How To Diagnose And Repair A Miss In Your Imperial's Engine


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Question from Anthony (1956):

I recently purchased a 56 imperial. I had it shipped in from California to West Virginia. i wanted a rust free southern car. I am having one problem. I have rebuilt the brake system converted it to silicone. I changed the oil and transmission fluid. Rebuilt the carb, new points plugs wires, changed the coil and ballist resistor, dropped the gas tank cleaned it etc.. I am having a problem with a miss under load. I checked the compression It is 145 lbs at all cylinders with one being 135. The shop manual states that it should be 150 to 180 lbs. I have put three sets of plugs. I have run one tank of gas through. In this fill up I put octane booster, lead substitute and 8 oz of marvel mystery oil. Does anyone have any ideas before I have the heads removed?


From Phil:

I see that you don't list having changed the cap or rotor. It could be, if you have a worn cap, or a crack or carbon track somewhere in it, that a plug is not firing, and is shorting out under load. One other thing, if the miss is just when you hit the throttle, it could be that your accelerator pump in the carburetor, is malfunctioning. Remove the air cleaner, and take a look down in the carburetor, while you pump the carburetor one or twice with the engine not running. It should give a good squirt of fuel. If not, maybe the accelerator pump linkage needs adjusted. Just cause the carburetor was rebuilt, doesn't mean it's working properly. 

From Kerry:

Can you put it in gear with the brake on and rev the engine to make it miss. If so you can figure out which sparkplug is missing. DO NOT stand in front of the car while in gear, no matter who is manning the brake pedal.

From Roy:

If the engine it self is sound, one of two things only can course a miss under a load:

1. fuel system = A dirty fuel filter, low fuel pump pressure or wrong metering rods in your carburetor.

2. electrical system = plug fouled, bad plug wire, coil getting to hot braking down. I first start with the electrical. At idle, one at a time pull each plug wire at the distributor to see if one makes NO change in the missing (removing one at a time and replacing it).  If one makes no change, then you have a bad plug or wire. Also with your carburetor choke set at a high idle at night (dark condition) look to see if the plug wires are sparking or shorting out from being grounded. 

Now knowing that the timing and points are correct, you find no trouble, then it must be a fuel problem. Check fuel pump pressure with a pressure gage (should be around 7lbs), change the fuel filter, clean the little fuel filter in the carburetor.

If you got the car second hand, they may have changed the carburetor or used parts from another carburetor and could have miss-matched or installed other METERING rods. Hopefully, they may only be out of adjustment. To correct them or check them, remove the two screws on the top cover and the cover at the top front area of the carburetor, now with the choke off and at high idle throttle rod closed, push down on the SRING T assembly, holding it with your finger using your other hand JUST start to pull back or open the throttle rod and as you do, you will see the metering rods lift (if not, loosen the large set screw and adjust it so it does just starts up as you pull on the throttle rod). That's it for adjustment of the metering rods (if the rods are correct for that engine).  As a matter of fact , I always loosen the screw and re-adjust them any ways.

Follow-up from Mikey:

Nice post, Roy.  Yes, you can adjust the metering rods on a Thermoquad, and the process does involve the threaded adjuster that moves both rods up or down. That DOES NOT apply to afb or avs carbs, the metering rods and the vacuum pistons they are actuated by have no mechanical adjustment, though you can get afb strip kits and replace rods and jets with different sizes.

Question from Chad (1972):

I have been having problems recently with my 73 Imp. It is a daily driver and I drive it 80 miles round trip to work every day. The car runs great while it is still a little cold. It has a new carburetor with electric choke which works great. Once the car has warmed up completely (about 10 miles or so) it will start to miss upon acceleration. I only notice this at speeds below 35 or so (most of my commute is on the highway)

At first I thought it was a leak in my AutoTemp cause I would usually turn the heat on after the car warmed up and this is when it would happen. Today I drove to work without turning on the heat and it ran great for the first 10 miles and then when ever I would stop for a toll booth or traffic it would idle and accelerate rough. I recently installed new plugs and wires. I made sure they were all on tight and have the new cap and rotor but have not yet put that on. Someone suggested the cap was the culprit. I don't think a worn out cap would only behave this way when the engine was warmed up.

Does anyone have any suggestions or ideas?


From Jim:

The ignition system is always a good place to look for the causes of acceleration problems. I have had them idle and accelerate terribly due to just one fouled plug of one bad wire. A good auto mechanic's text book or maybe even the shop manual will give you the procedure and what to look for in checking the plug wires with an ohm meter. It could be any of lots of ignition problems.

I once saw a 69 have idle & acceleration problems just because my Dad had Sears change the plugs and they forgot to tighten them. Sometimes its the obvious.

