help! SC popping problem

'83 sc · 03:49 PM

I have a '83 911sc and it just started having problems. I've had the car about a month with no problems. 98k miles stock but does have msd box on it. Everything was fine until one morning I went to drive the car and after I got out of the drive it started popping when I gave it gas and only would idle around 4-600. I drove it the day before without any problems I didn't drive it to hard but was running about 80mph for around 25 minutes but it's not the first time I've done that in the car. The guy I bought it from only put about 3k miles on it in 5 years and I already have put 700 on it. I've changed plugs, checked timing, checked plug wires but haven't found anything wrong with it. The thing was it was fine the day before. And now it starts and idles fine around 1200 for about 30 sec. then idles bad and just pops when you give it any gas. I can lift up on the fuel pluger a tiny bit and it idles better. The only other thing is that this was really the first colder (60deg F) day I've tried to drive the car.
If anyone has any ideas please let me know so I can get my 911 back on the road. Almost forgot it's an intake backfire and not out the exhaust.
Thanks

Scott R · Scott R's Garage

I can tell you from viewing a wideband O2 that popping is unburnt fuel in the exhaust most of the time. Why? over-rich condition. Sounds like you have a vacuum leak somewhere in the system, pop of valve maybe? Airbox? Did you experience any backfires recently?

'83 sc · 09-16-2007, 04:22 PM

the pop off valve isn't leaking and the lines I looked at look good I'm going to try spraying them with starting fluid or something when I get time to see if there are any leaks but, not backfires or anything. I just parked it running fine then the next day it started doing it. Like I said this was the first cooler day so I don't know if it's somthing with the cold start system.

fireant911 · 04:52 PM

There may possiby be an issue with the relay under the passenger seat - either a fuse is blown or the relay is not functioning.

Look at John Walker's response in the post below:
1980 CIS Problem

Paulporsche · 09-16-2007, 04:47 PM

As a general rule, popping through the intake is indicative of a mixture that is too lean. There could be several reasons for this, including an out of spec or malfunctioning AAR, vacuum leaks, or a mixture that has been simply set too lean. On later CIS cars like yours, there could also be problems w/ the O2 and various temp sensors.

It probably has nothing to do w/ your cold start system. In a nutshell it pretty well just starts the car or doesn't.

There are many many threads on this, and lots of info in the Bentley manual. Poster Souk has quite a few threads incl how he, and many other of us, adjust our mixture per seasonal change.

'83 sc · 09-16-2007, 05:03 PM

I did see the O2 sensor wire had come apart where someone must have put the wire together. I couldn't tell if I had done it while changing plugs or if it had fallen off. so maybe it caused some problems with the control unit or a fuse. I checked all the fuses under the hood and there were good or is there on under the seat?

Scott R · Scott R's Garage

Quote:

Originally Posted by Paulporsche

As a general rule, popping through the intake is indicative of a mixture that is too lean. There could be several reasons for this, including an out of spec or malfunctioning AAR, vacuum leaks, or a mixture that has been simply set too lean. On later CIS cars like yours, there could also be problems w/ the O2 and various temp sensors.

It probably has nothing to do w/ your cold start system. In a nutshell it pretty well just starts the car or doesn't.

There are many many threads on this, and lots of info in the Bentley manual. Poster Souk has quite a few threads incl how he, and many other of us, adjust our mixture per seasonal change.

Is it popping though the intake? I thought it was the exhaust, however I have no clue based on the post. I guess the best thing to do is get it on a set of CIS gauges, and a gas analyzer. Both can be added to your garage for a nominal price.

'83 sc · 09-16-2007, 05:16 PM

If it's not something simple like a fuse I think I'm just going to go with the Bitz efi kit. That is if it's not something wrong with the engine like head studs but I really think it's in the CIS
And yes it's in the intake I don't get anything out of the exhaust.

Scott R · Scott R's Garage

Quote:

Originally Posted by '83 sc

If it's not something simple like a fuse I think I'm just going to go with the Bitz efi kit. That is if it's not something wrong with the engine like head studs but I really think it's in the CIS
And yes it's in the intake I don't get anything out of the exhaust.

It would be hard for me to steer you away from Bitz

I get a perfect start, cold or hot, and a perfect 14:7 AFR at idle, not to mention the amazing throttle response.

Paulporsche · 09-16-2007, 06:23 PM

Scott,

Did you put that 02 wire back together? Was the popping before or after the wires were apart?

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by '83 sc

If it's not something simple like a fuse I think I'm just going to go with the Bitz efi kit. That is if it's not something wrong with the engine like head studs but I really think it's in the CIS
And yes it's in the intake I don't get anything out of the exhaust.

CIS maybe old and antiquated but very manageable. I'm with Scott and still looking for a CIS problem that's difficult to solve. Fuel distributor (FD) maybe one but it is durable and robust. The lack of knowledge and understanding how a Continous Injection System works resulted to its bad reputation. EFI is definitely a better and moderm fuel system than CIS no question about it.

Do you know what's your control and system pressures at this point? Don't give up yet.

