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Old 02-07-2013, 07:51 PM   #1
blownrunner
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Default coil replacement?

I have 1991 ZR-1. The engine still idles a little rough, replaced plugs and wires, and injectors the coils seem like the only thing left. Do the coils under the plenum frequently go bad requiring replacement? The injector change made a big difference in how it drives, but it still idles a little rough. It has been suggested to me that although a coil could still be working, that it could be delivering a poor quality spark, after all the car is over 20 years old. Curious as to how many people change their coils for preventative maintenance.

One interesting thing I notice though is that the chassis of the car has vibrations, but the engine does not, as if the engine mounts were designed to transfer vibrations away from itself. While on the subject, is it possible to get the LT5 to idle as smoothly as a new car, or is a little vibration normal?

Thanks everyone.
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Old 02-07-2013, 08:05 PM   #2
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When I replaced the injectors in my 91 years ago I replaced the coils with new G.M. One of the new coils died after 8000 miles so I replaced these with MSDs.As far as vibration goes,Ive read that when the LT5 was in development one of the engineers would place a nickel standing up on a running motor,this is what Ive read anyway.
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Old 02-07-2013, 08:08 PM   #3
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Quote:
Originally Posted by blownrunner View Post
I have 1991 ZR-1. The engine still idles a little rough, replaced plugs and wires, and injectors the coils seem like the only thing left. Do the coils under the plenum frequently go bad requiring replacement? The injector change made a big difference in how it drives, but it still idles a little rough. It has been suggested to me that although a coil could still be working, that it could be delivering a poor quality spark, after all the car is over 20 years old. Curious as to how many people change their coils for preventative maintenance.

One interesting thing I notice though is that the chassis of the car has vibrations, but the engine does not, as if the engine mounts were designed to transfer vibrations away from itself. While on the subject, is it possible to get the LT5 to idle as smoothly as a new car, or is a little vibration normal?



Thanks everyone.

Yes there is a "nickel test" results have been from a few seconds, to several seconds and some for a much longer duration. Google should land you many, many results.
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Old 02-07-2013, 08:17 PM   #4
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Originally Posted by Z51JEFF View Post
When I replaced the injectors in my 91 years ago I replaced the coils with new G.M. One of the new coils died after 8000 miles so I replaced these with MSDs.As far as vibration goes,Ive read that when the LT5 was in development one of the engineers would place a nickel standing up on a running motor,this is what Ive read anyway.
Did the coil fail all the time, or just occasionally or when the engine warmed up? This very well could be my problem then. Is the MSD a direct replacement or will I have to do a little work to install them? Thanks for the info on the nickel, I now know this can be improved upon.
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Old 02-07-2013, 08:24 PM   #5
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It is puzzling how then my engine could be smooth but the steering wheel has vibration and it could even be felt in the front tires. I have read how people change coils, I think a new set is in my future.
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Old 02-07-2013, 10:31 PM   #6
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Quote:
Originally Posted by blownrunner View Post
Did the coil fail all the time, or just occasionally or when the engine warmed up? This very well could be my problem then. Is the MSD a direct replacement or will I have to do a little work to install them? Thanks for the info on the nickel, I now know this can be improved upon.
The coil died altogether.Keep in mind that one coil fires 2 cylinders.
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Old 02-08-2013, 03:32 AM   #7
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If you feel vibration in steering it probably is not the engine if ut happens when moving but suspension parts...tie rod ends, wheel balance, failed shocks or possibly alignment I would guess. These engines are the smoothest engines you will find so if it isnt there is a cause as you are checking out

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Old 02-08-2013, 02:33 PM   #8
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Quote:
Originally Posted by blownrunner View Post
I have 1991 ZR-1. The engine still idles a little rough, replaced plugs and wires, and injectors the coils seem like the only thing left. Do the coils under the plenum frequently go bad requiring replacement? The injector change made a big difference in how it drives, but it still idles a little rough. It has been suggested to me that although a coil could still be working, that it could be delivering a poor quality spark, after all the car is over 20 years old. Curious as to how many people change their coils for preventative maintenance.

One interesting thing I notice though is that the chassis of the car has vibrations, but the engine does not, as if the engine mounts were designed to transfer vibrations away from itself. While on the subject, is it possible to get the LT5 to idle as smoothly as a new car, or is a little vibration normal?

Thanks everyone.
Hello,

Couple of things spring to mind - I've heard that when the injectors go bad they leak fuel over the coils and two I had a engine mount leak, they are filled with fluid and it behaved the way you describe. I replaced mine and all was OK.

Not saying these are the problem, but it is worth a look, plus the coils will be 22 years old.

Don't mean to hijack your thread, I am having a similar problem at the moment - my 1990 ZR-1 is idling ok-ish but when i raise the revs it idles rough and I can smell the neat smell of fuel, even at the rear of the car. Any ideas? Is this liable to be injectors and possibly coils?
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Old 02-08-2013, 06:13 PM   #9
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Last time I had running issues it was the 02 sensors. If your car still has the originals and the miles are getting up there it might be something to consider.
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Old 02-08-2013, 08:46 PM   #10
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I replaced my coils when I replaced the injectors. Before I replaced them, I was trying to find out if there was any way to test them. I couldn't find a way to test them ... does anyone know if this possible? Regardless, I decided that since the coils were 20+ years old, it was probably good preventive maintenance.


Replacing the O2 sensor is probably a good idea too.
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Old 02-08-2013, 09:59 PM   #11
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I replaced all four coils 2day. I think it is idling a little better, maybe it's just me. When I say I feel vibrations it is with the car stopped in neutral with the hood up. The vibrations I feel is when I put my hand on the tires or top of the motor. I have O2 sensors on order from corvette central since January, I should get them in a few weeks. Interesting, someone told me that a coil could be bad but only when it heats up/after the engine is running for a while. One of the original coils probably was bad only from time to time, fixed now.
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Old 02-08-2013, 11:38 PM   #12
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Quote:
Originally Posted by blownrunner View Post
I replaced all four coils 2day. I think it is idling a little better, maybe it's just me. When I say I feel vibrations it is with the car stopped in neutral with the hood up. The vibrations I feel is when I put my hand on the tires or top of the motor. I have O2 sensors on order from corvette central since January, I should get them in a few weeks. Interesting, someone told me that a coil could be bad but only when it heats up/after the engine is running for a while. One of the original coils probably was bad only from time to time, fixed now.
The pass side 02 sensor is a nightmare to replace.You need to remove the bracket under the motor mount so if your going to replace the motor mounts might look into doing both parts at the same time.
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Old 02-09-2013, 11:29 AM   #13
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Default Common mode failure

Could electronics fail with rising temperatures? Lemme put it this way: Faliure of electrical stuff with temperature increase is a CLASSIC "common mode" failure characteristic - especially for coil winding devices. CLASSIC!

Apparently, as temperature rises and falls, the windings expand and contract and two common failures occur: windings short one to another within the wire wraps, or windings short to the iorn cores (typical of ignition coils, etc).

As the insulation ages it begins to break down and become brittle. The expansion and contraction of the copper windings can exert sufficient force to eventually fracture the aging "varnish". The expansion and contraction results in a "scrubbing" action between windings that further breaks down the insulation.

The most common result of the insulation breaking down is the windings short to one another, as in the case of injectors. If the windings are shorting to one another, the obvious result is resistance measurement across coil will decrease. In electrical terms, the magnetic field the coil is suppose to be developing to acutate (the pintel, in the case of injectors), deminishes with deteriorating results for that device. As insulation continues to break down, more of the (copper) windings come in contact with one another, reflected in the reduced measurement seen in a resistance test (with a volt-ohm meter).

Typically, the resistance of the injectors is measured first when cold to pick off any "low hanging fruit".

(Depending on the make of the injectors, the average may be 12 or 14 ohms (typically). Usually the manufacture will provide that information.)


The typical resistance variance is in the realm of 1/2 Ω or a little more. Actually, reading an Ω higher than the average is less of a concern than reading the other way: low readings indicate potential insulation failure and internal shorting between some coil windings. Then, the resistance measurements are taken again after the engine is brought to operating temperature.

Once hot, the coils are in a thermally stressed situation, and insulation failures, if any, will commonly show up then. For a set of injector coils reading 12.5 Ω cold, an injector now reading 11 Ω or less is suspect, at that point. And, if reading of 10Ω or lower is detected, it is pretty much certain that injector is going bad. By the time an injector is reading 7-8Ω, it is starving that cylinder for fuel, and eventually valves can be injepardy, given enough time (trust me!). Nothing to shrug at, if you catch my drift.

Infant moretality aside, if one injector has deteriorated to the point of failure, replacing all of them is highly recommended. In truth, the OEM injectors in the 90 - 92 LT5s were not aclohol tollerant and suseptible to the insulation breaking down. (AND the factory replacement NOS are JUNK as well...Ask me how I know!!)

In spite of the 93-95s having alcohol tollerant injectors, age and termal cycles will break down 93-95 injector insulation as well. Regarless of how it happens, coils deteriorating will result in rough idle AND also show up when attempting to adjust fueling when tuning with a wideband O2 sensor. (Air-fuel ratios refuse or are sluggish to respond when increasing fuel to the tables, due to injectors inability to open properly.)

The flip side of injector failure is a leaking injector - due to various reasons. In some cases it has been known to fill the intake runner with fuel and can even cause hydra-locking, in severe cases. Fortunatly, the latter is rare. Usually, the symptoms are (again) rough idle, and sometimes hard starting accompanied with black smoke from the exhaust. A fuel rail pressure leak-down test will tell you if you have a problem with leakage - either an injector, or at the check valve at one of the fuel pumps (rare, but happens). A leaking injector will usually result in a fuel soaked or very sooty spark plug or two or...


Coils:

High voltage is a different animal than most DIA'er are equipped to deal with. Ingition coils are special in that voltages are elevated to the point where it can overcome or "punch through" the insulating material to short not only to other windings, but arc to the iron core as well! Whats more, such failures can go unnoticed when using standard digital volt-ohm meters (DVMs) to take direct measurements. For the sake of discussion, a device called a "Megar" is a special ohm meter capable of developing very high voltages used to tests for pinholes and the like in insulation; thus allowing the technician to make direct evaluations of the device under test (DUT).

However, sometimes a standard DVM will detect low winding resistance:

Resistance of some OEM AC Delco coils I have measure on average 0.4Ω* 0.05 on the primary (low voltage) side, and average 5.6kΩ across the secondary (plug wire) side, and primary to secondary = open in all cases. (Mabye someone with MDS or Accel coils can chime in here?)

*Note: When measuring very low resistances, it is important to "zero" out the test lead resistance, if the meter has that feature. Or, subtract the test lead resistance (attained by shorting the test leads together and noting the reading) from the device under test measurement. (My Fluke reads 0.2Ω with the test leads shorted, and the combined reading of the primary side of the coil read 0.6Ω. So, 0.6 - 0.2 = 0.4Ω)


However, lacking access to special test equipment to directly evaluate the coils, a bad coil can be identified indirectly, by process of elimination:
  1. By installing new spark plug pairs (the LT5 coils fire two plugs at the same time, i.e., 1-6, 2-3, 4-7, & 5-8) you eliminate plugs.
  2. By comparing the resistance of the plug wires, a bad wire might be located (keeping in mind the resistance of plug wires is proportional to their individual length, e.g. equal length wires should read approximately the same; say w/in about 5%).
  3. And by the THE CLASSIC failure...if spark fails when the engine heats up, you've got a far better than even chance of a coil failing. (ECM or DIS issues aside.)

I think this corrals the bulk of the common idle issues, but if not we can take next steps.

Good luck. See what comes up!

P.

Last edited by Paul Workman; 02-09-2013 at 11:40 AM. Reason: tpyo
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Old 02-09-2013, 12:04 PM   #14
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Quote:
Originally Posted by LoTu5 View Post
Hello,

Couple of things spring to mind - I've heard that when the injectors go bad they leak fuel over the coils ?
Not saying the O-rings never fail, but in the scope of typical injector failure, I'm guessing that kind of leaking is on the fringe. And, considering there is a drain hole in the valley, unless the O-ring is spraying fuel around the only way the coils would get wet is if the drain was plugged and the coils became flooded.

I've never heard of an injector O-ring failure to the point of spraying fuel on the coils. Could happen, I suppose, but usually the "leak" is internal to the intake runner and cylinder(s). Just sayin.


Quote:
Originally Posted by LoTu5 View Post
Don't mean to hijack your thread, I am having a similar problem at the moment - my 1990 ZR-1 is idling ok-ish but when i raise the revs it idles rough and I can smell the neat smell of fuel, even at the rear of the car. Any ideas? Is this liable to be injectors and possibly coils?
Ah! Something I failed to cover in my post re idle issues is, as someone mentioned, the O2 sensor/s can be bad.

