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79 NA will not inject fuel while cranking
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Mclaren924  



Joined: 13 Oct 2021
Posts: 171
Location: California

PostPosted: Sat Aug 05, 2023 4:13 pm    Post subject: 79 NA will not inject fuel while cranking Reply with quote

Got a 79 NA that I just rebuilt and cannot get the fuel injectors to flow when cranking. FD will flow when door is opened by hand through air box and will flow a consistent good amount across all injectors when doing so. I have 70 psi system pressure so more than enough to open when pressured, had my old man crank while I felt the FD door and noticed it was sporadically bouncing up and down. I'm loathed to think the FD died on the shelf in the span of 7 months but it wouldn't be the first time a toofah part has done that to me. Debating buying rebuild kit for FD but would like to be confident it is my issue before cracking it open... Mechanical timing has been triple checked and the intake provides enough vacuum to such a sheet of paper flat against the TB. Car managed to fire off yesterday for about 45 seconds across 3 different attempts. Sounded very rough and uneven, had no rhythm up until last 2 seconds than it died outright. We walked away thinking it was flooded as we tried revving it to get it going, that was yesterday and haven't gotten a peep since. Is this K-jet hell that so many speak about? sure feels like it lol, any advice and experiences would be awesome to hear. Cheers yall
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1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Sat Aug 05, 2023 9:16 pm    Post subject: Reply with quote

So if I understand correctly, if you have the fuel pump on and move the air metering arm in the fuel distributor, you see fuel coming out the injectors?

Then IMO while it doesn't rule out all fuel system issues, you are able to inject fuel enough to run.

If that were the issue, a shot of starting fluid into should get it to fire immediately.

It sounds to me like maybe you have a spark or timing issue, instead. Time to recheck all the marks, and check for spark on all 4 cylinders...
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Mclaren924  



Joined: 13 Oct 2021
Posts: 171
Location: California

PostPosted: Tue Aug 15, 2023 2:03 am    Post subject: Reply with quote

Well the fun continues, I assumed compression was going to be good. Well I Was wrong, HG seems to have not sealed at all, 0 compression in front 2 pistons and 20 psi in rear 2. Pulled the motor for a reseal 8 months ago and it had completely flat deck surfaces, the motor didn't mix oil and coolant and nothing was seeping out the side. Is it possible that it didn't seal the cylinders off from each other? I verified timing was right with taking valve cover off to confirm cam lobe location and used pistons stops to make sure the crank was timed correctly too. I checked and all bolts are tight, maybe I put it in backwards or something? lol. #### up part is my 77 that did blow a HG, sealed up great and it definitely had more distortion in the head. Such is life, motor is out and waiting for cometic HG to show up from piper, yeesh 200$ is steep for em. This was a valuable lesson not to make assumptions but to verify first, thankfully I only rebuilt the fuel distributor in the hunt to get the car to run, and it probably needed that anyways. Reason why it wasn't pulling the air meter down is because it had literally 0 compression. Sigh
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1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
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Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1353
Location: Washington DC area

PostPosted: Tue Aug 15, 2023 8:42 am    Post subject: Reply with quote

Bent valves will give you that type of lack of compression. Hard to believe that all four cylinders didn't seal unless you only had the head bolts finger tight.
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1979 924 NA race car
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Mclaren924  



Joined: 13 Oct 2021
Posts: 171
Location: California

PostPosted: Tue Aug 15, 2023 11:38 am    Post subject: Reply with quote

Fifty50Plus wrote:
Bent valves will give you that type of lack of compression. Hard to believe that all four cylinders didn't seal unless you only had the head bolts finger tight.

Yeah id normally agree with this train of thought but there's no way the valves are bent as the head was fine when disassembled and there's physically no way the valves could have been bent by cranking of motor as its an NA. To say the least im confused lol, cylinder 4 had about 30 psi and I never measured the 3rd cylinder as its a bitch to get on to and the other 3 are already far below passing. I ruled out timing as an issue as I had a running NA next to me to verify cam lobe position and piston position when at TDC. Meaning either the valves aren't closing or the the HG didn't seal. Either way head has to come off and inspection must begin but I cannot fathom an NA motor bending valves. Especially bending all 4... Will continue with updates and videos when the HG arrives and I'm back off vacation. No milkshakes in oil pan or radiator so if HG went it went between all 4 cylinders, also very strange. I dunno, very perplexing to say the least. Rewatched my video of assembly and I appear to have done everything to spec but I'll link it and hopefully somebody can point out where I went wrong. https://youtu.be/BNZElkJAzdU
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1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon
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safe  



Joined: 18 Mar 2017
Posts: 583
Location: Sweden

PostPosted: Tue Aug 15, 2023 4:40 pm    Post subject: Reply with quote

Don't bother with the cometics, use a stock head gasket. MLS gaskets have far higher demands on the surfaces being perfect, it likes a mirror finish surface.

