jerky

[retep]

hi to all the members of this forum. thanks for letting me join. ihave a 88 fleetwood brougham. the problem iam experiencing is that at a speed between 40-50 mph the car starts to buck and run rough. the trouble seems to be confined to this speed range. any help would be appreciated. thanks

[Cadillac Jim]

This is the 307 cid engine with distributor and 4-bbl carburetor. This car has an on-board diagnostic system that GM introduced in 1981 with Cadillac. It will have diagnostic codes that you can retrieve that will help us understand what is happening with your car. The method of getting the codes can be found on this page:

http://myweb.accessus.net/~090/81cfb.html#tc

Without these codes, I would use this information from your post:

• The problem is hesitation and stutter (engine miss).
• The problem is not evident at any throttle setting below 40 mph.
• The problem is not evident at any throttle setting above 50 mph.

If any of this is incorrect, let us know. You can eliminate the obvious by checking for very old distributor cap and rotor, spark plug wires, or spark plugs, and replacing as recommended in your owner's manual.

The mixture is controlled by three systems in carburetors -- idle, mid-range, and wide-open throttle (WOT). This speed range is characterized at WOT by a change in the mixture systems in the carburetor and possibly by opening of the vacuum-operated secondary throttles.

The fact that the problem doesn't occur below 40 mph at WOT and your checks of distributor cap and rotor, and wires and plugs, eliminates ignition as the likely cause of the problem, other than sticky centrifugal spark advance, which is a rare problem. There is some possibility that carbon deposits would cause this problem if 87 octane gasolines are used, but this isn't likely. Thus it seems that the problem is most likely sticking of the mixture regulation mechanisms in the carburetor. The vacuum-operated secondary isn't likely the problem if you don't see the symptoms above 50 mph.

We are left with a sticking internal carburetor mechanism that controls the mixture from idle through WOT, probably a sticking Carter WCFB needle-jet piston or equivalent component. Others may offer different opinions, but I would offer this advice:

1. Even though your car has an 8.0 compression ratio and is quite happy with 87 octane gas, put in a full tank of premium gasoline for the extra carburetor-cleaning additives.
2. Add an appropriate amount of STP Gas Treatment for your 25-gallon tank capacity.
3. Drive out this tank of gas in one or two days, preferably in a speed zone of 55 mph (100 kph) or higher. If you have a lead foot in you, this is the time to let it live, within legal limits.

[retep]

hey jims thanks for such a detailed and prompt reoly. this car has the blink system for reading fault codes. i will check all the things you suggested. one question. if it were an electrical problem, would the problem be speed sensitive? thanks again.

[Cadillac Jim]

A weak or intermittent spark problem can be speed sensitive. The mixture could be a little leaner in that speed range, particularly if you have a bad oxygen sensor or other sensor issue that prevents the fuel injection from running in the closed-loop mode. Or, arcing plug wires or misfiring plugs can give more problems when the centrifugal spark advance hasn't fully advanced the spark.

[KHE]

hey jims thanks for such a detailed and prompt reoly. this car has the blink system for reading fault codes. i will check all the things you suggested. one question. if it were an electrical problem, would the problem be speed sensitive? thanks again.

Simultaneously press and hold the OFF and WARMER buttons on the climate control panel - that sequence will enter diagnostics - you should see a -188 display (segment check) and then codes will be displayed on the climate control panel. When you see .7.0, the system is awaiting further input. Either press AUTO or turn off the ignition to exit diagnostics.

The speeds where the problem occurs may point to the torque converter clutch attempting to engage. Also, it may be exhaust gas recirculation (EGR) related - as there should be no EGR at idle or WOT but at partial throttle/cruising speed, there will be EGR into the air/fuel mix. If the spring on the EGR valve is weak, it may be allowing too much EGR and roughness may result.

[retep]

hey jims thanks for such a detailed and prompt reoly. this car has the blink system for reading fault codes. i will check all the things you suggested. one question. if it were an electrical problem, would the problem be speed sensitive? thanks again.

Simultaneously press and hold the OFF and WARMER buttons on the climate control panel - that sequence will enter diagnostics - you should see a -188 display (segment check) and then codes will be displayed on the climate control panel. When you see .7.0, the system is awaiting further input. Either press AUTO or turn off the ignition to exit diagnostics.

The speeds where the problem occurs may point to the torque converter clutch attempting to engage. Also, it may be exhaust gas recirculation (EGR) related - as there should be no EGR at idle or WOT but at partial throttle/cruising speed, there will be EGR into the air/fuel mix. If the spring on the EGR valve is weak, it may be allowing too much EGR and roughness may result.

to KHE and JIMS 97, hey fellas thanks alot. will look into all your suggestions. youall sure know your stuff. thanks again. p.s. jims i originally come from pt. pleasant, nj

[PAUL T]

hi to all the members of this forum. thanks for letting me join. ihave a 88 fleetwood brougham. the problem iam experiencing is that at a speed between 40-50 mph the car starts to buck and run rough. the trouble seems to be confined to this speed range. any help would be appreciated. thanks

Doesn't the carbureted 307 have a TV cable going from the transmission to the carburetor that controls the shift point? If so it could need adjusting or be binding. Does it feel like a shift problem?

[retep]

hi to all the members of this forum. thanks for letting me join. ihave a 88 fleetwood brougham. the problem iam experiencing is that at a speed between 40-50 mph the car starts to buck and run rough. the trouble seems to be confined to this speed range. any help would be appreciated. thanks

Doesn't the carbureted 307 have a TV cable going from the transmission to the carburetor that controls the shift point? If so it could need adjusting or be binding. Does it feel like a shift problem?

[retep]

hi to all the members of this forum. thanks for letting me join. ihave a 88 fleetwood brougham. the problem iam experiencing is that at a speed between 40-50 mph the car starts to buck and run rough. the trouble seems to be confined to this speed range. any help would be appreciated. thanks

Doesn't the carbureted 307 have a TV cable going from the transmission to the carburetor that controls the shift point? If so it could need adjusting or be binding. Does it feel like a shift problem?

hi paul, the problem seems to be a spark or fuel problem. however i did take jims advice and added a can of stp gas treatment and a tankful of hi test. i think theres a definite improvement as the trouble is not happening all the time now. i was wondering if i could get an explanation on how the auto. choke works in this car. is it mechanical or electrical? thanks for your interest.