1976 Eldorado missfiring problem

[Lukas S.]

Hi Everyone,

I'm a new member to this forum and a mechanic, i'm helping good friend to put his 1976 Eldorado Convertible V8 500 back on the road, the car was sleeping for many years but i manged to refresh the carburator and start the engine.
Unfortunately the car is idling and running bad, i've checked the compression and it is quite good on all cylinders approx. 10bars , spark plugs was replaced not long time ago, cables are poor quality but i did test with additional spark plug in each of the wires while cranking and i saw a spark on each cable, from inspecting the spark plugs after running a bit the engine i've noticed sparks on cylinders 3,5 and 8 were wet with fuel, other plugs burned dark color, so i think there is a problem on those cylinders (3,5,8) any idea or suggestion how to find the problem will be appreciated, thanks

["Cadillac Kid" Greg Surfas 15364]

I might suggest changing the plug wires.  They often look good but break down internally.  Ask me how I know.
Greg Surfas

[TJ Hopland]

Its a dual plane intake.    1,4,6,7 is the lower and 2,3,5,8 the upper.    You said 3,5, & 8 are wet?   Those are all on the same plane.  I think the even side is the upper and odd is the lower so look for things that could be making things uneven like vacuum devices or leaks especially things that are more on one side or the other of the carb or intake.   

Things like the level ride compressor could be stuck and causing a massive vacuum leak.   One of the AC compressor bracket bolts goes into an intake runner so if they ac has been removed there could be an open hole.  Charcoal canister purge could be stuck on and purging too much.  Also check the vacuum hoses from the canister for bits of charcoal.  The canister could have failed and you now have bits of charcoal in your clean carb.     

5,8,3 are also are all in line in the firing order.   

Note that these engines are physically laid out flipped from other American V8's of the era,  The odd side and #1 is on the right side of the car.     That can often cause issues with people looking at incorrect diagrams or not looking at all.

Another note is these had the plastic covered cam sprockets which at this age are more likely to be bad than good.   You can see enough of it by removing the distributor and looking down the hole or with a bore scope through the fuel pump hole.   You can also rock the engine back and fourth by hand while observing the distributor rotor for backlash.     

[V63]

In addition and Often overlooked with long term storage vehicles ...  Make sure you have fresh fuel!

Check distributor Internals for corrosion.

[Lukas S.]

Its a dual plane intake.    1,4,6,7 is the lower and 2,3,5,8 the upper.    You said 3,5, & 8 are wet?   Those are all on the same plane.  I think the even side is the upper and odd is the lower so look for things that could be making things uneven like vacuum devices or leaks especially things that are more on one side or the other of the carb or intake.   

Things like the level ride compressor could be stuck and causing a massive vacuum leak.   One of the AC compressor bracket bolts goes into an intake runner so if they ac has been removed there could be an open hole.  Charcoal canister purge could be stuck on and purging too much.  Also check the vacuum hoses from the canister for bits of charcoal.  The canister could have failed and you now have bits of charcoal in your clean carb.     

5,8,3 are also are all in line in the firing order.   

Note that these engines are physically laid out flipped from other American V8's of the era,  The odd side and #1 is on the right side of the car.     That can often cause issues with people looking at incorrect diagrams or not looking at all.

Another note is these had the plastic covered cam sprockets which at this age are more likely to be bad than good.   You can see enough of it by removing the distributor and looking down the hole or with a bore scope through the fuel pump hole.   You can also rock the engine back and fourth by hand while observing the distributor rotor for backlash.   

I've started this job from measuring the vacuum, and it was very weak, i removed the manifold, cleaned it and since i don't have easy access to new metal gasket underneath i cleaned it as well and used victor reinz high temperature seal maker to make sure there is no leak between the cylinder head and manifold, vacuum improved, i've checked also and sealed the bolts which as you mentioned are after ac compressor etc, than i've attacked the carb and i think this is where the problem is, i reassembled it already 3 times and had to cut out my own gaskets from "firefly" as the kit will arrive in some weeks. So here is my question, can a leaking carb cause problems like this?, i have to add that i'm still getting poor vacuum on idling (there is almost no idling) when keeping the engine revving the vacuum is in perfect range but engine still not firing properly, there is a vacuum operated valve, sort of EGR at the back of manifold, can it be also causing vacuum leaks ?

[Lukas S.]

