Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 924/944/968 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
fred cook's Avatar
 
Join Date: May 2001
Location: Deep South
Posts: 4,592
Garage
Cool 924S Exhaust backfire....

I'm helping a neighbor reassemble his 924S engine after a bout with bent valves. The car had been sitting for about 6 months since the valves were bent. We finally got it all together this afternoon and started the engine. After a few minutes of rattling lifters, they quieted down and I adjusted the idle down to about 1000 rpms. The engine had just started to get warm when it started an occasional backfire out the exhaust. About the same time, we noticed that the cat. converter was glowing. I shut off the engine and connected a fuel pressure guage to the fuel rail. Once restarted, it was reading about 25 lbs of pressure at idle. We then changed the fuel filter and fuel pressure came up to about 30 lbs at idle. The engine would still backfire thru the exhaust occasionally but the cat was no longer getting red hot. The engine fires up almost immediately, seems to idle fairly smoothly and revs up at the least movement of the throttle. Any ideas as to what is causing the exhaust backfire? If this were a carbureted engine, I would suspect a lean running condition. Any and all ideas or suggestions will be greatly appreciated.

Thanks,

Fred Cook
'88 924S (drove it for 6 years and swapped for a 911)
'80 911SC coupe (in the family for 3 1/2 years)
Old 06-15-2004, 05:38 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
scheistermeiste's Avatar
 
Join Date: Jul 2003
Location: Cincinnati, OH
Posts: 1,025
Send a message via AIM to scheistermeiste Send a message via Yahoo to scheistermeiste
could be that the cat is clogged, if it was glowing red and not all the gasses were burnt up the heat would probably light these gasses... on rx7s its normal to see flames shoot out the exhaust if there arent any cants and the michture is rich.

although i could be wrong.
__________________
Special Edition 924S 1988 <- SOLD!

Asking if you should replace that timing belt is like asking if you should re-use a condom that may have in hole in it with a hooker.
Old 06-15-2004, 06:23 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Moderator
 
AFJuvat's Avatar
 
Join Date: Feb 2001
Location: Viera FL
Posts: 5,643
Sounds like the cat is clogged - likely melted at this point - if you are in a emissions testing state, replace the car, otherwise, remove the cat pipe and core the cat out and\or install a test pipe

AFJuvat
__________________
Es geht nicht darum wie schnell man faehrt, sondern wie gut man schnell fahren kann.

Ihr Brunnen der nutzlosen Porsche Information
Old 06-15-2004, 06:49 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered User
 
Verruckt's Avatar
 
Join Date: May 2002
Location: Fairfax VA.
Posts: 409
Replace the car? He ment the cat. There are lots of exaust shops that will work and most any local muffler shop can weld one into place for about a 100 bucks. The test pipe is very loud...
__________________
The crashes people remember, but drivers remember the near misses. Mario Andretti

1991 944S2 Rally #951 Rally-America.com
Old 06-15-2004, 10:50 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
fast924S's Avatar
 
Join Date: Jun 2003
Location: New Haven, CT
Posts: 3,347
Garage
Send a message via AIM to fast924S
Why would a cloged cat make the car backfire?? I would check the cap/rotor and wires first. Maybe the spark is jumping the wires. Also the AFM could be out of adujstment and reading funny. Anyway check the plugs wire cap and rotor and also for vacumm leaks. Also is the timming set right??? What work was done to the head??
__________________
1986 951, Stock for now.
]87 924S Gaurds red- SOLD after 11 years of ownership
Old 06-16-2004, 07:36 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
scheistermeiste's Avatar
 
Join Date: Jul 2003
Location: Cincinnati, OH
Posts: 1,025
Send a message via AIM to scheistermeiste Send a message via Yahoo to scheistermeiste
well if the cat is glowing red hot from heat and the engine is spitting out unburn fuel the fuel will ignite. when the fuel ignites it goes POP!
__________________
Special Edition 924S 1988 <- SOLD!

Asking if you should replace that timing belt is like asking if you should re-use a condom that may have in hole in it with a hooker.
Old 06-16-2004, 08:37 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Registered User
 
fast924S's Avatar
 
Join Date: Jun 2003
Location: New Haven, CT
Posts: 3,347
Garage
Send a message via AIM to fast924S
Wouldnt it be hot from a very lean mixture??
__________________
1986 951, Stock for now.
]87 924S Gaurds red- SOLD after 11 years of ownership
Old 06-17-2004, 10:46 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
scheistermeiste's Avatar
 
Join Date: Jul 2003
Location: Cincinnati, OH
Posts: 1,025
Send a message via AIM to scheistermeiste Send a message via Yahoo to scheistermeiste
well if its clogged or all melted together there is more resistance and more resistance = more heat
__________________
Special Edition 924S 1988 <- SOLD!

