M-37 not starting opinions please

[Charles Talbert]

Saying 75PSI is not enough to run is quite the blanket statement, especially when you consider how many engines run around 75 brand new. Low compression could certainly make for a harder restart after dying, but I can`t see it causing the sudden shut-down, either.
I would say try to find the procedure for checking the coil with an ommeter, check it that way, and then look into fuel. I would not look to fuel myself, because of the sudden dying, it sounds more like spark to me.

I wonder about the coil also as I mentioned earlier.

Concerning 75PSI, I've worked on many a 230. I've found the rule of thumb with compression is this in these engines. 100PSI is border line, 90 PSI may run, but will be weak, most likely at 90 you will at best get intermitent skipping on the affected cylinder, More likely than not, it will be a dead hole much below that. Anything below 90 surely will not make the cut. Other engines you speak of running 75 as new, well that is not these engines. I've done enough of these to have a hat and t-shirt at least. I'm just trying to offer assitance to the guy with the issue here, 75 psi reading is an issue of major concern. I certainly have no reason to throw him a curve, this is proven factual info that I'm offering. It's now up to him however he choses to proceed. One thing for sure though, if he finds that the coil is the knocking off issue and replaces that, he still will not have a decent engine with compression readings that low. It might be worth mentioning also to be sure the test is being done correctly so accurate results are certain as test results are only as good as the person performing the test with known reliable equipment.

[Joe Flo]

I don't think it is a fuel problem since he said fuel pours out of the vacuum port on the manifold when he tries to restart it. It appears to be an ignition or compression problem. I would also check the grounding strap to the block and the connections at the coil, electronic ignition module and distributer. These parts are all realtively "new" to the engine and there may be some bad wiring here.

Speaking of the vacuum port. Isn't the PCV valve supposed to be hooked up to this? If the ventilation system is plugged off, this could lead to stalling.

The ignition timing should be checked too.

This is getting interesting...

[Gordon Maney]

I wonder about the coil also as I mentioned earlier.

I've found the rule of thumb with compression is this in these engines. 100PSI is border line, 90 PSI may run, but will be weak, most likely at 90 you will at best get intermitent skipping on the affected cylinder, More likely than not, it will be a dead hole much below that.

Variation is a no-no. I agree, an engine with normal compression readings in all cylinders but one having 75 will manifest a dead cylinder, even if it is making some theoretical contribution.

In this case, however, they are equal, and it does start and run, apparently [by what we are told, admittedly all we have to go on] running smoothly and fairly well. No measure of power there, obviously.

Two key issues. They are uniform, and it does start and run. Until it stops. I don't believe the compression suddenly becomes insufficient.

Added postscript:

I read again that he had run it on a separate, gravity feed supply, so yes, it is not a fuel supply issue. My initial outburst, excuse me, response, was more of a generality.

I was thinking about the 75 figure. I read this whole thread again. The first compression reading comments indicated an average of 90. Then when hot, it became 75.

I am wondering how many exhaust valves on this might be having insufficient clearance and opening up when it gets hot.

I am realizing I am now concerned about the drop from average 90 to consistently 75. What is the cause of that change? I struggle to believe every, single cylinder has insufficient clearance on at least one valve of its pair.

Hmmmm....

I think this is a good thread.

[jzimm]

WOW!!!

I've never had so many people talking about me before.

Some clarifications:

The original "avg. compression of 90 psi" was an approx. avg of all cylinders cold. I have a good (NAPA pro series kit) compression tester, but I am doing most of this testing alone, so I sit in the drivers seat and turn the engine over a few times then run out and check the gauge. I checked each cyl 2-3 times to get the 90 avg. There was very little variation among the cylinders. I might be getting a little pressure release from the time I get out of the drivers seat until I read the gauge, but I don't think it's much.

The 75 psi hot was on the 1st 2 cylinders only. Since it was consistant I didn't bother checking the rest.

I agree and understand that the compression may be an issue, but since I know this truck runs well and has good aceleration ( at least in 15 minute spurts), I tend to think the MAIN problem lies elsewhere. I plan on getting a new coil in the near future and trying that next, but the exsisting coil is only a few years old with almost no run time on it. I will also double check the wiring again.


PCV valve: I have the PCV valve, but removed it during testing because when trying to restart the hot engine, I would almost always end up flooding it and the PCV is where the fuel was collecting. I keep a brass plug in during the testing I'm doing.

One additional comment/concern: Should you be able to blow air through the PCV valve both ways? Mine rattles nicely and is clean but I can easily blow through it both ways........seems like it shouldn't be that way.

Thanks again for all the comments concerning my "issues". I consider myself to be a pretty good mechanic having grown up farming and using and working on less than new equipment. Most of what I work on now is diesels and my gas engine troubleshooting is probably a little rusty so I really appreciate the help.

[Gordon Maney]

Ohhhhhhh.........

Compression tests must be done correctly to get measurements that are valid and meaningful. Compression tests must be done to all cylinders.

Remove all spark plugs and leave them out for the duration of the test. I occasionally have heard from folks who took them out, one at a time. It is a good idea to disable the ignition system by disconnecting the battery lead to the distributor.

If you leave the ignition system enabled, and all the plug wires just lying about, the system will develop available voltage for each as you crank, and you [theoretical at best, perhaps, but still....] run the risk of developing carbon tracks on insulating surfaces that will bite you later under load. Whenever possible, I would disable the ignition system. It is just good form.

Prior to performing the test you must wire the throttle open and make sure the choke is wide open.

You want the throttle wide open for several reasons. First, it allows sufficient air to enter the cylinders to get best compression. Second, it prevents the idle circuit from bringing fuel into the cylinders. If fuel was to continue to enter the cylinder you run the risk of washing oil from the cylinder walls and reducing compression.

