Repair Information For Rough Running Imperials

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Question from Bill (413):

After taking my '59 Imperial to a couple of mechanics who deal with antique cars, the consensus was that it needs an engine rebuild, or I will only be able to use the car to go to the corner grocery store. My other alternative is to drive the car with two feet, one on the gas and the other on the brake, or putting it in neutral every time I stop. The engine is fine in neutral, but as soon as it is pulling it starts running very rough. Even when I pulled into the parking lot of the mechanic the car started stalling out all of the time. I decided to give the car to the mechanic who explained everything clear cut, and most of what he had to say was exactly the same responses I have gotten from members on this list. He could give the car a valve job, but that wasn't going to cure bad rings, and would only make the bottom of the engine blow out that much faster.


From Dave:

Have you had a compression test done ? It is possible that the Headgasket has blown between 2 of the Cylinders causing the rough running. Hence it runs rough under load.

From Paul:

Rebuilding the engine is definitely a good thing to do. Based on your post it sounds like it has needed that all along. Bad rings are bad rings, there is no getting around that.

I am still stumped though on why it would run perfectly smooth in neutral and really rough in gear. Burned valves effect the smoothness of the motor in both scenarios, although when pulling, the roughness would be more noticeable. The engine would run poorly all the time, well at least not good ever.

It is true that when in neutral, the engine RPM would be higher, and could cover up some symptoms of rough running, but I have not heard of a case where the difference was as drastic as you describe, unless the idle speed is set way too high. That would bring about other issues, though such as transmission damage when going into gear, using both feet on the brake to hold the car at stop lights, etc.

Although I would not dispute the fact that your engine should be overhauled, I think a few more ideas about what could be causing this problem might be in order. I have driven cars that had new motors and cars that were worn out, but short of having burned valves, broken pistons, blown head gaskets, cracked blocks, or some other catastrophe, I have always been able to make an engine run smoothly with fairly minor repairs.

If it were to turn out that your car had a burned valve, it would not be the end of the world to fix it and move on. True enough, it is always advisable to rebuild the entire engine if it needs to be rebuilt, but if you don't have or want to spend the money, there are alternatives. Do you plan on driving this car everyday, or occasionally? I have heard of people having their cylinder heads reconditioned without doing the bottom end, and their cars ran fine.

To get your car to run smoothly, I think more diagnosis my be in order. Taking your car to mechanic after mechanic is going to result in them telling you what they want to do, not necessarily what should be done. It still sounds like there are vacuum issues to me. Did you replace the hose to the booster? Did you check to see if the gasket at the base of the carburator is leaking? Are you sure that the intake manifold gaskets are not leaking? Have you done a compression test?

I would be willing to bet that many of us have cars that need engine overhauls, but that doesn't mean that the cars can't run smoothly. Of course your car could have one of those "catastrophic illnesses" mentioned earlier, but I think that would be obvious.

Question from Bill (413):

I got my car back from getting its brand new brake booster, and at idle the car runs very smooth, but once I put it in gear and hold the brakes, it starts running horrible, and usually will die when I am at a stop light, unless I throw it into neutral where it runs so smooth I'm not even sure if the engine is running. Any ideas on this one? I am also having bad overheating problems again after it was recently rodded out with a new thermostat. It seems as if the engine is not cooling at all. I left my other car near the mechanics today, and was thinking of taking the Imperial over to the radiator shop tomorrow to see if they can figure out the overheating problem. If anyone knows what steps I should take next I would welcome them, since I took a week off work, and it is going to be devoted to getting the Imperial running properly once again.


From Steve:

I’m going to guess carb problems in the idle circuit but that’s only a guess… As far as the overheating goes check to make sure the thermostat is working properly and that your fan clutch still works. With the engine hot turn the car off and see if there is resistance when you try to turn the fan.

If you have A/C you should have a clutch on the fan. If you have a non A/C car then there is no clutch and the fan is bolted directly to the water pump (thus it won't budge). If you don't have a clutch that only leaves you with the thermostat, water pump or radiator as the culprits.

The only valve I can think of on the front of the carb is the choke. The choke should be closed on a cold engine and fully open on a warm engine.

From Rob:

How is your gas tank? I had similar idling problems and it turned out my tank was rusted and spitting rust and sediment into the carb. I put an additional (temporary) glass filter in the line and you would not believe the junk that must have been running through the engine. Usually the sending unit to the gauge is the first thing to go when the rusty tank process begins.