I also saw a 74 have similar problems because the EGR valve was disconnected. I couldn't believe it. It seems that the engine needed the EGR to dilute the air/fuel charge and "cushion" the fuel burn. It acted as if the timing was radically advanced and stumbled all over itself. Put a new hose on the valve and it ran fine.

Years later, the same car developed a vacuum leak. It ran fine as long as the choke was on but once it heated up: terrible idle and stumble & bogging when it tried to accelerate. This particular instance was due to a leak at the intake manifold gasket but the same thing can be caused by a bad vacuum hose (especially the PCV hose) or a leak at the carburetor base. Anything that leans out the mixture when the choke isn't "choking" can do it. A decent vacuum gauge and a chart on how to interpret what the needle on the gauge does will show you if its a vacuum problem.

From Jay:

When I owned a '66 Crown it too had a miss at or around 35 mph as well, only under light to moderate acceleration. The miss was not noticeable at higher speeds.

The problem turned out to be the #5 & #7 spark plug wired were coupling, causing a cross-spark condition which was noticeable at low speed.

The solution was to not run the #5 & #7 plug wires near each other. Eliminate any parallel runs between the two wires and if the wires must cross, have them cross as close to 90 degrees from each other if possible.

I ended up running the #7 plug wire down the passenger side of the engine and along the firewall to the #7 plug. (the #7 wire is long enough to accommodate this routing) That seems to solve it for my '66 Imperial. Strange that my '62 Crown does not suffer the same condition even though the #5 & #7 wires are close ot each other. ??

Follow-up question from Chad:

That is what I had initially thought it might be since I had just redone the wires and made them look all nice and neat. Well I tried rerouting them so that none of them run near each other at all. That did not solve the problem so I guess I will check the heat riser as I think that might be the culprit? Does anyone have the passenger side exhaust manifold that I can send out and have the heat riser taken out of?

Reply from Leo:

If it only starts to miss after it's warmed up, the culprit might be a tired ignition coil.

Reply from Bob:

You mentioned not yet replacing the distributor cap. This should be done - I've had other cars where poor running was traced to a bad cap that was arcing when hot or damp. Hard for me to imagine how a heat riser could cause a miss.

Reply from Brooks:

Leo's right -- and don't overlook the condenser.

Reply from Dick:

If the heat riser is stuck in the closed position, so that all exhaust from one bank is routed through the passageway under the carburetor, it could cause boiling in the carburetor which would make the car run terrible, after it warmed up enough to overheat the carburetor. This would also show up as all or most exhaust coming out one pipe (assuming the car has dual exhaust) after it is warmed up.

Question from Kerry (1973):

Well, I'm approaching stumped. My 73 runs like crap and I've messed with it until I'm about out of ideas.

Here are the symptoms:

Timing set at 10 degrees BTDC with the Vacuum adv disconnected and plugged.

Vacuum gauge shows about 17, it used to be about 20.

I have adjusted on the carburetor and timing and made little change in the vacuum.

i have disconnected and plugged ALL vacuum lines with NO change on the vacuum gauge.

The car runs right now as well as it's run recently WITH THE BREATHER OFF. With the breather on it is very FLAT at high RPM and has a slight bog if you punch it at idle. Turning the breather lid upside down and letting more air in helps but not as much as removing the breather completely.

Vacuum leak you say. I've tried the starter spray, propane trick and get no RPM or engine noise change.

Compression is 130 per cylinder.

Now for the interesting stuff, under some conditions of timing, I can screw the idle jets all the way in and the car still runs which makes me think vacuum leak. Also the car will not idle down below about 600 RPM even with the screw completely backed out.

Plugs are about a year old but probably don't have a 1000 miles on the, same with wires, rotor button, and cap. Cap does not show signs of damage or wear.

At this point, the only think I can think of is that there is a vacuum leak around the intake manifold, probably on the bottom where I can't get to it.


From Leo:

Have you checked or replaced the PCV valve?

From Bob:

Sounds like you have accurately diagnosed the problem. I had a 413 that did the same thing. I replaced the manifold seals, and it was fixed.

From Allan:

Have you tried the old spray carburetor cleaner around the manifold to see if it increases RPM ? Have you checked to see if you have the right PCV valve? Try blocking off the PCV and brake booster lines and see if it runs any better. What carburetor is on it? Sounds more like you are running rich if you give it more air and it runs better. Have you checked for a plugged exhaust or collapsed muffler ( not likely but I've seen it happen) If you have a Thermoquad, I've seen these have a bowl problem that causes this, we worked on a state troopers car years ago that had this type of problem. Funny thing about that one was that it ran better at night than during the day. Drove us crazy till we changed the carburetor, even though we had rebuilt the old one a couple of times.

