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Porver9two4
Joined: 22 Jan 2004
Posts: 104
Location: United Kingdom
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 Posted: Wed Jun 01, 2016 6:25 pm Post subject: What on earth is wrong with my 924? Stumped! |
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Porsche brothers... I need help! I cannot get my 924 to run and am officially stumped.
THE PROBLEM:- My 924 is over-fueling. Catastrophically. To the point of hydraulic-lock. Needless to say, it won't start. The question is why has it suddenly started to over-fuel?
THE BACKGROUND:- I hadn't used my 924 for about 5 years. It had always been a good runner and starter. Following its slumber it was very difficult to start - the fuel had gone off, and once drained and refreshed with new petrol all was well. Hooray! It was driven into the garage for a full brake overhaul - new hard and soft lines, new slaves, full strip and rebuild and had lots of other love at the same time - new wheel bearings etc. - which took about 3 weeks to complete. Once complete I jumped in to drive it out of the garage again but it refused to start, refused to even fire...... which was odd.....
TROUBLESHOOTING SO FAR:- Initially I logically assumed it was no spark. Spark checked and confirmed as healthy - not that. Pulled out the plugs - all drenched in petrol - plugs cleaned, dried and cylinders left open overnight.
Next day, try again - still no start, not even a stutter, no firing at all. Pulled plugs again, all drenched in petrol. Spin engine on starter, petrol spewing out of plug holes. Its massively over-fueling. Assume 5th injector stuck open - check that (nope, fine) and disconnect electrical plug to avoid compounding issue.... Check airflow meter - plate moving freely. Reassemble everything and try again - still no firing, plugs still drenched.
So I have to suspect metering head - pull airflow assy. off and find, to my deep joy, that the piston in the metering head is sticky - AHA! Eureka, it must be that. Remove metering head to free piston (Polished with 1500grit wet & dry) and strip metering head to check for contamination. All as clean as a whistle and in good order - happy days. Carefully reassemble ensuring correct orientation of sandwich plate and ensuring fuel pressure regulating valve all free etc.
Reassemble to air flow meter and check piston operation with flap - all good. Reassemble to car confident it will now burst into life. Turn it over on the starter.... spins over but nothing, not even a firing before EUGHHHHH.... hydraulic lock! Remove plugs (all drenched), spin on starter and petrol geezers out of plug holes....
Assume I must have made a mistake on reassembly of metering head so strip off and strip down again to verify - nope all spot on. All assembled correctly, all o-rings fine, fuel pressure regulator working fine... nothing wrong. Reassemble to car, try again - still no firing and then hydraulic lock again.
Check fuel return line to tank isn't clogged - confirm all OK.
Check piston on metering head the right way up - confirm all OK.
Check WUR not clogged - confirm all OK (even though I knew this couldn't really be the problem, just going through all the components).
Check inlet manifold not blocked somehow - confirm all OK.
I have literally gone through everything each at least twice trying to get to the bottom of this and IT STILL WON'T START and is STILL catastrophically over-fueling..... I now have about 2 litres of fuel in the engine oil as an added bonus where it has washed past the rings.....
Can anybody suggest ANYTHING that might help..... I am at the point where I am stumped.
As I understand it, there is no way fuel can be reaching the injectors other than through the metering head via the metering slots as they are revealed by the movement of the piston, acted upon by the air flow meter flap. So how in the name of Ferry Porsche can a car that was driven into a garage 3 weeks ago with mechanical fuel injection spontaneously find itself defying the laws of physics and mechanical engineering????? |
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morghen
Joined: 21 Jan 2005
Posts: 9102
Location: Romania
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| Posted: Wed Jun 01, 2016 8:28 pm Post subject: |
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Just try out a different CIS metering head and another set of injectors.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Thu Jun 02, 2016 1:28 am Post subject: |
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Have you pulled the injectors from the head and put them into glass jars to see what is actually happening? This kind of over-fueling is typically one of three things:
- Leaking CSV (although hard to believe it produce that much fuel)
- Failure in the fuel distributor, typically the o-rings
- Failed injectors
Before you continue, you should do the injector test. Once you've done that, and before you attempt to start the car again, you MUST drain the oil and put fresh in, otherwise you will risk serious damage to bearings and other internals due to the fuel-diluted oil.
