curve are you running on the distributor? Stock tii?

I think you will probably need to recurve the distributor. As an experiment, try advancing the idle timing up a few degrees, to something like 7-10deg BTDC. Adjust the idle speed/mixture back down to a reasonable idle (900 or so). See if that helps eliminate the run-on. If it does, you'll need to recurve the distributor for less overall advance, so you can run more idle advance and yet max out at the same level.

Basically what you end up doing is trading ignition advance for air/fuel to maintain a stable idle. As for maintaining a stable idle with the A/C on, you may have to resort to some kind of solenoid to boost the idle when the compressor kicks in. I have enough trouble getting my stock tii to idle well at all times between having the A/C on and off.

[Guest Anonymous]

I'll try the timing suggestion. Thanks. Still, I can't help wondering how timing affects operation after the ignition is switched off?

[Guest Anonymous]

Maybe I should try W6DC, but I've been fighting fouled plugs due to cold weather and lack of use. Thanks.

[Guest Anonymous]

Timing has no effect after the ignition is turned off. What you're doing here is _advancing_ the timing to get the engine to idle higher/better on a given throttle opening, such that you can reduce that throttle opening for a given idle speed. This way, there's less air/fuel to maintain that idle and subsequently less air/fuel to continue igniting on the hot spots after the ignition is turned off.

Have you checked the curve of your 002? Most I've seen don't fall within spec for tii distributors.

[Guest Anonymous]

Food for thought on my engine too.

Jason