I was driving my '76 (L48, 4spd) home today and while accelerating from a stop it cut out for a moment, and then came back to life. It is running now, but backfiring a bit through the exhaust, and I can hear a miss at around 2500 rpm. It was running great before the cut-out. Judging from other posts I've read here, I think it's the ignition module. I'm going to start there anyway because it's cheap and easy and I've not done it in the four years I've had the car. I'll also probably check the timing and take a look at the wires (I did replace those and plugs, cap, rotor, etc. recently).
My question: is it worth it to spend the extra $$ for the performance, after-market modules, or should I just go parts-store OEM? The engine is basically stock except for a K&N filter and dual Magna-flow exhaust (no cats).
Also, could these symptoms be caused by a toasted EGR valve - it's still hooked up. What about the coil? Thanks,
I had the same problem - here is some advice I got that may be useful to you.
=============================================================== It is always advisable to change the ignition coil when replacing a module on an HEI system. Usually the primary of the coil intermittantly shorts out causing damage to the module. Another possibility is your pickup coil. Every time the vacuum advance pulls in or releases, the pickup coil leads flex. There is just so much flexing a 29 year old pair of wires can take, until the strands give out. Disconnect the pickup coil, and then put an ohm meter across the pickup coil leads. Connect a piece of vacuum hose to the vacuum advance and apply vacuum to the hose. This will make the coil rotate. Watch for any fluctuation in the ohm meter indicating broken wires. By doing this, you will also find out if your vacuum advance is working.
The module is probably the most often blamed, but seldom the culprit. All of the failed hei modules I've seen , and I've seen a lot (was a lic tech working in a busy shop in the 70's and 80's) all acted the same way. They always totally failed and engine quit. Intermittent problems and rough running were never found to be traced to a module. Unlike others in this post, i had never run across one that was toasted due to a bad coil. I've heard techs blame the coil when they had a second module fail in a short time, but I think ultimately the failure was traced to absence of heat sink grease in installation. Shorted coils usually manifest themselves elsewhere before they would ever burn a module, low secondary voltage resulting in misfire under load etc.
Broken/breaking wires on the pick-ip coil was a high probability for this kind of problem. Also, look for a burn mark in the underside centre of the rotor. If you have a plug(s) or plug wire(s) with high resistance the high secondary voltage will seek out it's easiest path to ground, often burning a hole through the centre of the rotor and grounding to the dist shaft. If you find burning in this spot don't stop there. Now find the cause of the high secondary resistance, check all the plugs and wires. In my opinion, not likely the coil, but it is possible. Running it on an old scope and pulling a plug wire off while running will show max coil output. If it's within spec keep looking, but again, yank on the pick-up wires first.
No on the egr. Failed and stuck open would give you an extremely rough idle and would smooth out at higher rpm. Failed closed and not opening may not even be noticeable, but can give you ignition "pinging". If you had previously had to retard the timing from factory settings to get rid of the ping it may be because of the egr not functioning. Not as prevelant on GM vehicle, but Ford v8's would ping like crazy when the egr quit functioning. A good working egr system allows you to run timing advanced quite a bit, increasing power and decreasing fuel consumption.
If the pick-up is determinded to be the problem, the dist must be removed and disassembled to replace it. Service all mech advance items and check the vacuum advance for proper function. I would also check cap, rotor, plugs and wires while I was at it, good time to do the whole tuneup thing. Good Luck, Steve G.
Thanks all for your responses. I waited a few days since I just finished pulling out the dist. I see a lot of that red dust around the advance mech and also the wires from the pickup are pretty brittle looking. I am considering now replacing the whole distributor. Ecklers and others on Ebay have a ~$100 aftermarket HEI - supposedly equivalent to an Accel. It includes 50K volt coil, cap, rotor and module. Definitely would be cheaper than replacing all those parts (or even some) individually. Not sure about the quality, you probably get what you pay for. If anyone has any experience with this distributor, let me know. Other options include rebuilding mine (at least with new pickup coil), a rebuilt OEM (about the same $$, but would have to buy coil and module seperately), or a name brand aftermarket (Accel, Mallory, etc. - much more $$).
As an aside, I checked the resistance of the plug wires. They were all at least in the same order of magnitude, 1K - 3K ohms. Sound OK? It at least tells me that none of them are cut or burned completely. (They're really only about a year old, with less than 1K miles). I'm assuming plugs are OK - replaced at same time with AC Delco's.
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