From Steve:

If you can screw the idle jets all the way in and it still runs then either the engine is idling fast and bypassing the idle circuit or you have an internal gas leak somewhere. I have had a very similar problem on a '65 GM carburetor where the float sunk a little bit allowing the gas to dribble into the intake. The problem was much worse With the air cleaner on the engine because it increased the vacuum at the gasket between the top and body of the carburetor.

From Phil:

If you can screw the idle jets all the way in without stalling the car, then you have an internal gas leak in the carburetor. Seeing that a vacuum leak allows more air into the intake system, leaning the carburetor by running the screws in, should make it quit idling sooner, instead of later. Either way, a car shouldn't idle with the screws all the way in, which makes me think that something has gone wrong in the carburetor.

From Dick:

The way to tell if you have a vacuum leak is to remove the air cleaner and then, with the engine idling, slowly put your hand or a piece of cardboard over the air horn such as to restrict the flow of air into the carburetor. Do this very slowly, progressively increasing the air restriction, while listening to the engine. If at ANY point between no restriction at all and stalling the engine, you hear an increase in RPM, or a smoothing out of a rough idle, you have a vacuum leak for sure. So try that, and let's hear your result before we go any further with the diagnosis.

From Pete:

This sounds like the dreaded Holley 4160 problem. What carburetor does the car have. Your description sounds familiar -- especially the part about the engine continuing to idle with the idle mixture screws run all the way in.

Question from Mark (1974):

Any insight into this condition would be greatly appreciated; and i have had other Mopars from the 70's that acted the same way..

Every once in awhile my '74 imp will start running somewhat rough; not bad enough to stall, just idling rough. If the a/c is on it becomes more pronounced...after a brief period, it quits and the idle smoothes out and it drives beautifully.....then it may start again, etc....

Is there a fix to this or something I just have to learn to accept?


From Dietmar:

A leaner mixture CO measured- and a warmer sparkplug may change a lot.

From Elijah:

Two possibilities come to mind (there may be others, of course)...

There may be some rust or other particles of crud in your gas tank. Once this stuff makes its way into your carburetor (yes, it is possible for stuff to go right through your fuel filter), it will clog up the idle jets, but then may pass on through -- this would cause the temporary rough idling. The best thing to do is to take the top off of your carburetor and see what's sitting in the fuel bowls. If there's any kind of dirt, grit, or rust, that's probably your problem. (Side note -- once got a used gas tank from Arizona, 'cause it was rust free. Started having the same problems your describing, and found that my carburetor was full of sand! Cleaned the carburetor and gas tank, and problem solved).

A second possibility would involve the fuel lines. You'll want to check ALL of the flexible rubber pieces of hose throughout the fuel lines. If you find pieces that are old, you should replace them. In fact, it's probably not a bad idea to just go ahead and replace them all anyway. Over time, fuel hose can develop small pinhole leaks. Depending on where the hose is located, you will either lose some fuel, or have a condition where the fuel pump is actually pulling air into the fuel system, which again will do odd things to the idle.

From Kerry:

I'd change the spark plugs. fought similar problem for 6 months on the 73, dang AC plugs were the culprit. Under load would not fire consistently. Worst case is dirty hands, sore back, and a spare set of plugs.

From Bill:

I had this problem on my '59, to the point I could see the hood wobbling from side to side, but I think my problem was my tranny fluid shooting up through the dipstick tube, and spewing out all over the spark plugs, and fowling them. Since I installed a clear hose over the dipstick tube which dumps the fluid back into the transmission, and not all over the engine, this problem has almost disappeared, and the rough idle is barely noticeable anymore. It also does not bog out like it used to before, during acceleration from a stop.

Question from Rolland (1982):

I need help to identify the cause of a low speed miss in the engine of my '81 Imperial with the factory EFI.

The engine misses at idle. The engine feels rough and when you listen at the exhaust there is a definite miss that sounds like exhaust blowing past the exhaust valve. It appears to continue to miss up through about 1500 rpm (my estimate) then smooth out. At hard acceleration and road load above 20 mph or so the engine seems to be hitting on all eight.

When I pull the plug wires one at a time, at idle, the engine will slow a little on all cylinders except #2 on the right bank. Removing the plug wire on this cylinder does not appear to affect idle speed.

When I remove the spark plug on #2 right it does not appear to be a typical fouled plug. It has a nice golden brown center insulator with perhaps a bit more soot than normal around the face of the housing. (this plug was new less that a thousand miles ago).

If I connect a spark plug to the #2 right wire, lay it on the exhaust manifold and run the engine a steady pink/lavender spark is observed.

So far I have: 

1. Replaced the spark plugs 

2. Replaced the spark plug wires 

3. Replaced the distributor cap and rotor. 

4. Checked and adjusted slightly, the gap between the distributor pick up coil and the shaft rotor. 

5. Checked compression on all cylinders - 125 psi plus or minus 10 psi on all cylinders. Nothing unusually high or low on #2 right. 

