Pre-ignition / starting problem. Ripping hair out.

[78 GMC Jimmy]

So I got this Blazer with a 400 motor. No points. Edelbrock intake and a 1406 carb.

Cold start: Perfect.
Hot start: Engine cranks VERY slowly and struggles to turn over for the first few seconds and then BARLEY starts (almost like its nearly hydro locked)
Hot start with distributor disabled: Cranks great and when you flip the switch to activate the distributor and it fires right up.

For what ever reason, its having a pre-ignition problem that I cant solve by turning the distributor. I tried 5, 10, 12, 15, degrees and then just kept spinning the distributor further and further to where a timing light would be useless since the timing marks are long gone. Problem gets worst with lower numbers.

Tried: Adjusting carb, switched to an electric fuel pump (because stock fuel was boiling and bubbling up into the clear fuel filter for 15 minutes with engine off), higher octane gas, different style air filter housing, confirmed that the correct plugs were installed, gapped plugs, turning distributor further and further by hand, cleaning distributor cap and rotor, switched vacuum ports on the 1406, rebuilt carburetor, changed thermostat, improved cooling fan type, carb cleaner in fuel tank, more powerful and faster starter. Problem remains. Some of the items I listed would not effect pre-ignition, but I posted the info anyway as it might be relevant some how.

Any ideas?

Note: cooling system is very dirty, but temps seem good. Seems like previous owner added lots of stop leak. Unrelated to the problem?

 

[Joel Wilson]

Maybe try some additional grounds and or another battery. Everything else you tried, worked for me. Definitely check electrical connections.

 

[78 GMC Jimmy]

Maybe try some additional grounds and or another battery. Everything else you tried, worked for me. Definitely check electrical connections.

Your thinking that its a power supply problem to the starter?

Its worth a shot, but when the rig is cold and the battery is cold, its cranks and starts great. Only has problems when hot.

I would expect the electrical problem to get worse when the battery is cold, but maybe not. I will throw a different battery in it tomorrow and try it out. I will try anything at this point.

 

[Wes Harden]

I think you are correct suspecting the timing. Need to prove the timing marks are correct and the balancer and timing pointer are matched. There are 2 different timing sets. 12 and 2 o'clock. If the outer ring on balancer has spun, then the timing marks useless. You could try timing by vacuum. Use a vacuum gauge reading manifold vacuum. retard the timing until rpm drops and the vacuum is low bouncing needle. Then advance timing till highest vacuum reading, you can stop there or retard till vacuum drops 1 inch. Then adjust idle air screws and idle rpm.

 

[78 GMC Jimmy]

I think you are correct suspecting the timing. Need to prove the timing marks are correct and the balancer and timing pointer are matched. There are 2 different timing sets. 12 and 2 o'clock. If the outer ring on balancer has spun, then the timing marks useless. You could try timing by vacuum. Use a vacuum gauge reading manifold vacuum. retard the timing until rpm drops and the vacuum is low bouncing needle. Then advance timing till highest vacuum reading, you can stop there or retard till vacuum drops 1 inch. Then adjust idle air screws and idle rpm.

I like what your saying. Thanks for the reply. You are the 3rd mechanic to suggest that the timing marks are useless. Me, my retired mechanic friend, and now you suggest that the timing marks might not be any help. My friend suggested that it might be timing chain slop or an aftermarket cam that requires different timing than stock. All I can say for sure is that setting the timing to 12 definitely does not work. So for timing with vacuum - I would like to try that timing method, but I have never tried that before. I get the general idea from your instructions, but some parts are a little hazy. Is there step by step guide? When you say "drops 1 inch" - is that a mark on the gauge or do I need a ruler to measure the needle movement by 1 inch?

 

[Blue85]

I think you are correct suspecting the timing. Need to prove the timing marks are correct and the balancer and timing pointer are matched. There are 2 different timing sets. 12 and 2 o'clock. If the outer ring on balancer has spun, then the timing marks useless.

Exactly what I was going to say. This fight will only be easier if you know the true timing. The only question is why the timing is advanced with a hot engine, but not with a cold one. What year is the Blazer and is it Federal or California? Some have thermal valves that route certain vacuum connections only when the coolant is hot, but I didn't think there was much vacuum during crank(?). Another real possibility is that the advance mechanism inside the distributor is sticking.

Another way to sort the timing is manually turn to #1 TDC using a straw or something in the spark plug hole (are straws banned there? ), then paint a line on the balancer to line up with your timing tab. You can also buy timing tape that gives you a wide range to read, without a dial timing light.

in Hg is a measurement of pressure that comes from a literal 1" change on a column of mercury, but now we have it printed on the gauge, so dropping 1 tick mark:

If for some reason you were connecting a mercurial barometer to the engine, then you would be looking for a literal inch of change with a ruler.

 

[500$k5]

Since you say that the problem gets worse, with smaller numbers, I think that you might actually be using after top dead center retarded timing numbers. This could be caused by a slipped balancer ring, or the wrong balancer installed. I would up towards the higher number, until the problem gets better. Ignore the numbers, and just turn it until the problem goes away. If it gets better, with larger numbers, turn it that direction.

