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last night i needed to get somewhere... jumped in the 412, hit the starter and it went rrrrrrrrrrrrrrrrrrmmmmmmm fwhump. then nothing. no backfire, no nasty metal sounds, no puddles under the car... just literally a fwhump sound. i haven't been able to check yet so it's time to play, "what just happened"! i will be checking it out when i get home tonight.

my guess is i launched a sparkplug.

If memory serves, that sound could be (a) a sparkplug as you think (b) a detonation in the manifold....excessive fuel usually...sometimes even caused by an overtight valve not closing or stopping the cranking and igniting cyle at just teh right point when overly rich cauing spent fuel to burn in teh exhaust manifold. ray

if it is the plug, anyone know what size helicoil i'll need?

that must have been a detonation, but not sure where as it blew the clap/hose off of the afm to the plenum, 3 other hoses and the oil breather off the tower! that is weird since it was such a muffled sound... really seemed nonviolent at the time.

everything is reconnected and seems right with the world again. oh, and as an aside... i've put enough miles on it with the new afm that all the fuel burned off in the case and oil level was at the bottom of full, not the top.

Hey, do you think you arched the points?? Maybe burnt them but usually it will still start but run ruff...Do you think the CHT sensor or vacuum leak at the vac can cause the fuel pressure to change?? You have a L-jet 1800 right??? If it's the 1700 D-jet I was thinking the FI point ground wire???
Good luck sorry you didn't get to cruize her!! bill

This backfiring issue....will kill an AFM. When its a cold start in cold weather....I have seen that excessive fuel can cause this. The issue to realyy source out....is what tehignition source is.

Vw Bill has a very good point. I have never seen "points' arc at teh wrong time. I suppose its possible...be he is thinking in the right direction. What caused the ignition of the fuel in the intake manifold?

Generally its one thing ....leaking valves.......caused by one of several issues. (a) poorly sealing valves due to wear/erosion/carbon. The once in a blue moon backfire is generally caused by carbon blocking a valve open and igniting the excessive fuel you have at start up...in the intake tract.(b) poorly sealing valves due to valve being bent....kind of out of the question or you would see this backfire constantly. (c) Valves too tight...causing one to be held open even a 1/2 thousandth.
Ray

Wait...forgot to read everything. This was a case detonation. Yes...I have seen those plenty. It is especially prevalent in the winter...when you may not be driving as much or as far....and the fuel from blow-by during rich start ups...saturates the oil.

Also...check that you do not have any injectors dripping or a leaking. These can put so much extra fuel into teh crankcase that you can get these detonations. They are usually at start up before the rings have fully seated. Ray

i think that my running rich for so long prior probably brought about the internal explosion. after my last run up on the car with the bad afm and the 9mpg i was getting, the oil was definitely rought with fuel. the return drive home with a proper FI system lowered that ratio and probably lulled me into a false sense of 'all is right'. I hadn't dumped the oil yet. Sound valid?

Oh it was a case ignition! I could see the leaking injector doing that!! Also what about the cold start injector??? Could it be a aux air issue too being a LJet?? I wonder what that oil will smell like? Have you pulled the dip stick to get a smell? I remember my brother's T3 Djet had two leaking injectors and the oil was ripe and was getting fuller while playing to find the leakers!!
Good luck, Bill

the injectors are not dripping/dribblilng... the excess fuel was due to the former bad AFM

Hey, so the AFM can cause fuel to leak?? I thought it was a air flow and pressure meter? Are you saying the meter was causing the injectors to run too long and dumping fuel in the cylinders or was it causing the fuel pressure at the regulator to be too high? Bill

it was causing the system to run at FULL RICH all the time. it wasn't causing a leak.

i think i found why i got the detonation... the timing had moved from 6 btdc idle (30 btdc total at 2800rpm) to 5 atdc at idle i found the culprit. it was one of the few non stock items on the car. the clamp was a cheap aftermarket unit and not cintching down correctly.

just some long term follow-up.

been getting a really consistent 24-25mpg on the L Jet system since putting the vanagon AFM on it. then, about 3 weeks ago, i started getting a mystery miss. very slight, but it happens about every 3 minutes. doesn't matter the load on the engine, the MPH or throttle position. i've tried grounding the AFM to the body and the engine to no avail. tried rechecking all the hoses and pushrod tubes for vacuum related issues with no change and then did a complete retune to see if something there was amiss. nothing makes a difference. so... I'm looking for another good used AFM to see if that is the issue. The vanagon unit was/is not my 1st choice, but i can't seem to locate one for a 412 L Jet that isn't either made of gold, or unknown in quality.

i'll post some more once i've made that change to the system. if that solves it, we know what happened. if it doesn't, i'll be scratching my head some more. the bummer is that it was getting enough MPG to not kill our income as a daily commuter.