It sounds like you have gotten something between the head and block when mounting it or your cam timing is off.
Have you checked valve lash? you haven't gotten some of the adjusters "between the snaps".
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Tue Aug 15, 2023 9:19 pm    Post subject: Reply with quote

Magnus has a good point there re: surface finish for Cometic - did you resurface the head and block? And anyway, a Cometic on a stock NA is a waste of money IMO; we've run stock headgaskets on the ITB car for so long, and they've never been an issue.

Had one briefly on my Turbo, but had to swap back to stock on an unplanned rebuild, and it's been fine with 15psi. Considering that you're running a lot less boost than that - none - you'll be fine on stock.

But since the head's coming off, check the valves for sealing this time then maybe lap them in while you're at it - costs virtually nothing but a little time.
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safe  



Joined: 18 Mar 2017
Posts: 583
Location: Sweden

PostPosted: Tue Aug 15, 2023 9:29 pm    Post subject: Reply with quote

924RACR wrote:
it's been fine with 15psi.


Mine is handling 29 psi at the moment

A leakdown test is whats required to figure out what is leaking compression. Reasonably cheap tool and very useful.
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morghen  



Joined: 21 Jan 2005
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PostPosted: Wed Aug 16, 2023 12:50 am    Post subject: Reply with quote

You cant have 0 compression, are you sure you are mesuring properly and the tool you are using is the right one?

I used a cometic gasket with a very rough head surface and it held just fine even with high boost.

Warped head cant cause such catastophic pressure loss.
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Beartooth  



Joined: 05 Apr 2022
Posts: 198
Location: Roberts, MT

PostPosted: Wed Aug 16, 2023 2:21 am    Post subject: Reply with quote

That's pretty crazy - definitely something went badly wrong for it to lose that much compression. Maybe a major defect in the head gasket? Anyway, I'd approach it fresh and make sure the problem isn't somewhere else before you pull the head. Of the top of my head, I'd re-re-check the cam timing, check valve lash, make sure your compression checker is working right (and you're able to get a good seal - sounds like access is a PITA, one nice thing about the 931 - so it's easy to see something happening there). I'd also try backing the head bolts off and re-torquing them as a last resort before you take the head off. It's hard to imagine it'd make a difference, but if compression changed after re-torquing, that would definitely point to the head gasket. Aren't cars fun!
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Fifty50Plus  



Joined: 28 Feb 2008
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PostPosted: Wed Aug 16, 2023 2:39 am    Post subject: Reply with quote

For external diagnostics before tearing into an engine, the leak down tester is THE tool. Put a piston to top dead center, inject 100 lbs of air and read gauge. Anything less than the mid to high 90s tells you that you have an issue. AND you can see/hear where the air is leaking from (intake valve, exhaust valve, rings, or head seal).
If you truly have 0 or 30 compression the leak down will tell you where the problem lies.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car
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Mclaren924  



Joined: 13 Oct 2021
Posts: 171
Location: California

PostPosted: Wed Aug 16, 2023 8:36 am    Post subject: Reply with quote

Well I appreciate the advice about the cometic HG use and will shelf it for a boosted motor build I have in the works. I'm reusing the same HG as I determined there was nothing wrong with it and not my cause for lack of compression. I took the head off and to my surprise the HG seemed to have sealed everywhere with no blowouts. I walk over to the head that I had set down on the bench and to my amusement (not really) I see this.

I like to learn my lessons the hard way but this one sucked. When installing the intake manifold I failed to remove the blue roll I used to keep it sealed from FOD. Promptly sucking the pieces straight into the valves and getting entangled in the valves. Before you ask, no it doesn't hurt being this stupid, it just makes it really hard to get compression lol. I fully disassembled the head and cleaned out all residue and inspected valves to make sure they weren't damaged. All looked good so I reassembled and then pulled the oil pan and pump to make sure the pistons didn't drag anything into the oil. Thankfully they were all clear so I sprayed them out for good measure and reinstalled. Currently all back together and motor will be installed when I'm back from vacation. As much as I would like to keep this mistake to myself, I figured you all deserve a laugh and I appreciate all your guys help. Will update hopefully with vid of it running when I'm back.
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1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon
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Mike9311  



Joined: 14 Dec 2004
Posts: 1678
Location: Chicago-ish

PostPosted: Wed Aug 16, 2023 11:03 am    Post subject: Reply with quote

Everyone does something like this every now and then. What I really appreciate is your sharing the story! I was reading this earlier today and said to myself I have nothing to add. At the very least there is an answer and you are back to putting it back together
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Thu Aug 17, 2023 1:19 am    Post subject: Reply with quote

OMG!!! Yeah, that explains that!

At least it's a zero-damage failure, and not a Turbo so a lot easier to pop the head off and fix!
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safe  



Joined: 18 Mar 2017
Posts: 583
Location: Sweden

PostPosted: Thu Aug 17, 2023 1:33 am    Post subject: Reply with quote

That explains a lot!
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