76 eldo rough idle

[hotrod095]

I've seen this posted here on the forum before and yes, I know there could be many reasons for this but I want to just narrow it down and rule out common issues. Checked and replaced all vacuum lines. The idle mixture screws were set as per shop manual. I put parking brake down, blocked wheels, put car in D and plugged hose for the parking brake release before setting screws. It seems to like a richer idle mixture. Pulls 21mm of vacuum with slight vibration 1mm hg. I also notice loss of power at the low end 1-2000 rpm. Car has 100k miles all original.

Tried propane to find a vacuum leak.. so far nothing. Every cylinder causes a drop in RPM when I pull spark plugs, so it seems like a global air/fuel issue. Distributor cap and rotor, plugs and wires are one year old with about 1-2k miles. Carb rebuilt last year too. Vacuum advance on distributor when pulled off makes idle worse. Could it be from a clogged EGR valve or timing chain issues? Any past experience is greatly appreciated.

[TJ Hopland]

Do the mixture screws seem to be fairly responsive?   If they are that is a good sign that you are not dealing with carb issues or vacuum leaks.     If not have you tried disconnecting or at least pinching off various vacuum devices to see if maybe one of them has a leak and is taking more vacuum than its supposed to be?   

The level ride compressors can do that.   The compressor I think is usually teed off the PCV line.    You have hydroboost brakes so its not the brake booster leaking.   Is your AC compressor still in place?   If not one of the rear mounting bolts goes into one of the runners so that would need to be plugged.     

I don't remember what the charcoal canister setup was like on these but some of them had a control valve that could get stuck so it was constantly sucking from the canister. 

If the EGR was leaking it would have an effect on idle but leaking isn't a common failure mode on these.  Its actually more common for them to plug up. 

I have been into several of these engines that have a kinda bad to incredible bad build up of carbon and goo on the back of the intake valves.  That goo kinda acts as sponge and absorbs the fuel.   It happens a lot on these engines because the valve guide seals crack and fall off and they don't tend to get driven that hard so it can't burn off.   

Timing chain you can get some clues by either pulling the distributor out and looking down the hole or rocking the crank back and fourth with the cap off observing low long it takes the rotor to change direction.   

Its interesting that you think the performance is kinda flat.   Whats the age of the exhaust system including the cat?   A plugged exhaust usually doesn't bother the idle much but will kill the performance.   Its very common at the age of these things to have internal failures in the muffler that lets the packing get loose and block the flow as well as increased chances over the years that some critter tried to move in. 

[MaR]

The quickest way to check for slack in the timing chain is to just look at the timing with a timing light. If it's wandering around, chances are that the chain has slack.

[hornetball]

Just as a reference point, these are the issues I found on my '74 (~80K miles) when I went through it:

1.  Timing chain had slack.  It had the original nylon timing gears.  They were intact, but developing cracks.
2.  Camshaft had 3 wiped lobes.
3.  I had rocker arm/valve tip wear on a few cylinders.  One was REALLY bad and traced to a blocked pushrod.
4.  The cruise control vacuum switch on the brake pedal leaked.
5.  The vacuum thermal switch on the front of the driver's side head leaked.

BTW, I prefer to use a vacuum gauge to set the idle screws.  I think the FSM procedure is designed for emissions compliance.

[Caddylackn]

Check to make sure that the mechanical advance weights are not stuck, springs are not missing, or the holes in the weights are not egged out.
If the springs are too weak you are getting some advance in around idle or well before it should come in. 

[hotrod095]

Mixture screws are very responsive. If this were a vacuum leak it would have to be very small which makes it more frustrating to find. I took off the ride level compressor and now have Gabriel shocks in back. I plugged off the hole leading down the runner with an adjustable rubber stopper last year.

I am going to an old school mechanic this week for some steering work and will mention this to him. I think hornetball is most likely correct, wiped cam lobe sounds feasible too. I have a loud lifter tick that improves with warm up and hard driving conditions. Timing chain probably is original too. I would hate to pull the balancer to check without diagnosing. Can a timing light be used to see if there is slack first before taking anything apart?

