Difficult black smoke problem

1980SC · 07:07 PM

I've been dealing with an annoying black smoke problem with my euro 1980SC ever since I've owned it. I'm running out of things to try. At full throttle from 5,000rpm - redline it blows a thick black cloud of smoke. Every gear, every time.

System and warm control pressures are perfectly in spec.

K-basic, so no lambda system is involved.

I've hooked a voltmeter up to an O2 sensor. I get .92-.94 volts at full throttle. That corresponds to about 12.4:1 air/fuel. I get the same reading with two different O2 sensors, so I feel that is accurate. That's actually not too far from a good power mixture, but I can't get the car any leaner.

I've had the CO level adjusted on a sniffer multiple times. I've also adjusted the mixture screw so lean the car will hardly run. Same black smoke, same rich O2 sensor readings.

I've varied system pressure from 70 - 85 psi. Same smoke, same O2 readings.

Plugs of the proper heat range stay black and sooty.

Seems like the car just goes too rich, and I can't stop it.

At my monthly autocrosses, I often have people asking me what's wrong with my car? Why does it blow black smoke?

Could the black smoke just be oil? I use plenty. Up to a quart in 300 miles. Some of that leaks out. Nice light blue smoke can be seen when I lift through portions of a autox course (valve guides), but dark black smoke when I'm on the throttle. Many of the above observations leave me to believe the smoke is fuel.

Fuel consumption is not that bad. Consistent 20-21 mpg.

Timing advances correctly. New rotor and cap. Wires are of unknown age, but the car exhibits no drivabilitly problems. Old wires creating a weak enough ignition that all the fuel can't be burned at high rpm?

I have not been inside the CIS system yet. Bent sensor plate?

I've thought about a leaky cold start valve, but it seems odd it would cause black smoke only at full throttle, high rpms...

The car runs great, I'm just sick of the black smoke following me around. Anyone have any ideas?

Let me add that the engine is stock, and has no broken head studs. I've also adjusted the valves recently.

Thanks,
Rob
1980SC

dean · 08-05-2002, 07:16 PM

I had a strange problem with my 930 motor. It ran great except at light throttle, high rpm. Then it wouldn't run. It turnd out to be the WUR. It was something I never would of supected. So I would guess the WUR or the fuel dist. I never got 21 mpg with my SC. I could only get 19 mpg. Now with the 930 motor in it I get 14 mpg. I would suggest swapping the WUR with a known good one and see what happens
Good luck

1980SC · 08-05-2002, 07:22 PM

Thanks for the response Blown,

I actually have two WUR's, and have spent quite a bit of time inside them getting my control pressure where I want it. I've learned all about them, how they work, and these are good.

Thanks !

Rob
1980SC

Superman · 08-05-2002, 07:24 PM

I don't know the solution but I think you can stop thinking it may be oil. It's fuel. I have had so much trouble with ignition wires over the years that I would recommend they be replaced. Seriously, I've seem A LOT of bad ignition wires. Found two on my car last week. And although ignition trouble is easiest to sense at idle, ignition systems work 100 times harder at 5000 rpm than at 1000 rpm. If you're losing a cylinder or two at high rpms, then your engine would blow black smoke. I'd replace the cap and rotor too. if you do, and these were the problem you'll know it before you look in your mirror.

Superman · 08-05-2002, 07:29 PM

When you said system and control pressures are perfectly in spec, I figured the WUR is fine. Still do. I have a new handful of those screw tips for the ends of the ignition wires, and I'm planning on using a solder method rather than the crimp method to attach them. Should be fun. If you need the part number for those little guys, I have it.

dean · 08-05-2002, 07:34 PM

I was so stumped by my problem that I was considering dumping the CIS. I still want to because I don't like it.

What about the fuel head? Can you swap that out?

I don't think it is ign. because you said that it runs good.

1980SC · 07:54 PM

Thanks Superman. I probably will change the wires soon. I think I'll buy an oem set though so I won't have to worry about whether I properly assembled the wires or not. What you said about losing a cylinder or two at high rpm makes sense.

Blown,
This makes me want to pull my CIS and chuck it into my neighbors back yard. I'm sure eventually I'll figure out the problem though. I do not have access to a spare fuel head. I thought a clogged or sticking fuel distributor usually meant you wouldn't get enough fuel, but it could very well be the problem.

masraum · 08-05-2002, 09:11 PM

I'm with superman. black smoke is gas. so either you're running rich (which you don't seem to be normally, or you are not burning the gas once you get up to 5K. I would think cap, rotor, and wires, and then I'd check the timing advance. I would think that if the timing was advancing too much or not enough(more likely I think) you might not be burning all of the fuel, and the car would seem to run fine at the other rpms.

So, I'm not sure of the cis car's setup, but I would check mechanical advance >5K and also vacuum adv if those cars have that.

beepbeep · 08-06-2002, 02:21 AM

CIS sucks...repeat after me "CIS sucks" ....

As soon as i stumble over enough money, that junk is going out...i'll try Megasquirt DIY MAP-based injection system with lambda sensor.

