M37 Still Won’t Start – Not Sure What To Look At Next

[m3751]

After months of work I still can’t get my M37 to start and could use some advice.

Prior to doing all this work the vehicle ran but would just quit after 15-30 minutes. (usually at a very inopportune time).
Here’s the parts I’ve changed so far:
1) Gas tank had rust – so removed and had it completely restored
2) Changed the fuel lines as a precaution
3) New Fuel Pump
4) New in-line Fuel Filter
5) New distributor
6) New spark plugs
7) New spark plug cables
New carburetor

So at this point, I know for certain I have fresh clean fuel at the carburetor (I can see it in my in-line glass fuel filter) , I have verified I have high voltage coming from each of the spark plug wires (spark jumps about ¾ of an inch without issue). I thought I may have installed the new distributor backwards to I took it out and rotated the shaft 180 degrees, no change. I guess I am now suspicious that my new carburetor may be at issue – I just don’t know what else it could be. I have a couple of small backfires – not sure if that is any indication of the issue. How can I verify I have a good air/fuel mix making into the cylinders?

 

[AN/GRC-46]

You question says it would run for a while before you did work on it. Are you saying that it now does the same, or that it won't start at all now?

When a vehicle which has all its parts in good working order runs for a while and then just quits, I would suspect that the gas tank is not properly vented. As fuel is used, a partial vacuum would be created in the tank which would be too powerful for the fuel pump to overcome. If it does run at all, try removing the gas cap and see if it will continue running.

A second possibility is that a fuel line is too close to the exhaust manifold, producing a vapor lock after running a while.

Given what you've changed, and that you get a few backfires, I would next suspect the timing is off. First recheck the gap at the points and that the points are actually opening and closing. Then double check that the plug wire order is correct at the distributor and that the right wires are going to the correct plugs.

You might also want to verify that the pushrod that operates the fuel pump is actually pushing. Either it, or the cam might be worn to the point where there is insufficient fuel pressure. You could measure the pressure going into the carb.

Wear on the camshaft could prevent sufficient fuel/air from entering one or more of the cylinders, although it wouldn't commonly affect multiple cylinders. At any rate you might want to check the valve adjustments.

Is the carburetor actually a new unit or is it used? If used, has it been rebuilt? Are you using 87 octane gasoline or something with a high ethenol content? I'm not sure of plastic parts in the fuel pump or carburetor but alcohol can destroy some plastics.

Verify that your choke is operating correctly. Each vehicle seems to have a different procedure for getting the engine to run. Mine required full choke and full throttle while cranking, then immediately pushing the choke full in and holding half throttle until it smoothes out. You might need full choke, half choke or even no choke. You might need too pump the gas or not.

 

[AN/GRC-46]

Xxx

 

[zak]

Verify that the timing is correct. In the MV world the M37 manuals are the best out there. If you don't have one, get it and follow the timing procedure. The advance adjusts from the plate on the bottom of the distributor.

 

[m3751]

The engine used to work (before I started messing with it - I know -- if it ain't broke don't touch it) but was not performing very well, sluggish and hesitant.

The first thing I did about a year ago was to put in new plugs and subsequently a new carburetor from VPW. This seemed to help a bit but I was still having very sluggish performance. I replaced the points with an electronic ignition and this completely solved the sluggish performance and the vehicle worked great for about six months.

Then the issue with the vehicle simply quitting with no forewarning started. It was at this point I started systematically started replacing parts hoping to find the one that was the culprit.

Your suggestion about the fuel line being too close to the exhaust creating a vapor lock may have been the problem, unfortunately now I can’t even get it to start at all.

I’ll recheck the plug wring tonight and verify the order is correct and maybe rotate the distributor shaft 180 degree one more time to make sure it’s not the firing orientation.

I think most of the other suggestions you make I’ve looked at, but probably I should go back and go through each one to make sure something I thought I checked/fixed hasn’t changed. Regarding the fuel, I think it may have up to 5% ethanol, but certainly not more than that.

Thanks for all the good suggestions.

 

[Capt.Marion]

When it quit without warning, was the engine warm when it would quit? A dying coil will frequently quit working when it gets warm. (At least on some vehicles and tractors.)

Have you checked and rechecked the timing of your distributor?

Have you tried removing the air filter and have someone spray starting fluid into the intake while cranking the engine over? A good, steady spray of starting fluid should get your engine to run enough for you to tell whether or not it is a fuel or spark issue.

Are you getting fuel to the fuel pump?

If you are getting fuel at the fuel pump, are you getting fuel at the carburetor? You should be able to check both of these using the fuel pump priming lever.

EDIT: A MAJOR suggestion: systematically replacing parts is not the way to fix the problem. What you should have done is started researching and trouble-shooting your problem, ultimately isolating the problem's cause and fixing that issue. By replacing a bunch of parts, you introduce that many more variables into your problem.

 

[ranger.41]

m3751,
If you have a military distributor the drive tang is offset to one side and can only go in one way.
ranger.41

 

[ranger.41]

m3751,
How is the compression on your engine? I now it ran before, but how well? If it is low a new carb,is not going to help.
You installed a new dist, if it is the military dist, the coil in inside it. It should have the cooling lines attached to the dist.
If you are removing the dist, the correct firing order is 1,5,3,6,2,4.
ranger.41

 

[Oldfart]

m3751,
If you have a military distributor the drive tang is offset to one side and can only go in one way.
ranger.41

It sounds like an aftermarket distributor so I am guessing either the oil pump has been replaced with a civilian pump or it is a civilian engine used to replace the military Dodge 230.