The reason you want the choke open is the same. If you are cranking with the choke closed, you will be causing a much richer mixture, causing fuel to wash the cylinder walls, reducing compression.

For each cylinder, make sure to utilize the same amount of cranking. For example, do each one for three compression strokes, or four. The point is, do them the same. I can't tell you how many people tell me their readings are the same, when later I learn that they cranked each one as long as they had to in order to get it up as high as it would go. They were equal, except the one got there in three strokes and the other one took 12 strokes.

You might say you don't know how to tell when the compression stroke occurs. Fine. Listen to the sounds. Rur rur rur. [Isn't that great? :-) ] Come up with a pattern for each cylinder that is the same, counting the audible features of the crank. Don't make them long; 4-5 cycles, perhaps, no more.

Do every cylinder, writing down the numbers. If you have one that is low, squirt some motor oil in the spark plug hole with your oil can, then repeat the test. The simple notion is that if it is a ring problem causing it to be low, that may bring it up. If it is a valve problem causing it to be low, it won't bring it up.

Look at your numbers, and look at variation.

If you have 120, 115, 121, 119, 90, 119.

That is way worse than 103, 102, 100, 98, 102, 103.

The first set indicates one weak cylinder. Likely a valve issue, though it could be broken rings.

The second set could be a fairly high mile engine that runs smoothly, but can't produce as much power as it did when new.

You can subtract the low from the high and calculate a percentage variation. My recollection fails me in this moment, but I am thinking you don't want more than 5% variation. I need to dig around and see if my memory is right. Edit: I am finding the figure to be 10%, not 5%.

[Charles Talbert]

Variation is a no-no. I agree, an engine with normal compression readings in all cylinders but one having 75 will manifest a dead cylinder, even if it is making some theoretical contribution.

In this case, however, they are equal, and it does start and run, apparently [by what we are told, admittedly all we have to go on] running smoothly and fairly well. No measure of power there, obviously.

Two key issues. They are uniform, and it does start and run. Until it stops. I don't believe the compression suddenly becomes insufficient.

Added postscript:

I read again that he had run it on a separate, gravity feed supply, so yes, it is not a fuel supply issue. My initial outburst, excuse me, response, was more of a generality.

I was thinking about the 75 figure. I read this whole thread again. The first compression reading comments indicated an average of 90. Then when hot, it became 75.

I am wondering how many exhaust valves on this might be having insufficient clearance and opening up when it gets hot.

I am realizing I am now concerned about the drop from average 90 to consistently 75. What is the cause of that change? I struggle to believe every, single cylinder has insufficient clearance on at least one valve of its pair.

Hmmmm....

I think this is a good thread.

Gordon, you may be onto something about valve lash being a little snug, certainly is a distinct possibility when 15PSI is lost between cold and hot. I've seen this happen in a few, usually when the valves were recently adjusted by someone who was not really in the know concerning valve adjustments. Also saw it a couple of times when someone got mixed up and used the intake lash spec for the exhaust valves.

I'm also in agreement with you concerning the way the test was performed, no way to get good #'s without performing a complete test, and being particular about how it is done.

[Gsmith]

I keep reading this thread and haven't felt as if I could add anything, but if fuel runs out of the manifold anywhere I'd say it's flooding out. Apparently the needle valve isn't doing it's job, or can't hold back the fuel pressure. Does it have a regular fuel pump or electric maybe? I certainly agree with valve adjustment but that should show up in starting to run funny I would think, not just killing it. When you let it run, does it just idle? Have you tried running it with the throttle holding it at maybe 1500 or so rpm? (At least more than just a fast idle just to make it consume the gas it's getting)

[Gordon Maney]

Gordon, you may be onto something about valve lash being a little snug, certainly is a distinct possibility when 15PSI is lost between cold and hot. I've seen this happen in a few, usually when the valves were recently adjusted by someone who was not really in the know concerning valve adjustments. Also saw it a couple of times when someone got mixed up and used the intake lash spec for the exhaust valves.

I'm also in agreement with you concerning the way the test was performed, no way to get good #'s without performing a complete test, and being particular about how it is done.

Yes, and the first sentence could explain the second sentence. :-)

Years ago I had a guy phone me about his Allis Chalmers tractor he had just rebuilt, and the fact that it would not start. The long story short was that all his valves were too tight and he had no compression.

[Gsmith]

I was hoping for an update by now. (I'm sure we all are).

[jzimm]

Sorry. I've had other things to do this week.

I have not touched the valves, so I doubt they are out of adjustment.

I did a correct compression test on a hot engine and got an avg. reading of a little over 100psi with a variation form 95-110.

Engine still dies after about 15-20 minutes and I still have spark after it dies. I'm rebuilding the distributor this week.

[Gsmith]

That's alright, I really meant I was hoping you had the problem resolved. Good luck with it!

[jzimm]

Thanks. I appreciate that. I'm getting pretty tired of this problem myself.

[Gordon Maney]

Sorry. I've had other things to do this week.

I have not touched the valves, so I doubt they are out of adjustment.

I did a correct compression test on a hot engine and got an avg. reading of a little over 100psi with a variation form 95-110.

Engine still dies after about 15-20 minutes and I still have spark after it dies. I'm rebuilding the distributor this week.

Interesting. That is more variation than I would like, but certainly is not the cause of your problem.

[Joe Flo]

"Thanks. I appreciate that. I'm getting pretty tired of this problem myself."

Hang in there, you'll get to the bottom of it.

Joe

[Joe Flo]

"Thanks. I appreciate that. I'm getting pretty tired of this problem myself."

Hang in there, you'll get to the bottom of it.

Joe

Keep us posted if you find anything