Sherman M4A2 Detroit Injectors
 

We’ve got the twin 6-71 Detroit’s out of Bart, one of our M4A2 Sherman’s. After replacing a bunch of seals and gaskets to fix horrible oil leakage, we test ran one of the engines on a stand and got lots of fuel knock and rough running. The injectors on Detroit 2-strokes are activated by additional lobes on the cam that operate rocker arms that depress a plunger in each cylinder injector that generates the 2000-3000 psi pressure required to spray atomized fuel into each cylinder at TDC.
There’s a good test you can do on these engines where you use a screw driver with the engine running to hold each injector plunger down away from the rocker arm so it’s doesn’t move up and down to inject fuel. It’s exactly like pulling a plug wire off a gas engine to monitor the rpm drop at idle.
We did that test and found 2 injectors that made virtually no difference to idle rpm when held down. And a third that made little difference, in other words, there was a problem with those injectors.
We are lucky enough to have a Detroit injector tester tool here, so we tested the worst of the injectors. It was pathetic. Instead of spraying an intense jet of atomized diesel out of each of the six tiny orifices in the tip, it squirted an anaemic solid jet out of each. I suspect that is why we get a lot of fuel knock from this engine as it’s warming up.
Even the injector that gave the most rpm drop was not atomizing the fuel like a new injector did.
I have a bunch of spare parts from N65 injectors but I’m not sure what would cause this. Worn plunger? Plugged tip orifices?
This is the needle-type injector with the needle in the tip that pops open about 2500 psi.
Malcolm

Hi Malcolm,
Are you running these N65's currently or are you using the older type injector?
It is a really good idea to have a fuel pressure gauge inline so you know exactly the fuel pressure going to the injectors and also what your by-pass cuts in at for the fuel pump. Also very critical to have your return line with the .080 thou,
(2mm ) restrictor hole on the end of the return line from the injectors as this holds the pressure for the injector, Cheers Andrew.

A drop of water in the fuel in the can do wonders on these injectors , a good teardown and clean will probably get you going. Great that you can check the spray after assembly, lots of pieces and timing involved. They may be coming back as I saw one on a table beside a Viper fighter, In Batttlestar Galactica.
Post some pics if you get a chance .

If you running the older type , the inj body is narrower and the tip set up is different as seen in the pic.

Regards K Armitage

If you want the best performance from the engines, don't waste your time on the worn injectors. Exchange the old ones for the GM Reliabuilt factory reconditioned ones. A little pricey, but you wont regret it.

Hi, Andrew, yes, these engines both have the brown N65 tag on the injectors. I’m not sure what the “brown” signifies but it seems to be important.
I don’t have a pressure gauge installed on the engine but I did run the fuel pump on the bench and confirmed that the pressure regulator opened at 75 psi, which is in the spec range.
I pulled the elbow fitting on the return gallery and confirmed it does have a restriction orifice. Looks more like the 0.055” orifice listed as one of the options rather than the 0.080” one.
Malcolm

Kirk, I notice from the Detroit manual there are two type on injectors, crown and needle. We have the needle type which I think is the later type. The needle valve pops open only after the injector pressure has reached a high value and gives a better spray pattern, I think.
Talking of spray pattern, I noticed something with our tester. The atomization of the 8 spray tip nozzles depends a lot on how quickly you bang the popping lever down on the injector follower. The faster you bang it down, the more atomization occurs. Considering the speed the injector is opened on a running engine, the atomization is probably better than I am seeing on the tester.
As you suggest, I built an injector from the best of the various parts we have and replaced the bad #3 injector with it. It worked far better and gave a good rpm drop when “shorted”.
Malcolm

Perry, we replaced the bad #1 injector with a Reliabuilt injector we (miraculously) found in stock. And then I replaced the bad #3 with one I rebuilt from parts. Both worked well. And the interesting thing is I found nothing obviously wrong with either of the removed ones except I saw a lot of debris when back flushing the injector inlet filters. I pulled the filter on the #3 injector and found lots of flaky debris that was way bigger than it should be, considering we have a fuel filter after the pump. I suspect a collapsed main filter or a total lack of main filter at some point in its life. So I think our main problem is plugged inlet filters. We need to check the filters in the other poorly-performing injectors.
We’re always a bit strapped for cash so just replacing injectors at around $300 a piece for the measly 5 miles a year we put on this tank is a hard option to sell!
Malcolm,

For those that don’t have a tester, I’ve seen a neat way of doing a quick check on an injector. With the injector in your hand, fill the inlet fitting with WD40 using the red straw. Stroke the follower up and down by hand. When it goes solid on you, give the follower a rap with a rubber mallet. You should see a misting spray from all the spray nozzles.
Of the 6 spare tips I have, I found 3 had at least one one nozzle plugged. The N65 tips have 8 nozzles. These nozzles are only about 0.006” diameter so I don’t have anything to poke them clear with. Detroit had a tool that I guess had a 0.005” wire attached to a handle but it must have been very fragile. And anything driven into the nozzle under 2000 psi pressure ain’t going to move very easily.
Malcolm

Also another little trick I have observed, is that if you have the return line from the injectors returning to the fuel tank right next to the intake line coming from
fuel tank , you will get air and bubbles sucked into the intake line and then onto the injectors , and it will sound like it is running on 5 cylinders and you think there is an injector problem when there is not. So these two pipes need to be apart in the fuel tank , and also make sure the fuel tank is at least 1/4 full. It is very surprising how much fuel returns to the tank via the 2mm restrictor and then into your normal fuel line, it is like a garden hose full on ! Cheers Andrew.

