Engine missing especially at low RPM

Z

zebra

Registered
My 93 Mark VIII has started missing. It started at low RPM, like right after a gentle upshift. I replaced the spark plugs and both coil packs and it helped a little, but it seems to be starting to get worse. Today I got on it a little and it continued to run rough, but in waves. The plug wires are not sparking in the dark and they aren't very old. No Check Engine light.

Could it be the Throttle Position Sensor?

The torque converter is giving trouble and I thought it might be related, it is a similar feeling, only very mild. Until today it was only noticeble while accelerating at low rpm. Once you got up to 2000rpm it would smooth out. Today it continued under heavy acceleration, but in a kind of wave pattern.

Does the torque converter act like that?
 
zebra said:
The plug wires are not sparking in the dark and they aren't very old.
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I got 5 bucks on a bad plug wire. Just because you don't see them arching in the dark, doesn't mean they didn't fail internally. And even a new part can fail.

If you're mechanically inclined enough to change the plug wires, then I'd diagnose the miss this way....

Loosen all the plug wires at the coil packs (you can do one pack at a time) but leave them in the coil pack ports, just unclipped. Start the car and make sure it is misfiring. As the car is misfiring, remove one plug wire at a time, thus preventing spark to go to the spark plug. If the car idles/runs WORSE then that spark plug wire is fine. If the car doesn't run any different, then that plug wire is BAD. Do this until you find the wire that is bad.

Once you find the bad wire, swap it to another cylinder and test it again. If the misfire follows the wire, you've got a bad wire.

If you don't want to go through the pain of this....just change the wires with some Beldon (Blue) from Napa.


zebra said:
Could it be the Throttle Position Sensor?
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Nope.

zebra said:
Does the torque converter act like that?
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Naw....you'd only get a rumble strip feeling when it goes into lock up, but not at all RPMs.
 
Can you verify if only one cylinder is missing, or more than one?

If just one, you can isolate the problem to the wire, plug, or an internal engine issue for that cylinder only. If more than one cylinder is missing, your coil may be going bad.

Aside from the ignition system itself, other things to check are the crank and cam position sensors. As you have a '93 which is OBD-1, I unfortunately do not know how to help you troubleshoot them. I'm sure others here do though...
 
J said:
I got 5 bucks on a bad plug wire. Just because you don't see them arching in the dark, doesn't mean they didn't fail internally. And even a new part can fail.
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+1

J said:
If you don't want to go through the pain of this....just change the wires with some Beldon (Blue) from Napa.
Click to expand...

Do you recommend the Beldon from NAPA over MSD's? I am about to buy some spark plug wires and i was leaning toward MSD's but if Beldons are just as good i will go that route, i have a hook up at NAPA.
 
JERM said:
Do you recommend the Beldon from NAPA over MSD's? I am about to buy some spark plug wires and i was leaning toward MSD's but if Beldons are just as good i will go that route, i have a hook up at NAPA.
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No man...MSD's will be better, but if you're doing regular/stock stuff Beldon's are GREAT! :D
 
Thanks guys! I'll work on isolating the miss...or maybe just replace the plug wires. If that doesn't work I'll let you know.
 
Sounds like an ignition problem to me, but don't eliminate sensors. And yes, a TPS sensor is possible, but it's not likely. The only thing I can think of for that would be if something is shorting power to the TPS circuit, like the adjacent wire feeding into the PCM has a tear in it or something. If the TPS gets a constant 3v or so from that, it could operate on a 3-5v range instead of the 0-5v range it's supposed to. It would run rough or miss at lower RPMs because the TPS would be telling the PCM to run richer than it should be, but would even out at higher RPMs because it would be reading accurately on that smaller scale. Again, I don't think it's likely, but don't eliminate it. It's really something that ought to be looked at if you've already verified that you have proper fuel and spark.

