I got some more suggestions from the engine builder today after ripping out more hair. He suggested mixing in some 93 octane, not sure the purpose or thinking behind that, and going to a lower temp thermostat. I’m not sure what’s currently in it, but the thought process is to go to a 160 degree high flow unit in an attempt to keep the engine running cooler. A cooler engine relieves stress on the starter because of less cylinder pressure. He was a bit confused on my testing as well, as some on this board has been confused on results I’m getting with how sometimes there is no problem, and it tests fine, and the next day I test it and it’s there. I’m here to assure everyone I’m not making these results up. It seems to be slow crank more often than not when it’s a hot engine. But I’ve had it do it before cold as well. Voltage drops do fluctuate a bit, but not enough to go above the recommended .5v drop. Advancing or retarding ignition doesn’t seem to help. But I also don’t remember a single instance where when I disconnected the coil lead it had the slow crank.

 

I've put on my orthopedic detective's hat, trying to make sense of why the slow cranking suddenly returned for you today. Heat definitely seems be be a factor, with higher compression making the starter work harder than when cool. There just doesn't seem to much margin between the cranking effort your starter system can supply vs. what is required. If that margin decreases, and demand exceeds supply, the slow crank rears its ugly head.

Then I caught your comment about 20 minutes' worth of cranking today (while cold?), and then the slow crank returned later today with the engine warm. Obviously you didn't grind on the starter for 20 straight minutes, but I'd have to suspect the poor starter's duty cycle was exceeded by a wide margin during that time, causing some heat damage. Was there any mention of the duty cycle included with the new starter's paperwork?

Something changed today, right? With this new starter, isn't this is the first time the slow crank appeared? You had previously mentioned a brief hesitation before the starter kicked in, but I don't remember you mentioning an actual slow crank. Part of troubleshooting is observing changes in symptoms and what preceded them, so this one has me scratching my head.

 

Per the specs I suggested, you're within limits. However, I got to wondering about that and did some searching today. Most sources agreed 0.5v max per leg (positive or negative) was acceptable. However, a few sources said 0.5v COMBINED was the limit.

I've always used 0.5v per leg (or 1.0v combined), but the closer to zero the better. Your two readings add up to 0.639v, so depending which source you prefer, you're over. Since this does seem to be a battle of minute increments, I'd say every teeny bit would help.

 

The 93 octane? I just had a eureka moment while typing. At first I was unsure of this logic, as isn't higher octane fuel harder to ignite? I was going to caution that making the engine start quicker isn't really a fix, but a band-aid. A good starter system, with an adequate reserve of cranking effort, should be able to crank the engine for at least 15 seconds at a shot without much noticeable slowing, regardless of whether the engine actually starts.

So let's say you invented an automatic ether spray system for starting assist, with pretty valves, sensors, etc. It works so well, the instant the starter kicks in, the engine roars to life immediately. You never have to engage the starter for more than a split second. Great, your engine is running, but is your starter system really adequate? That's what I mean by a band-aid of something that makes the engine start more easily, but hasn't really fixed the primary issue of an inadequate starter system.

But then the proverbial light bulb illuminated over my head, just like in the cartoons. Apparently some of the slow cranking is caused by having to fight the extra resistance of the combustion that takes place before TDC. You've tried retarding the timing and that seemed to help with the cranking speed, but then the engine ran poorly otherwise, so there's a practical limit to how much you can retard the timing.

That's when I was hit by the genius of trying some 93 octane, at least as an experiment. The higher octane is harder to ignite, and if my logic is correct, this could theoretically reduce the effort required to spin the engine because it won't ignite quite as readily. You might be able to get higher cranking speed out of your poor starter system. And if the stars align properly, you might get a more reliable start overall if the engine is cranking faster even with less combustible fuel vs. slower cranking with fuel that ignites more easily.

Is this a permanent fix? Not really, but the results could be interesting after trying a couple of tanks of 93 octane. This might give us an idea how best to proceed.

 

Nobody is doubting your findings, well, other than with that defective meter... It's more a case of sharing your frustration.

For the slow crank when cold, that was several Chinese starters and batteries ago, wasn't it? For the most part, we need to concentrate on the current (and recent) symptoms with the existing components.

So I gets to thinking about the hot restart problem and how compression is better as gaps decrease in a warm engine. What happens if you shut off the engine while warm and then try an immediate restart? No problem then, right?

