Help! 83SC - no power, backfiring, trying to diagnose

Mike Juzenas · 10-27-2001, 12:43 PM

Well, I need some suggestions to fix my car. My 83 911SC just started to show the following symptoms (within minutes after I made the decision to upgrade and sell it!):

- cold start is fine and seems to be okay until warmed up
- rough idle once warm
- lack of power with the occasional backfire (thank god for the pop-off valve!). Feels like its flooding. No overpowering smell of fuel though
- while its roughly idling there is a relay ticking under the dash (have no idea what that is).
- its has been intermittent but is now repeatable (that is good!)

Anyways, the ignition looks okay with a cursory check with the timing light. Timing is advancing on the distributor. Did not check to see if I get the full advance (still confused on that one anyways). I took out my trusty CIS pressure gauge and put it between the Control Pressure Regulator and the Fuel Distributor. In the following notes the valve I mention is the one on the test harness that stops the flow of fuel to the control pressure regulator. I also bypassed the fuel pump relay so the pump is running without the engine running. This is what I found so far:

- Fuel pump working, car not running - 72 PSI (within spec)
- Car idling with gauge valve open - 55 psi (within spec)
- Residual pressure 38 dropping to 34 in 6 minutes
- No affect if I disconnect either the thermo-time switch to shut off the cold-start valve, or the temperature switch that controls the ox sensor.
- Vaccum is 12 in Hg idling (taking off the vacuum side of the line to the vacuum advance on the distributor), and about 15 at medium engine speed

So,now what? Questions:

1 - Can it be the fuel distributor and how to I figure out that one?
2 - Other tests I can run?

Any ideas?

Thanks!

Mike
83 911SC

rstoll · 10-27-2001, 03:03 PM

Is relay under passenger seat ok?? No signal to the frequency valve without it.

------------------
Robert Stoll
83 SC
83 944

Mike Juzenas · 10-27-2001, 03:24 PM

Quote:

Originally posted by rstoll:
Is relay under passenger seat ok?? No signal to the frequency valve without it.

Not sure if I know how to test that. Let me check the Bentley. This may be the relay noise that I am hearing. It sounds like its from the underside of the dashboard, but it could be from under the passenger seat as well. I will listen more when I go back to work on the car.

I need to check the wiring diagrams in the manual to check the frequence valve. It appears to have a resistance of about 2-3 ohms, and should be buzzing when the engine is running. I can check that.

Cheers,

Mike

lagoon · 10-28-2001, 06:41 AM

If your O2 sensor relay is ok but your mixture is too rich, you will not hear the frequency valve buzz. Try lifting slightly up on the sensor arm when running. If engine smoothes out, you are too lean. If it stalls, you are too rich. try that. 81'SC

Mike Juzenas · 10-28-2001, 07:37 AM

Thanks guys!

Hmmm, I was slightly wrong on my initial assumptions.

Started the car cold this morning...its just as rough or rougher when cold. It appears the frequency valve is not operating, but I checked the resistance...its 2.7 Ohms, exactly on the money.

If I slightly lift up on the air plate it seems to smooth out.

The car does run and is drivable, but barely. It feels very sluggish and tend to backfire.

I have checked the voltage of the O2 sensor. Its very low. But if the mixture is way too rich then the O2 sensor voltage will be really low anyways, right? I took the sensor out and heated it with a torch...the voltage went to .9+ volts pretty quickly. But in the car it stays at .05-.09 volts. Pretty low perhaps because of the rich condition.

And I thought that if the O2 sensor does not work, it goes into safe mode which is a rich mixture anyways? However, the O2 sensor is not in the loop when the engine is cold, and the engine does not run properly then anyways, so maybe is not the sensor.

I am thinking of the fuel distributor. How do you know its going? What are the symptoms? How can I test it?

It really does sound like fuel but could this be an ignition problem?

Cheers,

Mike

Doug Zielke · 10-28-2001, 07:56 AM

Mike,
First of all you offended your SC's sensibilities by even thinking about selling her. You deserve what you get, you heartless bastarde!

Now the problem....
Are you sure you are getting good sparks to all 6 cylinders? The timing light does not tell you the whole story.
Check the multi-pin plug to the CDI. As a matter of fact, check all the electrical connections to ignition-related components.

Good luck (you traitor).

------------------
'81 SC Coupe (aka: "Blue Bomber")
Member: SCWDP
Canada West Region PCA
The Blue Bomber's Website
"If it ain't broke...we'll help you fix it 'til it is!"

lagoon · 10-28-2001, 08:43 AM

Mike, do you have access to a dwell meter? If so, i can walk you through on how to set your base mixture setting. I think you are set too LEAN. I had the same broblem and it was the CO mixture. 81'SC

Quote:

Originally posted by Mike Juzenas:

Thanks guys!

Hmmm, I was slightly wrong on my initial assumptions.

Started the car cold this morning...its just as rough or rougher when cold. It appears the frequency valve is not operating, but I checked the resistance...its 2.7 Ohms, exactly on the money.

If I slightly lift up on the air plate it seems to smooth out.

The car does run and is drivable, but barely. It feels very sluggish and tend to backfire.

I have checked the voltage of the O2 sensor. Its very low. But if the mixture is way too rich then the O2 sensor voltage will be really low anyways, right? I took the sensor out and heated it with a torch...the voltage went to .9+ volts pretty quickly. But in the car it stays at .05-.09 volts. Pretty low perhaps because of the rich condition.

And I thought that if the O2 sensor does not work, it goes into safe mode which is a rich mixture anyways? However, the O2 sensor is not in the loop when the engine is cold, and the engine does not run properly then anyways, so maybe is not the sensor.

I am thinking of the fuel distributor. How do you know its going? What are the symptoms? How can I test it?

It really does sound like fuel but could this be an ignition problem?

Cheers,

Mike

Doug Zielke · 10-28-2001, 09:12 AM

Mike,
I'm going out to the races in Mission this morning, but you can call me at home to-nite if you need to talk about the problem. I've got some other ideas. (Don't sell the car...yet

)

------------------
'81 SC Coupe (aka: "Blue Bomber")
Member: SCWDP
Canada West Region PCA
The Blue Bomber's Website
"If it ain't broke...we'll help you fix it 'til it is!"

[This message has been edited by Doug Zielke (edited 10-28-2001).]

john walker's workshop · 10-28-2001, 11:41 AM

the domelight fuse powers the relay under the right seat. unplug the relay. did the idle drop off? if not check the fuse or replace the relay. happens all the time.

Mike Juzenas · 10-28-2001, 06:25 PM

Aha! SUCCESS! Thanks for all the help. The guy closest to the answer was John Walker. Bang on John!

It turns out that the relay that controls the frequency valve is intermittent. I took it out and tried to get it to click with a 12V source. It would click only occasionally.

I then built a small harness the emulated the relay with a switch. Plugged that in, started the car, and turned the switch on. Started running smooth and nice! Turn it off and it runs like crap.

So, all I have to do is source the proper relay tomorrow and its fixed.

Thanks!

Mike

PS: Hey Doug. How can I be a 911 traitor if I am selling my car to get another 911 that may need my help? I have almost finished my journey with this one after 7 years and its going to get boring soon with all the upgrades done...I need a new source of grief and income dispersion!