How to tell if module is going bad? - Chevelle Tech
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  #1  
Old Aug 7th, 04, 9:17 PM
dirty_dawg dirty_dawg is offline
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Is there any way to tell if an ignition module is going bad?
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  #2  
Old Aug 7th, 04, 9:57 PM
Schurkey Schurkey is offline
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Usually, they just fail, and the engine won't run. Not always, though.

If you're talking about a GM HEI module, and the engine runs, start by checking to see if the dwell varies with RPM. Doesn't matter really, what the dwell is, just that you have more dwell as the rpm goes up.

Most intermittant problems with HEI tend to be with the pickup coils, in my experience. Next would be weak ignition coils that don't have enough snort to fire when heavily loaded.
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  #3  
Old Aug 7th, 04, 10:05 PM
dirty_dawg dirty_dawg is offline
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Would a defective ignition cause an engine to stumble and backfire thru the carb on heavy acceleration?
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  #4  
Old Aug 7th, 04, 10:21 PM
baddbob71 baddbob71 is offline
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yes it definately can but usually the problems I've experienced with HEI were on the high end of the rpm range, I could hear it breaking up. Also make sure the plug wires aren't arcing or crossfiring. Hei distributors also seam to wear the bushings out faster than the old style points so make sure there isn't much wobble in the shaft. The pickup units also go bad with age but are fairly easy to change with the distributor out and disassembled, usually the wires from the piskup break or the insulation protecting the windings deteriorates. Have you made any modifications to the car recently that could be related to the misfiring? Take a look at all the plugs and see if any look fouled, check the plug wires for resistance, check the distributor cap and rotor and if all are ok then I'd dig into the distributor. Also HEI's will misfire with less than 13 volts, a weak battery and weak changing system will cause misfires when the engine is under load. Running an HEI off a pre 74 resistor wire will cause mid and high rpms problems as well due to low voltage. Bob
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  #5  
Old Aug 8th, 04, 12:00 AM
wanarace wanarace is offline
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My car started missing above 4000rpm. I thought it was a fuel problem. I had the miss for about 2 weeks before the module died.

Steve
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  #6  
Old Aug 8th, 04, 1:47 AM
dirty_dawg dirty_dawg is offline
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Would anything in the ignition cause the engine to miss in the lower rpms under a load?
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  #7  
Old Aug 8th, 04, 12:36 PM
Schurkey Schurkey is offline
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Sure. ANYTHING is possible. I've seen ignition coils that were so weak the car would barely run--throw some throttle to it, and the engine just dies regardless of RPM. Did a good job of mimic-ing a bad accelerator pump.

I am NOT saying this is likely...but yeah, it's possible.

It would take about five minutes with an ignition oscilloscope to track that down. Got access to one?
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  #8  
Old Aug 8th, 04, 1:31 PM
dirty_dawg dirty_dawg is offline
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Nope. All I know is this engine in my truck sputters/backfires/etc. under load and it's making me mad.

About to put a for sale sign on it and wish it well unless I can get this problem fixed.
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  #9  
Old Aug 8th, 04, 1:53 PM
Mr.McFast Mr.McFast is offline
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dirty, my car started doing that same exact stuff i started to the mechanic with it and it backfired LOUD!!! and just quit so we got it hauled to the shop and i had burned the coil up the module up the pickup coil and condenser up......i put all new parts in it for about 80 or so and it started up "but my cam was ate up" so thats another reason it was backfireing threw the carb..... could be your cam also but it prolly would be a good idea to pull the cap and have the module checkd and pull the coil out and see if its bad too....you can have ur ignition stuff checked at autozone and orielys or something like that......
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  #10  
Old Aug 8th, 04, 10:07 PM
dirty_dawg dirty_dawg is offline
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Well, here's why I'm now wondering if it's an ignition problem though I kinda have my doubts:

I have been having a situation for awhile now I can't seem to figure out.

This is on my S-10 with a V8...with the trucks engine at 160 degrees or under it seems to run just fine. But once past 160, it runs fine until you floor it, which then puts a load on the engine...then it starts sputtering and backfiring thru the carb. In the past it seemed to only do this up to about 2800 and then go away but lately it seems to be doing it even past 3000 rpm. This also happens in neutral/park.

Everyone has told me this indicates a lean condition. The carb I'm using is a Holley 600 cfm 4160 series. This all started happing long long after I installed this carb.

So I took apart the carb, cleaned the carb inside and out and checked the accelerator pump (fuel does shoot from the nozzles) and the float level....didn't do any good. I swapped powervalves (from 6.5 to 8.5) and replaced the carb base gasket...still nothing. I checked for vacuum leaks....heck I even put on a brand new carb that flowed 150 cfm more than the old one....still does it.

