Low idle and rich exhaust

wcover91 · 08-22-2014, 05:08 PM

I have a 1983 911sc that has been giving me trouble today.

She started fine this morning. I drove her out of my complex and down the road just fine. I accelerated a little quickly onto the main road, and shortly after noticed she started gurgling and backfiring upon deceleration. I then noticed she had a low rough idle, around 600RPM. She isn't dying out but idles very very low and rough. I also noticed a gassy smell and gasoline misting out of the exhaust. However, when I get her going up and over 2000 RPM she runs and pulls totally fine just as normal. There is no noticeable power loss.

After calling and speaking with the guy who built my engine, I am assuming I have a stuck open injector over flooding a cylinder on idle. He recommended his usual method of running the car and loosening each fuel banjo one at a time on the main distributor, thus relieving the cylinder fuel pressure, until one of them shows an improvement in idle. I tried this but didn't get any noticeable results.

I then began pulling injectors and visually inspecting them. I was able to check 1, 2, 3, 4, and 5 injectors. I wasn't able to get to injector 6. None of the ones I looked at seemed to be visually stuck open.

I don't think it is the spark plugs or distributor as both are relatively new and the car has been driving absolutely fantastic for the past few weeks.

Any help or places to start would be very much appreciated.

DRACO A5OG · DRACO A5OG's Garage

sounds like a vacuum leak.

when warm, pull over leave her on and remove the oil filler cap, if she does not drop in idle then you need to find the vacuum leak.

Next would be is to check fuel presuure, HB has a $20 kit, you need a 3/8" hose x 12", connect and check. It may be the Fuel Pressure Regulator or Fuel Pump, but check first before swapping.

Jim

911SauCy · 911SauCy's Garage

if no vac leak perhaps lean idle screw a hair, counter cw, and see if it rIses.

i recently tuned my cis and aside from it "hunting" a bit at idle, it would drop like a brick when ipushed in the clutch, almost stall then idle really low. once in good tune that stopped, runs great now that its leaned out a hair.

wcover91 · 08-22-2014, 05:42 PM

I tried the oil cap trick and the idle did drop even more. I think this means its not a vacuum leak correct?

Another thing to note, I know the fuel accumulator is going out. The past month or so I have had to give her gas on warm start to get her going. Once she warms up and hits her idle sweet spot it is all good. It hasn't been too bad though so I have been putting off replacing it. I don't think this could cause my current issue could it?

DRACO A5OG · DRACO A5OG's Garage

Okay, no vacuum leak.

hmmm FA? yeah that could be it, damn. other CIS will chime in.

911SauCy · 911SauCy's Garage

i change my fuel accum but it fixed a hot start issue...

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by DRACO A5OG

sounds like a vacuum leak.

when warm, pull over leave her on and remove the oil filler cap, if she does not drop in idle then you need to find the vacuum leak.

Have to throw in a caveat. The big fat hose that is plugged into the back of the air-sensor to throttle body bridge jobbie feeds directly into the oil tank. There is a restrictor in there - oil tank end of hose - that is 1/4 - 3/8" inch in diameter that is literally in the hose. That makes for a big honkin' air leak when you pull the oil cap off.

I would not say that if the rpm's drop you have no air leaks. You could have a medium air leak that is causing problems. A gross air leak would cause the oil cap procedure to not respond.

The cap'n · 08-22-2014, 06:31 PM

If the idle is low and the tail pipe smells like fuel, a vacuum leak would be my last guess. Sounds to me like a bad plug or plug wire, because raw gas in the exhaust is unburned gas in the exhaust, and a no spark condition will do that. Make it idle slowly, too. One at a time, while the engine is idling, loosen each of the outlet fittings on the top of the fuel distributor. The idle should drop on any cylinder that's working. If it doesn't drop, tighten the fitting and move on to the next one.

The Cap'n

wcover91 · 08-22-2014, 07:18 PM

I don't see why a vacuum leak would just spontaneously occur while driving? Also the car has been running completely fine for the past few weeks. And the engine had a go over and cleanup a few months back. I wouldn't count on any rotting vacuum lines.

The Cap'n, I did exactly what you described. The only thing is that not a single one of the fittings made any noticeable change in the idle speed....

The cap'n · 08-22-2014, 09:01 PM

OK, that gives us something to go by. Next thing should be a test of the oxygen sensor. Unplug it and drive the car. If it runs well throughout the range, it's fixed. Buy a new one. If it remains the same, look at the plug. If the shield is grounded to the signal wire, it'll run REALLY rich. If that's OK, then you need to do a comprehensive fuel pressure test, and that requires a cis testing rig. Might want to set up an appointment with your favorite wrench for that, unless you have access to the setup.

The Cap'n

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by The cap'n

If the idle is low and the tail pipe smells like fuel, a vacuum leak would be my last guess. Sounds to me like a bad plug or plug wire, because raw gas in the exhaust is unburned gas in the exhaust, and a no spark condition will do that. Make it idle slowly, too. One at a time, while the engine is idling, loosen each of the outlet fittings on the top of the fuel distributor. The idle should drop on any cylinder that's working. If it doesn't drop, tighten the fitting and move on to the next one.

The Cap'n

Who in his right mind would intentionally cause a fuel leak on a running fuel injection engine!!!!!! Specially a CIS (constant injection system). The system fuel pressure is more than 70 psi. and opening these banjo fittings serve no purpose except leak raw fuel on a hot running engine. What are you trying to test? There is a much better and safer method than is ill advised procedure. Sorry, I have to disagree with the expert/s.

Tony

The cap'n · 08-22-2014, 09:17 PM

Well, I've been doing it since CIS school in early 1973, and I haven't had a fire yet. What's that, 41 years? Long enough, I think, for problems to show up. It's a useful diagnostic tool, approved by the training center, and you'd be surprised how little fuel bubbles out (and at the lack of pressure) when you do this. Might want to try it for yourself before you condemn a common industry practice. Would you care to share the "much better and safer method" to check cylinder balance in a work environment? Some folks don't own a scope with running compression and cylinder balance testing.

The Cap'n

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by The cap'n

Well, I've been doing it since CIS school in early 1973, and I haven't had a fire yet. What's that, 41 years? Long enough, I think, for problems to show up. It's a useful diagnostic tool, approved by the training center, and you'd be surprised how little fuel bubbles out (and at the lack of pressure) when you do this. Might want to try it for yourself before you condemn a common industry practice. Would you care to share the "much better and safer method" to check cylinder balance in a work environment? Some folks don't own a scope with running compression and cylinder balance testing.

The Cap'n

I was an engineer/scientist in my previous life with 50 some years work experience in R & D. And this is my toy:

Tony

The cap'n · 08-22-2014, 10:33 PM

Do us all a favor. Crack one of those banjo bolts while your system is powered up and watch the leak. I think you'll be surprised, and the you and folks here can see what a handy tool it can be. A sixth or a quarter of a turn will do.

The Cap'n

mysocal911 · 08-22-2014, 10:34 PM

Quote:

Originally Posted by Bob Kontak

Have to throw in a caveat. The big fat hose that is plugged into the back of the air-sensor to throttle body bridge jobbie feeds directly into the oil tank. There is a restrictor in there - oil tank end of hose - that is 1/4 - 3/8" inch in diameter that is literally in the hose. That makes for a big honkin' air leak when you pull the oil cap off.

I would not say that if the rpm's drop you have no air leaks. You could have a medium air leak that is causing problems. A gross air leak would cause the oil cap procedure to not respond.

Air leaks don't cause rich mixtures!

wcover91 · 08-23-2014, 08:31 AM

Well, today I went to fire her up and take her over to my engine guy. The power is drastically lower than before. I wasn't comfortable making the trip. So now she sits still in my garage.

I read about the relay under the passenger seat causing issues sometimes. I have noticed that when I unplug mine it doesn't appear to have any difference at all on the idle. Shouldn't my idle get worse? Is there a chance the relay went bad, wiring is bad, or frequency valve is going out?

What about the O2 sensor? Could it have fouled while I was driving it yesterday? I never got an O2 light but who knows if that even works...

I had been working on the interior prior to all these issues starting up. Is there some chance I accidentally knocked a wire loose somewhere that would cause all these symptoms?

wcover91 · 08-23-2014, 02:59 PM

Well, I have been working on the car all day with no success.

I started with the easy and obvious. I pulled all the spark plug wires to check for any failures. They all measured in right around 4k ohms, leading me to believe they are all fine. They are in pretty great condition as well. I then began pulling spark plugs. The plugs are brand new W3's from an engine cleanup a few months back. They all looked fantastic and clean with no heavy deposits. I did notice a small chip in the ceramic on the plug going to cylinder #4. Hmm? I suppose I should replace it...

Having checked the ignition system I am now fairly confident it is a CIS issue causing the rough idle and incredibly rich exhaust gasses being spit out.

After digging around, I also don't think my car has an oxygen sensor. It has SSI exhaust on it and I don't see the typical sensor bung on the top of the drivers side pipes... It must have been removed years back durring its engine rebuild...

timmy2 · timmy2's Garage

Sounds like you need to get a fuel gauge on there and check your pressures and then use a CO meter to set your mixture once you know your system, cold and hot fuel pressures are in spec
Sound like your WUR may be acting up.

The cap'n · 08-23-2014, 08:08 PM

"W3"? Really? An '83 911SC uses a "5" heat range plug. A "3" is WAY too cold. A W3 fits like a 2.4S with MFI. You don't want, or need a platinum plug in your 911, and you don't want a plug 2 heat ranges too cold. Fix that first, then get on with the fuel pressure tests.

The Cap'n

boyt911sc · boyt911sc's Garage

Check your control fuel pressures. Do you know what WUR is installed on your engine? It should have these numbers (0-438-140-xxx) and post the last 3 digits (critical information). While we are on the subject of WUR, measure the the heating resistance (Ohms) of the WUR. Keep us posted.

Tony