The problem is most prevalent when the engine is warm, but only after sitting shutdown for several minutes, wouldn't you say? Are the piston rings going to seal any better after sitting for just a few minutes? If anything, I'd think the compression seal would decrease slightly without the extreme heat of combustion. Regardless, I don't think the thermal expansion of the piston rings would change drastically in either direction in those few minutes, right at the center of mass of the warm engine block.

Meanwhile, that poor starter is soaking up all that ambient underhood heat, with no cooling airflow from the fan or vehicle movement. My TheoryDuJour(tm) is ambient heat hitting the starter is part of the problem. Try an immediate hot restart vs. sitting for a few minutes and see what happens. If the immediate hot restarts are good, I'd suggest a starter heat shield as part of the solution.

 

The 93 octane won't be harder to ignite it just burns slower and wont self detonate as easliy.

The slower burning may help alleviate the issue. He is not running high compression ratio's either.

The starter/elec system has been gone though to the point it is safe to say that is not the problem.

What I'm wondering is if it a tight engine since it s new rebuild this is a distinct possibility. It would fit the bill. if the engine was blueprinted it would be nice to see the specs for piston to bore clearance that were used and know the piston used.


I had to dig back in my memory to the 1970's as i had seen a similar issue on a 400 Pontiac in a Firebird, and it ended up being tight bores. The shop that rebuilt the engine used the factory specs for piston to bore clearance and not the specs provided by the piston manufacturer. When conditions were just right he would get a slow crank and sometimes slow enough to the point where the engine would barely turn over. Can't recall how many starters he burnt up, cables he melted and batteries he cooked but it was a pile. Eventually it scuffed a piston and broke a skirt it went to another shop for repair/reman and during the tear down they figured out the bores were too tight for the pistons used. It may have been cast piston clearances with forged pistons but I can't recall the specific details.any longer

 

Yes, but with a couple of caveats.

The first is the recent "20 minutes of cranking" which may have overheated the starter. The way I'm seeing the timeline, and please correct me if I'm wrong, the slow cranking didn't return until after this potential starter overheat.

As I mentioned, some sources say 0.5V drop combined total as a maximum, which is tighter than the 0.5V per leg I suggested. As maddening as this problem has been, it might not hurt to investigate a little deeper, especially on the positive leg.





This is an excellent theory, and well worth investigating. Would tight piston clearance fit with this hot restart scenario? Still waiting to hear back about the exact details, specifically if an immediate hot restart is fine, but not if the engine sits for a few minutes. Also, human nature might make this a tough one to investigate, as I suspect the engine builder will say his work was good and the problem must be elsewhere.

 

The engine reaches it highest temps after running then sitting for a few mins.

Once shut off there is no coolant circulating other than by convection, the latent heat in the block is not carried away and the engine will and does reach it max temps a few mins after shut off.. This is why most cooling hoses tend to fail a few mins after shut off. As this is when temps are the highest and pressures the greatest.

If clearances are tight this is when it is most likely to be experienced..
Here is the crappy part of this if it is tight clearances it can be real bitch to replicate. As the temp spread between clearances being to tight to drag the starter and not do it could be only a few degrees.

So the only way to really check for this is to work the engine get it good and hot. Then shut it off wait the few mins and see if you have slow crank. If you can consistently replicate it doing this then a tight engine may be on the table.

If the engine idles for just a few mins this may be just enough to cool the engine down before shut off to prevent the issue.


This thread dug up some old memories that were long filed away in the recesses of my brain.


When my bud's Firebird finally nuked a piston it was only after the cause was found by the engine shop, did the pattern of when this happen was put together.

It mostly happened when the motor had been worked, then almost immediately shut off and had sat for few mins then tried to be restarted like at the gas station, running in to the corner store for smokes (it was the 70's)
stopping at the beer store etc etc.


Almost exactly the same times you would commonly see with a starter heat soak issue. Starter heat soak was a common problem on GM's back in the day and this was long before high torque gear reduction starters were available.

Every tech, gear head, and my self, was convinced this was the problem, he even went so far as to install 2 temp gauges for the starter with one bulb hose clamped to the starter body and one to the solenoid.. This just confused the situation more to be honest as it would slow crank when the starter temp was low and high.
And it would start fine when the starter temp was low and high.


His headers were wrapped the starter was wrapped and this was when you could still get the heavy asbestos wrap, and that stuff worked far better at heat shielding than most of what we have today , plus a heat shield had been fabbed up and installed, 2/0 super flex battery cables installed, the starter grounded direct to mounting lugs, high amp battery, etc etc.

Timing was ruled out as he was able to get it to slow crank with the ignition disabled, he installed a temp toggle switch so he could disable the ignition system during the slow crank and it did not go away.