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morghen
Joined: 21 Jan 2005
Posts: 9102
Location: Romania
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| Posted: Thu Jun 02, 2016 1:49 am Post subject: |
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The oil change goes without saying...its obviously compromised beyound any use as oil.
Sorry to OT/hijack the topic.
Dan, have you recived the PM i sent you?
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POORscheMAN
Joined: 08 Sep 2003
Posts: 104
Location: Norwich NY
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| Posted: Thu Jun 02, 2016 3:30 am Post subject: |
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Its safe to assume the cylinder walls have been fuel washed as well.
You may want to put a small amount of oil in the cylinder or risk scuffing the walls and pistons.
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1978 924
Audi 5K Throttle Body - 10:1 comp
Stage 1 Cam -MSD coil- Crane XR700 ignition - Headers
260# springs -26mm rear TBars-
poly bushings -Weltmeister sway bars
1980 924 N/A -- stock
1982 931
1988 944
1994 968 M030 |
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Porver9two4
Joined: 22 Jan 2004
Posts: 104
Location: United Kingdom
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| Posted: Thu Jun 02, 2016 5:36 am Post subject: |
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Yes, goes without saying that oil will be changed.... but I have to get it going first! Or at least to fire!
I don't have access to another metering head to swap in.
I haven't pulled the injectors to see what is happening as it is very obvious they are just gushing fuel. It would be exceedingly odd for all the injectors to fail simultaneously though.... it drove fine just three weeks ago and has done nothing inbetween. I will pull them out and see what they are doing though.
It definitely isn't the Cold Start Valve - it is electrically disconnected while I troubleshoot and I have physically had it out and it's dry.
I have also had the metering head to bits and all the O-rings are good - still supple, pliable, tight fitting and undamaged.....
It is as if the control pressure has suddenly rocketed or the fuel is somehow bypassing the plunger and flooding straight through the metering slots. |
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brian19600
Joined: 28 Oct 2013
Posts: 375
Location: NJ/CT
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| Posted: Thu Jun 02, 2016 7:13 am Post subject: |
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There is a spec somewhere as to how much fuel should come out of an injector in 30 seconds.
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Ian89C4
Joined: 01 Apr 2011
Posts: 561
Location: North Carolina
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| Posted: Thu Jun 02, 2016 8:34 am Post subject: |
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Have you tried removing the pressure valve inside of the fuel distributor? It could be stuck and keeping the pressure too high. It's the little bolt on the side of the fuel distributor. Be careful when loosening the bolt, there is a string inside.
Good luck!
Cheers
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Ian Edgerly
North Carolina
Current:
1981 924 SCCA/ChampCar Weissach Race Car ("Serenity")
1987 924S ChampCar Endurance Car ("Der Weg")
1980 924 Euro ("Wahala") |
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morghen
Joined: 21 Jan 2005
Posts: 9102
Location: Romania
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| Posted: Thu Jun 02, 2016 4:51 pm Post subject: |
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You did something wrong when you re-assembled the metering head.
Re-check it part by part with a diagram in front of you until you find the fault.
As a holy rule, you NEVER take that thing apart...i mean NEVER. If it fails you replace it or send it to a specialist who rebuilds it for you...but now that you've done it...just try to find whats wrong inside it.
Maybe you did something wrong with the counter weights...control of the head.
Take the injectors out, put them in bottles, turn the ignition key and see what happens...my bet is that they will start pouring right away even without the engine started or cranking...when you see this try to operate the air meter plate to see if it makes a difference. My bet is it does not....so then you obviously did something wrong when reassembling.
ALSO....DONT CRANK IT ANYMORE WITHOUT SQUIRTING 5cc OF OIL IN THE CYLS AS YOU WILL DAMAGE THE CYL WALLS AND RINGS. Actually i would just flush the old oil and put in cheap one until the car runs if i were you.
