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Join Date: Feb 2005
Location: Midland, Texas
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Question - Backfire after long period of coasting?

I did not want to hijack the thread that I just read about backfiring. I was looking on the site for possible causes for my problem? I took my 87 turbo for its first long trip since I bought it a couple of months ago. The car has a 1 bar spring and a K27 turbo. I think the BOV is gone because when I let off the gas it burbles and "farts" a lot. I think it sounds cool, but I am a 930 newbie. The problem occurred during a long downhill stretch. I was out of the gas for +/- 20 seconds (the windows were down and the sunroof was open so I could not hear the engine very well). As I went to get back on the gas the car backfired (big puff of black smoke) and the tach needle was flopping around. I tried to get it to go, it did not want to go and would backfire if I got on it. I coasted with the car in neutral. The tach jumped around a couple more times and then the problem went away. I was scared &^%$ less that I had hurt the car. I stopped, checked the oil level, the car was not hot, fuel level over half full. The car restarted and I drove 200 miles home without issue? I hope it was some sort of mixture problem with the long downhill (if I could have heard the engine I bet it was "burbling and farting" like crazy down the long hill). Any ideas where to start?
Old 04-12-2008, 07:58 PM
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Dude, you need to hit more often, makes it easier to read...

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Originally Posted by david63 View Post
I think the BOV is gone because when I let off the gas it burbles and "farts" a lot. I think it sounds cool, but I am a 930 newbie.
CIS injects fuel all the time. The "C" stands for "continuous"...

All the CIS cars pop and bang on the overrun if you just let the throttle slam shut - it's the unburnt fuel building up in the headers/muffler. It's what lights off for the fireball when the turbo/wastegate is hot enough.

I don't think mine is any different since I converted to the 965-style BOV, but that's got me wondering now...

Quote:
The problem occurred during a long downhill stretch. I was out of the gas for +/- 20 seconds (the windows were down and the sunroof was open so I could not hear the engine very well). As I went to get back on the gas the car backfired (big puff of black smoke) and the tach needle was flopping around. I tried to get it to go, it did not want to go and would backfire if I got on it. I coasted with the car in neutral. The tach jumped around a couple more times and then the problem went away.
The "erratically jumping tach" needle is a classic sign of a bad voltage regulator.

If that's what it was (rather than the car just running ragged), it may have interfered with the proper operation of the CDI and prevented the car from firing cleanly.

A bad voltage regulator can boil the acid in your battery and make it puke acid out, or even buckle the plates and short the battery internally. The overvoltage can damage the CDI too. Keep an eye open for that.

It could just be an iffy connection, as it seemed to "fix" itself.

Quote:
I was scared &^%$ less that I had hurt the car. I stopped, checked the oil level, the car was not hot, fuel level over half full. The car restarted and I drove 200 miles home without issue? I hope it was some sort of mixture problem with the long downhill (if I could have heard the engine I bet it was "burbling and farting" like crazy down the long hill). Any ideas where to start?
Check the mixture by all means. It's good to baseline.

When I was trying to drive an open pipe in "stealth" mode, I found you could reduce the popping and banging (and the occasional backfire that sounded like a 12 gauge) to almost nothing by keeping the throttle open a tiny amount on the overrun - about as small an opening as you can manage off the idle stop works. Just a fraction more air seems to keep it burning.

Or just use more throttle, speed-match, downshift
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'77 S with '78 930 power and a few other things.
Old 04-14-2008, 11:08 PM
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Thanks for the hints, sorry for the long sentences. I have had Porsches since I was 16 (am 45 now) so not a newbie to Porsche, just new to the 930.
I appreciate the help. Need to do some looking into how the CIS works, the Turbo plumbing (BOV, etc). I seems that a good AFR gauge is a must. I will look into the charging system and consider a CIS tune up. Thanks again
Old 04-15-2008, 10:43 AM
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