1980 CIS Problem

[HopefullEngStdt]

Hey folks, long time no problems as I expect from my German Powerhouse =), but I sadly have to say my car let me down yesterday. It has been backfiring occasionally rarely after cold starts but nothing for me to really worry about. Then yesterday it started backfiring once warm while driving to such a degree, that I blew the seam on my muffler. It wouldn't do more than 40 mph and I just managed to limp her home. I pulled the plug from cylinder 1 and it was completely white. My first guess was that possibly the air flow sensor plate was sticking, took the rubber boot off and gave it a thorough washdown and cleanup with some carburetor cleaner. This showed no signs of helping so I took a 3mm allen and gave her more fuel, again, not helping. I heard some fuel pump noise that seemed more excessive than normal but not enough to cause this from what it seems. The weirdest thing is that it will fir right up when cold (within a second, no kidding) and will run fine for about 5 minutes until the problem starts to occur. This makes me think that the fuel pump is giving it enough fuel to run its cold-start cycle without a problem. It started completely suddenly and I am stumped to what it could be. I put some injector cleaner in the gas tank and let it idle before I killed it for a few minutes yesterday to see if it could be a clog in the fuel pump, then fired it up again today - yet again, fine when cold, not enough power to push itself up the driveway once warmed up. All you gurus of the horrendously outdated CIS out there, what am I doing wrong?

Thank you,

Moritz

[john walker's workshop]

when hot, remove the plug from the little O2 sensor system relay under the right seat and see if anything changes. when the relay fails or loses power from the domelight fuse, the system leans way out and it feels like you're dragging an anchor. the little relay, not the big computer. unplugging it should drop the idle way down and cause a rough run on a working system.

[HopefullEngStdt]

my O2 sensor is unpplugged and has been for a while, is it still gonna make a diff, i pulled the dome light fuse a while ago because it kept staying on and killed the battery overnight

[john walker's workshop]

there you have it. you da cause of da problem.

[rallyracer]

my 83 sc has a wire with a connector going to that fuse position for the dome light, etc. i was able to unplug it (had a short in the dome light that i couldn't find that kept blowing the fuse), kill power to those circuits, and still have power to the O2 brain frequency valve circuit. maybe you have this too?
David

[HopefullEngStdt]

well I have a hard time believing that this is the problem because I have had that fuse removed for about 8 months and have been driving the car daily without any issues, they just started Sunday. I believe the problem lies somewhere else.

[HopefullEngStdt]

just put the fuse back in, no difference

[john walker's workshop]

well, maybe the relay is bad. see if you can hear the frequency valve rattling/vibrating. it's behind the throttle housing, inline on a fuel line. pull the plug off the relay and see if the engine runs rough. warm engine.

[HopefullEngStdt]

I wish I could but it only has gotten worse, It will not fire up or run for more than 15 seconds now, even if I try to give it some throttle input.

[john walker's workshop]

are any of the airbox screws out or half way out? just wondering if the airbox is blown apart.

[HopefullEngStdt]

the airbox is on tight and secure

[Superman]

My O2 sensor is also not plugged in, but the system still needs the frequency valve relay working. If that relay fails, as John says, the car runs like a turd.

[HopefullEngStdt]

Well my car does not run at all right now, and if the lambda brain really needs the fuse for the dome light in to work, then my car has been running fine without it for 8 months, thats how long I have had the fuse out. I put it back in, and nothing has changed. That is not the problem

[john walker's workshop]

the fuse powers the relay. if the relay is dead, it doesn't care if it gets power or not.

[j_mancini]

Originally Posted by HopefullEngStdt

Hey folks, long time no problems as I expect from my German Powerhouse =), but I sadly have to say my car let me down yesterday. It has been backfiring occasionally rarely after cold starts but nothing for me to really worry about..............All you gurus of the horrendously outdated CIS out there, what am I doing wrong?

Thank you,

Moritz

My car (78 SC, so no o2 sensor) has exactly the same problem - seems like it's too rich, won't rev past 4500, weak power. A fuel pressure test kit is on the way (thanks JC Whitney), so hopefully some pressure data will shed light on the problem. It was running great when warm, just had a rough idle issue on warm-up (cold start was good).

What I do know:
- timing is correct
- distrib cap new about 5K ago
- plugs look good - clean white electrode
- new fuel filter, fuel lines 5K ago
- ignition wires appear original
- many of the vac hoses are relatively new

What I don't know:
- age/condition of fuel pump; tested 5k ago and was good then
- age/condition of fuel accumulator
- fuel check valve?
- aux air valve functioning ok? (rough idle at warm-up?)
- add'l air value ok?
- warm up regulator ok?
- system or control pressures?

I haven't torn into it, pending fuel pressure test - that seems to be the first place to start (yes?), unless someone has another suggestion, something easy to check.

[Ron.G]

I would pop the distributor cap and check for excessive play in your distributor. We had the same symptoms with a 79 SC. It ran pretty good when cool and got much worse as it warmed up.
Pretty violent back fires....