From Philippe:

One time I had this problem: everytime I pushed on the brakes, the car ran poor or died. If I released the brake pedal, it ran well. It was a problem on the brake booster (vacuum leak, the problem solved itself and never re-appeared). As you said that you install a new brake booster, perhaps take a look on this side.

From Demetrios:

Also, check for rust in the coolant. Brown rather than green coolant will get the engine hotter. Dilluting the coolant some with a bit of water will help reduce the temps some until you solve the problem.

Question from Mark (413):

I have a '60 Imperial, that has recently been put together. The car starts and idles GREAT! But as soon as I put it in Drive or reverse, the car dies! I have to contunious butterfly the gas peddle to get it to move. But it is just soo rough, when/if it will run.


From Kerry:

Have you put a vacuum guage on the car? This sounds like a major vacuum leak. Another possibility is timing but I doubt it.

From Dick:

If you want to be sure this is diagnosed correctly and promptly, have the car flatbedded to a mechanic who has an engine analyzer that can handle an old timer like this. There are many possibilities, some of which you can check yourself, but many that would probably take you way too much time to learn about and accomplish. If the engine is mechanically sound, a good man with an analyzer can get it running like a new car in a few hours.

From John:

This sounds like a bad fuel pump.

Question from Dale (413):

My 1964 Imperial Crown needs some help. Valve job? The car runs a little rough and is lacking power. I have not done a compression check, but I am assuming it will need a valve job at minimum. If I end up doing a valve job, is there more I should do when I have the engine apart?

Reply from Dick:

It is hard to guess what you might run into, but a big part of the information needed could come from the history of the car. Especially, how many miles on the engine, and what kind of previous care and repair has it had. The answer could range anywhere from "nothing" to "replace the timing set, camshaft and lifters, rocker shaft kit, rings and bearings, rebore and new pistons etc." or any of the above.

Certainly step one is to do a compression test, and step two, before you take it apart, is to do a leakdown test, to see where the leakage is escaping (intake valves, exhaust valves, rings, headgasket?).

The 413, when healthy, is the most responsive and delightful engine you will ever experience (yes, I like it better than my 440 equipped cars!), so put out a little effort in making it right, and you'll be rewarded with a real gem!

Question from Greg (440):

My '66 Imperial has started to run very rough at idle - like its going on 4 cylinders and produces a lot of black smoke. It smooths out when revved up or run over 25 mph but doesn't want to run slow. I suspect the carb but I'm not sure what to look for.


From Dan:

After it's warm check, make sure the chock is all the way open.

Does it smooth out once it is warmed up? if so, it could be the same problem I had some time ago with my '66's 440....turns out it was a bad choke-pull-off valve (it looks like a small flying saucer mounted to the side of the carb). It slowly open up the butterfly once the engine warms up, if working properly. Otherwise you get a very rough idle BUT only until the engin warms up (obviously this takes longer when its cold outside , like it is now!)

From Mark:

If the 60's choke spring is like the 70's choke spring, a good dousing of the spring with carb cleaner can help a stubborn choke too.

Question from Kerry (440):

Couple questions about my new '66.

The car runs rich but strong at speed. At idle it is very rough and lumpy. Picked up new distributor, points, condencer, wires, etc.

Timing was off a bit. Replaced dist points condencer, set to spec. Idles better in neutral but still rough when in gear. Vacuum is marginal and will not increase with idle mixture adjustment. Have a kit for the AFB but haven't done it yet.

Motor is just rebuilt by a known good local racer and sounds very strong. No knocks, rattles, or blue smoke. Will check compression tomorrow when I have the plugs out.

The plugs are champion and black with soot but do not appear to be fouled. Wires are relatively new (5 years and 500 miles)

Going into town to get new plugs. My plan is to try the plugs first, then wires, then pull the valve covers and see if the rockers are too tight. Failing all that, I'll go through the carb.


From John:

I'd get rid of the Champions for one thing & install Autolite #85's in it. Are you getting enough fuel pressure? I had a '66 that had a sometimes sticking choke pulloff that cause lumpy idle & black smoke. I'd check for vacumm leaks also.

From Dick:

Compression test should be the first step, before you spend any of your hard earned on buying parts it may not need.

Question from Robert (440):

I am having a problem with my '68 imperial. I have rebuilt the engine and I am having trouble getting it run right. It is hard to start and when I get it running it runs so ruff that it shacks the whole car. I do have two vacuum line tubes broken off on the atc in the right side kick panel will this give problems. Any help would be great.


From Brian:

Go through your plug wires and make sure that they are in the right spot. I rececently built a 440 for my 300 and it did the same thing. It took me 3 days to finally figure out that I had two plug wires mixed up. Fixed them and it ran great.

From John:

If you have any vacumm lines that are drawing vacumm, plug them. That would cause ruff running.

Follow-up from Robert:

I checked the firing order and it was right. I checked the point gap it was big so I reset the gap. Reset the timing and if I run the engine up to 1,200 rpm,s it smoothes right out. But when you set it at 600 rpm like the book calls for it shakes. I checked for vacuum leaks but could not find any.

Reply from Bill:

I recently had a rough running engine in one of my '68's, it got so bad that the engine would quit unless it was running at a high rpm. When a friend and I dug into the problem, found a leaking float in the Carter carburetor, it has now been replaced, so the engine is back to normal.

Question from Steve (440):

I have 1972 4-door Imperial with a 440. My problem is this: I can drive the car for short periods (about an hour or less) if driven any longer the car starts to run very rough and won't stay running at idle speeds. I have no clue what to do, i have tried adjusting the carb to no avail. If anyone has any suggestions, they would be much appreciated.


From Mike:

Might try changing out your fuel filters, might be crud in your tank.

From Dave:

If this car has sat for a number of years, your gas tank may be a mess inside. Chunks of gas nasties could be settling around the outlet. I had that problem with the '67 and took it in and had the tank steam cleaned. The guy said he was amazed at what came out of there. I was changing the fuel filter every other time I started the car and drove it. Didn't seem to matter how long I drove it either. But it had sat with the same gas in it for 10+ years.

From Rob:

I would second the thought that the tank might be dirty. When you changed the fuel filter was temporarily better? I had a similar problem and it turned out to be a dirty tank and a pinhole leak in the steel fuel line to the carb. Sometimes there would be no visible leak, but when I found it the spout of gas was at least 2 ' in the air. My problem seemed worse in the cold weather.

Question from Bruce (440):

After having my carb rebuilt by a carb shop that has always done well before, the mechanic there insists that there are no vacuum leaks he can find and that he suspects the rough running to be an electrical problem. The plugs, sparkplug wires, distributor, etc. were recently checked and wires and plugs replaced(by a different mechanic). Is there a way to check the lean burn computer to see if it is working properly? The car randomly and sporadically loses power and/or knocks terribly at 50 to 70 mph, then after a few seconds or after I back off the gas and step back on it, power returns to normal and knocking stops. I'm not talking about knocking on hard acceleration, which it normally doesn't do. Also, at low speed coasting, hitting the
gas occasionally causes a popping out the intake of the carb. Sometimes is blows off the air intake hose attached to the horn of the air cleaner assembly.


From Phil:

While your checking things, don't forget to check the timing chain! The way the symptom changes sounds like how timing will bounce all around with a loose chain. Easily checked by popping the distributor cap and turning the motor by hand one way, then the other. There should be almost no hesitation at the rotor when reverse direction at the crank. If there is much more than a quarter of an inch of slack at the damper before the distributor rotor reverses directions, odds are your chain is loose and stretched, or in some cases the nylon has came off the teeth. This will require dropping the pan and cleaning the pickup, luckily not that big of a job on a Mopar big block thanks to it's flat flange construction.

From Joe:

This doesn't sound like an electrical sounds like valve timing and it could be from the timing advance @ the distrbutor (although I'm not sure how the lean burn system handles timing advance it may be eletrical as in today's cars) or timing chain streach. Put an induction type timeing light on the car and fire it up and watch the timing mark. does it jump all over or stay in one place as long as the engine speed remains constant? ( If it's sporatic I'd check the chain) Rev
the engine and watch the timing mark it should smoothly clime toward advance as the ending speeds up and drop back down as engine speed decreases. If it doesn't move or jumps I start looking at the advance mechanism for wear, sticking, or electrical signal from the lean burn system.

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