View Full Version : Engine misfires

04-27-2006, 09:51 AM
I have a 97 Cobra. It has been runnig great, except for a water leak in a head that was fixed 21K ago. I now have 123K on it. Several months ago it started mis-firing at low RPM (under 2K) and under a load (accelerating). It idles fine. I have replaced the fuel filter and the fuel rail pressure regulator with no improvement. The pressure in the fuel rail, while driving is normal. Some days it's worse than others. It seams to run better when it's cold (first thing in the morning). While driving at constant speed with light thottle, it acts normal. As soon as I try to accelerate, is buck badly. Gas mileage does not seam to be affected. I am suspecting a fuel injector. I have run out of ideas.



Red Wedge
04-27-2006, 11:17 AM
When is the last time you changed your plugs?

04-27-2006, 04:11 PM
The plugs have about 50K on them. They are Bosch Platinum Plus 4. I removed and inspected all the plugs several months ago (this is one of the first things I did when the problem started) and all were very clean. Light grey in color with no evedence of fouling.



04-29-2006, 09:40 AM
This is a long one :( . Hopefully something in here would help. Some of what I listed here should cause a check engine light to turn on, but sometimes that may not happen if the bulb is burned out. In most cases if all is running as it is supposed to, it just may take a simple fix to cure the problem.

The issue may be related to the plugs but I have a reservations it may be something else. :rolleyes: I am sure that others would agree that there are better plugs available. It may be a matter of compression ratio, and if your motor is closer to 10:1 I would not use them (plug is too hot for that application).

There are several things I would consider;
First: have the codes read from the OBDII port to check for pending codes. Most scan tools will display information and a code if there are any faults found. A miss-fire would show up if there is an issue. Cause of miss-fire can be a spark plug, spark plug wire, coil, distributor if you have one, as well as fuel injector. Another issue may be if your vehicle is equiped with a knock sensor. If the sensor goes bad, any vibration would cause it to retard timming, or turn off fuel injectors. I do not believe the 97's were equiped with one (99 and 01 do have them). If it feels like you are hitting a brick wall when accelerating chances are you are getting detonation or you have a knock sensor playing havoc with your motor. Considering you had a head gasket leak that is usually caused by detonation. If your motor starts to buck, lay off the gas since this may be something other than ignition issues. It could be bad gas or carbon buildup in the cylinders but would also be evident on the plugs as well.

When was the last time you cleanded the throttle plate and IAC valve. Carbon build up on these two components may cause hesitation or poor performance. Just a thought but doubt this is the cause of any problems you describe.

(this I should not even mention but plausible) Dirty or clogged fuel injectors. Issues relating to fuel injectors will generate an ODB code. It is possible to remove the fuel injectors to clean them (if you do decide to do this, get the O rings ahead of time since they should be replaced when removing the fuel injectors, never reuse the old ones.) Definately use caution when servicing any fuel line especially on re-assembly of the fuel rails. It should go together without any resistance. I doubt it is the injectors but it could possibly be that. I would not recommend it since if not put back together properly it could cause an engine fire or worse.

Other than spark plugs, what is the condition of the plug boots? IF they seem dry or have any cracks in them that would cause a miss-fire especially once the motor heats up so do the plug boots and may allow energy to pass through the boot to the block. This will hold true to all motors including the later models with coil on plugs.

Here is where I get hazy, if the 97 has a distributor which I think it does, change the wires, along with the rotor and cap. That should improve throttle response and reduce any miss-fires if they occur. This is assuming that the distributor is not solid state (lacking any rotor or moving parts except for the distributor shaft gear). I am not quite sure what type of distributor is used on your motor.

Also, the coil may need to be changed since over time heat buildup within the coil does have a tendency to damage winding varnish which would result in a weak spark or lack of spark. At idle, the spark frequency is low but under acceleration and higher RPM the frequency increases and if the coil is loosing its inductance its frequency response will change and not hold the energy like it is designed to.

One thing to point out, the selection of spark plugs may effect the coil life. In some cases, too cold of a plug hinder performance on some coil on plug applications, and plugs lacking the required resistance or wires lacking the proper impedance may also have a similar effect. Typical life span of wires, boots and or distributor cap and rotor is about 40k miles or less.

The last thing would be to clean the MAF sensor. This may be the cause of poor throttle response but I doubt would cause a miss-fire.

Last, and almost forgotten, Oxygen sensor fault or stuck EGR. A faulty O2 sensor will definately send a message to the ODB system as well as light up the "check engine light" but not always. Also if the narrow band O2 sensor is out of whack it will cause the engine to run lean. It is supposed to operate in open loop if it fails and the motor will run rich. If your gas milage is dropping considerably that would be an indication of a failed O2 sensor. Could be a clogged cat if you still have them.

05-01-2006, 09:48 AM
We 96-98 do have Knock sensors according to the tuner at the Buckeye dyno pull. At least thats what he told me. No we don't have a rotor distributor set up we have dual coil packs. So using aussie's guide there I would lean towards the plug wire's. As you discribed the issue worsen's with heat that is a good sign the insulation in the electical system is breaking down. ie plug wires and boots as assie said above. And yes that would seem like misfire because the spark is happening prior to the plug, shorting on the valve cover. So that cyclinder dosen't fire cough cough.

05-01-2006, 06:05 PM
Change your plug wires, get some better plugs. Go with Autolite 764 (copper) gapped at .054. If that doesn't cure it (it probably will) you might want to look at your coil packs. If you need to replace just get stock replacements. NONE of the aftermarket ones have ever performed better than the stock ones.

05-02-2006, 06:49 AM
:twitchy: I keep forgetting what year ended the distributor. At least I kept it general. The more I think about it, and after reading the initial post, it almost sounds like a sick O2 sensor even though the fuel milage does not seem to be effected. The response of the O2 sensors may change over time and it may just be a matter of changing them. Some scan tools will have a means to run a diagonstic on the emissions system so that may help discover if there is an issue with the Oxygen sensors or not. If you have not changed the plug wires in a long time I would do that anyway. If it is a miss-fire the ODBII should at least trigger the "check engine light" unless it is pending. As I said before, check for any pending codes from the ODBII port as this may give you a clue as to what is happening. It almost sounds like the engine is running lean when you attempt to accelerate (bucking could be a lean condition, bogging could be a rich condition :scratch: ) but that will also happen if you get a misfire and loose a cylinder. A slow response on the MAF could be the issue caused by an air flow change that is if you had uninvited guests making a home out of the air box or engine compartment which could happen if the car sits for a long period of time.