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1987 Brougham idles rough but drives well

Started by JerRita, April 11, 2017, 08:31:09 PM

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JerRita

Hi all we have a 1987 Brougham wagon conversion w/ the 5.0 carburetor equipped Rebuilt carb, replaced intake manifold [leaking] No visible vacuum leaks found. Compression good on all cylinders, all plugs clean, wires, cap changed. Drives well but won't idle smoothly? Where to look now? Do these cars have a basic ECM? Thanks in advance JeRita
Jerry and Rita Trapani #15725 Caddymaniacs

TJ Hopland

They do have a full computer much like their EFI counterparts.  Only difference is instead of operating injectors they have limited control over the fuel mixture via a solenoid in the carb.   The rest of the stuff is just the same as an EFI car.   

One area that people seem to mess up is the timing and idle speeds.   Both have to be set to a specific setting with the computer put into specific modes.   If the process isn't followed and the factory settings are not used the computer gets confused because when it tries to make adjustments it doesn't see the results it expected so it tends to hunt around trying to find what it is expecting which it may never find because the base settings are wrong. 

At some point maybe 87?  Olds went from the electric choke back to an air heated one.  The air heated ones often seem to have issues working properly so the result is running most of the time with the choke closed.  Running with the choke closed eventually clogs the cat converter which usually lets it run ok but it runs hot and lacks power.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

cadman56

Don't know if this helps.  My 88 Brougham, 307 V8, somehow got carbon particles from the charcoal canister.  It got to idleing rough.  Had to replace the whole system from Cadillac and I rebuilt the carb myself.  My carb had the electric choke.  Car ran great after all that work. 
HTH, Larry
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville (sold)
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible (Sold)
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

JerRita

Thanks we think the ECM is acting up # 4 and # 1 cylinders die sporadically I found a working one and will swap out on Monday and see what happens JeRita
Jerry and Rita Trapani #15725 Caddymaniacs

TJ Hopland

Charcoal from the charcoal canister was a common problem in the late 70's and early 80's so its worth looking in the lines to see if there are any signs of it.  Those charcoal bits intermittently plug up the jets in the carb bowl.    The patch for that was to install an inline fuel filter in the line between the canister and carb.   

Let us know what happens with the ECU.   This system doesn't know the what cylinder is firing when, all it knows is a cylinder should be firing.  The ECU sends a timing signal to the module in the distributor which is then doing all the work to fire the coil.   The ECU thinks it knows what the timing is but that is based on someone following the correct procedure and using the correct settings.  The ECU just knows current RPM and that its saying go to 10, now 20, now 15 degrees.    If the distributor looses the timing signal it just goes to a fixed default setting which I think is the same mode it uses to set the base timing. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason