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tii dieseling - Any tips from the gurus?


wegweiser
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I've been chasing an errant dieseling issue on the F Bomb for weeks. I'll be installing an A/F meter and wideband Oxygen sensor this weekend, to verify idle mixture, etc... but was hoping for input - perhaps things I have not yet considered. Any thoughts?

 

10:1 Mahle pistons. Ran great for 14,000 miles, until about 10 weeks ago. Still runs great (and cool as a cucumber) at highway rpms.

 

Dieseling happens almost exclusively after 15+ minutes of city driving

 

Timing settings: 5.0-6.5 degrees at 900 rpm idle. 31.8 degrees at full advance.

 

Valves adjusted to .007" as I've done for years. Every 8,000 miles. 

 

Running 93 Octane fuel - have tried various gas stations to eliminate possibility of poor gasoline quality.

 

Fuel economy has dropped to 17-18 mpg versus previous 20-21 mpg (I run the car hard, with a 4 speed)

 

I've considered the possibility of carbon deposits staying hot, after shut off. Have run a can of BG 44k through the tank. No improvement yet. 

 

Plugs look good. Injectors checked out (including cold start injector and cold start / warm up system). No vacuum leaks (smoke tested, etc)

 

I'll be performing a leakdown test and installing the A/F meter this weekend, to rule out bad valve(s)

 

Throttle linkage all within spec and adjusted. 

 

If y'all think of anything I should add to my checklist - I'm all ears. 

 

Many Thanks!

Edited by wegweiser
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I believe with the mixture being too lean and timing it will cause run-on (dieseling) so your AF meter will confirm that. But it sounds like your timing is spot on. 

 

You can always shut'er down whilst in gear. 

 

Does it backfire? That'll be a cool effect for the bomb part of the F-car. 

Edited by conkitchen
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1 minute ago, conkitchen said:

I believe with the mixture being too lean and timing it will cause run-on (dieseling) so your AF meter will confirm that. But it sounds like your timing is spot on. 

 

You can always shut'er down whilst in gear. 

 

Does it backfire? That'll be a cool effect for the bomb part of the F-car. 

 

 

But what do you know?

 

:D

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The only way it continues to run is because you have something in the combustion chamber that is a combustion trigger. Ignition timing ( other than being so far off it is causing excessive heat when running) is not your problem. When you turn off the ignition the plugs no longer “spark”. The mechanical injection pump however does not know you have turned off the ignition so it keeps supplying fuel as long as the crank is turning. Try a slightly lower idle speed, and an Italian Tune Up to get the built up carbon out of the combustion chamber.  Maybe a heat range colder plug so it is not retaining as much heat. 

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47 minutes ago, Preyupy said:

The only way it continues to run is because you have something in the combustion chamber that is a combustion trigger. Ignition timing ( other than being so far off it is causing excessive heat when running) is not your problem. When you turn off the ignition the plugs no longer “spark”. The mechanical injection pump however does not know you have turned off the ignition so it keeps supplying fuel as long as the crank is turning. Try a slightly lower idle speed, and an Italian Tune Up to get the built up carbon out of the combustion chamber.  Maybe a heat range colder plug so it is not retaining as much heat. 

 

 

I've switched from BP5ES plugs to a cooler BP6ES plug. I've tinkered with idle speed - as low as 840 rpm...and the car gets an talian tune up every time  drive it....often 30 miles jaunts on the interstate at 85+ mph..... I think the AF meter will reveal some clues....as will a boroscope in the cylinders to check for carbon build up. 

 

My money is on carbon deposits.... but we'll see!

 

Thanks for all the feedback! I welcome all of it!

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3 hours ago, Preyupy said:

The mechanical injection pump however does not know you have turned off the ignition so it keeps supplying fuel as long as the crank is turning. Try a slightly lower idle speed, and an Italian Tune Up to get the built up carbon out of the combustion chamber.

 

It's usually idle speed, and at least the carbureted 02s are pretty sensitive to that.  But then on a carbureted car you can install an idle jet solenoid--and blow the carbon out by slowing pouring a half glass of water down the carb throat while running the engine at a fast (1500-2000 rpm) idle.

