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"Guess" that car problem.

Started by carl888, 27 September 2014, 11:39 PM

carl888

I thought I'd start a thread regarding diagnosing car problems, inspired by my own inability to fix my W116!  See if anyone can work this one out.  I ended up fixing the problem, but it took a lot of digging!

Case #1.

1976 Ferrari 308 GTB
Carburettored with the original points type twin ignition.

Problem:
Missfire at idle whilst in the warm up phase.  ie, drops a cylinder intermittently for the first three minutes of idle after start up from cold.  When warm, no issues.

Car came to me after the following had been performed:

Caburettors rebuilt, float levels set, idle circuit good and EG analysis confirms the engine is healthy.
Camshaft timing checked and valve clearances set (They were tight)
Plugs replaced, points gap set, coils replaced, distributor caps, rotor and leads replaced.  Old plugs look great.
Flies through a compression and leak down test.
Not consuming excessive oil.

Any guesses?

gf

could it be new plugs are different/ too cold- fouling them till they are hot enough?

KenM

Got to be ignition related rather than fuel, and there endeth my useful input I'm afraid.

koan

Everything tight in the distributor(s}?  Points and condenser screwed down, no loose connections.

Timing stable during warmup?
Boogity, Boogity, Boogity, Amen!

carl888

Here we go:

The car had a failed inlet manifold gasket on the front bank.  When cold, the inlet manifold allowed a small amount of water in between the water/air galleries under the manifold to be ingested by the engine.  When it was warm, I suppose it just expanded to the point where it sealed up!

You could hear the cylinder drop but the water consumption was such that the level decrease wasn't noticed.

Some pics:







And all back together:


wbrian63

What a beautiful engine....

If you'll permit me to toss one out to the group - nothing so grand as a miss on a Ferrari, but one that taught me a good lesson:

1972 Ford Pickup - F250, 360CID motor. When cold, press the accelerator once to set the choke, turn the key and it fires up after just a couple of revolutions. Idles around 1,000rpm until the choke pulls off. During that time, engine runs as smooth as one would expect for an American V8 of this vintage. Once it warms up fully and the choke is off, there's a slight miss at idle (about 600rpm). Consistent and coming from one cylinder. Step on the gas to bring the revs up off of idle and the miss goes away.

Finally figured out the problem quite by accident - what was the cause of the hot idle miss?
W. Brian Fogarty

'12 S550 (W221)
'76 450SEL 6.9 Euro #521
'02 S55 AMG (W220) - sold
'76 450SEL 6.9 Euro #1164 - parted out

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter V

gf

 I  wouldn't have guessed that one in a hurry! How long did it take  to find out about the leaking gasket?Now we have the answer to the problem - what's the best way for testing for this?

carl888

Quote from: wbrian63 on 31 October 2014, 06:46 AM
What a beautiful engine....

If you'll permit me to toss one out to the group - nothing so grand as a miss on a Ferrari, but one that taught me a good lesson:

1972 Ford Pickup - F250, 360CID motor. When cold, press the accelerator once to set the choke, turn the key and it fires up after just a couple of revolutions. Idles around 1,000rpm until the choke pulls off. During that time, engine runs as smooth as one would expect for an American V8 of this vintage. Once it warms up fully and the choke is off, there's a slight miss at idle (about 600rpm). Consistent and coming from one cylinder. Step on the gas to bring the revs up off of idle and the miss goes away.

Finally figured out the problem quite by accident - what was the cause of the hot idle miss?

yeah the little 3.0L V8 looks great with the airbox off.

Your issue is a hard one, what about a vacuum leak somewhere?  Loose spark plug?

carl888

Quote from: gf on 10 November 2014, 07:48 AM
I  wouldn't have guessed that one in a hurry! How long did it take  to find out about the leaking gasket?Now we have the answer to the problem - what's the best way for testing for this?

You may catch it by checking the water consumption, in this case, the 308 takes 13 liters so a few drops here and there take a while to notice.  I was pretty frustrated with it and in desperation pulled the carburettors off to have a peek at the back of the valves.  And there, sitting on the back of one of the inlet valves was a drop of water.  Just a fluke I looked really!

It took about a week to find, spending about an hour a day on it (Had to check from cold each time) but had I done the other work already performed first that then I would have been another 40 hours down the road with it.

wbrian63

Quote from: carl888 on 11 November 2014, 06:18 AM

yeah the little 3.0L V8 looks great with the airbox off.

Your issue is a hard one, what about a vacuum leak somewhere?  Loose spark plug?
Bingo - vacuum leak - a BIG vacuum leak - from the power brake booster.

The power brake booster pulled its vacuum via a port cast right into the intake manifold runner that fed the driver's side rear cylinder. With the choke on, the vacuum was nice and high due to the restricted air intake. Once the choke was off, then the large vacuum leak in that runner disrupted air/fuel flow to that cylinder, causing it to have a lean miss.

Oddly, the brake boost was just fine - never had a problem stopping the vehicle.

How I discovered it was quite by accident. Stopped at a traffic light one day, my cell phone rang. Buried in my pocket it was, so I had to unlatch the seatbelt and raise my hips off the seat to get the phone out. When I did so, I pressed extra hard on the brake pedal. When I did so, the miss went away.

I noticed it immediately. Just to make sure I wasn't imagining it, I released the pressure and the miss returned. Pressed hard on the brake pedal again - same results - a nice smooth idle.

The previous owner and his mechanic had chased this problem for ages and had finally given up. Since it was only missing at idle, there were no indications on the spark plugs.

Replaced the brake booster with a rebuilt unit and no more issues.

The BIG lesson learned here was to always check for vacuum leaks when chasing a hard-to-find miss at idle. I spent literally THOUSANDS of dollars to rebuild a 429cu in V8 in a 1966 Cadillac Fleetwood because it had a miss at idle. Same symptoms - no miss when cold, miss on one cylinder at hot idle. Shortly after I spent all that money and reinstalled the engine, the diaphragm in the power booster ruptured completely (hard brake pedal all the time). No proof that the booster had the same issues as the one in the Ford, but I'll always wonder if I really needed to spend all that money for something that was completely fixable for just a few hundred dollars.
W. Brian Fogarty

'12 S550 (W221)
'76 450SEL 6.9 Euro #521
'02 S55 AMG (W220) - sold
'76 450SEL 6.9 Euro #1164 - parted out

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter V

carl888

Jees that's a good find, well done.  Funny how after a rebuild you think, I wonder if that $2 parts was really the issue lol.

carl888

Spending a fair amount of time around 1970s Italian cars I've seen some horrendous things, mainly work performed when the cars were worth nothing, a few examples, see if you can have a guess:

1. 1971 Lamborghini Espada. V12
Light detonation under load but on the left bank of cylinders only.  This was a curly one.  (Hint, slightly higher compression on the left bank)

2. 1976 Lamborghini Urraco. V8
Excessive oil consumption but engine was fresh from a rebuild and produced not only good power but good compression too.  A leakdown test wasn't so good bearing in mind the fresh engine (That's the hint).

s class

For the Uracco, did they omit the oil ring, or reuse the old one?


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL