GM 6.2 and 6.5 injection advance settings

[Jake59]

* correction made for camshaft and crankshaft degrees, the latter being double!!

Hi,

Been searching for some clear and exact data on the injection advance settings for our 6.2 (and 6.5) diesel engines and haven't been able to find anything clarifying, other than"GM never published any advance data"
Perhaps I may have figured it out now...
Been reading in TM 34 about IP overhaul, adjustments and settings on the test bench and also in other GM documentation and here is what came up:

The reference line on top of the flange of the IP is set using the air-timing fixture, and marks the start of injection on cyl #1 +0.5 degrees (between 0.25 and 0.75 degrees is required) , which means 0.5 camshaft degrees advance.
Note that in this position on the IP, the center of the top gear bolt hole on the IP shaft is aligned (- 0.5 camshaft degrees) with the line scribed on the IP.

This information is from TM 34, 3-45. FUEL INJECTOR PUMP CALIBRATION (Con’t), g. CHECKING AND ADJUSTING TIMING MARK, page 246 and doing a number of surrogate air-fixture adjusting manipulations of the IP (without the actual tool, but managing a good match), to determine the actual "begin of high pressure pump action", so with the pump rollers just against the base of the pump lobe.

Next, the driven geart of the IP is set to TDC and this setting defines the position of the other reference line, which is being scribed on top of the distribution chain housing and which theoretically needs to be lined up with the line on the IP.
Looking at the offset on the driven gear, between the TDC hole and the top mounting hole for the gear of the IP, and after a few measurements (not 100% precise, but fairly close) on the gear mounting flange of the IP and some quick circumference and degrees calculations, it appears that the driven gear of the IP is designed so the IP starts building pressure about ~ 9.5 camshaft degrees before TDC or 19 degrees crankshaft BTDC.

This information is from the GM Product Service Training manual 16015.05-1C "The 6.2 liter Diesel Engine", 4B. High Pressure Fuel Delivery System, Static Timing FUEL SYSTEM ADJUSTMENT “MARKING TDC”
ON FRONT HOUSING 6.2L DIESEL ENGINE, using Tool #J33042, page 196.

The total advance thus being 10 camshaft degrees BTDC or 20 degrees crankshaft BTDC and which obviously is subject to cold start advance of 3 to 5 degrees and 1500 RPM and up retarding by 3 to 5 degrees.

Bearing in mind however, that the 20 degrees puts the IP merely at the very beginning of compressing the 2 pistons to send high pressure diesel to any given injector. It may take as much as 4 degrees before pressure has rizen sufficiently to have injection.

From reading information available, it appears that a luminosity probe in the glow plug hole shows that ignition starts/occurs usually 8 degrees later than the signal generated by the piezo receiver which clamps around the #1 high pressure injection line, near the injector (see J-33300-A Tach N Time, the little chart for various diesel engines and advance settings, shows the angle difference between piezo clamp on sensor and luminosity sensor is generally about 8 degrees).

We also need to take into account that if the IP begins to pump high pressure at 20 BTDC, it can take as much as 4 degrees crankshaft rotation, before opening pressure of the injector is reached, (4 degrees is for a Stanadyne IP with injectors set at 2500PSI opening pressure, where ours are set around ~1885PSI), so let's assume it's 2 to 4 degrees on our 6.2.
Also to consider is a few degrees time (lag) for the pump discharge pressure wave to pass through the fuel injection lines and reach the nozzle, good for 4 degrees.

Adding all this up, for our GM 6.2 Diesel, injection is initiated as off 20 degrees BTDC and actual powerstroke may start around 6 degrees or less BTDC...

But the main take away is that we have - or are supposed to have - ~20 degrees advance BTC when the scribed lines on the IP and on the housing are alined.
Any change or deviation from these 20 degrees will mainly be caused by timing chain wear and slack, which will likely explain why adjusting/offsetting the scribed lines for more advance is mostly necessary.

I'll be working on designing/welding a simple DIY tool which allows us to "measure" the deviation from the original scribed lines on our engines, possibly even serving as a reference for advancing the IP accordingly and preferably without having to remove the IP from the engine...

