1974 S3 6-pot Stuttering and misfiring

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kyamon

kyamon

Active Member
Posts
114
Location
Lausanne, Switzerland
My engine is driving me crazy, literally...
The problems seem to go all over the place, at least from my rookie perspective. The past few times I drove it, it had huge problems starting - it was stuttering all over the place and frequently just stalled when I let go of the accelerator pedal. Once warmed up, it ran better but still somewhat rough. Distributor is new, spark plugs replaced today, leads are new, coil is new, timing set to the best of my abilities, but without a strobe. Inlet valve clearances set after I had removed the cylinder head some time ago - it did run well after that, so the head gasket should be fine, I guess. The only thing I had never touched are the exhaust valves, but since the engine has been running fine some time ago, I thought they should be ok.

I took it for a spin today with the new plugs, hoping that this would sort things out. The engine seemed to calm down after a drive of about 10 km, but then suddenly started misfiring to the point where I had explosions out of the exhaust, each time followed by dark black clouds (made other drivers happy ;) ). This happened about 5-10 times during a 20 minute drive, and I could not establish a connection with high/low rev, speed, or anything. It just happened some times. At the same time, I lost power to the point where I could barely made it up a small hill in 2nd gear.

My assumption is that I am driving on substantially less than 6 cylinders, but what could be the cause? Does this sound like an exhaust valve issue? (I have shied away from adjusting them, because they are in such an inconvenient position on this engine, but I also know that this is a very bad argument...)

Any help and suggestions are appreciated, and I am fully aware that this may be an unacceptable question, given what I have and have not done on the engine...!
 
Are your rotor arm and condenser new? Are they genuine parts? Check your points gap. Were your plug leads of good quality? For the price of genuine items replace them so you know they are good.
 
Thanks wireman - I had the distributor replaced as one piece, so rotor arm and condenser are also new. I purchased them from Craddock, part # ERC1837. Leads were also purchased there.
In the meantime I did check the exhaust valve gaps, and they all were 12 thou. Adjusted them to the specified 10, but that did not help. Having the exhaust manifold off, however, did let me take a video (engine cold). It looks like cylinder 2 (and to some extent C5) spits out black smoke while the others either do nothing or run fine. Can anyone make something from this?
 
Is the head converted to run on unleaded or are you using the lead replacement additive. The symptoms sound like burnt out valve seats.

Col
 
Colthebrummie said:
Is the head converted to run on unleaded or are you using the lead replacement additive. The symptoms sound like burnt out valve seats.
Col
Click to expand...

I got the car a year ago, basically from a scrap yard, so I dont know if a conversion has been made and am running with additive. The engine had issues at first, but I believe to have solved most of them by redoing the fuel supply, fixing the carb, and redoing the distributor (including coil and leads). It was running fine for a while but then started acting up again.
Burnt valve seats sounds scary... If I understand it correctly, that would essentially mean that the valves dont seal any longer? So in principle, anything (other than burnt seats) that keeps them from properly closing would lead to similar symptoms?

Also - the next thing will be to measure the compression. If I find that one or more of the cylinders have insufficient compression that could either be a leak either in the head gasket, through one of the valves (faulty valve seat), or around the piston rings, is that correct?
 
kyamon said:
I got the car a year ago, basically from a scrap yard, so I dont know if a conversion has been made and am running with additive. The engine had issues at first, but I believe to have solved most of them by redoing the fuel supply, fixing the carb, and redoing the distributor (including coil and leads). It was running fine for a while but then started acting up again.
Burnt valve seats sounds scary... If I understand it correctly, that would essentially mean that the valves dont seal any longer? So in principle, anything (other than burnt seats) that keeps them from properly closing would lead to similar symptoms?

Also - the next thing will be to measure the compression. If I find that one or more of the cylinders have insufficient compression that could either be a leak either in the head gasket, through one of the valves (faulty valve seat), or around the piston rings, is that correct?
Click to expand...
Re compression test, yes, essentially, that is correct. A pressure test is better, because it usually tells you where the pressure is leaking from. Have you tried pulling a plug lead off whilst it is running to try to determine which particular cylinder is playing up.

Col
 
I did pull leads and got shocked ;)
But I could not really tell a difference, so I am assuming that multiple cylinders are bad, possibly for different reasons.

Sidenote - I always assumed that pressure and compression test are the same... If I hook up a manometer I measure the pressure, but that also tells me the compression, right?
 
With a pressure test, air under pressure is pumped into a cylinder and a guage tells how long it takes for it to escape. Depending where it is escaping, you can narrow down the problem. Test is repeated for all cylinders. Not a job for the average diy mechanic.

Col
 
Ah, OK - I believe I could do that, I have access to all the required equipment. But (as you might guess) I dont think I would be able to really diagnose anything.

But I would think that if I install a manometer and crank a cylinder to get maximum compression I should also see how quickly and where the pressure leaks - or does that not work? I should get roughly 6-7 bars overpressure (or the equivalent in PSI) that then drops back to zero (slowly, hopefully) while putting out an "audio signal" wherever it leaks (hissing of some sort). I probably imagine this much simpler than it is in reality...
 
I've never done a pressure test myself, there should be some old threads on here, it was often talked about in the days when series threads outnumbered rangie and disco threads. I've never found compression tests to be very reliable. Do you think you might have damaged the new head gasket when you changed it? Them 6 pot heads are heavy and difficult to man handle on your own. How many miles has you've engine done.

Col
 
Have you converted it to electronic ignition, judging by your posts, I guess not. New condensers are not good nowadays. I converted mine and it was the best thing I did. Mine used to run rough.

Col
 
It is, of course, possible that the head gasket has slipped a bit, or has been damaged when I reinstalled the cylinder head. It indeed is very heavy, and I installed it myself with the engine inside the car... But given that it did run fine for a while I assumed it was fine.

It reads a bit over 35000 km, and given the history of the car I am assuming this is more or less correct. The speedometer reads wrong, however (ca. 20% low), so it may well be that it has been replaced with a wrong one at some point, and that the car has more miles than indicated.
 
Colthebrummie said:
Have you converted it to electronic ignition, judging by your posts, I guess not. New condensers are not good nowadays. I converted mine and it was the best thing I did. Mine used to run rough.
Col
Click to expand...

You are right, I have not made the conversion. I will look into that once I know the compression is good.

Thanks for all your help!
 
The points conversion module costs about 40-50 Euros and is a 15 minute job to replace. Probably get the cost back in less than 1000 miles in saved petrol.

Col
 
Colthebrummie said:
The points conversion module costs about 40-50 Euros and is a 15 minute job to replace. Probably get the cost back in less than 1000 miles in saved petrol.

Col
Click to expand...

I will certainly consider this! But I think it is wise to get the rest fixed first (and I am tempted to just pull the engine from the bay and give it a good general overhaul...). Do you know of a good place to get the conversion kit?
 
I think mine is called Accuspark which I bought from eBay. It cost about £35 but mine is the 4 pot. I believe it just needs resetting for 6 pot but I might be wrong. It replaces the old points and condenser. You might also need to change the coil if yours has the resister.

Col
 
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