Misfire from cylinder 7

Phxpep

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Im having some problems with cylinder 7 misfiring. Ive changed spark plugs coil wires checked injector conectors where should i look next. The misfire is is hardly noticeable in park the min you put in drive it shakes. Engine seems to gather itself around mid to high rpm. 97 town car
 

Town

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Welcome to the Forum.

You must have had a DTC from the OBD2 system pointing to cylinder 7 so do you remember the code number? Was the check engine light flashing when the misfire (shaking engine is more than a misfire and likely more than one cylinder) was occurring? The flashing light would indicate detonation which is prone to do damage to your engine due to the fuel being fired before the spark is generated.

A fuel injector clicks when it pressures fuel into the intake. You can buy a noid light that fits the injector and the electrical connector. The light flashes when injector is fired so you can see if the circuit works. The other approach is to listen for the click with a screwdriver held to your ear while the metal end is pressed lightly to the injector body listening for the clicks that will occur at idle.

When you removed the spark plug from cylinder 7 at time of spark plug replacement, did it look different to the others?

When coming off idle do you notice any hesitation? Does your cruise control not maintain a stead speed on highway? The throttle position sensor can wear and not give a steady resistance to the PCM that can result in hesitation and as throttle is opened farther result in sudden engine acceleration. At cruise there may be a loss of throttle position and car speed hunts to find a steady rate but cannot.

Any water or coolant in the area that could get into the plug well?

Have you removed spark plug from cyl 7 to see what condition it is in?
 

Phxpep

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P0307 is the code check engine does not flash. Car shakes pretty bad at idle in drive but in park feels fine. I started it up this morning and it started perfectly fine felt fine I put it in drive and there was nothing wrong I drove it maybe an eighth of a mile and then the shaking came back. It was pretty violent I drove it for about 25 miles and the shake is hardly noticeable now and it does not seem to be dumping a lot of fuel I'm averaging around 17 miles per gallon. Have not checked the spark plug since the last time I changed it but the last time I changed it it did look like maybe the back 2 on the driver side might be running a little Rich but whenever I did the spark plugs there was about four of them in other cylinders that look bone dry and maybe to have some corrosion like they weren't getting enough fuel.
It almost looked as if on both sides the back plugs were running richer than the front. I was wondering if it could be the filter or the ecm seeing how its worse at times than others.
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Phxpep

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I also swapped the injector from 7 to another cylinder to see if the problem followed but it stayed in #7 i can see the spark tof the dist. Can hear the injector working
 

Roberto Gomez

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I also swapped the injector from 7 to another cylinder to see if the problem followed but it stayed in #7 i can see the spark tof the dist. Can hear the injector working
Have you checked the cable set that connects to the coils?, they could be having a short or they might be getting hot while driving.
 

Phxpep

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So i was driving home i accelerated from a light and the check engine light started blinking i pulled over restarted the car no more blinking but still shaking
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Phxpep

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I was going to go with a new fuel pump next im concidering injectors maybe that one is not closing correctly or somthing
 

Town

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Ottawa Ontario Canada
Fuel pumps are expensive and require dropping the gas tank to replace. So you need to get a fuel pressure reading before going that route. It should be around 45 psi or so.

Your description of the shaking occurring when the transmission is moved from Park to Drive is interesting. The torque converter puts a load on the engine when going into Drive so the PCM increases the engine rpm through the IAC (Idle air control) valve. These valves get carboned through use and stick so they don't move properly. Does engine speed increase on a cold start and slow as engine warms up? Ford's advice is to replace a sticky IAC and from my experience cleaning does not work for long if at all. I think you need to replace the IAC to fix that shaking problem.

It seems in your case the miss can also be detonation (when the check engine light blinks) and it occurs when accelerating. As you did, back off the accelerator to save the engine. Detonation usually occurs when the combustion chamber is overheated. So carbon deposits, lean mixture or wrong spark plugs would be common issues. What spark plugs did you put in?

An injector that has carbon deposits usually continues to fire (the click) but instead of a very fine atomised spray the fuel is larger droplets and not properly dispersed into the air stream. I think this would be a lean burn condition so the PCM may richen that bank with a longer injector on time to compensate. Gas has cleaners that remove deposits, the higher rated gas has more cleaners. You can buy fuel injector cleaners for the gas tank and your dealer can do a fuel injector cleaning through the injectors which is the best short term cleaning. Also the injectors can be removed (as you did) and checked for carbon buildup but use a new O ring on replacement. Injectors are not that expensive so replacement would be an option.

A compression test would be a good idea.
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Phxpep

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Fuel pumps are expensive and require dropping the gas tank to replace. So you need to get a fuel pressure reading before going that route. It should be around 45 psi or so.

Your description of the shaking occurring when the transmission is moved from Park to Drive is interesting. The torque converter puts a load on the engine when going into Drive so the PCM increases the engine rpm through the IAC (Idle air control) valve. These valves get carboned through use and stick so they don't move properly. Does engine speed increase on a cold start and slow as engine warms up? Ford's advice is to replace a sticky IAC and from my experience cleaning does not work for long if at all. I think you need to replace the IAC to fix that shaking problem.

