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1979 Mark V - Rough Idle, Hard Start....

Hi Bill,

Fantastic site, what a great resource with all your tips and tools!

I have been having a rather difficult problem with my 1979 Mark V with 70K original miles that I am desperate for help on. Long story short, my Lincoln does not seem to hold the adjustments after the carburetor is tuned. It will run like new then after a little while, will revert back to the same problem and we cant figure this out. It seems to do this in a matter or hours and goes back to the rough idle, hard start and will bog when pulling away from a stop sign. It seems this will only happen once the car is hot after stop and go around town driving.

The vacuum system lines have been checked and replaced as there where a leak was found in the engine compartment. I also had the fuel pump and filter changed. No holes in fuel supply lines, but car will behave as if its not getting fuel and/or a vapor lock kind of behavior. I have had the carburetor rebuilt and even bought a re-manufactured carburetor as well, still same issues.

I have the complete set of manuals and bought the extra wiring and vacuum diagrams as well. I supplied them to three different repair shops that have been using them to get this issue resolved to no avail. Plugs and wires have been checked and have no faults. Here is my laundry list of items below that have been repaired/replaced in an effort to solve this issue.

Re-torque intake manifold bolts (one was loose on driver side and sucking in air)
Ignition module
fuel pump (3 times)
fuel filter
EGR valve
Smog pump
Replaced both catalytic converters
New 3 row Radiator (also added electric fans to help with vapor lock symptoms)

Two of the three shops want to push for an electric fuel pump and swear its vapor locking. I know that would just be making the true problem. I have read the Internet till I am blue in the face, I see suggestions for a plugged Evap canister, temp sensor, faulty brake booster, carb float sticking, faulty vacuum switch and faulty timing advance. I am out of options and way over budget on repairs for this beauty.

Any ideas on what would cause a car to tune out great, then loose it?



AKA Givenchy in Glendale CA


Hello Joseph -

When you are a car lover, problems such as you are having sure can ruin your day or your summer. Not having your Mark V here for diagnosis I can only make suggestions of course based on what you have checked, repaired or replaced so far and it appears that everything has been replaced or inspected as you state. If the engine runs fine when tuned and then reverts back to bogging and roughness when at full hot engine operating temperature I would immediately suspect carburetor flooding. If the carburetor at that point (hot engine, rough idle and bogging) can then be adjusted back to a smoother idle with the mixture screws it could be flooding.

If you and your technicians suspect a fuel delivery problem or vapor lock, a fuel pressure and flow test must be performed as per the manual. This test must be performed on your engine after the poor engine performance begins. Therefore you must be prepared beforehand to do the test. We in some cases temporarily install a fuel pressure gauge to the engine and observe the gauge as the engine warms up. This type of diagnosis can uncover suspected problems or eliminate them. In your case the gauge would be observed carefully at the exact time when the roughness appears. These tests must be done Safely with the fuel lines properly plumbed and secured. These 400cid engines are not known to suffer from vapor lock but with the poor fuel that we are compelled to use today, vapor lock could be considered as a possibility.

You also report that the plugs and wires were checked and have no faults. Are they original and were they tested hot? If they are old or an off brand product I would replace them.

One other symptom that you mention is hard hot starting. This hard hot start condition can also be caused by a slightly flooded engine as well as a vapor locked fuel system. This makes me again turn towards a possible carburetor problem that delivers a mixture that slowly becomes too rich for the engine and slightly floods when the engine is hot. A cold engine requires a rich fuel mixture to operate properly when cold. This mixture must lean out as designed as the engine warms to operating temperature. A hot engine will run poorly with an over rich mixture. It is the carburetor's responsibility to deliver the correct mixture. If the choke is operating incorrectly or if the float circuit is faulty or maladjusted etc., the carburetor could eventually deliver an excessively rich mixture to a hot engine.

Electronic ignition systems when heated are also known to fail when hot and cause similar rough symptoms to a hot engine but with all of the new ignition components installed on your engine as stated ( interior distributor electronics and ignition module etc.) it is difficult to point a finger towards those items at this time. It is tough to consider that a new ignition part would have the exact same fault as the replaced one but it is not impossible. At Lincoln Land we have the luxury of being able to substitute "good known parts" temporarily for test purposes in some cases. Someone such as myself may also for tough problem situations drive your car daily until it is fully corrected.

To sum up, you appear to have considered and done everything but obviously something is not right. If an item is suspect it should be "properly" tested and proven faulty if possible ( no guessing ) therefore I would backtrack and carefully examine for a fuel issue first, especially from the carburetor with the engine at hot operating temperature. Keep us posted and let us know what you find.