924T popping and banging on over-run

[tref]

Turbo Cab came out to play today, after being laid up for the winter... started first time, running ok... passed it's MOT without issue, however... Idle is slightly low, and hunting a tad. It also pops and bangs on over-run. It was perfect before the winter, and in fact, each time it has been fired up to keep it running over that time... but the last "gap" between running was longer due to other electrical work.

I am guessing, therefore, it is any "settings" or timing out, but something somewhere has "stuck"... something which delivers extra fuel for idle...(correct me if I am wrong, but over-fueling would cause the over-run popping and banging in the exhaust?). Any suggestions?

Other than that, it is great... first time with the roof down, and it feels gooooood!!!

 

[924nutter]

The metering arm might not be going all the way up to its idle revs position. The anular gap around the disc is only about 0.2mm, from memory, in the resting position and crud and corrosion could easily have built up during its lay up. It is similar to the 924 but they differ in that the arm on the turbo is shorter and see-saw like whereas the arm on the 924 is hinged on the cantilever point and has the assistance of gravity because air entry is from below on the 924 but from above on the turbo.
Anything to do with fuel check that first.
The warm up regulator might not be in circuit due to a corroded connection, or it might not be operating correctly staying open.
An open regulator causes a reduction in fuel pressure which in turn causes richer running, by acting like a secondary pressure relief valve, and is indeed the way the metering system works because with the fuel pressure resisting the movement of the arm the lower pressure allows the arm to deflect further, exposing more of the outlet ports within the body to the waisted portion of the metering spindle.
The thermo time switch might not be disconnecting the earth to the cold start injector. All these things will cause over fuelling, as would incorrect setting of the co2 adjuster on the metering unit body.

Something as simple as leak on the exhaust will cause it because the exhaust sucks in air in the negative pulse phase and this extra air combines with the always present unburnt fuel to cause a pop, but having just passed the M O T you might rule that out.

It could be possible that the mixture is fractionally below the stoichiometric ideal (14.7:1 if you're asking) on a closed throttle, causing incomplete combustion due to over-lean mixture. This unburnt fuel vapour accumulates in the exhuast and as soon as the fuel/air ration is above the L.E.L combustion will ocur in the form of a pop when a hot particle of carbon can provide the necessary half a joule of energy to set the mixture alight.

Long story short, a lot of things to check.

 

[924nutter]

If the popping and banging is accompanied by a subsequent loss of power then check the condition of the multiplug on the DICT unit under the heater matrix. Both Geoff Ives and myself have had this plug work loose and you cant believe how bad the engine runs and sounds if this plug is even loose.

 

[tref]

Thanks John...

The multi-connector sounds like a good place to start, since the one thing I have been playing around with is all the wiring under the dash/centre console (but at least I have a heater blower fan working now!).

The engine runs fine... very nicely indeed in fact. Having given it further consideration, I had thought it more likely to be a loss of idle air rather than a gain in fuel. My reasoning being that a gain in fuel if anything would raise the tick-over speed not cause it to drop, and affect it further up the rev-range... I was thinking a spider crawling up an idle air passage somewhere and dying... or a sticking (closed) idle stabilisation valve... or would the fuel not increase if that were stuck?

As you say... lots to look at!

 

[924nutter]

I am not so sure that the 924t has an idlle stabilisation valve per se. There may well be idle stabilisation programmed into the DITC to advance the ignition at low rpm dependiing on the load on the altenator. FYI because of the 200 tooth flywheel pulse sensor ring the ignition can only advance in increments of 1.8 degrees. I stand to be corrected on idle stabilisation. If you alllude to the idle control valve that increases rpm when the engine is cold, then the answer is that all engine air goes via the metering head; the idle valve is nothing more than a tufnol disc with three different length slots in it. It's on a pivot and operated by a bimetalic spring. In essence it regulates the airflow through a bypass between the two sides of the throttle butterfly and only ocasionally needs adjusting. Any air leak on the pipes would raise idle speed and possibly lean the mixture out.

The installation is far less sophisticated that the 944. The turbo is oil cooled and for long life needs to be left to idle for about 30 seconds after a run to allow the turbocharger to normalise to its lowest operting temperature, to reduce oilway carbonising. There is a vacuum connection to the DITC and there could be a leak in this flexible hose or at either banjo coupling. It's been 4 years or so since I drove mine but I don't recall it popping except if snapping the thottle closed from over 5 and a half, which you don't often do.