But, rather than start throwing parts at it, I would strongly suggest getting the car scanned. That way if it is an O2, you'll know. (An O2 failing is a good point. I had one intermittenly go to zero volts, indicating to the ECM the bank had gone lean (95 LT1 Vette). The ECM reacted by flooding that side of the motor with fuel; doubling the injector dwell time compared to the other side, even at WOT! Talk about fuel smell and black smoke...Wow! New O2s fixed it, as indicated too by the scanner. (Best damn tool I ever bought for the Vettes!)



P.
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Old 02-09-2013, 07:59 PM   #15
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Originally Posted by Paul Workman View Post
Coils:
High voltage is a different animal than most DIA'er are equipped to deal with.

Great post Paul. Thanks.

I recently tried to test some coils on a different vehicle, but even though my DVM indicated it was OK, it wasn't. I suspect it had to do with what you indicated (high voltage, heat, etc)
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Old 02-11-2013, 05:24 AM   #16
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Quote:
Originally Posted by Paul Workman View Post
Not saying the O-rings never fail, but in the scope of typical injector failure, I'm guessing that kind of leaking is on the fringe. And, considering there is a drain hole in the valley, unless the O-ring is spraying fuel around the only way the coils would get wet is if the drain was plugged and the coils became flooded.

I've never heard of an injector O-ring failure to the point of spraying fuel on the coils. Could happen, I suppose, but usually the "leak" is internal to the intake runner and cylinder(s). Just sayin.




Ah! Something I failed to cover in my post re idle issues is, as someone mentioned, the O2 sensor/s can be bad.

But, rather than start throwing parts at it, I would strongly suggest getting the car scanned. That way if it is an O2, you'll know. (An O2 failing is a good point. I had one intermittenly go to zero volts, indicating to the ECM the bank had gone lean (95 LT1 Vette). The ECM reacted by flooding that side of the motor with fuel; doubling the injector dwell time compared to the other side, even at WOT! Talk about fuel smell and black smoke...Wow! New O2s fixed it, as indicated too by the scanner. (Best damn tool I ever bought for the Vettes!)



P.
Paul, thanks for the advice, I did have some exhaust work done on my car just before the hessitation. There's a chance a welders torch heated up the sensors.

I have heard ethanol eats into the original rubber O rings and you can now get the resistant O rings. As for the drain holes - in my experiences, you have the 8th wonder of the world if you have one that's not blocked. I've found in the past it takes very little to block these up and they're a right sob to unblock when they are.

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Old 06-30-2013, 04:01 AM   #17
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Just to update on my issue. Changed both 02 sensors and the problem was still there. In the end I purchased some Accel injectors and following Marc Haibeck's instructions, on his site, replaced them.


I only did the injectors while I was under the plenum, to make sure that was the problem. Will do the coil packs when they need it.

Injectors came from Summit and uprated O-rings from Jerry's gaskets.


Thanks to all those that advised.
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Old 06-30-2013, 10:22 AM   #18
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Re: OEM Coils

Basics:
The LT5 utilizes a waste spark ignition system, i.e. 2 cylinders are "paired" to the same coil and fire at the same time. Only one of the paired cylinders is on compression stroke, the other exhaust. When a coil fails completely, 2 cylinders are affected.

Coil Spark problems can range from "no-spark" to "weak-spark". Weak spark can occur on only one cylinder and probably related to a coil terminal, internal or external(corrosion).

Weak-spark symptoms are noticed as slightly rough idle, rough acceleration from 45mph in 6th gear (with stock cams) and a bad miss under WOT at higher rpm levels when it fails completely.

Detection of weak-spark is best noticed by
1. using a spark tool,
Click the image to open in full size.

OR
2. the more direct method, put an old plug in the plug wire terminal & arc to ground.

Testing should be performed on each cylinder, one at a time, running at idle. if using method #2, start with the plug close to ground and gradually increase the distance to ground 1/2" to 3/4".

By either method, the spark should be bright blue. If an orange spark is observed, it's weak. The problem may be in the plug wire or the coil and not readily verifiable which one, by resistance/ohm testing. Unfortunately, plenum removal is required to replace either or both. In the ones I have observed, the problem was rectified by coil replacement.

I have personally experienced weak spark on a single cylinder and witnessed it on a few other LT5's. It can and does occur, on one or both paired cylinders.

Last edited by A26B; 06-30-2013 at 10:25 AM.
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Old 06-30-2013, 10:22 AM
 
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