What causes backfires?

Brother · 02-07-2005, 05:20 PM

What are the common causes of backfires?

My car seems suceptible to backfires for the first five minutes after start each day. I get VERY slow acceleration and a few backfires under load.

The car runs great after a few minutes. What up?

176,600 miles on the clock and a well exercised pop off valve.

copper · copper's Garage

Are you running the lowest octane possible?

Brother · 02-07-2005, 05:37 PM

no. I run 89 octane. Mid grade.

wastintime · 02-07-2005, 05:57 PM

89 is still too low for a 911 engine... that might not be the problem, but if it were my car, 93 is the lowest I'd even consider putting in it.

Brother · 02-07-2005, 06:04 PM

But there is no problem once it warms up. I'm talking about a problem that only occurs for about 5 minutes or less.

I am pretty sure that 89 is ok for an SC.

wastintime · 02-07-2005, 06:10 PM

well, that's just my $.02 i wouldn't put 89 in a porsche... have you checked the plugs? sometimes colder heatrange plugs get fouled if you don't drive them hard enough and it'll backfire and not run all that well for a few minutes.

Joe Bob · Joe Bob's Garage

If on deceleration, it's an exhaust leak. On acceleration....could be an intake and/or vaccum line leak.

Minor leaks will seal up when the engine gets warm and components expand

Brother · 02-07-2005, 06:13 PM

I pulled a plug this weekend.

It was gray/white on the tip and the electrode was brown. The other surfaces were a little black and gummy. I have an MSD unit and Blaster coil. I think I was running rich for a while.

I might try the good stuff for a while and see what happens.

Brother · 02-07-2005, 06:16 PM

Quote:

Originally posted by mikez
If on deceleration, it's an exhaust leak. On acceleration....could be an intake and/or vaccum line leak.

Minor leaks will seal up when the engine gets warm and components expand

Dammit! You take that back! I refuse to hear this.

That is what I think. My idle increase slightly as the engine warms as well. I'll be hunting for a leak this weekend. Can you buy carb cleaner by the case?

copper · copper's Garage

A 1980 SC runs a compression ratio of either 8.6:1 or 9.3:1 depending on whether your model is US or ROW. Either way, the compression of these engines is low. Therefore, a higher octane will not provide any benefit, and can actually cause problems. You don't need any more than an 87 octane in that engine. Ask me how I know...

The fuel is the easiest thing to diagnose. Clean the plugs, fill her with a full tank of 87 and run her. Higher octanes need higher temps to burn completely. Therefore, you may be getting unburnt fuel (from a cold engine) igniting in the exhaust.

What you are describing is consistent with a number of things. The only consistency is that it stops once warm. Could be the fuel octane, could be a gasket or flange leak that closes once warm enough to expand and close the gap.

While running, put your hand near the flange where the exchangers meet the muffler and feel for exhaust pressure escaping.

At any rate, the only difference you'll see between 87 and 89 is the price...

Jim

TXScott · 02-07-2005, 07:30 PM

What Mike said

randywebb · 02-08-2005, 09:19 AM

Is it thru the muffler or thru the intake?

dhoward · dhoward's Garage

Backfire through intake - lean. Consistant with cold weather warm up.
Backfire through exhaust - rich mixture or leak.

David · David's Garage

Sounds like a fuel pressure problem when cold. Buy some CIS pressure gauges and check cold pressure.

Paulporsche · 02-08-2005, 10:49 AM

Brother,
Sounds like the all too common high cold control pressure causing a too lean mixture during warmup. Search WUR for info on the fuel gauge mentioned above and how to reset your WUR. That's usually it, but it is possible your AAR is also not functioning correctly, so you might want to search that too.

Brother · 02-08-2005, 06:22 PM

I started looking around and I think there is some gunk on the intake boots on the right side.

I originally thought it might be a pressure problem, but it does not happen for more than 1 minute or so. If I really milk the clutch and throttle out, i can avoid it all together. I think it is probably a vacuum leak.

I'll keep you posted.

al lkosmal · 02-08-2005, 10:14 PM

Intake leak

Aussie · Aussie's Garage

Too many baked beans!

Brother · 02-09-2005, 03:25 AM

Confirmed what MikeZ said last night. I'll post pictures later. Fuel residue, grease, and exposed wiring. The makings of a Porsche love story.

Anyone want a used CIS? Selling your PMO's?

Brother · 02-09-2005, 01:22 PM

Here is a picture of what is going on.

Lots of nasty residue under the boots. That wire pinned unter the bolt is the wire to the AAR. Lots of gunk back in there as well. This narrows it down substantially.

How much of a PITA is it to dismantle the CIS without dropping the engine?