It seems in your case the miss can also be detonation (when the check engine light blinks) and it occurs when accelerating. As you did, back off the accelerator to save the engine. Detonation usually occurs when the combustion chamber is overheated. So carbon deposits, lean mixture or wrong spark plugs would be common issues. What spark plugs did you put in?

An injector that has carbon deposits usually continues to fire (the click) but instead of a very fine atomised spray the fuel is larger droplets and not properly dispersed into the air stream. I think this would be a lean burn condition so the PCM may richen that bank with a longer injector on time to compensate. Gas has cleaners that remove deposits, the higher rated gas has more cleaners. You can buy fuel injector cleaners for the gas tank and your dealer can do a fuel injector cleaning through the injectors which is the best short term cleaning. Also the injectors can be removed (as you did) and checked for carbon buildup but use a new O ring on replacement. Injectors are not that expensive so replacement would be an option.

A compression test would be a good idea.
I never have any problems with the speed I always can keep a decent speed but it definitely has more hesitation whenever the engine is cold versus whenever it's warmed up. I already ran a bottle of seafoam through a full tank of gas and had no luck with that. if it is lacking compression what would cause of that be
 

Phxpep

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I never have any problems with the speed I always can keep a decent speed but it definitely has more hesitation whenever the engine is cold versus whenever it's warmed up. I already ran a bottle of seafoam through a full tank of gas and had no luck with that. if it is lacking compression what would cause of that be
I will say though that when this problem first started I was driving home one night and the car was going about 85 90 miles an hour and then out of nowhere it just dropped about 5 6 miles an hour and was still driving fine and then the next morning is whenever I had the shaking problem
 

Town

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Ottawa Ontario Canada
I never have any problems with the speed I always can keep a decent speed but it definitely has more hesitation whenever the engine is cold versus whenever it's warmed up. I already ran a bottle of seafoam through a full tank of gas and had no luck with that. if it is lacking compression what would cause of that be
Lack of compression on one cylinder would be a cause for a misfire and the DTC that you got. The PCM does not set a misfire code for a component problem, rather it measures continuously the effect on engine speed of each cylinder. There is a threshold for the number of times a cylinder under-performs before a misfire code is set for that cylinder. A detonation event causes a major slowing of the cylinder performance and the PCM identifies that as an immediate DTC and flashing light to warn driver to stop accelerating and slow down.

All cylinders should have a similar compression. General wear on the cylinder walls and rings will cause all cylinders to have reduced compression. The test for that is to do a general compression test and then put a little oil in the cylinder to bridge the ring gap and check the compression again. If the compression is significantly higher then the rings or walls are worn out. If it happens on just about all cylinders then the engine needs a rebuild. Valves can also get carboned causing the valve not to seat fully and cause a leak which reduces compression. In this case you need a leakdown test which pressurises the combustion chamber with air through a modified spark plug to allow air line fitting. The pressurised air will leak out from rings and pressurise the crankcase, or the intake valve causing air sound through intake, or through exhaust valve.

Where one or more cylinders have a significantly lower compression than the rest is a probable cause for a misfire DTC, since the low compression will not power the engine and add to engine rpm like the good compression cylinders. In these cases there may be a gasket leak between adjacent cylinders which causes the 2 cylinders to both read low. Ring failure would be checked by oil in cylinder to see if improvement occurs. valve problems would be diagnosed as tor the cylinder leakdown test. Cylinder head gasket failure could also occur with the oil ports and coolant ports but then you have additional factors such as oil colouration issues from carbon darkening oil or high crankcase pressures causing oil in intake/loss of oil. Coolant in oil or loss of coolant or bubbles in coolant or sweet coolant smell at exhaust tail pipe.
 

Town

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63
Ottawa Ontario Canada
I will say though that when this problem first started I was driving home one night and the car was going about 85 90 miles an hour and then out of nowhere it just dropped about 5 6 miles an hour and was still driving fine and then the next morning is whenever I had the shaking problem
The drop in speed may occur from the throttle position sensor losing resistance in a section. The test is to measure the resistance in the component as the throttle is slowly opened from closed to full throttle looking for a steady rise or fall of the resistance. A problem would be detected if the resistance value drops to zero at a particular point, usually off idle and normal cruising speed.

The problems with cold engine operation and idle may be affected by the throttle position sensor but more usually the idle air control if the engine does not quickly respond to engine load changes. I can get the replacement procedures for those components. Also the compression test. The compression test must be performed at full throttle so as to shut off the fuel injectors with all spark plugs removed and only a few turns of starter to get to full compression.
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Phxpep

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I took it to my buddies shop he said it gets fuel and spark but that’s it he said the rocker for that cylinder probably but he can’t take on a job this big in his small shop. But he said that cylinder was unresponsive and to take it to ford for best diagnosis
 
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