 

[Blue85]

Another simple test would be to disconnect the vacuum advance (and plug the line) to see if the issue is related to vacuum.

 

[78 GMC Jimmy]

So with the vacuum advance plugged in, I turned the distributor and get a maximum of 20 on the vacuum guage. The PCV valve is making a lot or racket. Rapid rattling. When cold ot starts fine. When hot it still struggles to start most of the time. No pinging during acceleration that I can hear. How to I test the distributor to see of the advance is ha going up or something? Should I replace the whole distributor or maybe part it? I have a few spares that I think would fit.

 

[78 GMC Jimmy]

So its running and I shut it off. Restarts very easily. Shut it off and try to restart, barely turns over. But starts after super slow cranking for a few seconds. Shut off and restart with same result. Super slow cranking. Retard the timing a little. Starts great. Then after that it barely starts. WTF is going on with my distributor?

 

[Bent77]

Is this an HEI distributor?

 

[78 GMC Jimmy]

Yes, it's a HEI desitributor. I pulled the cap. The rotor feels spring loaded, so I can use pressure to push it one way, and a spring returns it to the original position. However there is some free play in its natural resting position. You can rattle it back and fourth. No much, but its definitely there. The tip of the rotor moves back and fourth by just under 1/8 inch. So if it were a clock, it freely moves back and fourth between what... 11:55 and 12:00 I guess? What ever 1/8 inch would be. Is this normal and acceptable?

 

[Bent77]

Yeah that’s not awful. Returning means the mechanical advance is working and returning

How suspect is that battery?

 

[78 GMC Jimmy]

Yeah that’s not awful. Returning means the mechanical advance is working and returning

How suspect is that battery?

I mean, I guess if the battery is intermittently supplying different amounts of power, maybe. But when I disable the power wire to the distributor ita cranks over great. Then I plug the distributor back in and it intermittently cranks super slow. How much power does the distributor take? The battery is 8 months old. 840ca/700cca.

 

[Bent77]

For the battery, it’s more that you have been cranking a bunch more, so make sure it’s over 12v and charging

HEIs don’t like low voltage


You want to find the actual top dead center, and then “try” to check valve timing as best you can. If the timing gear skipped, so did the timing

 

[78 GMC Jimmy]

Tried bypassing the vacuum advance - no change. Tried timing using vacuum gauge - no change. Tried moving the distributor all over the place - no help.

How can it crank normally one time, and then 5 seconds later crank slow?

All else aside, it ALWAYS cranks great with the distributor kill switch turned off. So it MUST be per-ignition. It cant be the battery or battery connections. It has to be ignition related. Am I wrong? I am getting pretty fed up working on this thing. I have HOURS working on this problem. And a fair amount of new parts and money thrown at it.

Very frustrating.

Should I just give up and use the distributor kill switch to get it cranking good and then flip the switch on for it to start. Who would have to deal with that every time?

I'm open to any ideas guys.

Hell, I would pay a fee if someone can pinpoint the problem on this site.


Thanks for all your ideas so far guys, keep em coming.

Defective ignition control module? Is that possible? I though they either work or they dont type of thing.


Also, when I pulled the cap today the coil stopped working! No spark after putting the cap back on. Checked wiring, tested voltage, etc. Swapped out the cap/coil and it fired right up! ARG!

 

[Bent77]

You may have lost the center pin off the coil when you removed the cap


It is possibly doing pre-ignition. Or to state it more bluntly, igniting the cylinder so early that the piston is still rising, slowing down the cranking (too much timing, too early)

If the valve events changed due to the timing set moving, then the valves are out of sequence with the piston position, causing more problems


Start by proving TDC, then move to checking the valve open events as best as possible with the known TDC

 

[78 GMC Jimmy]

You may have lost the center pin off the coil when you removed the cap


It is possibly doing pre-ignition. Or to state it more bluntly, igniting the cylinder so early that the piston is still rising, slowing down the cranking (too much timing, too early)

If the valve events changed due to the timing set moving, then the valves are out of sequence with the piston position, causing more problems


Start by proving TDC, then move to checking the valve open events as best as possible with the known TDC

Timing set moving? So you thinking that the timing chain jumped a tooth? Or even assembled incorrectly, throwing off the piston valve relationship?

As for verifying TDC, that's just for verifying the valve positions, right? Since the timing marks don't matter when using a vacuum gauge to set the timing?

Note: Engine starts GREAT every morning. This problem is ONLY present when the engine is hot. The hotter it gets, the worse it gets. Once the thermostat is fully open its almost not able to start unless you kill the distributor and then crank and flip the switch on.

So if it only happens when hot, what does that mean? Could it still be the cam chain thing?

Also, I will check the cap for a center pin, good idea!

 

[bradshawtech]

There’s another variable I’d like to ask, automatic or standard transmission? I had a slow cranking engine whenever I had the clutch pedal pushed in because of a bad thrust bearing situation.

 

[78 GMC Jimmy]

Good suggestion. Mine is a manual. I always start the engine in neutral though.