Asking if you should replace that timing belt is like asking if you should re-use a condom that may have in hole in it with a hooker.
Old 06-18-2004, 04:22 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Registered User
 
fast924S's Avatar
 
Join Date: Jun 2003
Location: New Haven, CT
Posts: 3,347
Garage
Send a message via AIM to fast924S
Good point, But it would only get melted from a lean mixture, right??
__________________
1986 951, Stock for now.
]87 924S Gaurds red- SOLD after 11 years of ownership
Old 06-18-2004, 07:46 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Registered User
 
scheistermeiste's Avatar
 
Join Date: Jul 2003
Location: Cincinnati, OH
Posts: 1,025
Send a message via AIM to scheistermeiste Send a message via Yahoo to scheistermeiste
well if its original its old... old things dont work right. pluss that many years of heat is bound to melt it.
__________________
Special Edition 924S 1988 <- SOLD!

Asking if you should replace that timing belt is like asking if you should re-use a condom that may have in hole in it with a hooker.
Old 06-19-2004, 09:20 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
pokey's Avatar
 
Join Date: Feb 2004
Location: Long Beach, CA
Posts: 1,041
Send a message via AIM to pokey
Cats will get red hot if raw unburnt gas gets to them. You are running rich. I'd check the O2 sensor. Then the AFM. But you will ned a new cat, as they do not get along well with raw fuel, and self destruct when red hot. Please, pardon any typos, as I am quite drunk after seeing a friends band tonight.
__________________
Still looking for the right deal on another P-car
1968 BMW 1600 2 liter
1956 Ford F250
1955 BMW R69
1999 Range Rover Callaway #011 of 220 (Yeah, as in Callaway Corvette...)
Old 06-20-2004, 12:33 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
scheistermeiste's Avatar
 
Join Date: Jul 2003
Location: Cincinnati, OH
Posts: 1,025
Send a message via AIM to scheistermeiste Send a message via Yahoo to scheistermeiste
lol, drunken responses are great! fast924s i do see what you are saying tho, but i dont think it would make the cat glow. running lean = hotter combustion = hotter exhaust gasses. but i think youd have pinging before youd have a glowing cat from it.
__________________
Special Edition 924S 1988 <- SOLD!

Asking if you should replace that timing belt is like asking if you should re-use a condom that may have in hole in it with a hooker.
Old 06-20-2004, 05:48 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
Registered User
 
fast924S's Avatar
 
Join Date: Jun 2003
Location: New Haven, CT
Posts: 3,347
Garage
Send a message via AIM to fast924S
Yea I think your right, If it was running that hot to melt the cat then I think it would be pinging too. Also like Pokey said if its running really rich I could see how that would kill the cat too. As far as the rich mixtue goes, I was running really rich at one time to the point that when I shifted from 2nd at 6500rpm to third flames shot out of the exhaust (kinda cool) I removed my cat like 2 years ago and the inside was still in good shape but the outside was cracked. Anyway we need to find out if he is running lean or rich
__________________
1986 951, Stock for now.
]87 924S Gaurds red- SOLD after 11 years of ownership
Old 06-20-2004, 06:41 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Registered User
 
fred cook's Avatar
 
Join Date: May 2001
Location: Deep South
Posts: 4,592
Garage
Cool Exhausted....finally!....

Ok, I finally figured it out! The plug wires for cylinders 2 and 3 somehow got reversed! So, 2 was firing when 3 should have been and vice versa. I was fairly certain that it had to be something simple as there just is not too much that affects mixture on these cars. Anyway, it turns out that the exhaust was running rich with the extra fuel igniting in the cat.

Thanks for all the replys. Maybe next time I will be able to count to four!

Fred Cook
'88 924S (sold)
'80 911SC coupe (getting better every day!)
Old 06-20-2004, 01:24 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Registered User
 
Join Date: May 2004
Location: Delaware
Posts: 200
Your exhaust will also backfire if there is a leak before the cat because of the extra oxygen mixing with the unburnt fuel and heat.
__________________
1984 Porsche 944 na

Black on Black leather
Digital Dash
Auto dimming rear view mirror
Enkei 15x7's
Fog/headlight mod to come

AIM - ShawnMacAnanny
Old 06-21-2004, 04:28 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 11:33 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.