In addition and Often overlooked with long term storage vehicles ...  Make sure you have fresh fuel!

Check distributor Internals for corrosion.

Yes, fresh fuel, and checked and cleaned the distributor, the thing is i'm getting the spark, as the engine starts but looks like few cylinders are literally flooded with fuel, smell of the fumes killed all mosquitos in 5km range

[The Tassie Devil(le)]

G'day Lukas,

You have a second email that I sent that requires your attention.

Bruce.

[TJ Hopland]

The EGR ports are pretty evenly distributed so I would not expect that to only effect one side.  A minor EGR leak would have a greater effect on idle.   Disconnect the vacuum line going to it to make sure its not being activated, at idle it should have no vacuum and be closed.   The can get something stuck in em and leak a bit but it wasn't that common of a problem.

Since its all on one side there does seem like a really good chance its an issue with the carb.  Can you tell if the needle and float are working?   Maybe its not for some reason and its just dumping fuel in there when its running.  Not sure why it would favor the one side but if you had to improvise gaskets that isn't likely helping.   Also when making up gaskets there is a chance of missing some hole that is or isn't supposed to be there.   As you saw there are tons of passages that go from between all 3 main sections so its pretty easy to have 2 passages connected that are not supposed to be or one blocked.

Since the wires are suspect I assume you have a new set on the way?    How about new plugs?  If you don't have new ones try swapping with cylinders that are working?

And since (seem to be using that word a lot) its a flooding issue at this point I would be leaving it sitting with the plugs out to increase airflow and let the gas that  is in there now evaporate. 

[79 Eldorado]

Hi Lukas,
To me it sounds like it could be a needle and seat issue. I don't know the 500 well but the way I interpreted the TJ made, intake design having different planes, to say a stuck needle and seat dumping fuel could favor certain cylinders. I've had needle and seat issues a couple of times. Once the rubber tip of the needle had taken the form of where it was sitting in the seat for an extended time of storing the car. It worked fine, for a couple of days, until it didn't. I was able to see fuel over flowing through the top of the carb. When I took it apart I could see the form the tip of the needle had taken. Another time I had a small sliver of metal embedded in the rubber tip which prevented sealing against the seat. I was relieved to actually find a root cause.

Regarding the spark plug wires I've used a trick to determine if a single wire is bad while running which doesn't involve disconnecting anything. I've mentioned it before but I think it's worth describing again. I use a timing light with an integrated tachometer. Take the pick-up which normally goes on the reference wire for setting timing (#1 cylinder on most cars) and, instead of just the reference wire, move it from wire-to-wire and note the RPM. I once found a bad wire in a brand new set. In that case the RPM on that wire read about half the RPM of the other wires. The symptom was simply running rough.

One other thing I've found is once you glaze a spark plug it's sometimes difficult to start the car short of changing the plugs. The vehicle I was working on turned out to have a lower heat range plug than it should have though so that was likely a contributing factor.

With respect to plug selection, and availability, it would probably be a good addition to ask which plug you are using and which plugs are good solutions today. Again I don't know the 500 but there was an extensive discussion on plug gap in general for HEI and out of it my conclusion was GM was experimenting with gaps during early HEI ignitions and chasing fuel economy. At some point the factory gap got pretty small and a lot of the documentation including service manuals showed values which resulted in a lot of warranty and trouble; gaps too small. My car is a '79 and I've always used ~0.060" (1.52mm) gap and I've never had any issues but someone connected to the actual GM HEI situation and development who was referenced from a source outside the forum seemed to be recommending 0.080" (2.03mm). I'm not making a recommendation for the 500 I'm just suggesting possibly that should be part of this discussion because in some cases the original recommended plug isn't even available.

Pozdrawiam,
Scott

[bcroe]

If that car has the original timing chain set, I would change it out with a
new (all steel, no plastic) true roller set.  Timing is everything.  Bruce Roe

[TJ Hopland]

Yes the stock intakes on the 472/500 were dual plane and his issues are all on one plane.   

The quadrajet only has one needle and float so its not like a Carter AFB that had 2 and they are left and right vs. front and back like a Holley.    If it was overflowing its possible that its just happening to all end up in one side.   A clogged jet or passage is much more likely and would only effect one side.  If you look at how long and complex both the primary and even secondary passages are on these its crazy that they don't have more issues. 