I am aware that these engines respond best to manual advance setting "by feel and ear" which is what I will also be doing, but knowing what the actual factory numbers and settings are and hopefully being able to determine how big our "wear factor" is and which needs to be compensated, is always a good starting point for any adjustment by feeling I would think...

Cheers,
Jake

 

[Sharecropper]

Jake; Thanks for your very informative post. I've been fooling with timing the 6.2 for as long as it has been in existence. What I do is to time the warm engine by feel & ear as you describe, and then run it down the road and back. There is definitely a sweet spot of power and economy, which is usually no more than a matchstick-thickness advance of the factory line. Usually just short of a matchstick width. Anyway, once I find the sweet spot, I then use the timing light driven by the #1 line and see where the timing hits on the crank scale. I write this down in my book so it can be easily repeated if necessary in the future. Having said all this, I have no idea where the sweet spot will be on my new supercharged P400 drivetrain, still on the stand awaiting to be installed. It could be somewhere different, I'll just have to play with it until it hits and runs the best. I have also felt that the temperature of the fuel when it gets to the pump impacts performance. I believe that is the reason the original designers of the engine ran the 3/8" hard line along the head in the valley, to preheat the fuel before routing up the factory filter, with it's own heater, and then to the pump. If the factory heater in the filter becomes inoperative for some reason, in cold weather the fuel may not be at an ideal temperature when entering the pump. I seem to remember an outstanding thread by someone here on SS several years ago about the effects of fuel temperature entering the pump. I wish I had saved that thread.

 

[Jake59]

Jake; Thanks for your very informative post. I've been fooling with timing the 6.2 for as long as it has been in existence. What I do is to time the warm engine by feel & ear as you describe, and then run it down the road and back. There is definitely a sweet spot of power and economy, which is usually no more than a matchstick-thickness advance of the factory line. Usually just short of a matchstick width. Anyway, once I find the sweet spot, I then use the timing light driven by the #1 line and see where the timing hits on the crank scale. I write this down in my book so it can be easily repeated if necessary in the future. Having said all this, I have no idea where the sweet spot will be on my new supercharged P400 drivetrain, still on the stand awaiting to be installed. It could be somewhere different, I'll just have to play with it until it hits and runs the best. I have also felt that the temperature of the fuel when it gets to the pump impacts performance. I believe that is the reason the original designers of the engine ran the 3/8" hard line along the head in the valley, to preheat the fuel before routing up the factory filter, with it's own heater, and then to the pump. If the factory heater in the filter becomes inoperative for some reason, in cold weather the fuel may not be at an ideal temperature when entering the pump. I seem to remember an outstanding thread by someone here on SS several years ago about the effects of fuel temperature entering the pump. I wish I had saved that thread.

Hi,
And thank you!
Curious to hear whereabouts the sweet spot on your truck may be... I would guesstimate, using the piezo sensor on injector #1 fuel line and a timing light, it will flash between 10 and 14 degrees BTDC... ?

Best,
Jake

 

[Jake59]

Hi all,

Been fiddeling with the advance of my IP.
My 1985 M1009 has a DB2-829-4879 which is said to be 1.2cSt specific.
This pump was only released in 1990 and specifcally for HMMWV, so is definitely not the original on my 6.2.

I have been advancing the timing from line thikness all the way to 1/8", with no satisfactory result.
For the first time I have my engine somewhat running as one may expect, although there is still too much rattle in my opinion.
I had to change the advance to nearly 1/16" later or retard timing; i.e. the line on the pump is nearly 1/16" turned toward the passenger side. 1/16" roughly corresponds to 4-5 degrees timing retard on the cranckshaft!

I cannot yet do a road test to see how she handles, because I am not allowed until I have registration, but the exhaust at idle seems quite good... I'll check again tomorrow.

Strangely one would assume that advancing is usually necessary, mainly due to timing chain wear and to compensate for chain slack.
In my case, I had to retard timing considerably.

I mentioned my IP model specifically, because I think that must have something to do with it.
The DB2 4879 is for HMMWV and use in very cold climate environment hence the "1.2cSt" mention.

I'm wondering if this IP model 4879 has perhaps been set with more timing advance than the otherwise standard setting of 0.5 camshaft degrees for our CUCVs, to compensate for use in artic climate situations, where diesel fuel will burn slower due to the lower air temperature and all other elements related to cold climate?

Work in progress...

Jake