Getting back to ignition, it's possible the primary side of the coils aren't receiving full voltage either, which could produce a weak spark regardless of replacing the coils, wires, plugs, etc.. Check your wiring harnesses.

And like I always seem to say... Do you have access to a scan tool? Check to see which cylinder(s) is misfiring if you can, check to see if there are any codes on it that may not trigger a check engine light, and maybe run a snap test on the MAF and TPS if all else fails.

EDIT: My grammar sucks.
 
Hey Guys,

I posting here because I have the very same problem as in the subject of this post.

My difference .. the car seems to run fine at idle.. and I'm not able to test by switching around my wires.

By fine idle, I mean .. if there is a bottle of water in the passenger seat, I can see it ripple. But there is no audible miss, and reeving the engine in neutral from idle and the car seems very smooth.

BUT, just like the OP, my car will miss especially when cold, especially at low RPM and high load. Just after shifting into 3rd OD and going up the first hill of my commute to work, I can only give say 25% throttle.. 35% will cause a consistent miss. I notice it is definitely getting worse as time goes on as well.

I changed the plug wires not even 10k ago. 200 miles ago I replaced all sparkplugs with autolite copper's, and gapped them as best I could (with the gapper puck) to OEM spec. The plugs I took out looked OK with a light clay coloring. If it makes any difference, I have been running 87 for the last 4-5 fill ups. (Will feed it 92 next)



I am having trouble diagnosing this problem because the miss is only at low RPM and higher load.. I can't just sit there at idle and pull the wires to each cyl.

Maybe I should go a bit smaller in gap? Maybe re-clean the MAF?

Any other ideas where I could start troubleshooting, before throwing another set of 120$ blue wires from napa?
 
You CAN still test it the way I described, you'll just have to "powerbrake" the car to do it.

If you're having a miss under load, I'm 95% positive its going to be a bad wire or a bad coil pack. Seriously.

The best thing you can do at this point (if you don't have another set of wires to throw on) is to get a scan tool and see which cylinder is misfiring, then diagnose from there. Even though you don't have a Check Engine light on, it may (probably will) have codes. Go get one of those 30 dollar scanner tool for OBD1 Fords.
 
AssFoe said:
And yes, a TPS sensor is possible, but it's not likely.

It's really something that ought to be looked at if you've already verified that you have proper fuel and spark.
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How does a bad TPS cause a misfire? A TPS is not isolated to each cylinder/bank so it wouldn't cause a significant miss. It would only cause a steady flat spot or maybe even a complete dead spot to where the ECU would run off a base setting. I think you're just going to confuse the poor guy with all your technical garble. :)

AssFoe said:
Getting back to ignition, it's possible the primary side of the coils aren't receiving full voltage either, which could produce a weak spark regardless of replacing the coils, wires, plugs, etc.. Check your wiring harnesses.
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Again....lets get him started on the EASY stuff and the more likely issue instead of pointing him off on some tangent. If the coil packs had a weak power source (which could be a plethera of things) his symptoms would be greatly different. If one coil pack was weak, he would have a very rough running engine, if both were weak, the car would just perform very poorly or hardly at all.

AssFoe said:
And like I always seem to say... Do you have access to a scan tool? Check to see which cylinder(s) is misfiring if you can, check to see if there are any codes on it that may not trigger a check engine light, and maybe run a snap test on the MAF and TPS if all else fails.
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I thought you don't like to start by checking codes first. Hehe! :D I agree though...go get one of those cheapy code readers and see what comes up. but ONLY after you replace your BAD spark plug wires! Five bucks man!!
 
thanks much for the advice J

Waiting at my first stop light of the day this morning, I tried power braking it ... it doesnt seem to miss nearly as consistently as it does when in overdrive and climbing a hill. I guess the engine RPM can get lower with the lockup or something


anyway, I will check codes next. You say buy a code reader, but I can just ground the STI pin under the hood and check the blinks, right? i.e. .. will I miss any sections containing codes if I do it that way?
 
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