Tony

rberry951 · 09-16-2007, 07:41 PM

I read about a similar problem a while back, and the PO of my 911 had the same issue. In both cases it was an intake gasket. PO said it didn't take him long at all to do it on a '86. Maybe worth having a look.

Regards,
Russell

'83 sc · 09-17-2007, 02:59 AM

I really don't know when the o2 wire came apart I just happen to see it. But it didn't seem to help when I put it back. As for the Bitz kit how hard was it to install? does it come with any base maps for the SC or do you have to do all the tuning. I'm thinking of going that way because I like to be able to jump in my 911 and go sometimes putting about 500 miles a week on one and I really don't want to be screwing with the CIS all the time.

psalt · 09-17-2007, 04:23 AM

really don't know when the o2 wire came apart I just happen to see it. But it didn't seem to help when I put it back. As for the Bitz kit how hard was it to install? does it come with any base maps for the SC or do you have to do all the tuning. I'm thinking of going that way because I like to be able to jump in my 911 and go sometimes putting about 500 miles a week on one and I really don't want to be screwing with the CIS all the time.

Reliability is not an issue with CIS, once it is operating properly and adjusted, it can go 300K. Understanding how the system works, especially the final version, is more of an issue. Many people assume that since the engine will run OK with the O2 sensor disconnected, that the lambda system is not necessary. This is false, the 83 SC requires the lambda brain and FV to function, at least in the default mode, to get a proper fuel curve. The first thing you should do is diagnose the lambda system. This is covered several times in the archives. It is easy to do with a analog dwell meter. The test plug is under the plastic cover on the left side of the engine compartment. Hook up the meter to the green/white strip wire, start the car and let the system tell you what is happening. The needle should show a steady 58 dwell when in open loop and switch to a fluctuating 40-55 dwell when it switches over to closed loop after a few minutes of warm up. If the system is not working, no amount of parts swapping or fiddling with basic CIS parts will solve the problem. If the FV is not working, check the relay under the passenger seat and the two control box connections. The FV duty cycle is a proxy for the A/F mixture and you can easily dial in the best mixture adjustment on an 83SC with an allen wrench. Most cars run best with the open loop mixture set to 30-40 dwell. If you are having cold start problems, the vacuum retard may be contributing to the problem and the engine runs better and cooler without it. These cars also behave better on regular Bosch plugs not the platinum versions.

Paul

Paulporsche · 09-17-2007, 05:55 AM

I agree w/ psalt. Sounds like someone unhooked the O2 sensor because of a problem, which may be easily remedied.

OTOH, 83 sc, if you want to modify your car it's your right. I don't see where you are located, so I don't know if emissions requirements have to be met.

Superman · 09-17-2007, 06:47 AM

Intake popping suggests a lean mixture, or an exhaust leak, which also causes a lean mixture. Exhaust popping is generally an exhaust gasket leak, allowing air into that hot exhaust manifold, igniting unburned fuel.

CIS malfunctions should be determined by following the standard CIS troubleshooting procedures.

mca · 09-17-2007, 06:50 AM

Quote:

Originally Posted by psalt

the 83 SC requires the lambda brain and FV to function, at least in the default mode, to get a proper fuel curve. The first thing you should do is diagnose the lambda system.

Paul - forgive my ignorance. Are you saying that the lamda brain affects running conditions even when the O2 sensor is disconnected?

'83 sc · 09-17-2007, 06:50 AM

I'm going to check all the 02 stuff after work today to see if there are any problems, but the thing that gets me is that it was running perfect the day before when I parked it. Then the next day it just started having all the problems. And the car starts fine then idles down and pops when you try to give it gas. I'd rather keep the car with CIS but I really don't feel like screwing with it too much, It takes me back to my old 911t MFI I tuned on that thing for about a year and never did get it running right. So that's why I'm thinking of doing the EFI kit.

Scott R · Scott R's Garage

Quote:

Originally Posted by '83 sc

I'm going to check all the 02 stuff after work today to see if there are any problems, but the thing that gets me is that it was running perfect the day before when I parked it. Then the next day it just started having all the problems. And the car starts fine then idles down and pops when you try to give it gas. I'd rather keep the car with CIS but I really don't feel like screwing with it too much, It takes me back to my old 911t MFI I tuned on that thing for about a year and never did get it running right. So that's why I'm thinking of doing the EFI kit.

You'll be tuning quite a bit with EFI as well. I still have to go in from time to time to adjust something for a given temperature or RPM range. The nice thing is that all I need to diagnose is my laptop, other than that you're going to be adjusting from time to time on either system.

'83 sc · 09-17-2007, 07:28 AM

I don't mind doing a little tuning from time to time but to go from running "perfect" to not even being able to drive it over night without anything being done to it isn't what I'm looking for. I'm going to check all the vacuum lines with some starting fluid and see if there is something going on I can't see. I also noticed a small oil leak that had been getting worse coming from the oil level sender could that cause the problem since vacuum lines go to the oil tank? Should I be worried about heads studs at all I thought the 83 sc's had that problem fixed? If I can't find an problems when I get home I might order the CIS testing stuff to see if that helps. I just really want to get this thing back on the road has soon as possible.