6. Removed both valve covers to observe for worn cam lobes. 

7. Installed a second new wire and spark plug to #2 right. 

8. Removed the EGR valve hose and observed no change. Then removed and inspected the EGR valve. (It is located between #2 and #3 right.) 

9. Added a quart of Rislone to the crankcase on the outside chance that it was a stuck valve.

All of the above have had no detectable impact on the miss. The car has 90,000 miles, takes very little oil, and has run well except for the usual EFI quirks.

My current thinking is that I may have a vacuum leak at the intake manifold gasket where #2 right cylinder feeds, but before I remove the intake manifold (or drive the car off a cliff) I would like to tap the expertise of the IML gurus. (driving it off the cliff has a slight edge at the moment).


From Mike:

I would suspect a lean condition on one or more cylinders. Because it works well under load and at higher revs, the ignition probably is not the cause. The fact that the #2 cylinder seems to have no effect at idle points toward mixture in that cylinder.

To check for the intake manifold vacuum leak, you could carefully spray some starting fluid in the vicinity of that intake runner while the engine is idling. If there's much of a vacuum leak some of the starting fluid will get through and the idle should change noticeably. Also apply a vacuum gauge to a manifold port and see fi the needle vibrates excessively. This is a multi-port fuel injection system, right? If so, one of the injectors might be clogged.

From Bryan:

I would suggest if you must spray a flammable fluid on the engine, to help isolate the problem, I would use a much less combustible fluid than starting fluid. Last winter, I had a similar situation, and we sprayed a little carburetor cleaner on the intake and then next minute I had a customers engine momentarily covered in flames...I've done it many times over the 15 years I've been in the business, but I'll think twice before I do it again. It's kind of like burning out a short.......

From Dick:

A way to do this is to use an unlit propane torch, set for very low gas flow. If you pass it near a leak, the intake vacuum will pull it into the engine and the engine will respond. You can pinpoint the leak pretty well with this technique, and even if something does light it off (which I have never had happen), you'll only have a small finger of flame to deal with, and you can quickly pull the propane torch out and shut it off.

Addition from Jim:

Attach a rubber hose to it and that keeps the bottle out of the way of hot and moving parts. It's called a propane enrichment test. SAP for diagnostics.

From Doug:

I know that this may sound strange, but if your car has a Throttle Position Sensor, which it might, you may want to check and/or replace it.... My grandfather owned a '97 Dodge Shadow for years, and repeatedly took it back into the dealer because of the exact problem you describe... Except for the #2 cylinder not changing idle. Then again, it was a 4 cylinder, and any unplugged wire makes a miss..... The dealership never could figure the problem out, even when pulling the computer codes. My parents bought the car after my grandfather passed away, and dad tried for a year or so to find the problem, then asked me for my advice. I listened to what he described, looked up the code that the computer displayed, and suggested that he look at the TPS.... He couldn't get a good reading on it, so he checked prices. Too expensive through his local AutoZone to try as a "maybe"... I checked local here (about 1.5 hours away) and AutoZone had the same part for about 1/2 the price. We swapped out the TPS, and all is well.

Might be worth a look.

I agree with Byran, you might not want to use a highly flammable liquid, and if you do put something down that will burn, do the following. I have always used an unlit propane torch to "feed" propane into areas where I thought there might be vacuum leaks, etc...

1) Do it outside. 

2) Have someone there to turn off the engine right away if things get wild. 

3) Wear leather gloves, goggles, etc. SAFETY FIRST - We all want you safe. 

4) Have a coat, blanket, etc.. handy to smother any fires. - Don't use the garden hose... It won't put out a fuel/gasoline fire... - Just makes it worse. 

5) Have a fire extinguisher handy in case the blanket won't put it out... 

If you have ever cleaned up after an extinguisher has been used, you will want to try #4 first.


I spent the day replacing the intake manifold gasket and this corrected the problem. The gasket had pulled away at the top of the intake port and was drawing in air to the extent that the mixture was too lean to fire at low speed. The car now idles great without the puff from the exhaust and the rough idle.

I tried the propane test for air leaks and did not get results. It may be that my torch was running low on gas or maybe I didn't hold it near the manifold long enough. (Propane being heavier than air scares me). You never know where it is going to settle and ignite.

I did spray a contact cleaner near the number four intake port and the engine slowed down considerably. This was my clue that the gasket was bad. I sprayed it on several of the other ports and nothing happened. The cleaner had some combustible contents for the cleaning but the propellant was carbon dioxide which is probably the reason the engine slowed when the CO2 got into the manifold.

In any case after two months of chasing down the miss the car runs fine again

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