 

[glcaines]

Start with the basics.
1. Confirm correct valve timing
2. Confirm correct ignition timing
3. Spray starter fluid or gasoline into throat of carburetor while cranking

If 1 - 2 are correct, the engine will run if it has fuel. If it runs after #3, you have a fuel problem. Then confirm that the fuel pump is pumping if the float bowl is full of fuel, you have a carburetor problem. Troubleshoot the problem - don't replace any parts until you have isolated the problem.

 

[Capt.Marion]

Start with the basics.
1. Confirm correct valve timing

??? How would one change the valve timing on a T245?

Do you mean to check the compression to verify that the valves are closing properly?

 

[littlebob]

Check the fuel itself. We got a golf cart in the shop today that had already been looked at by a few other shops.
Owner said it ran out of gas and they refilled it. It hasn't run since. Contaminated with water, runs fine with an alternate fuel tank.

 

[Oldfart]

If the distributor can be stabbed 180 out, something is not military. Assuming the engine is a vintage Chrysler (now called MoPar) 6 cyl flat head, Top Dead Center can be determined by removing the pipe plug over the center of the #6 cylinder. Use a piece of gas welding rod or something similar, and long enough to reach through the hole and rest on the top of the #6 piston. Rotate the engine until the wire is as far out of the hole as it will go. That should be TDC.

 

[plym49]

First, confirm the distributor timing since you spun it and it does not run. Let's be sure it is right. Take out the #1 plug and slowly spin the motor with your finger in the hole. When you start to feel compression, that hole is on the compression stroke. Now line up the timing marks. Now you are at TDC on the compression stroke on #1 cylinder. Now we have a frame of reference.

Remove the cap and see which terminal the rotor is pointing to. If it is not exactly pointing right, lossen and rotate the dizzy until it is. That is now your #1 position on the cap (and it does not matter which one it is).

Now notice which way your rotor spins as you crank the motor. Wire the remaining plugs according to the firing order: 1-5-3-6-2-4.

Now see if it starts. If it does not, leave the dizzy alone.

Items to check are the condensor, the points clean and square, the points gap around .020 (you can set them by eye - they should just open a bit. Not too wide, not barely at all). Make sure the pigtail wire from the points to the coil is not broken (open) or shorting to ground.

With this done you should have a decent spark at the plugs and even a little spark at the points. If this does not happen, let us know and we will move on to Phase 2.

John

 

[natem]

Check your coil,a bad coil can work well cold. When it quits running pull the cap and you can hear it sizzle, like fried eggs.

Nate

 

[glcaines]

??? How would one change the valve timing on a T245?

Do you mean to check the compression to verify that the valves are closing properly?

M37s have a timing chain and any vehicle with a timing chain can skip a tooth if the chain or sprockets are worn. I have had this happen on two different vehicles. It's easy enough to check and eliminates one possibility. You said you had spark, so that eliminates everything in the ignition system, so check the ignition timing as well. However, check the ignition timing after confirming the valve timing is correct. If you happen to find the valve timing is off, don't correct the problem by moving the chain to the correct position. Replace the chain and sprockets.

 

[m3751]

OK - Well today I started systematically going through all the various suggestions, and it looks like my number one issue is a major problem with compression (or the lack thereof). I measured each of the cylinders and the results were really disappointing.

Cylinder 1 90 psi
Cylinder 2 70 psi
Cylinder 3 90 psi
Cylinder 4 60 psi
Cylinder 5 70 psi
Cylinder 6 60 psi

According to the manual it should be between 90 and 130 psi with no more than 10 psi variation.

Wow – time for a big decision, pull the engine and rebuild it or to perform the operation outlined in Chapter 4 Section 2 paragraph 290 of TM 9-8030.

 

[Capt.Marion]

I'd just pop the cylinder head off and replace the head gasket, and then try the compression check again.

I seem to recall that these engines are bad about leaking around/blowing head gaskets. You should be able to see soot and maybe water where the head gasket is leaking if that is the case.

If anything, replacing the head gasket is a good thing to do, even if it doesn't fully solve your compression problem. (If anything it will eliminate one major variable.)

 

[91W350]

Have you tried pull starting it? Sometimes the higher rpm of being towed will get it to fire, especially if the compression is weak. Not very M-37 savvy, but I have worked on a few Continental flat heads. They could be very tough to start with low compression. It seems worse after sitting for extended periods.

I agree with the others that originally it was probably a coil or vapor lock causing it to die while running. Been a while since I over hauled a flat motor, the last one was a Chrysler or Mopar six in a Clark airplane tug. Glen

 

[natem]

Add a little oil to each cylinder and try your compression test again. If it has been sitting without running for some time your rings have dried out and are not forming a good seal. (best case) Also how long do they take to leak down?
Is there coolant in oil?
Is there oil in coolant?
Have you used the proper verbiage while trying to start?

Nate