Is an N65 a big enough injector size?

I have a few Injectors from Australian Grant Tanks, and these were the earlier type, but a Number 80. I realise that you can improve the performance, and efficiency (and emissions!) by upgrading to later versions of injectors and also the Cylinder liners, but would a N65 size Injector give equivalent power and performance to an earlier 80? I understand the earlier Injector has a different nozzle pattern that gave less efficient atomisation, but fuel volume per injection cycle is what makes power.

Andrew, yes, I noticed that when test running this engine on the stand. We have a clear return line and saw bubbles occasionally. So we ran with separate supply and return jerry cans.
The tanks fuel tanks are well designed that way. Return comes in at the top of the top tank and has a long path to deaerate before it reaches the pickup at the bottom of the lower tank.
Malcolm

Tony, neat photos!
I checked through the manuals I have but I couldn’t confirm what injectors these tanks came with originally. All I know is the Detroit manual for the 71 series of engines lists the N65 as an option.
I found out today that one of the injectors I removed from this engine was actually an N60. It was the only one with the ID tag on the body missing. Somebody must have installed it hoping it was close enough! The part numbers stamped on the injector tip and the plunger gave it away.

The engine is running much more smoothly, with easier starting and much less vibration, smoke, and eye-watering fumes after replacing these two injectors. #6 is the only one left to be replaced as it doesn’t cause any significant rpm drop when shorted.

Our other Sherman has N95 injectors in both engines. That size isn’t even listed as an option in the Detroit manual! The manual is a lot more modern than the tank so probably emission concerns left it off the option list. A previous member here must have decided, to hell with the smoke, let’s get some real power.
Ironically, it’s no more powerful than this tank, but of course there are a multitude of reasons why that may be so.
Malcolm

From what I have discovered, it is not always that simple to just keep putting bigger injectors in. From the old HV series to N series, things like injector rack have to be changed and compression ratio and type of liners have to all have consideration , as there are several options available, Cheers Andrew.

Interesting, Andrew. Any idea why liners would have to be changed?

I would have thought blower output would be a factor too. Dumping more fuel in would require more air, at least at wide open throttle when the engine doesn’t have excess air. It’s gotta be able to burn all that extra fuel, not just belch it out as black smoke!

Each injector type doesn’t just squirt a controlled amount of fuel in over the throttle range, it controls the timing of the injection too by varying the shape of the helix cutout in the plunger.

Never worked on a diesel before. There’s a lot to learn!

Malcolm

You are right in the fact that more diesel requires more air. There are the standard old liners that had a couple of rows of small holes all the around the liner and then for the bigger injectors there are liners with large e-elongated holes. You can view what type of liner you have by removing one of the little side covers on the side of the engine, and that will indicate whether the engine will appear to have been built matched to the injectors. There are quite a few different configurations that can interchange on these motors , from injectors to liners to different heads, 4 valve to the old "low block" 2 valve head. To get the power with bigger injectors the 4 valve head is needed. Cheers Andrew

And don't forget, too, Malcolm, that despite the presence of the Exhaust Valves in the head, the 6-71 is still a 2 stroke engine and the liners (specifically the port shapes and heights) control the timing of the Intake Cycle and cylinder scavenging, in much the same way as a camshaft controls valve timing.

Over the many years of the GM/Detroit Diesel engines, a wide variety of liners were developed to give different operating parameters, just as many 4 stoke engines had a range of camshafts for differing applications. The 6-71 has been used in Trucks, Earthmoving Plant, Locomotives, Boats, Gensets, Waterpumps (and even Tanks!). Each application had a unique need for power, torque and rpm range. The port shape in the liner is modified to optimise engine output to application.

We’ve got the liners with large holes. We had those airbox covers off to check out the pistons, ring, and bores. Pretty handy!

This tank has 4-valve heads on the original 2-valve blocks. Strangely enough, the other tank with the larger N95 injectors still has the original 2-valve heads.
Malcolm

Ah, yes, good point, Tony. So I should have said above that it’s effectively the ignition timing the varying injectors types can change.

Talking of 2-strokes, we have these two tanks and a bunch of military vehicles with Detroit 6V53’s, also 2-strokes. I knew they should all idle around 500 rpm, but it always sounded to me like they were idling way too fast. My light bulb moment was when I remembered that they were firing twice as fast at 500 rpm as the old rumbly 4 stroke gas engines I was used to!
Great old engines. We’ll never see the likes again.
Malcolm