Hi, newbie here, I have had my 89 928 for a year now and have thoroughly enjoyed my first year (except for the 4k servicing bill which included a new cambelt). My car has covered 46000 miles from new and is in fantastic condition, she has been my daily driver all over winter and has not missed a beat. But tonight after a ten mile drive back from the kart track I stopped of at my showroom for 10 minutes and when I went to restart the old girl she back fired under the bonnet, a puff of smoke emanated from under the bonnet and after lifting the bonnet I noticed the rubber cap from the round thing on top of the plenum had been blown off. Due to it being dark I could not make a full inspection, but the symptoms are now that she will start and run for about 2-3 seconds then die. If I pump the throttle she runs a bit longer but horribly lumpy. Any ideas of what I need to look for in the morning???

 

Welcome to the forum.

Post some pics of your car - inside and out, please.

Sounds like you may have disconnected the intake from the MAF body due to the backfire.

I'm sure others will provide additional things to check.

 

Welcome to the forum Tony.

It sounds like the rubber boot connecting the intake plenum from the MAF (the alloy 'round thing') under the air box may have come loose. Remove the airbox and MAF, make sure the rubber boot is firmly and evenly seated on the intake, refit MAF and see if it will run.

Lots of great advice and knowledge here, highly recommend you join the mail list on 928.org.uk which is also a great source of info for UK based owners.

 

Do you have any pics of the MAF?
What is the round thing in the centre of the plenum on top?

 

MAF is the mass air flow sensor. It's a semi-cylindrical device, hollow with a sensor in the middle, gray with metal meshings on both ends.

Pictured here: http://www.jdsporsche.com/main.html

It's connected to the engine via a rubber boot which also connects to the air box. It should be located under the air box.

 

The thing on top of the intake is the flappy valve bearing cover - probably a little blow-by blew it off - not great - but the gas has to go somewhere. The flappy valve is a vacuum actuator controlled throttle like plate that opens & closes at different RPM's to link the two chambers (sides) of the intake through the middle - this changes the intake resonant freq for better performance characteristics.

The car will run without it operational - so not critical, vacuum leakage there could be a minor issue - but ignore it for now - monitor it later.

Alan

 

before you crank the engine anymore,
you need to inspect the timing belt to make sure that the cams and crank are in time.
A backfire from a S4 engine isnt a common occurrence .

if the timing is good then you may be in store for an intake removal
as there are a few things that can not be reached.
You may also need new spark plug wires as the ones you have may be leaking

 

Thanks for your help so far, i have unplugged the MAF and the car fires up and runs albeit fast and a tad lumpy but she runs. What does this suggest??

 

pay attention to post #7
dont run the engine till you have inspected the timing

 

Seems like they ignore the advise and then something really expensive happens, plus 2 on the timing!

 

That it runs fast suggests you now have some vacuum leaks into the intake - probably cracks or blown off connectors - likely under the intake. Sounds like you will need to take it somewhere that knows what they are doing to diagnose/fix.

It is indeed a very rare occurrence for a 928 to backfire - something was very wrong - you need to fix that so it doesn't happen again - you got off easy this time, (even though seems there was some minor damage) it could have been much much worse.

Alan

 

There are several rubber hoses under the intake that can age to the point where they split and cause various idling and runnning issues. Unfortunately the only way to determine if this is the case is to pressurise the intake and do some detective work. If indeed this is the issue the intake will have to be removed and then perform a full refresh.

As mr merlin has mentioned twice.....verify cam timing before starting the engine further as you wont be worrying about a few hoses but rather a replacement engine if you bend valves, score pistons/cylinders etc etc.

 

I'll reiterate Stan's post above.

Backfire on an S4 is *very* unusual.

It can be caused by incorrectly connecting the plug wires to the wrong cylinders, or it can be caused by cam timing.

The first thing I'd check is that your cam gears have not jumped teeth (don't rely on the belt warning system unless you've had it apart yourself and verified it - too many cars have had it electrically disabled by idiot PO's and their mechanics so the warning never goes off). The backfire event itself can cause the gears to jump belt teeth (or worse, strip them off the belt completely).

After verifiying your cam gear notches align with the backing plate notch at TDC, I'd then check the ignition system (wires/plugs) and belt tension, before cranking the car or starting it again.

 

What is the best way to hand crank the engine to check the timing?
Not a lot of room at the front of the enginewithoutpulling fans etc, wanted to check through the vent holes first.

 

27mm deep socket, only turn crank clockwise.