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Hate to say it, but another problem, SBC only starts sometimes

The Griff

The Griff

High drag, low speed
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Well, I hate to start another thread about my annoyances, but, oh well

My Blazer has had a problem for as long as I can remember, It only starts when It feels like it.

I have got all the other problems taken care of, no more ticks or pinging, Idling nice at 1100, and kick down to 750, and feels like it has quite a lot more power, enough to put you back in the seat.

But the same problem I have had since my dad passed away just won't go away.

It only starts when it feels like it.

I have had four straight 90 degree days, so the top is off, and I have been able to drive it once this week. Monday, I go out, and it fires up first try. Tuesday morning, it doesn't feel like it. Tuesday night, Fires up no problems. This morning, Nope. Now, at 8:00 PM, still not having it.


I have never been able to figure out why it is so tempermental. A good friend of mine who completely rebuilt a 94 heavy half Chevy from an $500 plow truck to a brand new truck, says it is just a bad bowtie.

Another good friend who has 3 Chevy trucks, and I helped him on a motor swap in his 72 C20, says it must be a 307 to be this tempermental.

It doesn't matter if it has been run lately or not, On saturday, when I took the top off, I ran around down in the bottoms near the flooded creeks, then went to town to put some gas in it, and when I came back out, it had lost interest in starting. A friend of mine stopped by when he saw me there, and we couldn't figure out what was wrong, so we killed some time trying to figure out why his truck was stuck in 4-hi, (92 F-250, 61,000 miles, it is basically a new truck, but he is a dumbass.) And after about 45 minutes it started up again. I went home, shut if off, and after about 5 minutes, tried it, and it fired up.

It has brand new plugs, (less than 3 months old) a dizzy that is less than a year, it is getting fuel to the carb, the air filter is only a couple of years old.
Like I said, when it does start, it runs very, very good.

I can't figure out what is wrong, It only starts when it feels like it.
 
86 K5

Jasper 350
Rochester 2 barrel
mechanical fuel pump


Tell me what you need to know.
 
What does it do?
Does it do anything when you turn the key? Does it click? Does it turn over and just not fire up?
 
It just sits there turning over, never fires, just cranks and cranks.

Also this is a truck that usually doesn't need any pumping at all, just get in and go.

it floods on about 2 pumps.
 
Ignition module in the dizzy.

I hate to keep throwing that out there for everything, but it is just so often the problem for intermittents like that.
Don't bother trying to test it, just see if you can find a genuine GM one, and be sure to use the heat sink goop under it when you replace it.

It certainly could be something else, coil breaking down, stuff like that, but its so common.......
About the only test that might tell anything, is to check to see if you have spark when it won't crank.
Since it does it cold sometimes, vapor lock is not likely.
 
ive had one do that that was the ignition switch on the collum. the switch was engaging the starter but not allowing spark for some reason.

actually my burb right now has a slight issue where i have to turn it just far enough to turn over, then it starts right up. if i turn it forward hard all the way itll crank until i let up to try again. then starts as Im letting up..:dunno: Im to lazy to fix it:rolleyes:
 
What brand distributor did you buy last year?

shady said:
ive had one do that that was the ignition switch on the collum. the switch was engaging the starter but not allowing spark for some reason.
Click to expand...

Jup, check the power wire going to the distributor. Those times it doesn't start, pull the wire off the distributor and check for voltage. Could be pinched, loose, frayed, corroded, etc., and sometimes it supplies power, sometimes it doesn't.

Though if that were the case, I would assume it would periodically shut the truck off if you're driving and a bump cut power on that wire.

But it does sound electrical related since there's no pattern, like never starts hot, hard start cold.

Fordum said:
Ignition module in the dizzy.

I hate to keep throwing that out there for everything, but it is just so often the problem for intermittents like that.
Don't bother trying to test it, just see if you can find a genuine GM one, and be sure to use the heat sink goop under it when you replace it.
Click to expand...

Yup, or a known good one from a friend's truck, only takes a few minutes to swap it and check. No joke on that head sink, don't forget that step! When I had that problem, it wasn't intermittent, it just gave out. Had a spare distributor swapped in the ignition module and good to go. And yeah, don't bother trying to test, there's only one test you can do at home, and it pretty much proves nothing.

