I've been having some problems with the 350 in my 70 Cutlass. I feel bad asking since this seems to be a very common thread topic. I have tried many of the things I have read here in other threads going way back, but have not found the cause yet. My case is what I call a hybrid. In my case, the motor will start and idle, and rev up fine. It will even drive a little while just fine. Then, a few sputters, and it dies. Almost like you turned off the key. Then a quick restart and it runs fine again. Maybe for 30 seconds...maybe for 10 minutes, but always the same result...it instantly dies after a random time period. It does seem that this process does not happen when it's cold though. Usually happens after it warms up for 5 min or so. It's tough because every time I've think I fixed it, it will idle and tease me....then die 10 minutes later.

So in the thought of you need two things for a motor to run, fuel and spark, as often noted here, it perplexes me since I have both, and they seem to be working fine for long periods. This motor has been rebuilt and I have been driving it for several years without issues, so this is a new issue with mostly new parts all around. I tried the standard easy fixes so far...new fuel pump and filter, looked for vacuum leaks, checked the vacuum lines, but nothing jumps out. Anyone seen this before?

As to the motor, it is a mildly built Olds 350 with Edelbrock Performer intake and a mild cam, with a Quadrajet on top. It has an MSD HEI distributor and coil, and headers. As part of restoring the car over the years, pretty much everything to and from the motor is new. Fuel tank, sending unit, fuel lines, fuel pump, fuel filter, and the Qjet was new/rebuilt.

Any ideas? Thanks in advance!

 

I was following along until you said this: "they seem to be working fine for long periods" because I thought I had just read you stated the opposite - it runs, sputters & shuts down after maybe 30 seconds or maybe 10 minutes. Therefore, I'm not following your logic here.

IMO, it sounds like a fuel delivery issue - this is just my SWAG but it's where I'd put my money first on diagnosing the issue.

Fuel flow sounds like it's interrupted for whatever reason. Indigestion in the fuel tank and/or lines. Vented or Non-Vented gas cap? Fuel flows until you develop an air pocket which starves the fuel pump/carburetor until it (more or less) burps and then begins flowing again.
Fuel filter (the type with the rubber end plunger) inserted backwards and/or the spring inserted on the wrong side of the fuel filter or there is no spring or a very weak spring? Fuel filter buggered up with some fuel gum while under pressure (during fuel delivery), engine stops, the gum falls of the filter, engine starts, then the gum gets right back into blocking flow again. Carburetor float level adjusted incorrectly or defective float - carburetor fuel bowl fills, engine runs awhile, fuel bowl begins losing fuel, carburetor fuel float hangs and reduces demand for fuel.

 

If you have been reading lately, HEI's have been mentioned a lot. I would also check out the coil and HEI stuff. Perhaps check for loose wires.

 

Next time it dies on you install a new module in the HEI distributor on the side of the road. I bet that fixes your problem. Also, make sure you're using a good pigtail on the distributor to supply 12V IGNition:

 

When was the last time you dropped the gas tank? It's not uncommon for crud in the tank to get sucked up against the sock on the fuel pickup, starving the motor. Of course, once the motor stalls out the crud drops back into the tank and the process repeats. A fuel pressure gauge at the carb inlet would help diagnose this problem.

 

Sorry for that confusion. I was trying to compare to the posts where they will ask you to check to see if you actually have spark, or have fuel. I was trying to note that I do have spark and fuel, for long periods since the engine will start and run.

Thanks for your comments about the fuel items. . I may try the old demo derby trick and run the fuel line straight to the pump from a new gas can, bypassing the tank/sending unit/lines as a test. Seems that heat is also an issue, since I tried again tonight and it ran fine for 20 minutes....then started the random shutting off as it was then hot. Perhaps this is related to the float issue you mention? I have not taken the carb apart yet, but your point of the float is valid. Maybe as it gets hot is not moving as it should, and when its cold. I'll also check the fuel filter again. I have had those rubber check valves get twisted up sometimes. I'll keep looking.....thanks so far

 

Thanks for the HEI comments and the link to the connector. Although I am not sure that would work for my MSD unit? I'll try to attach a picture of the engine so you can see what I'm working with.

