L72 Dies at Idle - Magnetic Pickup Coil Resistance

[Scott H.]

My recently minted L72 runs great for the first 10 minutes after a cold start. After that it will idle at ~900 rpm for a second or two and then quickly lose rpm's until it dies. When it dies it will not start again until cool. No idea if it is related but the car also dies on an incline. My driveway has about a 10% slope and the engine tries to die on this slope. The warmer the engine, the more likely it is to die. Once again it will not restart until cool.

The engine is about as stock as I could make it.

Carb is a two year old Holley 4150, List 3247. Not a rebuild. Cam is stock grind (SpeedPro CS-165R). Heads, intake and block are all date and part number correct. Valves are stock size. Compression ratio is calculated 11:1. Exhaust manifolds are reproduction. The carb worked great on the previous engine, a 427/390 hp. The carb did sit for several months while I assembled the L72. The fuel (I used 10% ethanol in the 390hp) was not drained while before storage.

Carb and fuel: idle mixture screws are set to maximum vacuum, steady 14 inches Hg. Floats are set so fuel falls out of the fuel bowl openings when the fender is bumped. Fuel is straight avgas. Fuel filter is clean. The acceleration jets always deliver a steady stream of fuel. Fuel pump pressure is 7.5 psig at idle. Fuel line between the rubber hose near the gas tank and the fuel pump have been blown out with compressed air. Gas cap never makes a pressure relief sound when removed (I think that means the vent works).

Ignition: Distributor is a TI repro from Zip, ignition coil is a TI repro from Zip (0.65 ohms across the terminals and 12,500 ohms between the + and coil wire terminal when cold). TI amplifier is a repro from Zip. Wire harnesses are 1 year old Lectric Limited. Distributor has a B28 vacuum advance canister with full time vacuum per Duke's recommendation. Spark plug wires are one year old Lectric limited dated reproductions. Initial spark advance is 8-12 degrees BTDC (I have tried several settings without positive effect).

Engine coolant temperature is 180F.

Here's my question. The resistance on the magnetic pickup coil in the distributor is ~705 ohms cold and ~830 ohms hot (like when the engine just died). Delco Service Bulletin 1D-155 lists the acceptable resistance as 550-750 ohms without a reference to temperature. Does anybody have experience as to how well the car runs at 800+ ohms? Zip Products has been great trying to help solve this mystery. Their position is that because the pickup is in spec at 75F (the Service Manual lists the ignition coil resistance at 75F) then the resistance is not the problem so they are continuing with their own investigation.

Any suggestions, especially with the pickup resistance? I think I have checked everything else but am open to any suggestions. Thanks for reading this long post.

[Duke W.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Put a spare spark plug in your pocket and a jumper wire with alligator clips. Next time it dies pull off any plug wire, attach it to the loose spark plug and hook the jumper between the spark plug ground electrode and any decent chassis ground to the battery. Have someone crank and determine if you see a spark. I expect you will not. Next step is to check ignition coil primary and secondary resistance and pickup coil resistance. Take your time and be patient. These types of electrical glitches that are likely temperature related can be tough to isolate.

Good write up on the problem and what you've done so far. A 12" B26 will pass the Two Inch Rul4 (14" @ 900 is right on for the OE cam) but if it doesn't detonate with the more aggressive B28 no big deal.

Duke

[Richard M.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Scott,

Yes I suspect you're losing spark. Verify as Duke suggested.

I've been through similar issues many times with TI.

The PU coil spec from GM for their coils may not apply to yours as it may be a reproduction PU with a different spec. I would not consider that your problem as its very close to spec.

What I would suspect is your Ignition Coil. Symptoms are consistent with many TI cars I've done. I suspect if you put a IR Gun on the repro IGN coil when it fails you'll be in the 250* range. I've had enourmous trouble with those coils.

TI will run with a stock points coil, but won't yield best high RPM performance. Use one as a diagnosis tool. Get yourself a Napa IC12. Try it under the same time and conditions.

I also feel its the coil as on incline its failing under load.

If it turns out to be the Ign coil, try to find a good used or NOS GM 207 service replacement.

Rich

[David H.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Scott

What engine wiring harness is installed? TI specific?