I wouldn't try that with a tii or any injected engine.

 

mike

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Obviously as Preyup says the process is not a normal combustion process and it probably more common with carbs than injection for a number of reasons.  Any combustion needs a source of ignition, air and fuel. Without one it won't work obviously. A slight increase in one or more when the engine is shut down will cause it. 

 

Since the fuel pump on a tii is constantly circulating fuel, when it stops the system pressure should drop rapidly. Could anything have happened to this process? Increased system pressure?

 

If the plugs are at the right heat range to burn off deposits then they won't be far off causing ignition. A few degrees higher cylinder temperature for any reason can contribute to that. Carbon deposits give a further opportunity for ignition but given the way you drive the car it it hard to see how that would occur. Electronic ignition (you don't have that!) gives the opportunity that the ignition continues to be powered after shut off by leaking current or a back feed. This is genuine ignition and with the right back feed can run permanently, powered by the alternator. Ignition timing is often blamed but this is not a direct thing as the ignition is switched off so the timing shouldn't make any difference. What it can do, if retarded, is contribute to higher cylinder temperatures and encourage the carb butterflies to be wound out more to get the desired idle speed. 

 

As Mike points out, on carbed engines, the cause is generally a high idle speed with the car idling on the transition and a bit more rotational energy stored in the flywheel.  What could be the equivalent with a tii? If the engine is idling at 900 rpm? How could the engine be getting 'more air' at idle?

 

what I have learned from researching fixes for a chronic dieselling carbed car (it was the idle speed). 

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The best idea has already been said:  you got "something" in the cylinder sufficiently hot to ignite a stoichiometrically-correct air/fuel mixture.

 

My bizarre idea, and I'm not even sure it makes sense from a system operability viewpoint that it could even happen:  the Cold Start Valve is leaking and you're getting "just enough" air/fuel into a cylinder or two that gets ignited in a "last gasp" dieseling sort of way.

 

Cheers,

 

Carl

 

Edited by OriginalOwner
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I can appreciate your concerns with having too much spark advance... but 31.8 degrees seems low.

 

The one thing I have read (and can remember) about the risk of running too little advance is elevated head temperatures.

 

Is it possible that a bit more advance would make it run cooler up top?

 

edit:  I am guessing that elevated head temps would be due to extremely retarded timing situations... which yours is obviously not.

 

I think I recall you saying once that you requested that your distributor be rebuilt for a bit more advance at idle.  Am I imagining this?  Or, has something changed in the distributor?  (I too am running 6 degrees at idle, but much more all-in, due to my low compression carby-car).

Edited by '76mintgrun'02
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As mentioned, try a lower idle speed, ~800ish. 

I've had dieseling before and a lower idle helped. 

 

Also, many regions may have switched over to their "winter" gas blends. I've read that they result in lower overall mileage (and you stated your mileage dropped). For myself, I switched to non-ethanol gas (pure-gas.org). No cheap or convenient but worth it IMHO.  

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Paul,

Are you sure the vertical linkage rod at the clamp hasn't moved somehow- or the dizzy clamp isn't tight- or a plug wire has crapped out on you - or one of the crimped wires at the coil has broken/come loose?  

 

I'm only 10 years in tii ownership and new stuff always amazes me.  I chased my tail for 3 months back in 2009 only to find my Bosch lifetime plug wires were junk and I needed to adjust the vertical linkage joint more than the factory setting.

 

WB 02 sensor is your friend here.  

 

Jim 

ps: see you in Pittsburgh in 2018

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2 hours ago, OriginalOwner said:

The best idea has already been said:  you got "something" in the cylinder sufficiently hot to ignite a stoichiometrically-correct air/fuel mixture.

 

My bizarre idea, and I'm not even sure it makes sense from a system operability viewpoint that it could even happen:  the Cold Start Valve is leaking and you're getting "just enough" air/fuel into a cylinder or two that gets ignited in a "last gasp" dieseling sort of way.

 

Cheers,

 

Carl

 

Cold start valve swapped for one that I *KNOW* does not dribble or leak. The last one remained slightly slightly slightly moist, while the engine was running... so I think I've ruled that out. Not suspecting wonky cold start electronics, yet.

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