Weird sounds when engine shuts down

[Joe In Montana]

I used to have this problem with my 75 with a neutral idle speed of 800 RPM or more. My base timing is 10*btdc, not too low and not too high. I always used 91 from Shell or Chevron, and the engine was squeeky clean inside, no carbon build up, confirmed with bore scope. The throttle plates of the qjet were just opening too wide at that idle RPM causing the dieseling effect. I found a working anti dieseling solenoid from a Corvette, installed it on the 4MV Qjet in my truck and Bingo! Problem solved.

GM used anti-dieseling tricks for a while - especially in 1968-1975 or so.

In those years the emphasis was on HC and CO emissions with little regard for NOx.

Does anyone remember the extreme heat they would feel when they touched their hoods or opened the hood and had the blast furnace heated air hit them in the face? That was an attempt to control only two parts of the smog problems in SoCal.

This heat was from the retarded timing and ridiculously lean fuel and other engine settings.

One trick was to turn the AC compressor ON for 15 seconds with a delay-relay. This was an attempt to 'drag the engine' to kill it once the ignition was turned OFF.

Another was that the carb had a solenoid or vacuum diaphragm that allowed the idle speed to drop when the ignition was turned OFF.

A few transmissions had the output from the front pump, diverted to a very small orifice that substantially gave a lot of hydraulic resistance to the front pump turning, resulting in dragging the engine to a halt.​

It all helped - some!

OK - the reasons why those engines of that period were afterrunning was from

1) increased idle speeds. (typically ONLY in Drive/Reverse, but basically held there by a anti-dieseling solenoid on the carburetor)
2) lean air:fuel mixtures. (up to 17:1 - originally the optimal A:F ratio is about 8:1 or so)
3) 195-205 F engine coolant thermostats. (originally 160 F)
4) retarded timing (up to about 55 MPH when ignition timing was allowed to advance a few degrees).
5) lowered octane values in the gasoline, also there was no tetraethyl lead in the fuel any more. Lead-Free was the new gasoline, ca: 1973 in California areas of Non-Compliance)

Here comes a technical lecture - I warned you!

The following information is scholastic - as it has had an impact on the problems in the OP. I think it might be interesting to read about the genesis of the vehicle emission control systems of today.

[soapbox]

(then).... there was the California 1966-1970 Retrofit NOx systems that everyone screamed and yelled about.

The fact that there were used-car dealer 'Kar-Kits' that cost $6.95 and consisted of two rubber vacuum nipple plugs, some green RTV and stickers to seal the increased idle speed and ultra lean mixture adjustment, retarded ignition timing except on Ford CBZ-style vacuum advance ONLY type- distributors, and a sticker on the speedometer that warned the driver to NOT drive at speeds of 55 MPH for more than a few minutes or overheating might occur.

Used car dealers and unscrupulous CalARB installers didn't put the warning sticker on the speedometer.​

These Kar-Kits were super el-cheapo for the used car dealer trade --- but there were some very good and therefore much more expensive units such as: the Pure Power and the Contignitron and perhaps I'll include the Carter Retro-NOx Device here too.

The first two were actually capacitor-discharge ignition (CDI) systems that actually created an EXEMPTION for the vehicles upon which they were installed and made the engines run much better!

The Carter Kit however, was not an EXEMPTION, but was intelligent enough to actually measure the upper radiator coolant temperature and advance the timing override to help return the engine to a lower and safer temperature before it self-immolated.

The Kar-Kits could not sense the engine temperature and they were singularly responsible for the whole '66-'70 Retrofit NOx System failure I believe.

Well - sorry for digressing there with that tirade. I was a California certified Installer-Inspector-Adjuster from the beginning of the 'SMOG' or Clean-Air pact that California had declared war upon. I originally installed the (also) much hated CV/KV Crankcase Control System in 1963.

We had to drill holes into the vacuum side of the intake manifold using a pencil magnet to pull the drill shavings out of the manifold --- and add a diaphragm-controlled 'smog valve' --- to suck the blowby out of the crankcase.

It originally was a US Army idea to capture and send-to-combustion the escaped fuel and fumes that would normally be vented to the atmosphere. It was a fuel economy device - believe it?

Bottom line here:

After-running or 'dieseling' is from just a few reasons - most are controllable - by using a better grade of fuel, setting the idle in Neutral or Park, adjusting the carb to a decently rich mixture of about 7:1 at idle only, and keep the timing at a decent value.

Ergo: it's a product of timing, mixture, idle speed, fuel octane rating.

Engine compression figures are in here too - but not logically since most engines built after 1970 or so, are relatively low compression.

Remember this though: the timing MIGHT read incorrectly --- and that's possible if the timing chain is badly stretched, causing a false timing reading.

In the meantime - and I know this has been said already: shut the engine off while the transmission is in any gear - forward or reverse - to have the torque converter 'drag the engine to a halt' *.

* The parts inside the transmission don't actually form much or any drag when they are standing still. It's the torque converter that's doing all the slipping and dragging. If the rear wheels aren't moving - there are no parts except the front pump and a stator and maybe one of the concentric stator supports that are in motion - depending on the transmission type of course!