Step One: when it doesn't start, pull a plug wire off a spark plug, shove a short 1/4 extention in the wire, hold it up to the plug and have someone crank, check to see if there's spark.

Step Two: (If no spark is found) Check power wire to distributor (key in on position)

Step Three (If power is found at wire) Coil or Ignition Module, there's checks for the coil, but not really for the ignition module.


If spark is found, or if power to the distributor is not found, we can go from there.
 
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Many Gm's around that vintage would flood easily when you'd go to start it cold,or the choke pull off failed to open the choke enough after starting--then the spark plugs would get gas washed and refuse to fire after stalling,and wont,until the engine either sits long enough to evaporate all the excess fuel,or more often,not start again until all 8 plugs were replaced...Champions used to be the worst for this,but AC plugs do it too,they do seem to recover better ,Champions were "dead" for good usually after being fouled that way..

My friend had a '86 305 that did this repeatedly until I replaced the choke pull off and choke thermostat...some years when GM had plugs gapped wider than .060 with HEI's were very prone to having this happen,and they soon went back to reccomending plugs that were gapped at .045..

The ignition modules are common failures too,for intermittent starting troubles,as well as the pick up coil in the distributor...the wires on them flex every time the vacuum advance moves ,so eventually they can break inside and only make good contact "sometimes"...if its often damp and humid when this happens it could be old spark plug wires letting the spark jump to ground before it gets to the plugs..cracked distributor cap or rotor can do it too..
 
Well, I have checked all of the wiring on the distributor, and after making no progress, I am getting angry.

My inner diesel guy is getting annoyed with these feeble things.
 
It could be many things doing it,you have to determine if it loses spark when it refuses to start...it could be the ignition module as Fordum suggested,or the pick up coil inside the distributor,they often fail when the two wires get frayed inside under the insulation and fail to make contact all the time,they can make it run perfect one minute and dead the next,and go back and forth that way,same as a bad module can...

--and if it loses spark,the plugs will load up with raw fuel,adding to the misery,once they get gas washed like I described before,you can crank that engine till you kill several batteries or the starter,and it wont even poop once..put a new set of 8 plugs of proper heat range in it and see if it fires up,that might be all it needs--if it floods again ,stalls,etc,check for spark right away,if it has spark its a fuel related flooding problem,if not,its the lack of spark..if the plugs get fouled no matter how strong the spark looks,they will not fire usually,or you'll get a lot of spitting back thru the carb or exhaust..

I'd inspect the cap and rotor for carbon tracking or cracks too,those can make the spark jump to ground,rather than fire the plugs too,and it might not do it on a nice dry day,but when its damp,or raining,you wont be able to fire it up...or it might run great in park or neutral,but sputter and stall when you try taking off...I've seen many HEI rotors burn thru under the spring that contacts the coil...make sure someone didn't forget to put the metal ground strap to the coil under the cap too,and the coil mounting screws havent poked thru the cap too..
 
I know that right now it is not getting spark.

And my ignrance is starting to get the better of me, Where is the ignition module at?

The plugs didn't seem fouled, but I would have to check all of them.
 
On Hei the ignition module is under the rotor mounted inside the cap. Has either 2 or 3 wires on each end. One set will be what goes out and plugs into the cap for the coil. It has 2 screws holding it down. Like said before make sure you use heat sink grease between it and the distributor or it won't last anytime at all.
 
The easiest way to test a module on the car is to ensure that the wiring & pickup coil are OK. The module is essentially an amplifier with a built in analog to digital convertor.

Easy tests for HEIsystem:

1. Test for power at the pink BAT terminal. You should have battery voltage w/ the key in the start and RUN positions.

2. Connect the ground side of your test lamp to the battery positive cable. Probe the TACH terminal on the dist. cap while a helper attempts to start the engine. The test lamp should blink repeatedly as the engine cranks. No blink=bad module or pickup coil. Further testing is required to pinpoint the problem. Blink but no spark = bad ignition coil.