IIRC he was just about to rule out the starting circuit and consider a mechanical interference issue when the piston went for a dump and was clacking with that tell tail broken skirt noise

I'm not saying this is the issue but it might be something to consider and test for.

 

Ding! Ding! Ding! We have a winner. I've been wracking my poor brain, trying to understand why the hot restart typically only appears at the gas station, but can't be reliably duplicated at home. Then your comment appeared. How about this scenario:

At the gas station, you pull in off the busy street, meaning the engine may have been worked a little bit. Then you shut off the engine right away.

At home, the engine is typically loafing driving through your neighborhood. You may even let the engine idle briefly before shutdown. Any subsequent testing you do at home is only going to be after idling, not working the engine hard.

So other than my previous suggestion to try a different gas station, how about this as an experiment:

Pick a place close beside an open road where you can safely park away from traffic. Work the engine hard, and then shut it down with an absolute minimum of idling. Wait five minutes (or however long it normally takes to fill the tanks) and restart. See if the slow crank returns.

If the fault is now repeatable, you'd still have to deduce exactly why, but you'd be on the right path.

 

LOL, that was my next suggestion. I was even looking on Amazon for a suitable remote thermometer...

 

And yup the trick is to be able to reliable replicate it. If he can get to the point when A and B are done slow crank appears. Then the field of possible causes can be narrowed down and then ruled out one by one.

 

I guess I’m a little late to the party today! First things first, 20 minutes of cranking, obviously not continuous, 10-15 second cranks, about 2 minutes in between.

i will agree, the cold start problem was with the Chinese starters, so the problem at hand is a hot restart.

as far as how tight the engine is, I guess I had no reason to concern myself with writing it down on a proper build sheet, I did use plasti gage on allbearing tolerances and have several pieces of scratch paper I wrote everything down on. The only thing I concerned myself with was returning to STOCK tolerances. The engine bottom end was rebuilt several years ago, and to my disgrace, put back on the weighted flywheel on an internal balanced engine....insert nasty comments here.

engine had about 2k miles before realizing this so I tore it back apart replaced mains and rods, decked heads and reassembled with new cam, valves etc.
The current problem, which I guess we need to focus on, is a hot restart after sitting a few minutes. Immediately after shut down it seems to crank fine, the theory of the heatsoak seems to be prevalent, hence the cooler t-star and higher octane.

fact is after **** off, and sitting a few minutes, it has the sluggish start. I have proved this many times in the past 2days. But that was not driving it either, that was letting it idle and shutting it off, not working it hard, I could only safely assume it would be harder if it was driven and worked hard. I can test that this weekend. I can agree that what I know in the past is irrelevant now to a certain extent, and I need to cross the bridges right in front of me and not dwell on what it did with Chinese starters and bad batteries and diodes in the alternator.

 

“Fact is, after SHUT off”. Not the other word....

maybe it is time to invest in one of those ir thermometers to check the temp of the engine, starter etc?

 

To reiterate, even while the engine is idling for 10-15 minutes, a hot restart happens. My testing limited to time the past few days has had me starting the truck, letting it idle until it was warm/hot, shutting it off. I can do an immediate restart with no problem, but any sort of pause greater than a few minutes is when I get the slow start.

With the cards on the table, I’m going to forget that I had slow cranks at cold engine temps because I have not had that problem since this new setup I’m having. I’m also wanting to rule out electrical. I believe it is safe to say that’s not the issue. IThe problem seems to point at the fact that the engine is hot. Retarding the timing did NOT help starting, and also diminished idle and performance, but one piece of that puzzle is the recurved distributor. If I put the factory one back in, would I notice a difference?

today I’m also letting it warm up to temps (idling, with a few revs here and there) and immediately pulling the distributor cap to see if the advance is sticking. Or anything....

 

Of course I couldn’t replicate it today....

i did reach up and grab the starter body several times, the actual starter body is, believe it or not, cool to the touch at all times. solenoid gets ambient temperature, to a little on the warm side after starting, obviously. It doesn’t seem like it’s getting excessively hot at all compared to the engine.

also, had that awful sound during one startup after sitting a while, so it wasn’t my mind playing tricks on me. it was a combination of grinding metal. I pulled the starter again to see if teeth had sheered off or wear on the starter teeth or flywheel. Didn’t see anything wrong with either, but it almost seemed like that one instance the bendix wasn’t throwing out the starter completely to engage the flywheel. That’s only how I could think it make that sound...