My fuel pump is a Holley 110 gpm mechanical fuel pump. When I had a pressure gauge attached it showed 5-7 psi and the pressure didn't change with the winging of the carb.

I tried swapping the stock power valve from a 6.5 to an 8.5. The engine at the time was drawing 15 hg at idle at 750 rpm in park/13 hg's in gear at 500 rpm in idle.

Then I bought a new 4150 750 cfm Holley for the Chevelle but threw it on the truck just to see if it would make any difference. This carb came from the factory with a 4.5 power valve and I knew the truck needed a bigger power valve so I switched to a 6.5. I did this because I've always heard the power valve needs to be 1/2 of what the engine vacuum (in gear) is. (13 divided by 2=6.5) Of course now the engine vacuum has gone up to 25 in park at 750 rpm/20 in gear at 500 rpm.....if the power valve is supposed to be 2 inches less than idle vacuum is in idle does that mean I'll need an 18 power valve? (do they even make them that high?)

Well....needless to say it didn't cure the problem. As a matter of fact I was coming back from the classic car cruise in last night and was getting on it and it starts sputtering, backfiring thru the carb....at first I began to think the gas in the bowls was boiling in the bowls so I pulled over but yet most of the engine compartment was only mildly warm. So then I got back on the road and I discovereed that the backfiring and sputtering seems to happen when the engine is under load. I downshifted to 2nd and the problem seemed to go away. However in the lower rpms in 1st (taking off from a dead stop) it does this.

Now I'm begining to wonder if there is a leak in the intake somewhere? I tried spraying all around with carb cleaner to see if the engine will rev up (as I've been told this is how to trace a vac. leak). I didn't find a leak but I did discover I think the back intake to block gasket has started leaking as there seems to be some oil coming from this area.....this wouldn't cause an inake leak would it?

Since this same thing happened on my previous Holley (and it ran fine when first bolted on) I'm beginning to think it's something other than the carb.

But remember this stumble happens under load while engine is warm-hot. If bone cold it won't do it at all.
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  #11  
Old Aug 8th, 04, 11:12 PM
baddbob71 baddbob71 is offline
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Heat will effect the ignition performance, coil, module, and pickup. Borrow a distributor and plop it in to see if it fixes the problem. Just a thought. Bob
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  #12  
Old Aug 9th, 04, 4:52 AM
Cameano Cameano is offline
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I had the same problem with an HEI on a 3.8 Buick V6 years ago. Your symptoms are exactly what mine were, too. Drove me crazy troubleshooting it, turned out to be the distributor pickup. You can check it with a meter. It should have between 500-1500 ohms, and no continuity to ground through the housing.


Darren
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  #13  
Old Aug 9th, 04, 11:36 AM
wanarace wanarace is offline
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If it is actually backfiring through the carb, then I suspect timming or cam problem. If it's coughing through the carb that means it is lean.

The last two cams I had go flat had very similar symptoms. Car ran fine at idle and cold. But as it warmed up the car started backfiring through the carb under load and surging at higher speeds.

Pull the valve covers and see if any of the rockers move less then the others.

Steve
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  #14  
Old Aug 9th, 04, 1:30 PM
sidlev sidlev is offline
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I'm pretty sure you have an ignition problem. From the sound of things your pick up coil is going bad, and the heat aggrevates the problem, which is a classis symptom. You can force the failure mode by thermal shocking the pick up coil and the module to determine which one is causing the problem, but my money is on the pick up coil. Do this:

1. Let the engine get cold, so tha the problem is not happening.

2. Pull the cap, get a heat gun and heat up the pick up coil. Try to heat the pick up coil and not the module. Put an omh meter across the pick up coil leads and see if the resitance makes a dramatic change while you heat the coil. Copper has a thermal coeffcicent of resistance, but ti is veyr small, so you should see very small changes. If you see changes on the order of 100's of ohms or higher, you have a bad pick up coil.

OR, just change the thing and be done with it. Cams don't get worse as the engine heats up, and you have tried enough good carbs to eleminate that problem. Heat induced probelems are almost always electrical related, and heat induced ignition problems on electronic ignitions are almost always the pick up coil, followed by the module.

Good luck.
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  #15  
Old Aug 9th, 04, 2:38 PM
wanarace wanarace is offline
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Quote:
Cams don't get worse as the engine heats up,
No trust me they can. I would never have believed it myself but both cams I had go flat in completley different motors, cars, and ignitions did the same thing. Under normal temp they worked pretty good except WOT. When temp started going over 200 the car would start to buck like crazy and would barley run untill it cooled off for abit. I will admit in the 305 where I lost 3 lobes it was much worse and more noticable then the 350 where I only lost 1 lobe. Put new cams in each motor with the exact same parts and settings, and everything worked perfect.

Steve
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