I'm thinking about my carb reply earlier and heat. If the engine is getting hot and then making something fail, I could see it possibly in a carb fuel level? But also, is it possible for the heat to impact the coil? I suppose the distributor could be heating up too? Not sure now to check that, but I guess a generic coil would be cost effective test.

 

 

Regarding your statements pertaining to engine heat. If it were the HEI IGN module and/or the distributor, I'd suspect the issue would manifest itself during startup and at all points during engine operation - both cold & hot. With that said however, one item I might suspect is the electric choke. I'm curious if instead of fuel starvation, the choke is closing down & you have a too much fuel condition - enriching the carburetor and causing the sputtering & shut down. When the issue is occurring, you might pull the plug on the choke effectively removing the choke from the equation. Or, you might simply pull the electrical plug entirely on the choke - you should be able to start up the engine w/o the choke (weather is pretty nice here in the Carolinas and doubtful you need the choke right now). You might have to bump & pump the throttle a little to get it started in a cold condition, but it should fire up rather easily. With the choke removed from the equation, take it for a spin and see if it sputters and shuts down with no choke hooked up. You see any smoke emanate from the exhaust during these sputtering shutdown occurrences?

 

I mentioned the HEI module because I've experienced this before with stock GM HEI. When the engine is hot it causes the HEI module in the distributor to fail, solid state electronics... I was able to get the car started again by swapping in a spare HEI module I kept in my glove box. Interestingly, I had the failed HEI module tested later at the auto parts store and it passed.

All the comments above about testing the fuel system are right on track. I'm just letting you know what I've experienced before.

 

I had a similar issue with the factory HEI unit in my 1990 camaro. It would start and run fine for awhile then just die. I would disconnect the battery and let the car sit for awhile an cool off then connect everything back up. And it would start and run for a short time then die. It was in fact a faulty ignition control module. I swapped it out and the car ran fine for about a month then the module failed again. I chopped it up to a cheap replacement part failing and got another one and installed it ran for another two weeks then failed again. So I started to trouble shoot even further because that many modules shouldn't fail like that. So i replaced the wire going from the coil which i also replaced with a new one. To the control module. I also used a silver based heat sink compound used for computer processors and then I never had an issue with it again. Moral of the story is check for shorts and make sure you use good heat sink compound on the control module to transfer the heat away from it.

 

Excellent!

You get thermal paste or dielectric grease when you buy an HEI module but chances are there are better products out there. Here's some stuff I found with a quick search.

 

In the mess of sorting all this out and looking at fuel issues, I had another problem pop up. When trying to start the engine, it wouldn't. I would turn the key and just hear a click from the starter. It would engage the flywheel but not turn. After turning the key off and back on, same thing. Then I wiggled the gear shift (column mounted) and it started fine. I thought maybe it was a one off, but it repeated. SO long and short of that is I replaced the ignition switch, and while upside down and one with the column, I replaced the neutral safety switch too. Unfortunately, that did not fix the problem.

Soooo...looking at the electrical diagrams I found a possible link in all this. I'm wondering if the solenoid is going bad on the starter, with these new problems? And...the link is that the +12V wire for the ignition coil comes from the starter solenoid (the yellow wire on a 1970). Perhaps the solenoid is doing something funny when it heats up, or it has a cracked terminal, etc. And that in turn kills the 12V, even if for a moment, to the coil? Does this sound like something you guys have seen before?

 

The wire from the starter to the coil provides 12 Volts only when cranking the engine, then the voltage is disconnected when the key is turned from Start to Run. That wire is used with the points distributor to provide full 12 Volts while cranking and it is not needed with the HEI distributor, so you can disconnect it to check if it's causing the issue, then leave it disconnected.