Does harness have resistor wire incorporated?

Dave

[Jim D.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Rich is right on the money, as usual. Those reproduction coils are absolute garbage. Every single one I've ever seen failed when it got warm.

[Scott H.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

It is a TI specific Lectric Limited wiring harness. It's about 18 months old. There is a resistor in the harness. Many thanks to all!

[Danny P.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Scott, find a use coil that you know it been working for awhile , the new Delco box GM coils are junk made in china ,you may find one of eight that might work as soon as it warms up the coil get too hot and stall and dies out , like you said when cool down it restarts.

[Scott H.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Tying all of the advice together my plan is to reinstall the distributor and buy a UC12X coil to verify that my reproduction coil is the problem. The X appears to have a lifetime warranty as opposed to the UC12 so I assume it is a more robust coil? Also the UC12X is $10 cheaper at O'Rielly's than the UC12 at NAPA.

Two questions: 1) How long can I leave the temporary coil in my car? It only goes on back road outings on the weekend and it will never see the high side of 4000 rpm (3.08 rear end). I would like to take my time getting an NOS GM coil. 2) Am I missing something by using the UC12X instead of the UC12? I so appreciate everyone's advice and want follow your recommendations correctly.

Thanks again. The Discussion Board is the BEST! I will report back NLT 26 July.

[David H.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

[QUOTE=Scott Harrison (64741);897913]Tying all of the advice together my plan is to .....[QUOTE]

Scott

Don't throw parts at a problem - sort out what's wrong first. Danny's advice is good - get a coil you KNOW works. A new O'Reilly's, AutoZone, NAPA, etc coil that doesn't fix your problem leaves you in the same spot - bad coil or something else?

Dave

"... find a use coil that you know it been working for awhile , the new Delco box GM coils are junk ..."

[Richard M.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Tying all of the advice together my plan is to reinstall the distributor and buy a UC12X coil to verify that my reproduction coil is the problem. The X appears to have a lifetime warranty as opposed to the UC12 so I assume it is a more robust coil? Also the UC12X is $10 cheaper at O'Rielly's than the UC12 at NAPA.

Two questions: 1) How long can I leave the temporary coil in my car? It only goes on back road outings on the weekend and it will never see the high side of 4000 rpm (3.08 rear end). I would like to take my time getting an NOS GM coil. 2) Am I missing something by using the UC12X instead of the UC12? I so appreciate everyone's advice and want follow your recommendations correctly.

Thanks again. The Discussion Board is the BEST! I will report back NLT 26 July.

Scott,

1. I left it in for a entire summer driving season in my friends L71 in NH. He couldn't feel any difference. Over that winter I located a NOS Delco 207. The following July when I returned, I installed the NOS 207. This was 4 years ago. Car has been perfect since. He keeps the points coil in the storage well as a spare, along with the spare K&B TI module circuit board I got him.

One caveat... The original Delco TI Circuit board may not play long term with the points coil. It shouldn't harm it as the primary resistance is a bit higher. Since yours in a new replacement from Dave Fiedler(I believe Zip gets theirs from Dave, maybe Mike Zamora, also a TI expert), it's likely the K&B. BTW, Dave was the advisor of the points coil swap to me back then when I had TI issues. He was right, and I'vecontinued to advise others of his knowledge.

2. Typically, parts are identical, but the more expensive is usually to cover additional initial testing procedures and additional warranty period. I'd go with the non-X UC12.

For reference if some are confused by the numbers...

IC12 is Napa Echlin brand
UC12 is Standard Motor Products brand

Either of these would work.

QUOTE=David Houlihan (36425);897915][QUOTE=Scott Harrison (64741);897913]Tying all of the advice together my plan is to .....

Scott

Don't throw parts at a problem - sort out what's wrong first. Danny's advice is good - get a coil you KNOW works. A new O'Reilly's, AutoZone, NAPA, etc coil that doesn't fix your problem leaves you in the same spot - bad coil or something else?

Dave

"... find a use coil that you know it been working for awhile , the new Delco box GM coils are junk ..."

True, ideally he could borrow a known good, but if not at his disposal the next best option IMO is the new part.

