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Sudden power loss and backfiring between shifts and decel- video

Had a great drive up to Charlie Griffith's in north NJ for a complete AC system. Some beautiful farm country and rolling hills up in that Rt 94 / Rt 516 area. Unfortunately on the way home, traveling about 70MPH the car suddenly sagged, the exhaust note changed, and I was getting huge backfires whenever I left off the gas. It takes more accelerator pedal than it should to maintain a cruise. At idle it nearly dies, stumbling below 500RPM and recovering. In between shifts, it backfires.

I parked it and will get into it tomorrow when it (and I) cool down. In between then and now, I thought I'd post a brief youtube for anyone who wanted to guess at what it might be. When I figure it out I post up who was right.
One hint- I have already replaced the decel diaphragm in the car a few years ago, as I was getting popping and mine didn't hold vacuum. This is way more popping then what I experienced back then, if memory can be trusted. And i don't recall the car running like crap at that time, just too much popping on decel.

Popping on decel - YouTube



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Ken
1986 930, 1969 Mach1, 2003 540 Sport, 2016 R1200RS

Last edited by gsxrken; 08-02-2012 at 02:58 PM.. Reason: embed video
Old 08-02-2012, 02:52 PM
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Sounds just like mine the day my ignition box died. Car was hot, running the AC, suddenly felt very lean. Either no spark or no gas.
I thought a fuel pump fried, switched relays etc. ended up being the box.
Also, double check that they didn't dislodge something during the AC install.
GL
c

Last edited by Tt surgeon; 08-02-2012 at 03:39 PM..
Old 08-02-2012, 03:11 PM
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IC O'ring blew out...my guess. Got a huge air leak that'll suck in air and be lean when off throttle, and rich as a pig when on throttle trying to build boost. This presuming you still run with CIS fueling.
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Old 08-02-2012, 03:47 PM
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mine sounds like that when the number 3 spark plug wire pops loose. not sure why but it's always that same one.
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Old 08-02-2012, 05:13 PM
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Does it idle well or barely at all? You said no I think so sounds like a vacuum leak.
If you have a vacuum leak it should not idle well if at all.
Could be a broken exhaust header or leak there too.
You need to start checking the obvious and move into the difficult in steps.
Check fuel pump relays, leaks in the intake or exhaust, and then start looking at ignition items.
Whatever it is you said it failed suddenly without warning so start looking at what you disturbed when you last touched the engine.

Fred
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
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Old 08-02-2012, 05:25 PM
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What Mark said.
Old 08-02-2012, 05:32 PM
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Loose Red fuel pump relay! My fault, too. I had pulled back my carpet in the bonnet to get at the brake light senders and left it folded back b/c I knew I was running up to Griffiths and he might want to peek in there at the smugglers box.
The carpet had knocked off the rectangular fuse cover, which must have taken the relay with it.
Fred, go buy yourself a good cigar.

PS- Charlie is blameless. I purchased the parts from him but I'm strictly a DIY type. I'll start another post when I get to the installing his full 930 kit.
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Old 08-03-2012, 07:48 AM
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Mine has always popped a lot on decell, without power loss. maybe I'll change my diaphram too.
Where did you source the part, pelican?
c
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Last edited by Tt surgeon; 08-03-2012 at 08:48 AM..
Old 08-03-2012, 08:28 AM
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..so hows the AC workin?

Charlie's been a big help to me even though I never bought anything from him.
Old 08-03-2012, 08:38 AM
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Quote:
Originally Posted by Tt surgeon View Post
Mine has always popped a lot on decell, without power loss. maybe I'll change my diaphram too.
Where did you source the part, pelican?
c
I just had an eBay and CL search agent running for +Porsche +decel or +limiter. They come up now and then. EFI guys ditching CIS are a good source too. Problem is, they're just as old as yours and might be bad too. Check yours with a mityvac and see if it holds a vacuum.

Quote:
Originally Posted by JFairman View Post
..so hows the AC workin?

Charlie's been a big help to me even though I never bought anything from him.
Charlie is a walking encyclopedia of what works and what doesnt for these cars. Im glad he's within an hour's drive. I got a tour of the shop, a very detail oriented walkthrough of each part i need to install, and the instructions have to be seen to be believed.
I havent started the install. 8 hours under the car routing AC lines (Charlie's estimate) is psyching me out. I need to man up right after I finish this latte.
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Old 08-03-2012, 09:04 AM
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I did his center vent, makes a huge difference. I just converted to r134 w his cannister thing, works great!
Not sure how to test that diaphram, I'll do a search see what I come up with....
Old 08-03-2012, 09:17 AM
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Quote:
Originally Posted by gsxrken View Post
Loose Red fuel pump relay! My fault, too. I had pulled back my carpet in the bonnet to get at the brake light senders and left it folded back b/c I knew I was running up to Griffiths and he might want to peek in there at the smugglers box.
The carpet had knocked off the rectangular fuse cover, which must have taken the relay with it.
Fred, go buy yourself a good cigar.

PS- Charlie is blameless. I purchased the parts from him but I'm strictly a DIY type. I'll start another post when I get to the installing his full 930 kit.
Nah, I think I will have a few beers though.....next time your driving thru Baltimore drop off a six pack of Corona, ha,ha.

Lucky it was something simple to fix.

Fred
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 08-03-2012, 09:53 AM
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Is there fuel in your air box?

Old 08-03-2012, 12:05 PM
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