mj1pate

In advance of taking delivery of my 86+1/2, I was reading tech articles in the 928 Tech Area, including one about backfiring within a 86+1/2 L-Jetronic equipped 928. Featured was Wally Plumley's response. The reader's enquiry featured the following info:

"This afternoon I got a frantic call from my wife telling me the "Beast" just blew up! I'll get to the point... upon inspection, the air cleaner box top was blown into a few pieces, the entire induction system (86.5) was separated from virtually every rubber connector, including the throttle body, and the explosion went off with such force, that we now have two "power bulges" in the hood where the 2 metal air boxes came in contact with it. This all occurred upon an attempted cold start. My wife doesn't even remember hearing the engine crank... it just went off upon turning the key. "

Wally's response included descriptions about the relevance of incidental valve positioning contributing to this problem. I'm confident in the technical accuracy of Wally's response, and intend not to take issue. I do want to convey that serious backfiring has been a problem with varying severity with many L-Jetronic type equipped motors from different manufacturers. Alfa Romeo owners have been (mostly successfully) dealing with this problem over the years.

Alfa L-jetronic guru Greg Gordon published tech articles concerning the effects of air leaks http://www.hiperformancestore.com/Ljetronic.htm, and noted:
"Air leaks are easily the most common problem on the GTV6. *They can cause all kinds of trouble including difficult starting, lack of power, irregular idle, high emissions, and the famous engines starts, then stalls right away syndrome. A big air leak can cause an explosion in the intake system which can blow the plenum right off the motor, or in some cases blow the main air intake hose off. "

Internet searches reveal how flapper valves can be installed on Porsche L-Jetronic equipped air boxes to allow a safe egress for backfire gasses, preventing what Greg Gordon described above.

In essence, searching for and eradicating air leaks is the first step to avoiding little explosions. Its hard to believe any 20+ year old car can reliably operate on original, factory rubber. These measures would also help avoid the requisite hostility from our significant others, whose understanding we depend on to stay in relationships with our (mostly) aluminum friends.

Bill51sdr

Interesting theory. That was my old car btw, and my post. The only problem I have with the air leak theory in this case is that both times (yes, twice) this occurred the backfire began just as the key was turned to engage the starter. The engine barely cranked, if at all. All the rubber parts in that engine had been replaced and the car had no other symptoms of an air leak. Somehow the fuel in one of the combustion chambers was being ignited while an intake valve was open. This seems to be a physical impossibility unless perhaps a valve was hanging open on that particular cylinder. On that car, I eventually checked/set the cam timing and removed an aftermarket chipset from the ECU's, the problem never recurred. Which solved the problem, who knows?

This may be nit-picking, but the F.I. system in the 32V engines is LH, not L-Jet, a significant difference. I know you may be using the term "L-Jet" as a generalized statement.

FlyingDog

Brett928S2

Hi

For what its worth....on my 16 V S2...that explosion was EXACTLY what occured when I accidentally activated my Nitrous while engine was turned off....
As soon as I turned the key...BOOM....and half my intake system blew apart..including blowing my air filter casing into a lot of bits....I even managed to BEND the throttle plate and blow the maf gauzes into little ***** of mesh....

The point here is....I know EXACTLY what occured......there was gaseous Nitrous sitting in my spider legs and inlet plenum...
When I turned the key......the first spark ignited it all and it all flashed backwards.....and I suspect thats what happened to first poster but on petrol not Nitrous.....not quite sure HOW it can happen on petrol though....

All the best Brett

PORSCHE 928 S2 AUTO V8 4.7 LTR 1986 IRIS METALLIC BLUE WITH X-PIPES ,RMB ,PROMAX CHIPS, KICKDOWN SWITCH, K&N, WIZARDS OF NOS - NITROUS KIT 150 HP JETTING - 505 BHP / 500 FT LBS TORQUE
PCGB UK 928 FORUM MODERATOR - Email managingdirector@pchealthcare.co.uk

Bill51sdr

Brett, what in the intake tract ignites the fuel?? There are no spark plugs in the intake tract.

heinrich

86 cars, 86.5 cars, 85 cars (non-euro) are LH-Jetronic, not flapper L-jetronic. They do, however, backfire as you describe and destroy the intake.

Bill51sdr

In my case, I was a little lucky. Only the airbox was destroyed and had to be replaced. The plenums and throttle body separated at the rubber tubes, which is how the plenums made the two dents in the hood. They were easily reattached. Hmmmm... almost as if Porsche knew this could be a problem

FlyingDog

Open valves.I don't know why you put "(non-Euro)" all 84+ Euro/ROW 928s are LH-jet.

Bill51sdr

During the combustion cycle?? How? Only way I can think of is if an intake valve is leaking a little and allowing burning gasses to escape into the intake tract. Why does this only seem to happen to the 85-86 engine? Perhaps the camshafts have a bit more overlap (just speculating, I do not know)?

928SS

I blew up my airbox, dented the hood, etc w/my OEM motor/intake years ago. I managed to do it by revving the motor and cutting the ignition off at about 2400 or so rpms... I think it loaded the cyls w/fuel, some vapor leaked into the intake overnight and w/the cam timing off a bit or the wasted spark - bang!

wasnt' pretty

jpitman2

I had a little explosion once, where the fuel vapour ran donw the vacuum advance line into the dizzy through a perforated diaphragm, and it blew the dizzy cap off, split the rotor button in half....tow home job. In another make, but no reason it couldnt happen to an OB.
jp 83 Euro S AT 50k

mark kibort

it usually happens with fuel vapors in the intake runners, and if you have a wet NOS system and the intake is hot enough, you can have ignition. not an issue with Ljet, but a problem per se.

mk

Bill51sdr

Both times mine did it was during a cold start. Stone-cold engine/intake.

heinrich

my one explosion was ice cold too

928SS

yep, cold start on mine too... I wonder if the cold start map has something to do with it.