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Porver9two4
Joined: 22 Jan 2004
Posts: 104
Location: United Kingdom
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| Posted: Thu Jun 02, 2016 8:51 pm Post subject: |
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| morghen wrote: | You did something wrong when you re-assembled the metering head.
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I agree with you.... I have been thinking about this ALOT over the last few days as I have worked through the remaining components in the FI and checked they are working correctly and concluded that the problem must be in the metering head.
I actually think I might have worked out what has happened and am planning to dive in again tonight to rip it all apart again and check. I think what has happened is that, right at the outset, I had a stuck piston in the metering head which produced an over-fueling symptom.....
In fixing the stuck piston I think I have created another fault which is giving the apparently same over-fueling symptom - except they are actually different faults .... not a continuation.
I think I know what the fault is too. I think what might have happened is that the circular metering slot body has partially rotated in the metering head as I have taken it apart and I didn't notice the movement. On reassembly that has misaligned the metering slots with the upper chamber fuel inlet ports (which lead to the injectors) and simultaneously exposed those ports to primary fuel pressure from the lower chambers. Hence the metering head isn't actually doing any metering at all... just flowing open circuit as soon as the fuel pump runs....
I will find out if I am right when I strip it back down....
As an aside, there is nothing to be afraid of in the Bosch K-Jetronic and no reason not to take the metering head apart and investigate. I have had these apart numerous times off various vehicles before and they aren't complex - fiddly, yes and very accurately machined but not beyond home mechanics. My hunch is I have made a mistake here, which I didn't spot (hey, it happens!) and its been masked by producing the same symptoms as the original fault leading me to continue to overlook it.... we'll see!
Thank you all for the suggestions above though, they've all been very helpful in distilling thought. I am getting closer  |
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morghen
Joined: 21 Jan 2005
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Location: Romania
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Rasta Monsta
Joined: 12 Jul 2006
Posts: 11733
Location: PacNW
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| Posted: Fri Jun 03, 2016 7:43 am Post subject: |
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Epiphany.
The control pressure orifice in the metering head is blocked.
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Porver9two4
Joined: 22 Jan 2004
Posts: 104
Location: United Kingdom
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| Posted: Fri Jun 03, 2016 4:53 pm Post subject: |
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FIXED! 
Pleased to report my 924 is now running sweetly again. I found the problem which was, I had suspected yesterday, a problem of my own making created within the metering head in the process of fixing the original non-starting over-fueling issue (caused by a stuck piston in the metering head). Turned out the metering slit barrel had rotated about 30° within the metering head body, exposing the injector chambers to full primary fuel pressure - I didn't remove the barrel when I stripped the metering head to free off the piston and didn't notice that it had rotated before I put it all back together..... a poor observation and mistake on my part.
Because it gave the same symptoms as the original stuck piston problem I (incorrectly) assumed I hadn't fixed the original problem and went off on a tangent trying to run down a phantom issue..... we live and learn!
Anyway, the bonus is I now know my fuel system intimately and can also be confident it is all in good, clean condition and free of any pending issues
Thank you all for your suggestions and help - it was much appreciated and does help to distil the thinking process in order to reach the right conclusion.
Happy days are here again ....  |
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morghen
Joined: 21 Jan 2005
Posts: 9102
Location: Romania
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| Posted: Fri Jun 03, 2016 5:04 pm Post subject: |
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Change the oil or you'll know the rotating assembly intimately as well.
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Porver9two4
Joined: 22 Jan 2004
Posts: 104
Location: United Kingdom
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| Posted: Fri Jun 03, 2016 5:31 pm Post subject: |
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| morghen wrote: | | Change the oil or you'll know the rotating assembly intimately as well. |
Sorry, should have said, all done - new oil, new filter. Plus the bonus of a nice clean, petrol washed engine internally
(I took your suggestion and squirted oil down the bores via the plug holes before attempting any further starting - sound advice, thank you.) |
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