These cars should idle smooth as glass and have tons of low end power. I think if timing is in range and there is no vacuum leak I am looking at an engine rebuild soon.

[MaR]

Mixture screws are very responsive. If this were a vacuum leak it would have to be very small which makes it more frustrating to find. I took off the ride level compressor and now have Gabriel shocks in back. I plugged off the hole leading down the runner with an adjustable rubber stopper last year.

I am going to an old school mechanic this week for some steering work and will mention this to him. I think hornetball is most likely correct, wiped cam lobe sounds feasible too. I have a loud lifter tick that improves with warm up and hard driving conditions. Timing chain probably is original too. I would hate to pull the balancer to check without diagnosing. Can a timing light be used to see if there is slack first before taking anything apart?

These cars should idle smooth as glass and have tons of low end power. I think if timing is in range and there is no vacuum leak I am looking at an engine rebuild soon.

I had one wiped lobe in my '74 Eldorado (~50k miles) and most of my rockers were worn as well as I had about an inch of slack in the timing chain. I replaced the cam, rockers, pedestals, and the timing set and now the timing is rock solid and the valve train is very quiet.

[Steve Lomas]

I'd 2nd TJ-  plug or pinch all the vacuum lines to eliminate those-  my ride leveler was leaking internally (75 eldo)-    fascinating engineering in there but it looked like the diaphragm for the air pump was made of cardboard..

and this maybe obvious but if it seems like it is idling rich-   is that both cold and warm? are the choke pull-off and electric choke working properly?-   I always found the electric choke had to be moved a couple notches off the center mark so that the throttle would be just barely closed when cold.

[hotrod095]

Pinched vacuum lines coming off of manifold... nothing yet. Also I noticed the choke closes as it should when cold and opens gradually. Vacuum to choke pull off seems to be working as well without issues. Fast idle system works as expected, too. This engine pulls 22mm steady vacuum so it is definitely not dying by any means, but I can't seem to smooth out the idle. Any other ideas??? At this point I am still thinking vacuum leak because the engine seems to run better with a richer idle screw mixture.

[79 Eldorado]

If you're planning to check the timing and your timing light is one which can read RPM I would read RPM on each wire (place the pick-up coil on the wire you want to read instead of just the nr 1). I had one bad wire in a new set and I caught it by checking each wire. The bad wire read about half the RPM as the others. That's probably not the issue but it's super easy to do if you have the light out anyway.

Scott

[hotrod095]

SOLVED: Just wanted to thank everyone for all their help. I pinpointed the issue and narrowed it down to a small vacuum leak coming from the intake manifold bolt hole that bolts down the AC compressor bracket. The bolt hole has stripped threads, most likely due to over-tightening from a previous mechanic's work. I have an expandable rubber stopper in for now, but plan on just covering the hole up permanently. This vacuum leak was causing Cylinder 2 to misfire and run lean. She runs very well now and has plenty of power without any hesitation. 

[76eldo]

That's great!
Thanks for posting the solution.
My 76 Eldorado convertible also has 100,000 mikes on it and runs great and idles smoothly. That's the way they should be.
Glad you got yours fixed.

[bcroe]

The quickest way to check for slack in the timing chain is to just look at the timing with a timing light. If it's wandering around, chances are that the chain has slack.

I would add, move the timing light 4 plugs down the timing order,
and it should read just the same.  NOT, time to work on that. 
good luck, Bruce Roe

[MaR]

SOLVED: Just wanted to thank everyone for all their help. I pinpointed the issue and narrowed it down to a small vacuum leak coming from the intake manifold bolt hole that bolts down the AC compressor bracket. The bolt hole has stripped threads, most likely due to over-tightening from a previous mechanic's work. I have an expandable rubber stopper in for now, but plan on just covering the hole up permanently. This vacuum leak was causing Cylinder 2 to misfire and run lean. She runs very well now and has plenty of power without any hesitation. 

I bet I know which one is stripped. I have a stripped hole in my intake for the AC compressor bracket also.

[hotrod095]

I have seen this posted about somewhere else on this forum years ago. IMO it's a very poor design choice to put a bolt into a runner like that. The wall of the manifold is only about 3/8" thick and there is not much to grab onto the bolt.