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1970 Cadillac Coupe Deville engine shudder, HELP!

Tiny
CADILLAC1970
  • MEMBER
  • 1970 CADILLAC DEVILLE
Engine Mechanical problem
1970 Cadillac Deville V8 Two Wheel Drive Automatic

Hello,
Maybe you guys can help me with my frustrating engine problem on my caddy. It's a 1970 Coupe with a 472 ci. V-8 and has about 115,000 miles on it. The engine has been freshened up quite a bit since I got it including a top end re-build, full tune up, new exhaust system, etc. Recently I noticed that the engine has begun stuttering. It still starts perfectly, but seems to have this skip and shakes throughout the throttle range like it has dropped a cylinder. I did a compression test, it has ample in all cylinders. I put in a new set of lifters thinking maybe it had a collapsed lifter, but that didn't change the problem at all. I checked the cap and rotor, plugs and wires (all fairly new) and it has ample fire throughout. What am I missing here? When I changed the lifters none of them looked cupped or deformed on the bottoms so I really don't believe it's the cam. Could it possibly be a rocker or pushrod? The tips of the rockers where they contact the valve stems did look well worn. The pushrods looked straight from what I could see. I looked all over and didn't find any sort of vacuum leak or anything around the crb or intake. I'm at a loss. Any suggestions would be really, really appreciated at this point. THANKS!
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Sunday, June 21st, 2009 AT 8:42 PM

7 Replies

Tiny
BUDDYCRAIGG
  • MECHANIC
  • 2,262 POSTS
I'm betting on a bad plug.

Do a load balance test.

You can buy a dwell/tach meter at most auto part stores for about 10 bucks.
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Sunday, June 21st, 2009 AT 8:59 PM
Tiny
CADILLAC1970
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I didn't check the plugs themselves because they are fairly new AC Delcos, meaning I didn't actually pull them out to inspect them or visually check for fire. I did however take a spare plug and make sure there was fire on each of the wires. When you hold the throttle at a medium idle you can hear a ticking in the exhaust which coincides with the shudder. Not sure that a plug would account for that but at this point I'm open to anything. I'll definately check them. Any other suggestions?
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Sunday, June 21st, 2009 AT 9:22 PM
Tiny
BUDDYCRAIGG
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Forget checking how the plugs look.

Do a load balance test.
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Monday, June 22nd, 2009 AT 12:56 AM
Tiny
CADILLAC1970
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I have never performed a load balance test so I just went ahead and pulled the plugs out. All 8 were black with carbon so I bought a new set and put them in. This did not effect the shudder / missing in any way. I told you it was a frustrating problem, I'm really pulling my hair out on this one. Any other thoughts?
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Tuesday, June 23rd, 2009 AT 3:01 PM
Tiny
BUDDYCRAIGG
  • MECHANIC
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This is a how to do a load balance test.
Get the car good a warm and at an idle.
Hook up a low RPM tach (you can buy a cheap dwell/ tach from most auto parts stores for $10)

Note the RPM
Pull each plug wire off.
Note the RPM.
Reinstall the plug wire.

Do this for all of the cylinders.

The dead cylinder will show no change in RPM
while the others will drop down a little bit.
***************************
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but with all of your plugs fouled, then you have a carb problem of dumping too much gas.
Or the ignition is weak or off time.

Points and condensers are cheap. How old are they?
Maybe the coil is weak or isn't getting a good supply of 12 volts.
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Tuesday, June 23rd, 2009 AT 9:46 PM
Tiny
CADILLAC1970
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Well here is the disheartening update for today. I opened up the distributor and looked at the points, which I actually had put in when I first got the car, just forgot about it. The block that rides on the distributor was obviously worn and had decreased the gap. I re-gapped them, started the car, no effect on the shaking. I then took them out, cleaned the point surfaces themselves, which were a little bit burnt, tried it, no effect. Then I went to the store, got a brand new set, tried them, no effect. I also played around with the gap a little just to see, which didn't do anything either. I checked the timing and firing order, all are spot on. The only thing that has changed is that the exhaust smokes ever so slightly, which it didn't do before, and you can hear the ticking and sputtering in the exhaust more prominently now it seems. Am I wrong for wanting to stick a rag in this thing's fuel tank and light it? Any more ideas?
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Wednesday, June 24th, 2009 AT 8:12 PM
Tiny
BUDDYCRAIGG
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It does, it actually effects the dwell, but we wont talk about that.

Have you done any of the tests i've asked for?
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Wednesday, June 24th, 2009 AT 8:27 PM

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