Superman · 08-06-2002, 08:12 AM

There are those who would disagree. A properly working CIS system makes as much power as any other system and is as reliable as any other system, as long as you put clean gas through it. IN fact, a properly running CIS system will start and run as smooth as silk regardless of temperature, altitude, etc. etc. My CIS systems over the past twenty-five years have virtually never failed to start in less than one engine revolution.

dickster · 08-06-2002, 10:10 AM

just i thought, how does it run from 5k to redline?

if its still pulling strong you probably haven't got a dead cylinder.

sounds to me like theres just too much fuel going in constantly. when you first floor the throttle you should get a little black smoke (just a puff) as the mixture is rich, but that should disappear rapidly.

if its generally running ok, it sounds fuel related to me. (assuming its definitely black soot on the plugs, and not oily). if you drive around at less than 5k do the plugs still soot up?

just mho.

Bob W. · Bob W.'s Garage

A clogged fuel filter will cause the system to go rich. At idle the pressure will be correct because only a small amount of fuel must flow through the filter. As the engine speeds up, more fuel must flow through the filter and if it is restricted (clogged) the pressure on the outlet side of the filter will drop, causing the CIS system to go rich. You should be replacing the fuel filter in a CIS system at least at every tune up.

Bob W.

1980SC · 01:08 PM

Thanks for all of the advice guys.

The engine does run very well at high rpm's. If I didn't have a sideview mirror, I wouldn't know anything was wrong.

Fuel filter, along with the pump, are new. The fuel tank screen is clean.

I will replace the wires soon, and if that doesn't stop the smoke, then I'll start pulling CIS components apart.

Thanks !

Rob
1980SC

wckrause · wckrause's Garage

Have you tried clamping off the vacuum lines to the WUR to see if you still get black smoke?

Seems like that would prevent WOT enrichment. Maybe your WUR is too sensitive to vacuum. I've never seen specs for control pressure vs. vacuum for a WUR.

Just a guess.

1980SC · 01:49 PM

That's good thinking Bill, but it seems to work opposite of that.

My warm control pressures :

Vaccum line attached 51 psi.
Vaccum line off 45 psi.

At idle and cruise the engine's vaccum raises the control pressure, and leans the mixture slightly. At full throttle, vaccum decreases, and control pressure is reduced, thus richening the mixture slightly.

If the vaccum line were blocked, then control pressure would remain at 45 psi, and the mixture would stay rich during idle and cruise, while remaining unchanged at full throttle.

As you can see with my particular engine, there is only a 6 psi difference in control pressure.

The specs are as follows :

Vaccum line attached 46-53 psi.
Vaccum line off 39-45 psi.

wckrause · wckrause's Garage

Ah, I see. You don't think that extra 6 psi of control pressure drop at WOT is causing the black smoke?

Could you block the vacuum lines, re-set your mixture, and then see if you get black smoke?

Where did you get the specs for control pressure vs. vacuum???

1980SC · 03:17 PM

I'm not saying that the extra 6 psi isn't causing the black smoke, I'm saying there isn't anything I can do about it. At atmospheric pressure, my WUR creates 45 psi control pressure. In order to increase the control pressure those 6 psi, I need vacuum applied to the WUR. That vacuum is only available at low throttle inputs. Once I go to full throttle, I lose the vacuum.

I've already set the mixture so lean that the car could no longer be made to run at idle. Still blew black smoke and unburnt gas according to the O2 sensor.

The specs are in Bentley's manual. p 240-14.

I'm leaning toward bad spark plug wires as the cause...

1980SC · 08-06-2002, 03:46 PM

Okay. If I remove and plug the vacuum line, the idle mixture will be too rich. I can then lean out the mixture screw, and leave the the vacuum blocked off. This should eliminate the full throttle enrichment function of the WUR.

I'll try it tomorrow.

911X2C · 08-06-2002, 05:10 PM

Check your fuel pressure regulator. Use a fuel pressure gauge to monitor the pressure throught the RPM range.....then watch the pressure at the higher RPM's. How is the pressure building up? You should not see any rapid spikes in pressure buildup. If you do, it may indicate a bad fuel pressure regulator. Also check voltage to to fuel pump, verify upper limit is within specs.

If that is not the problem, then check your injectors, ideally you would want to run a flow test on the injectors. It could be some of your injectors are stuck open at the higher RPMs, and/or possibly leaking too. Also look at your plugs, I suspect the ones that are getting extra fuel being dumped into the cylinder would be easy to tell.

Good Luck

Lee

Early_S_Man · 08-06-2002, 05:45 PM

It doesn't take very long to test the spark plug wires!

1. Do an 'end-to-end' resistance check. 3000 Ohms to 4000 Ohms is the expected range.

2. In addition to a static test ... just sitting on the bench, you need to do a lateral 'shake test.' Unscrew the wire from the Beru connector. FIRMLY gripping the test leads AND the Beru connector ... shake the connector and leads up and down in front of you, vigorously ... while an assistant watches the multimeter for any momentary 'opens.' The Beru connectors can be intermittent, and that is the reason for the shake test!