3. Remove the cap & rotor. Remove the green & white leads from the module. Connect your ohmmeter to the green & white leads. You should have approx. 800-1500 ohms depending on the ambient temperature. Open circuit (infinite ohms) =bad pickup coil.

Wiggle the green & white leads as you test. Ohm reading should remain constant if the leads are good. If the reading varies as the leads are wiggled, the pickup coil is bad. You'll often find broken pickup coil leads this way.

4. DVOM (meter) still connected to green & white leads. Set your DVOM to AC VOLTS. Have a helper crank the engine as you watch the AC VOLTS reading. A good pickup coil will produce about 3V AC when cranking. Less than approx. 2V AC indicates a bad pickup coil.
 
I swear this truck hates me, I messed with the connections on that module and now we get spark, yay! well, not so fast, now it has decided not to send any fuel to the carb, my little in-line filter is completely empty. It can crank all it wants and no fuel reaches my filter.

It hates me.
 
Are you sure the filter isn't full of fuel and just looks empty? I don't know how messing with an electrical connection could affect a mech pump. It isn't out of gas by chance is it?
 
You are having a lot of the same issues I had with the stock 305 in mine before I decided to just put my new engine in. Try cranking it with the throttle wide open see if it changes anything.
 
My patience is completely gone now.

It is so damn hot out that I can only mess with it for about 15 minutes at a time.

I checked, and whoever said that the in-line filter was actually full, was right, so now I have no focking idea what is wrong.

My friend who said it is a 307 has a wrecked 85 G10 van, with a 305 that hasn't run since 06, and we mess with it for 15 minutes, and it f*cking starts.

he said he would sell it to me for $75.
I'm strongly considering it. My Blazer had a 305 when it was born anyway.

What pisses me off is that the Blazer has no rust, anywhere, only a couple of dents, and the engine should only have 65K on it. and it won't start.
 
If it is in fact a 307,the last year for thase was 1973,and they all had points...someone swapped an HEI distributor in it evidently...if the truck was made before 1974 and had points,you need to run a new wire that gets full 12V from the fuse box that is only energized with the key on...the reason is the point style ignition coil used a special resistance wire to power the coil so it only got about 9v at limited amperage in order to keep the contacts on the points from burning up rapidly...an HEI distributor module wont like running on lower voltage and amps,and it can make it fail in short order,start hard,and have a stumble on acceleration..
This may not apply to your truck,but I thought I'd add this just in case it applies...

I have had several 307 engines and they all ran OK,I know some folks dont like them much,but they are not the "worst" engine chevy ever had..

There are a lot of things that can make an engine start OK,and then refuse too...one straight six my friends uncle had stumped me,all the other professional mechanics and back yard car doctors--the thing would start and run fine for several minutes,then act up--sometimes it would just stall and refuse to fire up,just crank until the battery dies...other times it lost power gradually,then sputtered out..then sometimes it would spit back thru the carb or exhaust,then it would restart instantly and run great again...everything you could replace on the engine was removed,rebuilt,tinkered with,he rebuilt the carb,tried a new fuel pump,filter,ignition coil,module,pick up coil,spark plugs,and he finally had enough and GAVE the truck away to his neighbor..

He took it to an old guy in another town who found the trouble almost instantly...he took the distributor out,and held the rotor still with one hand,and turned the drive gear with his other hand...the little roll pin that held the gear on broke,and sometimes the gear would slip on the shaft,throwing the distributor out of time!...he replaced the roll pin,and the darn thing never gave another lick of trouble the 3 years he used it afterwards...:doah:
 
Well, when it would start it would run fine, very good actually. Just when I shut it off, I have no idea if it will start again.


Thing is, now it hasn't started since Monday night.

over the last weekend, I took the top off, then rode around, and then I went to town and put some gas in it. Came back out from paying, and it wouldn't start. After about 45 minutes it started again. I took it home and couldn't find anything wrong. (it has been tempermental, on and off for about 9 months) Then I drove it to school on Monday, with no problems, it sat for about an hour and a half over at my friend's house (same friend mentioned before) and started up no problem, went home and parked it.
Next day, it woudln't start, and hasn't since, now on Sunday.
 
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