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Diagnosing a Misfire on my 3000GT

shima

shima

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Hey guys,

Despite me claiming that I was going to sell this car (on about 3 separate occasions), I still own it. I did put it up for sale, and a father-son duo had a pre-purchase inspection done on it. Well, the shop that performed the inspection diagnosed it with a few issues. One of those issues is a misfire under light load. As the driver, you can feel the misfire occur when you're accelerating from a stop or when you're coming to stop. It feels like a "skip" / "lurch" / etc. At no other time can you feel a misfire.

The car has 125k on the odometer and I doubt that the plugs have ever been changed. My guess is that I should just go ahead and do the plugs and see whether that fixes it. The front bank is really easy to change, the back bank is obviously a bit harder to get to. But, it should probably just be done.

If you guys were me, would you go straight into changing the plugs or would you check some other stuff first? Since this misfire occurs only under light load, my noobie guess is that it has to be worn plugs - they work fine under most conditions but fail to fire correctly under a light load. This makes me think that it wouldn't be distributor/coil pack or the wires, because those items seem to "work all the time or not at all".

What do you all think, am I on the right track? Thanks in advance for any input, I appreciate you guys.
 
LOL I bother Justin often enough already.
 
I would assume it needs a general tune-up. Plugs, wires, etc...
 
shima said:
Hey guys,

Despite me claiming that I was going to sell this car (on about 3 separate occasions), I still own it. I did put it up for sale, and a father-son duo had a pre-purchase inspection done on it. Well, the shop that performed the inspection diagnosed it with a few issues. One of those issues is a misfire under light load. As the driver, you can feel the misfire occur when you're accelerating from a stop or when you're coming to stop. It feels like a "skip" / "lurch" / etc. At no other time can you feel a misfire.

The car has 125k on the odometer and I doubt that the plugs have ever been changed. My guess is that I should just go ahead and do the plugs and see whether that fixes it. The front bank is really easy to change, the back bank is obviously a bit harder to get to. But, it should probably just be done.

If you guys were me, would you go straight into changing the plugs or would you check some other stuff first? Since this misfire occurs only under light load, my noobie guess is that it has to be worn plugs - they work fine under most conditions but fail to fire correctly under a light load. This makes me think that it wouldn't be distributor/coil pack or the wires, because those items seem to "work all the time or not at all".

What do you all think, am I on the right track? Thanks in advance for any input, I appreciate you guys.
Click to expand...
I worked on a Mitsubishi eclipse with the 3.0 v6 and it had the same symptoms, I checked the wires on the easy 3 no change happened but the wires looked bad so I went ahead and changed the other 3 and the miss went away.
I did the plugs since I was in there it had 130k.
 
@imiceman44 yeah, I think I'll just do plugs and wires across the board. Seems the right way to go.
 
I'm not sure what the 'whole' state of tune is on it, but I'd start by making sure everything is in good working order, plugs/wires/filters/injectors ect if it's got a vacuum leak it would show up at low rpm, check for that. Make sure all the basics, easy to check things are good. I'm not sure on that engine how good the data system is on it, (obd1 or 2) but can it be watched for specific cylinders that may be misfiring?
 
x2 on wires and plugs, just due to the general age of the car and how awesome plug wire insulation ages over time.
 
If the plugs are original, they need to be changed. Iridium are about the best I've seen they go 90 to 100k, depending on fuel strategies and engine condition. Wires is 50 50 chance will be damage during plug change, so yeah I normally plan on those too. Give all the vac line a good inspection, clean the throttle bore, use an old tooth brush and brake/carb clean. check PCV system for function.
should run like a top after. Good luck.

I don't know that car well at all, but on some fwd v6, if you disconnect the front upper mount you can sometimes pry the motor forward to get to rear plugs, some gm v6 cars had a second hole in dog bone mount to hold the motor away from firewall for service.
 
Wes Harden said:
If the plugs are original, they need to be changed. Iridium are about the best I've seen they go 90 to 100k, depending on fuel strategies and engine condition. Wires is 50 50 chance will be damage during plug change, so yeah I normally plan on those too. Give all the vac line a good inspection, clean the throttle bore, use an old tooth brush and brake/carb clean. check PCV system for function.
should run like a top after. Good luck.

I don't know that car well at all, but on some fwd v6, if you disconnect the front upper mount you can sometimes pry the motor forward to get to rear plugs, some gm v6 cars had a second hole in dog bone mount to hold the motor away from firewall for service.
Click to expand...
On the Mitsubishi v6 the 3 plugs are under the intake.
Can not get to from the back
 
Hey guys, just wanted to say thanks for responding. As always I appreciate you all immensely.

I bought plugs first and took a risk... changed only those. It worked out, the car runs much better now!

Here is a picture of the original plugs I took out: https://imgur.com/zzE8zhc

I'm new to reading spark plug wear but I think it was definitely their time. Thanks again.
 
The lower left and right a different shape to the electrode - worn? kinda hard to tell?
 
Yeah kinda hard to tell. It looks so flat compared to the rest of them. I still have the spark plugs sitting in the garage, I can take a closer look.
 
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