[/soapbox]
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[Blue85]

On gasoline, you can't idle at 7:1 or even 8:1. Maybe on ethanol, but that hasn't been discussed here. You're proposing to make the engine stall by making the mixture super fat after ignition is stopped? We're talking about basically an 80's gas truck, so it doesn't have to be re-invented. With a clean engine all you need is the idle speed set low enough, timing set properly and the engine temp under control.

Quadrajets have been known to leak fuel from the bottom which can screw up your idle mixture (among other problems), but probably wouldn't be single-handedly responsible for dieseling.

 

[Joe In Montana]

Rich idle, in all my years (50+) as a mechanic... has never been a causative agent to post-ignition afterrun. There are conditions where it was - yet was not - a factor to another problem however.

Indeed, a lean idle has been a most likely agent of after-run because it raises combustion chamber temperatures near to- or over- acceptable cylinder stoichiometric levels. Other factors must be in evidence too, but lean-idle immediately after having just run in a general lean run at 'normal engine temps' is the largest mitigating factor.

Remember: 'normal operating temperatures' for these NOx engines far exceeded what had been considered normal for engines just a few years prior.​

One of the primary factors was manifold/slash/runner 'wetting' from the high vacuum from deceleration with a closed throttle coupled with the non-atomizing of the fuel (again: at idle) because of lack of turbulence and flow inside the intake manifold and idle circuit.

Remember 'GULP' valves* on A.I.R systems to keep from blowing your muffler to pieces during deceleration? (* commonly called 'anti-backfire valves')

How about the Ford Pinto 2.0 and 2.3L answer in their wonderful Decel-Valve? I've still got the official tool to adjust it.

These were all attempts to mitigate a raucously (read: dangerously) rich 8:1 idle AFR while the engine was coming down from higher RPM with a closed throttle and (now) high vacuum and heat and retarded timing, etc, etc.

Add in glowing oil or fuel deposits in the combustion chamber and Ka-BOOM!
​

This non-turbulation factor had to somehow take into account and consideration that the engine has just been running and therefor has gone over the dynamic temperature of 'fuel ambient temperature flashpoint'.

This usually happened from having just exited a freeway or been in cruise mode where the engine was run at highway speeds and with closed throttle it now was 'cooking' from lowered coolant flow and cooling fan speed at idle/low RPM as it sat at an intersection waiting for a green light.

Add in air conditioning compressors running and the resulting heat collection on a hot day with little to no ram air under the hood and heat rapidly rising in the cooling system in general, normally deplorable engine operating conditions now grew critical.

The fuel in the intake system where the pressure/temps are normally conducive to rapidly liberating the fuel into vapor just didn't happen. Not in the field anyway!

AFRs went whack-o. Thermo-quads didn't help much. Neither did insulating spacers under the carb.

The fuel inside the carb just boiled - making it even MORE uncontrollable.

The problem we found was that (the fuel that was actually available, that is) just 'wetted' the intake runners and intake valve pockets because of their relatively cooler temperatures.

Exhaust crossover/heated manifolds were somewhat successful in ceasing laminar airflow. Somewhat.

NOTE: Intake valves were cooled by the rapid expansion of the gasoline as it vaporised when the valve opened and allowed the gas to enter a seriously lower air pressure zone - resulting in refrigeration from that rapid expansion. This is why the intake area immediately near the valve head itself was cooler than the general whole intake zone.​

Puddles of collected liquid gasoline do not flow in a weakened idle air flow, and therefore the actual AFR is more like 22:1 from the lack of entrained fuel. With a largely unsustainable and correct AFR of the desired 14.7:1, the engine could not idle at all.

The only way - (and this is 'way before modern oxygen sensor-support of dynamically metered fuel injection directly into the air flow at the intake ports) - was to adjust the idle mixture to 8 or 9:1 AFR.

With the advent of the CalEPA standards - the idle mixture was now critically hobbled - the AFR that was legislated was now to achieve 16:1-17:1 at idle.

Only one engine typically met this standard - the Honda CVCC engines.

IIRC - the Honda could survive on 22:1 because of the pre-combustion chambers where (why - looky here!) the AFR was 6:1!​

This bollixed new CalEPA standard lead to severe stumble and hesitation when the throttle was then opened, allowing the fuel that was in puddles inside the intake system to be gathered up in the increased airflow, and a momentary extremely very rich AFR condition occurred.

The engineers got all hunched over in their individual cubicles as they tried this-n-that to get to where Sacramento wanted the emission standards to be - no matter how much it cost vehicle makers and THEIR engineers who were actively trying to anticipate what somebody's brother-in-law in California was going to sell to the CalEPA next.

But - yes --- and no --- idle mixtures of 8:1 were not totally without merit at that time.

5% CO was The Golden Fleece!

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NOTE TO SELF: Leaking floorplates and defective metering rod machining plugs in Q-jets are an exception. ... and don't EVEN start me up on Holleys with ruptured power valve diaphragms!