 

Just to close the loop on this post, I thought I would note what ended up being the root cause. It was the distributor, and I assume the electronics inside. After trying to work on this for some time, the car eventually stopped running while I had it on, and it never started after that. So I sank the money into a new distributor and presto, it runs. Unfortunately, the ready-to-run MSD distributor is not serviceable, which I did not know when I bought it, so you can not replace the electronics inside. Well, actually you can, but, they do not sell that part. That requires a return to the factory for service. Not sure if I will do that or not. But at least it's running again. Thanks guys for all the comments! - Al

 

I apologize in advance if this topic has already been posted. I couldn't find it via search, but let me know if it exists. I have developed a problem with my 70 Cutlass with a 350, where the engine will stay running when the key is turned off. I have read other posts about ways to test for this issue, and that led me to alternator feedback causing the issue. If I unplug the 2-pin connector from the alternator voltage regulator, the problem goes away. The solution they say is to put a diode in this line to stop the back feed. While I understand the electrical concept, I have two questions...
1) Why does this happen in the first place? Is the alternator/voltage regulator bad?
2) If this is common practice, what size diode should be put in, and on which wire of the 2-wire connector?

Thanks.!

 

What do you mean by “the problem goes away”?

 

This does not happen with stock wiring configurations. I merged your new and old thread together as its pertinent to your problem. Which distributor did you buy as a replacement?

 

Is the alternator and voltage regulator wiring stock right now, or did you replace your alternator with an aftermarket unit?

 

Well I did not know that. I suspect if this were widely known folks would be more inclined to use a factory HEI distributor as replacement parts are readily available from just about every auto parts retailer.

 

When you guys suspect a module is failing from heat related issues, ask the guy using the tester to run the test at least 10 times. I used to test them like this regularly. The module will get hot from being tested, many times the module will start showing failure after the 7th or 8th round.

 

Are you running a MSD Ignition Box?

 

Hey guys...let me see if I can answer all the questions...

VintageChief...When I say the problem goes away, if I unplug the voltage regulator at the alternator, I can then turn the car on and off normally. If I plug it back in, I can not turn the car off, I guess due to backfeed form the regulator.

oldcutlass...I ended up buying the same distributor I had, which is a MSD 8529. After several discussions with their tech group I was able to get a little bit off the price of a new one.

Olds64...The alternator is a newer style GM unit, with the internal voltage regulator, with the 2-pin flat connector. But I kept the original wiring, so the old voltage regulator was removed and a jumper installed. It's a simple retrofit kit...like the one sold here...https://www.speedwaymotors.com/Alter...ss,116911.html

Troys Toy 70...I do not have an ignition box. The nice thing I liked about the 8529 series is that it is 'ready to run' and does not require a box. They say in the paperwork you can use one, but it is not required.



.

 

So to help clarify, in all this process of troubleshooting, it was noted here that the wiring from the starter is not really needed when using a HEI system. So I disconnected the yellow wire form the starter solenoid that used to go to the coil. I also undid the pink/black resistance wire from the ignition that went to the coil as well, since this can prevent a full 12V from going to the coil, and the MSD coil requires a full 12V. I ran a wire direct from the fuseblock that is switched with the key the same as the pink/black wire. I wired the system as recommended by MSD, which you can see in the attached PDF.

So the only link here from the car's electrical system is the wire I'm using from the ignition switch. When I turn the key off, and the engine keeps running, there is 6.25V on this wire, which in my case keeps the relay pulled in and the car running. Soooo I'm not sure if the voltage regulator is bad, or this is a common issue, with this type of wiring, to have to add a diode to prevent it?

 

There is a terminal in the fuse box marked IGN (its a 1/4 male spade), connect your power wire there.

 

I have an MSD box with my MSD distributor. So this is not an apple to apple comparison. However, the diode basically acts as a check valve to only allow current to flow one way . In my case, the MSD box has a constant 12 volts, and it takes very little current from the electrical system to excite the box to send the power to coil and distributor.

The regulator in the alternator regulates maximum but does not set any minimums. Thus, a small amount of current could run through the system to possibly keep the ignition excited.

I know another guy that had a similar problem. I’ll see if I can get ahold of him.

 

So I found a neat way to stop leakage current from the alternator that was causing my issue with not being able to turn the motor off once it got running. Putting a diode in line with the voltage regulator output is one way. And I questioned how to mount this diode exactly, but found this nifty thing on Ebay...



It plugs into the newer style GM alternator voltage regulator, and then your existing wire plugs into the other connector. I saw different versions of this around online, but all for around $5. Provides a neat solution to an annoying problem. With this diode inline, my car now shuts off!

Thanks for all the comments trying to help me figure this out....your guys input is awesome when trying to solve these weird things! - Al