Rich
PS I'm planning a experiment while up in NH on my friends L71. I have 2 repro 263 TI coils. These failed identically to Scott's failure. I'm going to reinstall each one and record temperature in real time and actual time-to-fail and will document the failure mode. Ill report back with a separate analysis when I get to this sometime this summer.

I may open one and analyze where the internal short occurs. Likely deep in the windings. I just need to be careful of the oil which is not healthy(rubber gloves, safety glasses, etc.)

This is my stash that travelled with me for my upcoming experiments. The 2 repro coils to re-test. The TI Module is a working original Delco I repaired a few years ago. It was given to me from a friend which I then restored, repainted and had the backplate replated, a original stamped-on-flange 1115005. One output transistor was bad. Also spare transistors and a new M&H manufactured encapsulated module. I had a K&B(green ckt board) but passed that to my friend for his L71 spare.

[Scott H.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

I got a UC12X. The car started right off but ran very rough for 2-3 minutes until I turned it off. The distributor was off by a gear tooth so I blame the rough running on that? When I fixed the timing the car would not even try to start. I put a spark test light in series with the #1 plug wire and got a light during cranking. My timing light also worked too. This led me to believe that I was getting spark. However, when I installed the old reproduction coil the car started right off.

The resistance on the UC12X was 1.4 ohms. I'm thinking I need to get a GM NOS 207 coil as Rich touched on. Thoughts?

[Danny P.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

I got a UC12X. The car started right off but ran very rough for 2-3 minutes until I turned it off. The distributor was off by a gear tooth so I blame the rough running on that? When I fixed the timing the car would not even try to start. I put a spark test light in series with the #1 plug wire and got a light during cranking. My timing light also worked too. This led me to believe that I was getting spark. However, when I installed the old reproduction coil the car started right off.

The resistance on the UC12X was 1.4 ohms. I'm thinking I need to get a GM NOS 207 coil as Rich touched on. Thoughts?

Scott, I keep telling you to get a used old coil that you know it works and try it and see how it works, but you keep using the repro. Coil again,

[Richard M.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Scott,

This is what I believe is the reason for your odd restart issue. I've gone through exactly the same scenario.

Since you were off a tooth the engine was disturbed and fuel was in the system erratically. Some cylinders had none and some were flooded. The BB engine needs a lot of fuel, in the right places. You tried a restart with it now in time and but hadn't recovered. I've gone through similar scenarios many times, scratching my head when diagnosing TI ignitions.

I suspect either coil would've given you the same result after you got the distributor set properly.

After the time it took you to swap coils the second time, things had normalized and it now ran.

As long as the TI board is the K&B, the UC12 will work fine, both as a diagnostic tool and continued operation, albeit as mentioned before, not to high performance specifications. All bets are off if it's a original Delco circuit board.

I would have swapped the UC12 back in and retried.

Yes the ideal choice is a 207 but since a bit tough to locate, and since you still have the possibility that a used part is still a unknown(will it fail hot? is that the reason it was on someone's shelf for years?), the easiest, most economical choice as a diagnostic tool is the UC12.

The best scenario is borrowing someone's known good, runs fine hot, etc, used 207, but reality states the odds of this happening can be sparse.

Rich

[Scott H.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Danny, I didn't mean to ignore your advise. I just don't have access to a known good coil.

As for the UC12 I got another one today. Car would not begin to start. Put the repro TI coil back in and it tried to start immediately and did start after 3 attempts. It does have a K&B TI amp board.

Could the use of resistor spark plugs make the UC12 unusable?

Maybe Dave Fielder has a good coil. I will contact him tomorrow. Nothing on ebay right now.

Thank you all. More to come.

Scott

[Richard M.]

Re: L72 Dies at Idle - Magnetic Pickup Coil Resistance

Scott I used a UC12 for a entire year while searching for a nos 207.

I'm thinking not enough fuel on first attempts. Next time check you're getting enough pump shots out of the squirters after choke is set. Darn BBs need a big gulp at first start.

Dave is the expert. He's the one that gave me the advice many years ago to use the UC12 to diagnose TI coil issues.

Not sure about resistor plugs, I'd think not a problem though.

Rich