[Koa]

Hi TJ

do You have any picture to this:
"Its a dual plane intake.    1,4,6,7 is the lower and 2,3,5,8 the upper."  ?

[TJ Hopland]

This is the Edelbrock but its the same as the stock one and a bit easier to see especially at the angle this photo was taken from.    Front of the engine is the left, rear right.   Top row L to R is 1-3-5-7   Bottom is 2-4-6-8.   IF you look in the carb hole you can see the floor in the top section and then follow the runners.  You can see which runners are higher and lower.    The factory intake has the same layout but its sort of squished down into the V more so that many of the height differences are sticking down into the V rather than sticking up high like this one.   Likely was done for hood clearance of the FWD Eldo that came out at the same time and they didn't see any reason to have a taller intake on the other models. 

["Cadillac Kid" Greg Surfas 15364]

Lukas,
I will again suggest replacing the plug wires.  A bad wire will show a spark when you ground it, but when it is connected to the plug inside the combustion chamber the wire will not flow sufficient current to igniter the plug.  In your first post you say the wires are "of poor quality"  These wires age and should be replaced periodically.
Just saying
Greg Surfas

[TJ Hopland]

Other thing about the plug wires is the 3 problem ones are all in line in the firing order so maybe its something like those 3 wires were against the radiator hose and got cooked or just rubbed on something.

[Koa]

To TJ:

Do You mean, its essential for the wires to be separated like this (picture).

And
what is the 100 % way to proof the wires? When can I decide, the wire is good or ready to replace?

[TJ Hopland]

I was just pointing out that the 3 cylinders that were identified as having issues happen to be in line in the firing order so that may be another clue, look for something that could have damaged those wires where they are all together which would be near the distributor.     The areas highlighted in the photo are fairly typical where they get grouped together and look to have a fairly typical separator clamp sort of thing so I don't see a problem there. 

There really isn't a 100% for sure test a normal person can do on plug wires.  Movement and heat eventually start to damage them to the point where all the way to the spark plug isn't the most attractive path for the spark so it either doesn't make it or it takes a detour along the way. 

One thing to remember about the spark is jumping gaps is just one of the things it does so small breaks in the wire don't stop it at first.   Think of the spark as a little athlete that wants to get to the finish line which is the spark plug in this case.   As long as it can see the other side so to speak it will make the jump. 

Moving along a unbroken path causes very little damage or wear to the path and doesn't take any extra energy from the athlete so that is the ideal way to get to the finish line.  If there are gaps no big deal especially narrow ones our athlete can jump them but it takes a little more energy and does a little damage to the launch and landing site. 

Each jump causes a little more damage which may increase the gap.  As the gap increases it takes that much more energy out of our spark and causes more damage faster.   As the gaps increase there gets to be times where maybe our athlete doesn't quite make the gap and splatters near the landing site.  Next one maybe makes it but slips on the mess and doesn't make it.   Maybe the next one makes it but after slipping just doesn't have the energy to make it to the finish line.   

Points systems had a limit of what they could handle and create so how strong an athlete you could get was limited so it didn't take that much to stop them.   Electronics like HEI got us much better athletes that could overcome more obstacles.   In the 80's when we started to get rid of distributors that eliminated the high jump from near the starting line so there was a lot more energy still available at the finish line.    This meant you could have some pretty terrible spark plugs and the engine still ran pretty well.         

[Daryl Chesterman]

A YouTube video on testing spark plug wires.

https://www.youtube.com/watch/4_EFGAFxi1w

I believe you could use the multimeter and test the wires while they are in their respective loom holders by disconnecting the spark plug end and the distributor cap end of each wire and testing them while installed on the car.  If the misfiring is consistent, the checking under the hood after dark is a good way to see if there is external arcing of the spark.

Daryl Chesterman

[Steve Lomas]

..could well be more than one contributing problem- so just a couple of other things to check if not mentioned already

choke pull off-  has rubber vacuum hose and diaphragm that could be bad causing flooding
also the electric choke- might need adjusted (turned clockwise a couple of notches) to open properly and avoid running rich.
break booster line? basically anything with original rubber

TJ mentioned the ride leveler also- its just in front of the blower motor- the leak can be inside and easy to miss

[TJ Hopland]

76 Eldo should have a hydroboost brake system so one less place for a vacuum leak unless however the factory blocked the port on the back of the carb has failed or maybe someone swapped carbs and didn't notice there wasn't a line to connect there?