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Random misfire - low RPM under load

10K views 11 replies 5 participants last post by  jrfintex 
#1 ·
My 97 GM has another misfire problem. I don't have any codes yet but I can feel it. In drive at hot idle the engine will be idling as smooth as glass and suddenly misfire one time, and again several seconds later. I can feel it throughout the car as if a cylinder went 100% dead for one cycle. Also, at slow speed if I ease down on the accelerator to climb a hill or resume highway speed it will jerk 3-4 times. If I nail the gas it runs like a champ.

I've recently replaced the intake (cracked plastic), plugs, fuel filter. All was good until a few days ago.

I had this problem before and someone suggested it was a bad plug, plug wire or coil. I agree, but after replacing the plugs the problem went away. I don't recall the part number of the plugs I pulled but they looked very conventional and nothing like the Motorcraft Finewire SP432 plugs I put back in.

Seems like if it was a coil or wire problem it would break up under acceleration. This is 100% a load at low RPM issue but I don't know what would be causing it. Is there anything else I could check before I start throwing more parts at it?

I've read about this problem elsewhere and it seems to be common but no one has offered a solution. Thanks.
 
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#3 ·
My 97 GM has another misfire problem. I don't have any codes yet but I can feel it. In drive at hot idle the engine will be idling as smooth as glass and suddenly misfire one time, and again several seconds later. I can feel it throughout the car as if a cylinder went 100% dead for one cycle. Also, at slow speed if I ease down on the accelerator to climb a hill or resume highway speed it will jerk 3-4 times. If I nail the gas it runs like a champ.

I've recently replaced the intake (cracked plastic), plugs, fuel filter. All was good until a few days ago.

I had this problem before and someone suggested it was a bad plug, plug wire or coil. I agree, but after replacing the plugs the problem went away. I don't recall the part number of the plugs I pulled but they looked very conventional and nothing like the Motorcraft Finewire SP432 plugs I put back in.

Seems like if it was a coil or wire problem it would break up under acceleration. This is 100% a load at low RPM issue but I don't know what would be causing it. Is there anything else I could check before I start throwing more parts at it?

I've read about this problem elsewhere and it seems to be common but no one has offered a solution. Thanks.
I would bet on the coil or wires. If it is just missing once in a while, it may just be more noticeable at low rpms.
 
#4 ·

Also make sure the wires are seperated and not cross firing or laying on the exhaust manifold. I had that happen recently myself and it caused the car to shake more. A set of wire seperators and I was good to go.


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#5 ·
Good info. I'll check it out the ignition this Sunday. It's intermittent and new to me. I can generally "feel" the cause of a problem but not this time.
Typically, an ignition problem is more noticeable at high RPM under load. This is most noticeable in gear at idle or low RPM under load.
 
#6 ·
I think you have the last year 4.6 BEFORE COP (?) If so, your ignition wires should be well separated by the plastic wire guides (wire separators) that run over the engine from the coil packs, and run nowhere near exhaust manifolds or any excessive heat source. I had the same problem as you (low RPM miss under load) with several 4.6s before COP, especially after changing plugs. The rubber plug boots tend to stick in the deep spark plug wells and crack when removed. You can't see the cracks or deterioration, but it happens and the spark takes the path of least resistance to the aluminum surrounding the boot. Replace your boots and the springs inside (make sure they connect firmly with the plug tip) with an OEM or other high quality "boot kit". Lightly grease the new boots with dielectric grease before installing. Maybe consider replacing the wires also, and pay particular attention to the firing order as the coil packs can confuse.
 
#7 ·
All of you were correct! Bad plug wire on #1 cylinder.

According to Champion's website, a plug wire should have a resistance of about 12K per foot. I checked all the wires with my multimeter and #1 would drift to the extremes, so I checked the boot and found a fair amount of dielectric grease on the inside. I cleaned it with carb cleaner, went for a long test drive and now the engine runs like new again.

The wires have probably never been changed so I'm going to order a new set from RA just to be safe.

Thanks again for the help.
 
#9 ·
All of you were correct! Bad plug wire on #1 cylinder.

According to Champion's website, a plug wire should have a resistance of about 12K per foot. I checked all the wires with my multimeter and #1 would drift to the extremes, so I checked the boot and found a fair amount of dielectric grease on the inside. I cleaned it with carb cleaner, went for a long test drive and now the engine runs like new again.

The wires have probably never been changed so I'm going to order a new set from RA just to be safe.

Thanks again for the help.
I would get the motorcraft. If the ones you have have lasted 23 years they must be pretty darned good.
 
#10 ·
AMCLARK -
You are right about Motorcraft but due to my odd work schedule I needed to get them swapped out ASAP and RA had a better delivery date on a set of Denso class E wires, which are rated very high if you believe ratings. They arrived in 48 hours and were installed today. They appear well made, fit perfectly, and the car runs great. The only negative I could find in the reviews were complaints about the lack of number markings. The OEM wires are marked with the cylinder number on them but these are not. However, since the coils are clearly marked it's pretty hard to screw up.

I didn't think cleaning the grease and gunk out of the old ones was really the problem and I was right. The car ran great for a day and went back to throwing misfire codes. I knew it was due a set of wires and went straight to the RA site and placed an order.
Thanks.
 
#11 ·
AMCLARK -
You are right about Motorcraft but due to my odd work schedule I needed to get them swapped out ASAP and RA had a better delivery date on a set of Denso class E wires, which are rated very high if you believe ratings. They arrived in 48 hours and were installed today. They appear well made, fit perfectly, and the car runs great. The only negative I could find in the reviews were complaints about the lack of number markings. The OEM wires are marked with the cylinder number on them but these are not. However, since the coils are clearly marked it's pretty hard to screw up.

I didn't think cleaning the grease and gunk out of the old ones was really the problem and I was right. The car ran great for a day and went back to throwing misfire codes. I knew it was due a set of wires and went straight to the RA site and placed an order.
Thanks.
You had me a little worried about the grease causing problems since I had just put it on my new coil packs when I put them on my 2000 grand marquis. Glad to hear you have your problem fixed.
 
#12 ·
AMCLARK
I've read that it's not good to use dielectric grease inside the boot but I've packed electrical connectors full of that stuff and never had a problem, Supposedly it can increase the resistance causing a weak spark and even damage to the coil over time. I briefly thought that was the problem, but common sense and good advice eventually kicked in and I figured the the 23 year old wires were most likely the cause for the stumbling and stuttering.

The grease is non-conductive so I can see how it might cause a misfire but I 've done it for years until recently. Now days I try to use just a smidge on the porcelain part of the plug and then grease the outside of the boot pretty good. I don't know if it really makes a difference though.
 
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